wdiff rfc8489.original rfc8489.txt

TRAM
Internet Engineering Task Force (IETF) M. Petit-Huguenin
Internet-Draft
Request for Comments: 8489 Impedance Mismatch
Obsoletes: 5389 (if approved) G. Salgueiro
Intended status:
Category: Standards Track Cisco
Expires: September 22, 2019
ISSN: 2070-1721 J. Rosenberg
Five9
D. Wing
Citrix
R. Mahy
Unaffiliated
P. Matthews
Nokia
March 21,
December 2019

Session Traversal Utilities for NAT (STUN)
draft-ietf-tram-stunbis-21

Abstract

Session Traversal Utilities for NAT (STUN) is a protocol that serves
as a tool for other protocols in dealing with Network Address
Translator (NAT) NAT traversal. It can
be used by an endpoint to determine the IP address and port allocated
to it by a NAT. It can also be used to check connectivity between
two endpoints, endpoints and as a keep-alive protocol to maintain NAT bindings.
STUN works with many existing NATs, NATs and does not require any special
behavior from them.

STUN is not a NAT traversal solution by itself. Rather, it is a tool
to be used in the context of a NAT traversal solution.

This document obsoletes RFC 5389.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 22, 2019.
https://www.rfc-editor.org/info/rfc8489.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 4
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 6 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8 7
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 8 7
5. STUN Message Structure . . . . . . . . . . . . . . . . . . . 10 9
6. Base Protocol Procedures . . . . . . . . . . . . . . . . . . 12 11
6.1. Forming a Request or an Indication . . . . . . . . . . . 12 11
6.2. Sending the Request or Indication . . . . . . . . . . . . 13 12
6.2.1. Sending over UDP or DTLS-over-UDP . . . . . . . . . . 14 13
6.2.2. Sending over TCP or TLS-over-TCP . . . . . . . . . . 15 14
6.2.3. Sending over TLS-over-TCP or DTLS-over-UDP . . . . . 16 15
6.3. Receiving a STUN Message . . . . . . . . . . . . . . . . 17 16
6.3.1. Processing a Request . . . . . . . . . . . . . . . . 18 17
6.3.1.1. Forming a Success or Error Response . . . . . . . 18 17
6.3.1.2. Sending the Success or Error Response . . . . . . 19 18
6.3.2. Processing an Indication . . . . . . . . . . . . . . 19 18
6.3.3. Processing a Success Response . . . . . . . . . . . . 20 19
6.3.4. Processing an Error Response . . . . . . . . . . . . 20 19
7. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 21 20
8. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 21 20
8.1. STUN URI Scheme Semantics . . . . . . . . . . . . . . . . 22 21
9. Authentication and Message-Integrity Mechanisms . . . . . . . 23 22
9.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 23
9.1.1. HMAC Key . . . . . . . . . . . . . . . . . . . . . . 24 23
9.1.2. Forming a Request or Indication . . . . . . . . . . . 24 23
9.1.3. Receiving a Request or Indication . . . . . . . . . . 24 23
9.1.4. Receiving a Response . . . . . . . . . . . . . . . . 25
9.1.5. Sending Subsequent Requests . . . . . . . . . . . . . 26 25
9.2. Long-Term Credential Mechanism . . . . . . . . . . . . . 26 25
9.2.1. Bid Down Bid-Down Attack Prevention . . . . . . . . . . . . . 28 27
9.2.2. HMAC Key . . . . . . . . . . . . . . . . . . . . . . 28 27
9.2.3. Forming a Request . . . . . . . . . . . . . . . . . . 29 28
9.2.3.1. First Request . . . . . . . . . . . . . . . . . . 29 28
9.2.3.2. Subsequent Requests . . . . . . . . . . . . . . . 29 28
9.2.4. Receiving a Request . . . . . . . . . . . . . . . . . 30 29
9.2.5. Receiving a Response . . . . . . . . . . . . . . . . 32 31
10. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . 33 32
11. Backwards Compatibility with RFC 3489 . . . . . . . . . . . . 34 33
12. Basic Server Behavior . . . . . . . . . . . . . . . . . . . . 35 33
13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 36 34
14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 37 36
14.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 38 37
14.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 39 37
14.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 40 38
14.4. USERHASH . . . . . . . . . . . . . . . . . . . . . . . . 40 39
14.5. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . 40
14.6. MESSAGE-INTEGRITY-SHA256 . . . . . . . . . . . . . . . . 41 40
14.7. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . 42 41
14.8. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 42
14.9. REALM . . . . . . . . . . . . . . . . . . . . . . . . . 44 43
14.10. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . 44
14.11. PASSWORD-ALGORITHMS . . . . . . . . . . . . . . . . . . 44
14.12. PASSWORD-ALGORITHM . . . . . . . . . . . . . . . . . . . 45
14.13. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 46 45
14.14. SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . . 46
14.15. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 46
14.16. ALTERNATE-DOMAIN . . . . . . . . . . . . . . . . . . . . 47 46
15. Operational Considerations . . . . . . . . . . . . . . . . . 47
16. Security Considerations . . . . . . . . . . . . . . . . . . . 47
16.1. Attacks against the Protocol . . . . . . . . . . . . . . 47
16.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . 47
16.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . 48
16.1.3. Bid-Down Attacks . . . . . . . . . . . . . . . . . . 49 48
16.2. Attacks Affecting the Usage . . . . . . . . . . . . . . 51 50
16.2.1. Attack I: Distributed DoS (DDoS) against a Target . 51
16.2.2. Attack II: Silencing a Client . . . . . . . . . . . 52 51
16.2.3. Attack III: Assuming the Identity of a Client . . . 52
16.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . 52
16.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . 52
17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 53
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54 53
18.1. STUN Security Features Registry . . . . . . . . . . . . 54 53
18.2. STUN Methods Registry . . . . . . . . . . . . . . . . . 54
18.3. STUN Attribute Attributes Registry . . . . . . . . . . . . . . . . 54
18.3.1. Updated Attributes . . . . . . . . . . . . . . . . . 54 55
18.3.2. New Attributes . . . . . . . . . . . . . . . . . . . 55
18.4. STUN Error Code Codes Registry . . . . . . . . . . . . . . . . 55 56
18.5. STUN Password Algorithm Algorithms Registry . . . . . . . . . . . . 55 56
18.5.1. Password Algorithms . . . . . . . . . . . . . . . . 56
18.5.1.1. MD5 . . . . . . . . . . . . . . . . . . . . . . 56
18.5.1.2. SHA-256 . . . . . . . . . . . . . . . . . . . . 56 57
18.6. STUN UDP and TCP Port Numbers . . . . . . . . . . . . . 56 57
19. Changes Since since RFC 5389 . . . . . . . . . . . . . . . . . . . 57
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 57 58
20.1. Normative References . . . . . . . . . . . . . . . . . . 57 58
20.2. Informative References . . . . . . . . . . . . . . . . . 60
Appendix A. C Snippet to Determine STUN Message Types . . . . . 62 63
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 63 64
B.1. Sample Request with Long-Term Authentication with
MESSAGE-INTEGRITY-SHA256 and USERHASH . . . . . . . . . . 63
Appendix C. Release notes 64
Acknowledgements . . . . . . . . . . . . . . . . . . . 65
C.1. Modifications between draft-ietf-tram-stunbis-21 and
draft-ietf-tram-stunbis-20 . . . . . 65
Contributors . . . . . . . . . . 65
C.2. Modifications between draft-ietf-tram-stunbis-20 and
draft-ietf-tram-stunbis-19 . . . . . . . . . . . . . . . 65
C.3. Modifications between draft-ietf-tram-stunbis-19 and
draft-ietf-tram-stunbis-18 . . . . . . . . . . . . . . . 65
C.4. Modifications between draft-ietf-tram-stunbis-18 and
draft-ietf-tram-stunbis-17 . . . . . . . . . . . . . . . 65
C.5. Modifications between draft-ietf-tram-stunbis-17 and
draft-ietf-tram-stunbis-16 . . . . . . . . . . . . . . . 65
C.6. Modifications between draft-ietf-tram-stunbis-16 and
draft-ietf-tram-stunbis-15 . . . . . . . . . . . . . . . 65
C.7. Modifications between draft-ietf-tram-stunbis-15 and
draft-ietf-tram-stunbis-14 . . . . . . . 66
Authors' Addresses . . . . . . . . 66
C.8. Modifications between draft-ietf-tram-stunbis-14 and
draft-ietf-tram-stunbis-13 . . . . . . . . . . . . . . . 66
C.9. Modifications between draft-ietf-tram-stunbis-13

1. Introduction

The protocol defined in this specification, Session Traversal
Utilities for NAT (STUN), provides a tool for dealing with Network
Address Translators (NATs). It provides a means for an endpoint to
determine the IP address and
draft-ietf-tram-stunbis-12 . . . . . . . . . . . . . . . 67
C.10. Modifications between draft-ietf-tram-stunbis-12 port allocated by a NAT that corresponds
to its private IP address and
draft-ietf-tram-stunbis-11 . . . . . . . . . . . . . . . 67
C.11. Modifications port. It also provides a way for an
endpoint to keep a NAT binding alive. With some extensions, the
protocol can be used to do connectivity checks between draft-ietf-tram-stunbis-11 and
draft-ietf-tram-stunbis-10 . . . . . . . . . . . . . . . 67
C.12. Modifications two endpoints
[RFC8445] or to relay packets between draft-ietf-tram-stunbis-10 two endpoints [RFC5766].

In keeping with its tool nature, this specification defines an
extensible packet format, defines operation over several transport
protocols, and
draft-ietf-tram-stunbis-09 . . . . . . . . . . . . . . . 68
C.13. Modifications between draft-ietf-tram-stunbis-09 and
draft-ietf-tram-stunbis-08 . . . . . . . . . . . . . . . 68
C.14. Modifications between draft-ietf-tram-stunbis-08 and
draft-ietf-tram-stunbis-07 . . . . . . . . . . . . . . . 69
C.15. Modifications between draft-ietf-tram-stunbis-07 and
draft-ietf-tram-stunbis-06 . . . . . . . . . . . . . . . 69
C.16. Modifications between draft-ietf-tram-stunbis-06 and
draft-ietf-tram-stunbis-05 . . . . . . . . . . . . . . . 69
C.17. Modifications between draft-ietf-tram-stunbis-05 and
draft-ietf-tram-stunbis-04 . . . . . . . . . . . . . . . 69
C.18. Modifications between draft-ietf-tram-stunbis-04 and
draft-ietf-tram-stunbis-03 . . . . . . . . . . . . . . . 70
C.19. Modifications between draft-ietf-tram-stunbis-03 and
draft-ietf-tram-stunbis-02 . . . . . . . . . . . . . . . 70
C.20. Modifications between draft-ietf-tram-stunbis-02 and
draft-ietf-tram-stunbis-01 . . . . . . . . . . . . . . . 70
C.21. Modifications between draft-ietf-tram-stunbis-01 and
draft-ietf-tram-stunbis-00 . . . . . . . . . . . . . . . 71
C.22. Modifications between draft-salgueiro-tram-stunbis-02 and
draft-ietf-tram-stunbis-00 . . . . . . . . . . . . . . . 72
C.23. Modifications between draft-salgueiro-tram-stunbis-02 and
draft-salgueiro-tram-stunbis-01 . . . . . . . . . . . . . 72
C.24. Modifications between draft-salgueiro-tram-stunbis-01 and
draft-salgueiro-tram-stunbis-00 . . . . . . . . . . . . . 72
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 72
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 73

1. Introduction

The protocol defined in this specification, Session Traversal
Utilities for NAT, provides a tool for dealing with NATs. It
provides a means for an endpoint to determine the IP address and port
allocated by a NAT that corresponds to its private IP address and
port. It also provides a way for an endpoint to keep a NAT binding
alive. With some extensions, the protocol can be used to do
connectivity checks between two endpoints [RFC8445], or to relay
packets between two endpoints [RFC5766].

In keeping with its tool nature, this specification defines an
extensible packet format, defines operation over several transport
protocols, and provides for two forms of authentication.

STUN is intended to be used in the context of one or more NAT
traversal solutions. These solutions are known as STUN usages. Each
usage describes how STUN is utilized to achieve the NAT traversal
solution. Typically, a usage indicates when STUN messages get sent,
which optional attributes to include, what server is used, and what
authentication mechanism is to be used. Interactive Connectivity
Establishment (ICE) [RFC8445] is one usage of STUN. SIP Outbound
[RFC5626] is another usage of STUN. In some cases, a usage will
require extensions to STUN. A STUN extension can be in the form of
new methods, attributes, or error response codes. More information
on STUN usages can be found in Section 13.

2. Overview of Operation

This section is descriptive only.

/-----\
// STUN \\
| Server |
\\ //
\-----/

+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+

+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+

/-----\
// STUN \\
| Client |
\\ // Private NET 1
\-----/

Figure 1: One Possible STUN Configuration

One possible STUN configuration is shown in Figure 1. In this
configuration, there are two entities (called STUN agents) that
implement the STUN protocol. The lower agent in the figure is the
client, and is connected to private network 1. This network connects
to private network 2 through NAT 1. Private network 2 connects to
the public Internet through NAT 2. The upper agent in the figure is
the server, and resides on the public Internet.

STUN is a client-server protocol. It supports two types of
transactions. One is a request/response transaction in which a
client sends a request to a server, and the server returns a
response. The second is an indication transaction in which either
agent -- client or server -- sends an indication that generates no
response. Both types of transactions include a transaction ID, which
is a randomly selected 96-bit number. For request/response
transactions, this transaction ID allows the client to associate the
response with the request that generated it; for indications, the
transaction ID serves as a debugging aid.

All STUN messages start with a fixed header that includes a method, a
class, and the transaction ID. The method indicates which of the
various requests or indications this is; this specification defines
just one method, Binding, but other methods are expected to be
defined in other documents. The class indicates whether this is a
request, a success response, an error response, or an indication.
Following the fixed header comes zero or more attributes, which are
Type-Length-Value extensions that convey additional information for
the specific message.

This document defines a single method called Binding. The Binding
method can be used either in request/response transactions or in
indication transactions. When used in request/response transactions,
the Binding method can be used to determine the particular "binding"
a NAT has allocated to a STUN client. When used in either request/
response or in indication transactions, the Binding method can also
be used to keep these "bindings" alive.

In the Binding request/response transaction, a Binding request is
sent from a STUN client to a STUN server. When the Binding request
arrives at the STUN server, it may have passed through one or more
NATs between the STUN client and the STUN server (in Figure 1, there
were two such NATs). As the Binding request message passes through a
NAT, the NAT will modify the source transport address (that is, the
source IP address and the source port) of the packet. As a result,
the source transport address of the request received by the server
will be the public IP address and port created by the NAT closest to
the server. This is called a reflexive transport address. The STUN
server copies that source transport address into an XOR-MAPPED-
ADDRESS attribute in the STUN Binding response and sends the Binding
response back to the STUN client. As this packet passes back through
a NAT, the NAT will modify the destination transport address in the
IP header, but the transport address in the XOR-MAPPED-ADDRESS
attribute within the body of the STUN response will remain untouched.
In this way, the client can learn its reflexive transport address
allocated by the outermost NAT with respect to the STUN server.

In some usages, STUN must be multiplexed with other protocols (e.g.,
[RFC8445], [RFC5626]). In these usages, there must be a way to
inspect a packet and determine if it is a STUN packet or not. STUN
provides three fields in the STUN header with fixed values that can
be used for this purpose. If this is not sufficient, then STUN
packets can also contain a FINGERPRINT value, which can further be
used to distinguish the packets.

STUN defines a set of optional procedures that a usage can decide to
use, called mechanisms. These mechanisms include DNS discovery, a
redirection technique to an alternate server, a fingerprint attribute
for demultiplexing, and two authentication and message-integrity
exchanges. The authentication mechanisms revolve around the use of a
username, password, and message-integrity value. Two authentication
mechanisms, the long-term credential mechanism and the short-term
credential mechanism, are defined in this specification. Each usage
specifies the mechanisms allowed with that usage.

In the long-term credential mechanism, the client and server share a
pre-provisioned username and password and perform a digest challenge/
response exchange inspired by (but differing in details) to the one
defined for HTTP [RFC7616]. In the short-term credential mechanism,
the client and the server exchange a username and password through
some out-of-band method prior to the STUN exchange. For example, in
the ICE usage [RFC8445] the two endpoints use out-of-band signaling
to exchange a username and password. These are used to integrity
protect and authenticate the request and response. There is no
challenge or nonce used.

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.

4. Definitions

STUN Agent: A STUN agent is an entity that implements the STUN
protocol. The entity can be either a STUN client or a STUN
server.

STUN Client: A STUN client is an entity that sends STUN requests and
receives STUN responses and STUN indications. A STUN client can
also send indications. In this specification, the terms STUN
client and client are synonymous.

STUN Server: A STUN server is an entity that receives STUN requests
and STUN indications, and sends STUN responses. A STUN server can
also send indications. In this specification, the terms STUN
server and server are synonymous.

Transport Address: The combination of an IP address and port number
(such as a UDP or TCP port number).

Reflexive Transport Address: A transport address learned by a client
that identifies that client as seen by another host on an IP
network, typically a STUN server. When there is an intervening
NAT between the client and the other host, the reflexive transport
address represents the mapped address allocated to the client on
the public side of the NAT. Reflexive transport addresses are
learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
MAPPED-ADDRESS) in STUN responses.

Mapped Address: Same meaning as reflexive address. This term is
retained only for historic reasons and due to the naming of the
MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.

Long-Term Credential: A username and associated password that
represent a shared secret between client and server. Long-term
credentials are generally granted to the client when a subscriber
enrolls in a service and persist until the subscriber leaves the
service or explicitly changes the credential.

Long-Term Password: The password from a long-term credential.

Short-Term Credential: A temporary username and associated password
that represent a shared secret between client and server. Short-
term credentials are obtained through some kind of protocol
mechanism between the client and server, preceding the STUN
exchange. A short-term credential has an explicit temporal scope,
which may be based on a specific amount of time (such as 5
minutes) or on an event (such as termination of a Session
Initiation Protocol (SIP [RFC3261]) dialog). The specific scope
of a short-term credential is defined by the application usage.

Short-Term Password: The password component of a short-term
credential.

STUN Indication: A STUN message that does not receive a response.

Attribute: The STUN term for a Type-Length-Value (TLV) object that
can be added to a STUN message. Attributes are divided into two
types: comprehension-required and comprehension-optional. STUN
agents can safely ignore comprehension-optional attributes they
don't understand, but cannot successfully process a message if it
contains comprehension-required attributes that are not
understood.

RTO: Retransmission TimeOut, which defines the initial period of
time between transmission of a request and the first retransmit of
that request.

5. STUN Message Structure

STUN messages are encoded in binary using network-oriented format
(most significant byte or octet first, also commonly known as big-
endian). The transmission order is described in detail in Appendix B
of [RFC0791]. Unless otherwise noted, numeric constants are in
decimal (base 10).

All STUN messages comprise a 20-byte header followed by zero or more
Attributes. The STUN header contains a STUN message type, message
length, magic cookie, and transaction ID.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0| STUN Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Transaction ID (96 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Format of STUN Message Header

The most significant 2 bits of every STUN message MUST be zeroes.
This can be used to differentiate STUN packets from other protocols
when STUN is multiplexed with other protocols on the same port.

The message type defines the message class (request, success
response, error response, or indication) and the message method (the
primary function) of the STUN message. Although there are four
message classes, there are only two types of transactions in STUN:
request/response transactions (which consist of a request message and
a response message) and indication transactions (which consist of a
single indication message). Response classes are split into error
and success responses to aid in quickly processing the STUN message.

The message type field is decomposed further into the following
structure:

0 1
2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
|M |M |M|M|M|C|M|M|M|C|M|M|M|M|
|11|10|9|8|7|1|6|5|4|0|3|2|1|0|
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Format of STUN Message Type Field

Here the bits in the message type field are shown as most significant
(M11) through least significant (M0). M11 through M0 represent a
12-bit encoding of the method. C1 and C0 represent a 2-bit encoding
of the class. A class of 0b00 is a request, a class of 0b01 is an
indication, a class of 0b10 is a success response, and a class of
0b11 is an error response. This specification defines a single
method, Binding. The method and class are orthogonal, so that for
each method, a request, success response, error response, and
indication are possible for that method. Extensions defining new
methods MUST indicate which classes are permitted for that method.

For example, a Binding request has class=0b00 (request) and
method=0b000000000001 (Binding) and is encoded into the first 16 bits
as 0x0001. A Binding response has class=0b10 (success response) and
method=0b000000000001, and is encoded into the first 16 bits as
0x0101.

Note: This unfortunate encoding is due to assignment of values in
[RFC3489] that did not consider encoding Indications, Success, and
Errors using bit fields.

The magic cookie field MUST contain the fixed value 0x2112A442 in
network byte order. In [RFC3489], this field was part of the
transaction ID; placing the magic cookie in this location allows a
server to detect if the client will understand certain attributes
that were added to STUN by [RFC5389]. In addition, it aids in
distinguishing STUN packets from packets of other protocols when STUN
is multiplexed with those other protocols on the same port.

The transaction ID is a 96-bit identifier, used to uniquely identify
STUN transactions. For request/response transactions, the
transaction ID is chosen by the STUN client for the request and
echoed by the server in the response. For indications, it is chosen
by the agent sending the indication. It primarily serves to
correlate requests with responses, though it also plays a small role
in helping to prevent certain types of attacks. The server also uses
the transaction ID as a key to identify each transaction uniquely
across all clients. As such, the transaction ID MUST be uniformly
and randomly chosen from the interval 0 .. 2**96-1, and MUST be
cryptographically random. Resends of the same request reuse the same
transaction ID, but the client MUST choose a new transaction ID for
new transactions unless the new request is bit-wise identical to the
previous request and sent from the same transport address to the same
IP address. Success and error responses MUST carry the same
transaction ID as their corresponding request. When an agent is
acting as a STUN server and STUN client on the same port, the
transaction IDs in requests sent by the agent have no relationship to
the transaction IDs in requests received by the agent.

The message length MUST contain the size, in bytes, of the message
not including the 20-byte STUN header. Since all STUN attributes are
padded to a multiple of 4 bytes, the last 2 bits of this field are
always zero. This provides another way to distinguish STUN packets
from packets of other protocols.

Following the STUN fixed portion of the header are zero or more
attributes. Each attribute is TLV (Type-Length-Value) encoded. The
details of the encoding, and of the attributes themselves are given
in Section 14.

6. Base Protocol Procedures

This section defines the base procedures of the STUN protocol. It
describes how messages are formed, how they are sent, and how they
are processed when they are received. It also defines the detailed
processing of the Binding method. Other sections in this document
describe optional procedures that a usage may elect to use in certain
situations. Other documents may define other extensions to STUN, by
adding new methods, new attributes, or new error response codes.

6.1. Forming a Request or an Indication

When formulating a request or indication message, the agent MUST
follow the rules in Section 5 when creating the header. In addition,
the message class MUST be either "Request" or "Indication" (as
appropriate), and the method must be either Binding or some method
defined in another document.

The agent then adds any attributes specified by the method or the
usage. For example, some usages may specify that the agent use an
authentication method (Section 9) or the FINGERPRINT attribute
(Section 7).

If the agent is sending a request, it SHOULD add a SOFTWARE attribute
to the request. Agents MAY include a SOFTWARE attribute in
indications, depending on the method. Extensions to STUN should
discuss whether SOFTWARE is useful in new indications. Note that the
inclusion of a SOFTWARE attribute may have security implications; see
Section 16.1.2 for details.

For the Binding method with no authentication, no attributes are
required unless the usage specifies otherwise.

All STUN messages sent over UDP or DTLS-over-UDP [RFC6347] SHOULD be
less than the path MTU, if known.

If the path MTU is unknown for UDP, messages SHOULD be the smaller of
576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
IPv6 [RFC8200]. This value corresponds to the overall size of the IP
packet. Consequently, for IPv4, the actual STUN message would need
to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
UDP header, assuming no IP options are used).

If the path MTU is unknown for DTLS-over-UDP, the rules described in
the previous paragraph need to be adjusted to take into account the
size of the (13-byte) DTLS Record header, the MAC size, and the
padding size.

STUN provides no ability to handle the case where the request is
under the MTU but the response would be larger than the MTU. It is
not envisioned that this limitation will be an issue for STUN. The
MTU limitation is a SHOULD, and not a MUST, to account for cases
where STUN itself is being used to probe for MTU characteristics
[RFC5780]. See also [I-D.ietf-tram-stun-pmtud] for a framework that
uses STUN to add Path MTU Discovery to protocols that lack one.
Outside of this or similar applications, the MTU constraint MUST be
followed.

6.2. Sending the Request or Indication

The agent then sends the request or indication. This document
specifies how to send STUN messages over UDP, TCP, TLS-over-TCP, or
DTLS-over-UDP; other transport protocols may be added in the future.
The STUN usage must specify which transport protocol is used, and how
the agent determines the IP address and port of the recipient.
Section 8 describes a DNS-based method of determining the IP address
and port of a server that a usage may elect to use.

At any time, a client MAY have multiple outstanding STUN requests
with the same STUN server (that is, multiple transactions in
progress, with different transaction IDs). Absent other limits to
the rate of new transactions (such as those specified by ICE for
connectivity checks or when STUN is run over TCP), a client SHOULD
limit itself to ten outstanding transactions to the same server.

6.2.1. Sending over UDP or DTLS-over-UDP

When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
is possible that the STUN message might be dropped by the network.
Reliability of STUN request/response transactions is accomplished
through retransmissions of the request message by the client
application itself. STUN indications are not retransmitted; thus,
indication transactions over UDP or DTLS-over-UDP are not reliable.

A client SHOULD retransmit a STUN request message starting with an
interval of RTO ("Retransmission TimeOut"), doubling after each
retransmission. The RTO is an estimate of the round-trip time (RTT),
and is computed as described in [RFC6298], with two exceptions.
First, the initial value for RTO SHOULD be greater or equal to 500
ms. The exception cases for this "SHOULD" are when other mechanisms
are used to derive congestion thresholds (such as the ones defined in
ICE for fixed rate streams), or when STUN is used in non-Internet
environments with known network capacities. In fixed-line access
links, a value of 500 ms is RECOMMENDED. Second, the value of RTO
SHOULD NOT be rounded up to the nearest second. Rather, a 1 ms
accuracy SHOULD be maintained. As with TCP, the usage of Karn's
algorithm is RECOMMENDED [KARN87]. When applied to STUN, it means
that RTT estimates SHOULD NOT be computed from STUN transactions that
result in the retransmission of a request.

The value for RTO SHOULD be cached by a client after the completion
of the transaction, and used as the starting value for RTO for the
next transaction to the same server (based on equality of IP
address). The value SHOULD be considered stale and discarded if no
transactions have occurred to the same server in the last 10 minutes.

Retransmissions continue until a response is received, or until a
total of Rc requests have been sent. Rc SHOULD be configurable and
SHOULD have a default of 7. If, after the last request, a duration
equal to Rm times the RTO has passed without a response (providing
ample time to get a response if only this final request actually
succeeds), the client SHOULD consider the transaction to have failed.
Rm SHOULD be configurable and SHOULD have a default of 16. A STUN
transaction over UDP or DTLS-over-UDP is also considered failed if
there has been a hard ICMP error [RFC1122]. For example, assuming an
RTO of 500ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
3500 ms, 7500 ms, 15500 ms, and 31500 ms. If the client has not
received a response after 39500 ms, the client will consider the
transaction to have timed out.

6.2.2. Sending over TCP or TLS-over-TCP

For TCP and TLS-over-TCP [RFC5246], the client opens a TCP connection
to the server.

In some usages of STUN, STUN is sent as the only protocol over the
TCP connection. In this case, it can be sent without the aid of any
additional framing or demultiplexing. In other usages, or with other
extensions, it may be multiplexed with other data over a TCP
connection. In that case, STUN MUST be run on top of some kind of
framing protocol, specified by the usage or extension, which allows
for the agent to extract complete STUN messages and complete
application layer messages. The STUN service running on the well-
known port or ports discovered through the DNS procedures in
Section 8 is for STUN alone, and not provides for STUN multiplexed with other
data. Consequently, no framing protocols are two forms of authentication.

STUN is intended to be used in connections to
those servers. When additional framing is utilized, the context of one or more NAT
traversal solutions. These solutions are known as "STUN Usages".
Each usage will
specify describes how STUN is utilized to achieve the client knows NAT
traversal solution. Typically, a usage indicates when STUN messages
get sent, which optional attributes to apply it include, what server is used,
and what port authentication mechanism is to connect to.
For example, be used. Interactive
Connectivity Establishment (ICE) [RFC8445] is one usage of STUN. SIP
Outbound [RFC5626] is another usage of STUN. In some cases, a usage
will require extensions to STUN. A STUN extension can be in the case form
of ICE connectivity checks, this new methods, attributes, or error response codes. More
information
is learned through out-of-band negotiation between client and server.

Reliability on STUN Usages can be found in Section 13.

2. Overview of Operation

This section is descriptive only.

/-----\
// STUN over TCP and TLS-over-TCP \\
| Server |
\\ //
\-----/

+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+

+--------------+ Private Network 2
................| NAT 1 |.......................
+--------------+

/-----\
// STUN \\
| Client |
\\ // Private Network 1
\-----/

Figure 1: One Possible STUN Configuration

One possible STUN configuration is handled by TCP
itself, and shown in Figure 1. In this
configuration, there are no retransmissions at the two entities (called STUN protocol level.
However, for a request/response transaction, if agents) that
implement the client has not
received a response by Ti seconds after it sent STUN protocol. The lower agent in the request message,
it considers figure is the transaction
client, which is connected to have timed out. Ti SHOULD be
configurable and SHOULD have a default of 39.5s. private network 1. This value has been
chosen network
connects to private network 2 through NAT 1. Private network 2
connects to equalize the TCP and UDP timeouts for the default initial
RTO.

In addition, if public Internet through NAT 2. The upper agent in
the client figure is unable to establish the TCP connection,
or server, which resides on the TCP connection public Internet.

STUN is reset or fails before a response client-server protocol. It supports two types of
transactions. One is
received, any a request/response transaction in progress is considered
to have failed.

The which a
client MAY send multiple transactions over sends a single TCP (or TLS-
over-TCP) connection, and it MAY send another request before
receiving a response to the previous request. The client SHOULD keep
the connection open until it:

o has no further STUN requests or indications to send over that
connection, and

o has no plans to use any resources (such as a mapped address
(MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
[RFC5766]) that were learned though STUN requests sent over that
connection, and
o if multiplexing other application protocols over that port, has
finished using those other protocols, server, and

o if using that learned port with the server returns a remote peer, has established
communications with that remote peer, as is required by some TCP
NAT traversal techniques (e.g., [RFC6544]).
response. The details of second is an eventual keep-alive mechanism are left to each STUN
Usage. In any case if indication transaction in which either
agent -- client or server -- sends an indication that generates no
response. Both types of transactions include a transaction fails because an idle TCP
connection doesn't work anymore ID, which
is a randomly selected 96-bit number. For request/response
transactions, this transaction ID allows the client SHOULD send an RST and try to open a new TCP connection.

At associate the server end,
response with the server SHOULD keep request that generated it; for indications, the connection open,
transaction ID serves as a debugging aid.

All STUN messages start with a fixed header that includes a method, a
class, and
let the client close it, unless the server has determined that transaction ID. The method indicates which of the
connection has timed out (for example, due
various requests or indications this is; this specification defines
just one method, Binding, but other methods are expected to be
defined in other documents. The class indicates whether this is a
request, a success response, an error response, or an indication.
Following the client
disconnecting from the network). Bindings learned by fixed header comes zero or more attributes, which are
Type-Length-Value extensions that convey additional information for
the client will
remain valid specific message.

This document defines a single method called "Binding". The Binding
method can be used either in intervening NATs only while the connection remains
open. Only request/response transactions or in
indication transactions. When used in request/response transactions,
the client knows how long it needs Binding method can be used to determine the binding. The
server SHOULD NOT close particular binding a connection if
NAT has allocated to a STUN client. When used in either request/
response or in indication transactions, the Binding method can also
be used to keep these bindings alive.

In the Binding request/response transaction, a Binding request was received over
that connection for which a response was not sent. A server MUST NOT
ever open is
sent from a connection back towards the STUN client in order to send a
response. Servers SHOULD follow best practices regarding connection
management in cases of overload.

6.2.3. Sending over TLS-over-TCP or DTLS-over-UDP STUN server. When the Binding request
arrives at the STUN is run by itself over TLS-over-TCP server, it may have passed through one or DTLS-over-UDP, more
NATs between the
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 cipher suites MUST be
implemented STUN client and other cipher suites MAY be implemented. Perfect
Forward Secrecy (PFS) cipher suites MUST be preferred over non-PFS
cipher suites. Cipher suites with known weaknesses, the STUN server (in Figure 1, there
are two such as those
based on (single) DES NATs). As the Binding request message passes through a
NAT, the NAT will modify the source transport address (that is, the
source IP address and RC4, MUST NOT be used. Implementations
MUST disable TLS-level compression.

These recommendations are just the source port) of the packet. As a part result,
the source transport address of the recommendations in
[BCP195] that implementations request received by the server
will be the public IP address and deployments of port created by the NAT closest to
the server. This is called a "reflexive transport address". The
STUN Usage using
TLS or DTLS MUST follow.

When it receives server copies that source transport address into an XOR-MAPPED-
ADDRESS attribute in the TLS Certificate message, STUN Binding response and sends the client MUST verify Binding
response back to the certificate and inspect STUN client. As this packet passes back through
a NAT, the NAT will modify the site identified by destination transport address in the certificate.
If
IP header, but the certificate is invalid or revoked, or if it does not identify transport address in the appropriate party, XOR-MAPPED-ADDRESS
attribute within the client MUST NOT send body of the STUN message or
otherwise proceed with response will remain untouched.
In this way, the STUN transaction. The client MUST verify can learn its reflexive transport address
allocated by the identity of outermost NAT with respect to the STUN server. To do that, it follows the
identification procedures defined in [RFC6125], with a certificate
containing an identifier of type DNS-ID or CN-ID, optionally with a
wildcard character as leftmost label, but not of type SRV-ID or URI-
ID.

When

In some usages, STUN is run must be multiplexed with other protocols over (e.g.,
[RFC8445] and [RFC5626]). In these usages, there must be a TLS-over-TCP
connection or way to
inspect a DTLS-over-UDP association, the mandatory ciphersuites packet and TLS handling procedures operate as defined by those protocols.

6.3. Receiving determine if it is a STUN Message

This section specifies packet or not. STUN
provides three fields in the processing of a STUN message. The
processing specified here is header with fixed values that can
be used for STUN messages as defined in this
specification; additional rules for backwards compatibility are
defined in Section 11. Those additional procedures are optional, and
usages can elect to utilize them. First, a set of processing
operations is applied that is independent of the class. This purpose. If this is
followed by class-specific processing, described in the subsections
that follow.

When not sufficient, then STUN
packets can also contain a FINGERPRINT value, which can further be
used to distinguish the packets.

STUN agent receives defines a STUN message, it first checks set of optional procedures that a usage can decide to
use, called "mechanisms". These mechanisms include DNS discovery, a
redirection technique to an alternate server, a fingerprint attribute
for demultiplexing, and two authentication and message-integrity
exchanges. The authentication mechanisms revolve around the
message obeys the rules use of Section 5. It checks that a
username, password, and message-integrity value. Two authentication
mechanisms, the first two
bits long-term credential mechanism and the short-term
credential mechanism, are 0, defined in this specification. Each usage
specifies the mechanisms allowed with that usage.

In the magic cookie field has long-term credential mechanism, the correct value, that client and server share a
pre-provisioned username and password and perform a digest challenge/
response exchange inspired by the message length is sensible, one defined for HTTP [RFC7616] but
differing in details. In the short-term credential mechanism, the
client and that the method value is server exchange a
supported method. It checks that username and password through some
out-of-band method prior to the message class is allowed for STUN exchange. For example, in the particular method. If
ICE usage [RFC8445], the message class two endpoints use out-of-band signaling to
exchange a username and password. These are used to integrity
protect and authenticate the request and response. There is "Success Response" no
challenge or
"Error Response", the nonce used.

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

4. Definitions

STUN Agent: A STUN agent checks is an entity that implements the transaction ID matches STUN
protocol. The entity can be either a
transaction that is still in progress. If the FINGERPRINT extension STUN client or a STUN
server.

STUN Client: A STUN client is being used, the agent checks an entity that the FINGERPRINT attribute is
present sends STUN requests and contains the correct value. If any errors are detected,
the message is silently discarded.
receives STUN responses and STUN indications. A STUN client can
also send indications. In this specification, the case when terms "STUN
client" and "client" are synonymous.

STUN Server: A STUN server is being
multiplexed with another protocol, an error may indicate entity that this is
not really a receives STUN message; in requests
and STUN indications and that sends STUN responses. A STUN server
can also send indications. In this case, the agent should try to
parse specification, the message terms "STUN
server" and "server" are synonymous.

Transport Address: The combination of an IP address and port number
(such as a different protocol.

The STUN agent then does any checks that are required UDP or TCP port number).

Reflexive Transport Address: A transport address learned by a
authentication mechanism client
that the usage has specified (see
Section 9).

Once the authentication checks are done, the STUN agent checks for
unknown attributes and known-but-unexpected attributes in the
message. Unknown comprehension-optional attributes MUST be ignored
by the agent. Known-but-unexpected attributes SHOULD be ignored by
the agent. Unknown comprehension-required attributes cause
processing identifies that depends client as seen by another host on an IP
network, typically a STUN server. When there is an intervening
NAT between the message class client and is described below.

At this point, further processing depends the other host, the reflexive transport
address represents the mapped address allocated to the client on
the message class public side of the
request.

6.3.1. Processing a Request

If NAT. Reflexive transport addresses are
learned from the request contains one mapped address attribute (MAPPED-ADDRESS or more unknown comprehension-required
attributes, XOR-
MAPPED-ADDRESS) in STUN responses.

Mapped Address: Same meaning as reflexive address. This term is
retained only for historic reasons and due to the server replies with an error response with an error
code naming of 420 (Unknown Attribute), and includes an UNKNOWN-ATTRIBUTES
attribute in the response
MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.

Long-Term Credential: A username and associated password that lists
represent a shared secret between client and server. Long-term
credentials are generally granted to the unknown comprehension-
required attributes.

Otherwise client when a subscriber
enrolls in a service and persist until the server then does any additional checking that subscriber leaves the
method
service or explicitly changes the specific usage requires. If all credential.

Long-Term Password: The password from a long-term credential.

Short-Term Credential: A temporary username and associated password
that represent a shared secret between client and server. Short-
term credentials are obtained through some kind of protocol
mechanism between the checks succeed, client and server, preceding the server formulates STUN
exchange. A short-term credential has an explicit temporal scope,
which may be based on a success response specific amount of time (such as described below.

When run over UDP 5
minutes) or DTLS-over-UDP, on an event (such as termination of a request received Session
Initiation Protocol (SIP) [RFC3261] dialog). The specific scope
of a short-term credential is defined by the server
could be the first request application usage.

Short-Term Password: The password component of a transaction, or short-term
credential.

STUN Indication: A STUN message that does not receive a retransmission. response.

Attribute: The server MUST respond to retransmissions such STUN term for a Type-Length-Value (TLV) object that the following
property is preserved: if the client receives the response
can be added to the
retransmission a STUN message. Attributes are divided into two
types: comprehension-required and not the response comprehension-optional. STUN
agents can safely ignore comprehension-optional attributes they
don't understand but cannot successfully process a message if it
contains comprehension-required attributes that was sent to the original
request, the overall state on the client and server is identical to
the case where only the response to the original retransmission is
received, or where both responses are received (in not
understood.

RTO: Retransmission TimeOut, which case defines the
client will use initial period of
time between transmission of a request and the first). first retransmit of
that request.

5. STUN Message Structure

STUN messages are encoded in binary using network-oriented format
(most significant byte or octet first, also commonly known as big-
endian). The easiest way to meet this requirement transmission order is for the server to remember all transaction IDs received over UDP
or DTLS-over-UDP and their corresponding responses described in the last 40
seconds. However, this requires the server to hold state, and will
be inappropriate for any requests which detail in Appendix B
of [RFC0791]. Unless otherwise noted, numeric constants are not authenticated.
Another way is to reprocess the request in
decimal (base 10).

All STUN messages comprise a 20-byte header followed by zero or more
attributes. The STUN header contains a STUN message type, message
length, magic cookie, and recompute the response. transaction ID.

Figure 2: Format of STUN Message Header

The latter technique most significant 2 bits of every STUN message MUST only be applied zeroes.
This can be used to requests that are
idempotent (a request is considered idempotent differentiate STUN packets from other protocols
when STUN is multiplexed with other protocols on the same request
can be safely repeated without impacting the overall state of the
system) and result in port.

The message type defines the same message class (request, success response for
response, error response, or indication) and the same request.
The Binding message method is considered to be idempotent. Note that (the
primary function) of the STUN message. Although there are certain rare network events that could cause the reflexive
transport address value to change, resulting in a different mapped
address four
message classes, there are only two types of transactions in different success responses. Extensions to STUN MUST
discuss the implications STUN:
request/response transactions (which consist of a request retransmissions on servers that
do not store transaction state.

6.3.1.1. Forming message and
a response message) and indication transactions (which consist of a Success or Error
single indication message). Response

When forming the response (success or error), the server follows classes are split into error
and success responses to aid in quickly processing the
rules of Section 6. STUN message.

The method STUN Message Type field is decomposed further into the following
structure:

0 1
2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
|M |M |M|M|M|C|M|M|M|C|M|M|M|M|
|11|10|9|8|7|1|6|5|4|0|3|2|1|0|
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Format of STUN Message Type Field

Here the response is bits in the same STUN Message Type field are shown as that most
significant (M11) through least significant (M0). M11 through M0
represent a 12-bit encoding of the request, method. C1 and C0 represent a
2-bit encoding of the message class. A class of 0b00 is a request, a class
of 0b01 is either "Success Response" or
"Error Response".

For an error indication, a class of 0b10 is a success response, the server MUST add and
a class of 0b11 is an ERROR-CODE attribute
containing the error code specified in the processing above. response. This specification defines a
single method, Binding. The
reason phrase is not fixed, but SHOULD be something suitable method and class are orthogonal, so that
for the each method, a request, success response, error code. For certain errors, additional attributes response, and
indication are added to
the message. These attributes possible for that method. Extensions defining new
methods MUST indicate which classes are spelled out in the description
where permitted for that method.

For example, a Binding request has class=0b00 (request) and
method=0b000000000001 (Binding) and is encoded into the error code first 16 bits
as 0x0001. A Binding response has class=0b10 (success response) and
method=0b000000000001 and is specified. For example, for an error code of
420 (Unknown Attribute), encoded into the server MUST include an UNKNOWN-
ATTRIBUTES attribute. Certain authentication errors also cause
attributes first 16 bits as
0x0101.

Note: This unfortunate encoding is due to be added (see Section 9). Extensions may define other assignment of values in
[RFC3489] that did not consider encoding indication messages,
success responses, and errors and/or additional attributes to add responses using bit fields.

The Magic Cookie field MUST contain the fixed value 0x2112A442 in error cases.

If
network byte order. In [RFC3489], the server authenticated 32 bits comprising the request using an authentication
mechanism, then Magic
Cookie field were part of the server SHOULD add transaction ID; placing the appropriate authentication
attributes magic
cookie in this location allows a server to detect if the response (see Section 9).

The server also adds any client will
understand certain attributes required that were added to STUN by the specific method
or usage. [RFC5389].
In addition, it aids in distinguishing STUN packets from packets of
other protocols when STUN is multiplexed with those other protocols
on the server SHOULD add same port.

The transaction ID is a SOFTWARE attribute 96-bit identifier, used to
the message. uniquely identify
STUN transactions. For request/response transactions, the Binding method, no additional checking
transaction ID is required unless chosen by the
usage specifies otherwise. When forming STUN client for the success response, request and
echoed by the server in the response. For indications, it is chosen
by the agent sending the indication. It primarily serves to
correlate requests with responses, though it also plays a small role
in helping to prevent certain types of attacks. The server adds also uses
the transaction ID as a XOR-MAPPED-ADDRESS attribute key to identify each transaction uniquely
across all clients. As such, the response, where transaction ID MUST be uniformly
and randomly chosen from the
contents interval 0 .. 2**96-1 and MUST be
cryptographically random. Resends of the attribute are same request reuse the source transport address of same
transaction ID, but the client MUST choose a new transaction ID for
new transactions unless the new request message. For UDP or DTLS-over-UDP this is bit-wise identical to the source IP
address and source UDP port of the
previous request message. For TCP and TLS-
over-TCP, this is sent from the source IP same transport address to the same
IP address. Success and source TCP port of error responses MUST carry the
TCP connection same
transaction ID as seen by their corresponding request. When an agent is
acting as a STUN server and STUN client on the server.

6.3.1.2. Sending same port, the Success or Error Response

The response (success or error) is
transaction IDs in requests sent over by the same transport as agent have no relationship to
the request was transaction IDs in requests received on. If by the request was received over UDP or
DTLS-over-UDP agent.

The message length MUST contain the destination IP address and port size of the response are message in bytes, not
including the source IP address and port 20-byte STUN header. Since all STUN attributes are
padded to a multiple of 4 bytes, the received request message, and
the source IP address and port last 2 bits of the response this field are equal
always zero. This provides another way to the
destination IP address and port distinguish STUN packets
from packets of other protocols.

Following the received request message. If STUN fixed portion of the request was received over TCP header are zero or TLS-over-TCP, the response more
attributes. Each attribute is
sent back on TLV (Type-Length-Value) encoded.
Details of the same TCP connection as encoding and the request was received on.

The server is allowed to send responses attributes themselves are given in a different order than it
received the requests.

6.3.2. Processing an Indication

If
Section 14.

6. Base Protocol Procedures

This section defines the indication contains unknown comprehension-required attributes, base procedures of the indication is discarded STUN protocol. It
describes how messages are formed, how they are sent, and how they
are processed when they are received. It also defines the detailed
processing ceases.

Otherwise of the agent then does any additional checking Binding method. Other sections in this document
describe optional procedures that the method
or the specific a usage requires. If all the checks succeed, the agent
then processes the indication. No may elect to use in certain
situations. Other documents may define other extensions to STUN, by
adding new methods, new attributes, or new error response is generated for codes.

6.1. Forming a Request or an
indication.

For the Binding method, no additional checking Indication

When formulating a request or processing is
required, unless the usage specifies otherwise. The mere receipt of
the message by indication message, the agent has refreshed MUST
follow the "bindings" rules in Section 5 when creating the
intervening NATs.

Since indications are not re-transmitted over UDP or DTLS-over-UDP
(unlike requests), there is no need to handle re-transmissions of
indications at the sending agent.

6.3.3. Processing a Success Response

If the success response contains unknown comprehension-required
attributes, the response is discarded and header. In addition,
the transaction is
considered to have failed.

Otherwise message class MUST be either "Request" or "Indication" (as
appropriate), and the client method must be either Binding or some method
defined in another document.

The agent then does adds any additional checking that attributes specified by the method or the specific usage requires. If all the checks succeed,
the client then processes the success response.
usage. For the Binding method, the client checks example, some usages may specify that the XOR-MAPPED-ADDRESS agent use an
authentication method (Section 9) or the FINGERPRINT attribute
(Section 7).

If the agent is present in sending a request, it SHOULD add a SOFTWARE attribute
to the response. The client checks request. Agents MAY include a SOFTWARE attribute in
indications, depending on the address
family specified. If it method. Extensions to STUN should
discuss whether SOFTWARE is an unsupported address family, useful in new indications. Note that the
inclusion of a SOFTWARE attribute SHOULD be ignored. If it is an unexpected but supported
address family (for example, may have security implications; see
Section 16.1.2 for details.

For the Binding transaction was method with no authentication, no attributes are
required unless the usage specifies otherwise.

All STUN messages sent over
IPv4, but UDP or DTLS-over-UDP [RFC6347] SHOULD be
less than the address family specified path MTU, if known.

If the path MTU is IPv6), then unknown for UDP, messages SHOULD be the client MAY
accept smaller of
576 bytes and use the value.

6.3.4. Processing an Error Response first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
IPv6 [RFC8200]. This value corresponds to the overall size of the IP
packet. Consequently, for IPv4, the actual STUN message would need
to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
UDP header, assuming no IP options are used).

If the error response contains path MTU is unknown comprehension-required
attributes, or if for DTLS-over-UDP, the error response does not contain an ERROR-CODE
attribute, then rules described in
the transaction is simply considered previous paragraph need to have failed.

Otherwise the client then does any processing specified by be adjusted to take into account the
authentication mechanism (see Section 9). This may result in a new
transaction attempt.

The processing at this point depends on
size of the error code, (13-byte) DTLS Record header, the method, Message Authentication
Code (MAC) size, and the usage; the following are padding size.

STUN provides no ability to handle the default rules:

o If case where the error code request is 300 through 399,
smaller than the client SHOULD consider MTU but the transaction as failed unless response is larger than the ALTERNATE-SERVER extension
(Section 10) MTU. It is
not envisioned that this limitation will be an issue for STUN. The
MTU limitation is a SHOULD, not a MUST, to account for cases where
STUN itself is being used.

o If used to probe for MTU characteristics [RFC5780].
See also [STUN-PMTUD] for a framework that uses STUN to add Path MTU
Discovery to protocols that lack such a mechanism. Outside of this
or similar applications, the MTU constraint MUST be followed.

6.2. Sending the Request or Indication

The agent then sends the request or indication. This document
specifies how to send STUN messages over UDP, TCP, TLS-over-TCP, or
DTLS-over-UDP; other transport protocols may be added in the error code future.
The STUN Usage must specify which transport protocol is 400 through 499, used and how
the client declares agent determines the
transaction failed; in IP address and port of the case recipient.
Section 8 describes a DNS-based method of 420 (Unknown Attribute), determining the
response should contain IP address
and port of a UNKNOWN-ATTRIBUTES attribute server that gives
additional information.

o If the error code is 500 through 599, the a usage may elect to use.

At any time, a client MAY resend the
request; clients that do so MUST limit have multiple outstanding STUN requests
with the number of times they do
this. Unless a specific error code specifies a same STUN server (that is, multiple transactions in
progress, with different value, transaction IDs). Absent other limits to
the number rate of retransmissions new transactions (such as those specified by ICE for
connectivity checks or when STUN is run over TCP), a client SHOULD be limited
limit itself to 4.

Any other error code causes the client ten outstanding transactions to consider the transaction
failed.

7. FINGERPRINT Mechanism

This section describes an optional mechanism for same server.

6.2.1. Sending over UDP or DTLS-over-UDP

When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
is possible that aids in
distinguishing the STUN messages from packets message might be dropped by the network.
Reliability of STUN request/response transactions is accomplished
through retransmissions of other protocols when the
two are multiplexed on request message by the same transport address. This mechanism is
optional, and client
application itself. STUN indications are not retransmitted; thus,
indication transactions over UDP or DTLS-over-UDP are not reliable.

A client SHOULD retransmit a STUN usage must describe if and when it is used. request message starting with an
interval of RTO ("Retransmission TimeOut"), doubling after each
retransmission. The
FINGERPRINT mechanism RTO is not backwards compatible with RFC3489, an estimate of the round-trip time (RTT)
and
cannot be used in environments where such compatibility is required.

In some usages, STUN messages are multiplexed on the same transport
address as other protocols, such computed as the Real Time Transport Protocol
(RTP). In order to apply the processing described in Section 6, STUN
messages must first [RFC6298], with two exceptions.
First, the initial value for RTO SHOULD be separated from greater than or equal to
500 ms. The exception cases for this "SHOULD" are when other
mechanisms are used to derive congestion thresholds (such as the application packets.

Section 5 describes three fixed fields ones
defined in the ICE for fixed-rate streams) or when STUN header that can be is used for this purpose. However, in some cases, these three fixed
fields may not be sufficient.

When the FINGERPRINT extension non-
Internet environments with known network capacities. In fixed-line
access links, a value of 500 ms is used, an agent includes RECOMMENDED. Second, the
FINGERPRINT attribute in messages it sends value of
RTO SHOULD NOT be rounded up to another agent.
Section 14.7 describes the placement and value nearest second. Rather, a 1 ms
accuracy SHOULD be maintained. As with TCP, the usage of this attribute. Karn's
algorithm is RECOMMENDED [KARN87]. When the agent receives what applied to STUN, it believes is means
that RTT estimates SHOULD NOT be computed from STUN transactions that
result in the retransmission of a request.

The value for RTO SHOULD be cached by a STUN message, then, in
addition to other basic checks, client after the agent also checks that completion
of the
message contains a FINGERPRINT attribute transaction and that used as the attribute
contains starting value for RTO for the correct value. Section 6.3 describes when in
next transaction to the
overall processing same server (based on equality of a STUN message IP
address). The value SHOULD be considered stale and discarded if no
transactions have occurred to the FINGERPRINT check is
performed. This additional check helps same server in the agent detect messages last 10 minutes.

Retransmissions continue until a response is received or until a
total of
other protocols that might otherwise seem to Rc requests have been sent. Rc SHOULD be STUN messages.

8. DNS Discovery of configurable and
SHOULD have a Server

This section describes an optional procedure for STUN that allows default of 7. If, after the last request, a
client to use DNS duration
equal to determine Rm times the IP address and port of RTO has passed without a server.
A STUN usage must describe response (providing
ample time to get a response if and when this extension is used. To
use only this procedure, final request actually
succeeds), the client must know SHOULD consider the transaction to have failed.
Rm SHOULD be configurable and SHOULD have a default of 16. A STUN URI [RFC7064]; the
usage must
transaction over UDP or DTLS-over-UDP is also describe how considered failed if
there has been a hard ICMP error [RFC1122]. For example, assuming an
RTO of 500 ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
3500 ms, 7500 ms, 15500 ms, and 31500 ms. If the client obtains this URI. Hard-
coding has not
received a STUN URI into software is NOT RECOMMENDED in case response after 39500 ms, the domain
name is lost or needs to change for legal or other reasons.

When a client wishes to locate a STUN server on the public Internet
that accepts Binding request/response transactions, will consider the STUN URI
scheme is "stun". When it wishes
transaction to locate a STUN server that
accepts Binding request/response transactions have timed out.

6.2.2. Sending over a TLS, TCP or DTLS
session, the URI scheme is "stuns".

The syntax of the "stun" TLS-over-TCP

For TCP and "stuns" URIs are defined in Section 3.1
of [RFC7064]. STUN usages MAY define additional URI schemes.

8.1. STUN URI Scheme Semantics

If TLS-over-TCP [RFC8446], the <host> part of client opens a "stun" URI contains an IP address, then this
IP address is used directly TCP connection
to contact the server. A "stuns" URI
containing an IP address MUST be rejected. A future

In some usages of STUN, STUN extension
or usage may relax is the only protocol over the TCP
connection. In this requirement provided case, it demonstrates how to
authenticate can be sent without the aid of any
additional framing or demultiplexing. In other usages, or with other
extensions, it may be multiplexed with other data over a TCP
connection. In that case, STUN server and prevent man in the middle attacks.

If the URI does not contain an IP address, the domain name contained
in MUST be run on top of some kind of
framing protocol, specified by the <host> part is resolved to a transport address using usage or extension, which allows
for the SRV
procedures specified in [RFC2782]. agent to extract complete STUN messages and complete
application-layer messages. The DNS SRV STUN service name is running on the
content of well-
known port or ports discovered through the <scheme> part. The protocol DNS procedures in the SRV lookup
Section 8 is the
transport protocol the client will run STUN over: "udp" for UDP STUN alone, and
"tcp" not for TCP.

The procedures of RFC 2782 STUN multiplexed with other
data. Consequently, no framing protocols are followed to determine the server used in connections to
contact. RFC 2782 spells out
those servers. When additional framing is utilized, the details of usage will
specify how a set of SRV records
is sorted and then tried. However, RFC 2782 only states that the client should "try to connect knows to the (protocol, address, service)"
without giving any details on apply it and what happens port to connect to.
For example, in the event case of failure.
When following these procedures, if ICE connectivity checks, this information
is learned through out-of-band negotiation between client and server.

Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
itself, and there are no retransmissions at the STUN transaction times out
without receipt of protocol level.
However, for a response, request/response transaction, if the client SHOULD retry has not
received a response by Ti seconds after it sent the request to
the next server in message,
it considers the ordered defined by RFC 2782. Such transaction to have timed out. Ti SHOULD be
configurable and SHOULD have a retry is
only possible for request/response transmissions, since indication
transactions generate no response or timeout. default of 39.5 s. This value has
been chosen to equalize the TCP and UDP timeouts for the default
initial RTO.

In addition, instead of querying either the A or if the AAAA resource
records for a domain name, a dual-stack IPv4/IPv6 client MUST query
both and try is unable to establish the requests with all TCP connection,
or the IP addresses TCP connection is reset or fails before a response is
received, as
specified any request/response transaction in [RFC8305].

The default port for STUN requests progress is 3478, for both TCP and UDP. considered
to have failed.

The default port for STUN client MAY send multiple transactions over TLS a single TCP (or TLS-
over-TCP) connection, and it MAY send another request before
receiving a response to the previous request. The client SHOULD keep
the connection open until it:

o has no further STUN over DTLS requests is
5349. Servers can run STUN or indications to send over DTLS on the same port that
connection,

o has no plans to use any resources (such as STUN over
UDP if the server software supports determining whether the initial
message is a DTLS mapped address
(MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
[RFC5766]) that were learned though STUN message. Servers can run STUN requests sent over TLS on
the same that
connection,
o if multiplexing other application protocols over that port, has
finished using those other protocols,

o if using that learned port with a remote peer, has established
communications with that remote peer, as is required by some TCP
NAT traversal techniques (e.g., [RFC6544]).

The details of an eventual keep-alive mechanism are left to each STUN over
Usage. In any case, if a transaction fails because an idle TCP
connection doesn't work anymore, the client SHOULD send a RST and try
to open a new TCP if connection.

At the server software supports
determining whether end, the initial message is a TLS or STUN message.

Administrators of STUN servers server SHOULD use these ports in their SRV
records for UDP and TCP. In all cases, keep the port in DNS MUST reflect connection open and let
the one on which client close it, unless the server is listening.

If no SRV records were found, has determined that the
connection has timed out (for example, due to the client performs both an A and AAAA
record lookup of
disconnecting from the domain name, as described in [RFC8305]. The
result network). Bindings learned by the client will be a list of IP addresses, each of which can be
simultaneously contacted at
remain valid in intervening NATs only while the default port using UDP or TCP,
independent of connection remains
open. Only the STUN usage. For usages client knows how long it needs the binding. The
server SHOULD NOT close a connection if a request was received over
that require TLS, connection for which a response was not sent. A server MUST NOT
ever open a connection back towards the client connects in order to the IP addresses using the default send a
response. Servers SHOULD follow best practices regarding connection
management in cases of overload.

6.2.3. Sending over TLS-over-TCP or DTLS-over-UDP

When STUN is run by itself over TLS
port. For usages TLS-over-TCP or DTLS-over-UDP, the
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 ciphersuites MUST be
implemented (for compatibility with older versions of this protocol),
except if deprecated by rules of a specific STUN usage. Other
ciphersuites MAY be implemented. Note that require DTLS, the client connects to the IP
addresses using the default STUN over DTLS port.

9. Authentication clients and Message-Integrity Mechanisms

This section defines two mechanisms for STUN servers
that a client and server
can use implement TLS version 1.3 [RFC8446] or subsequent versions are
also required to provide authentication implement mandatory ciphersuites from those
specifications and message integrity; these two
mechanisms are SHOULD disable usage of deprecated ciphersuites
when they detect support for those specifications. Perfect Forward
Secrecy (PFS) ciphersuites MUST be preferred over non-PFS
ciphersuites. Ciphersuites with known weaknesses, such as the short-term credential mechanism those
based on (single) DES and the
long-term credential mechanism. RC4, MUST NOT be used. Implementations
MUST disable TLS-level compression.

These two mechanisms are optional,
and each usage must specify if and when these mechanisms recommendations are used.
Consequently, both clients and servers will know which mechanism (if
any) to follow based on knowledge of which usage applies. For
example, just a STUN server on the public Internet supporting ICE would
have no authentication, whereas the STUN server functionality in an
agent supporting connectivity checks would utilize short-term
credentials. An overview part of these two mechanisms is given in
Section 2.

Each mechanism specifies the additional processing required to use
that mechanism, extending the processing specified in Section 6. The
additional processing occurs recommendations in three different places: when forming
a message, when receiving a message immediately after the basic
checks have been performed,
[BCP195] that implementations and when doing the detailed processing deployments of
error responses.

Note that agents a STUN Usage using
TLS or DTLS MUST ignore all attributes that follow MESSAGE-
INTEGRITY, with follow.

When it receives the exception of TLS Certificate message, the MESSAGE-INTEGRITY-SHA256 and
FINGERPRINT attributes. Similarly agents client MUST ignore all attributes
that follow verify
the MESSAGE-INTEGRITY-SHA256 attribute if certificate and inspect the MESSAGE-
INTEGRITY attribute site identified by the certificate.
If the certificate is invalid or revoked, or if it does not present, identify
the appropriate party, the client MUST NOT send the STUN message or
otherwise proceed with the exception STUN transaction. The client MUST verify
the identity of the
FINGERPRINT attribute.

9.1. Short-Term Credential Mechanism

The short-term credential mechanism assumes server. To do that, prior to it follows the
identification procedures defined in [RFC6125], with a certificate
containing an identifier of type DNS-ID or CN-ID, optionally with a
wildcard character as the leftmost label, but not of type SRV-ID or
URI-ID.

When STUN
transaction, is run multiplexed with other protocols over a TLS-over-TCP
connection or a DTLS-over-UDP association, the client mandatory ciphersuites
and server have used some other protocol to
exchange TLS handling procedures operate as defined by those protocols.

6.3. Receiving a credential in STUN Message

This section specifies the form processing of a username and password. This
credential is time-limited. STUN message. The time limit
processing specified here is for STUN messages as defined by the usage.

As an example, in the ICE usage [RFC8445], the two endpoints use out-
of-band signaling to agree on a username and password, and this
username and password are applicable
specification; additional rules for the duration of the media
session.

This credential is used to form a message-integrity check backwards compatibility are
defined in each
request Section 11. Those additional procedures are optional, and in many responses. There
usages can elect to utilize them. First, a set of processing
operations is no challenge and response as
in the long-term mechanism; consequently, replay applied that is limited by virtue
of the time-limited nature independent of the credential.

9.1.1. HMAC Key

For short-term credentials the HMAC key is defined as follow:

key = OpaqueString(password)

where the OpaqueString profile class. This is defined
followed by class-specific processing, described in [RFC8265]. The encoding
used is UTF-8 [RFC3629].

9.1.2. Forming the subsections
that follow.

When a Request or Indication

For STUN agent receives a request or indication STUN message, it first checks that the agent MUST include
message obeys the
USERNAME, MESSAGE-INTEGRITY-SHA256, and MESSAGE-INTEGRITY attributes
in rules of Section 5. It checks that the message unless first two
bits are 0, that the agent knows from an external indication
which message integrity algorithm is supported by both agents. In
this case either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST
be included in addition to USERNAME. The HMAC for Magic Cookie field has the MESSAGE-
INTEGRITY attribute correct value, that
the message length is computed as described in Section 14.5 sensible, and that the
HMAC for the MESSAGE-INTEGRITY-SHA256 attributes method value is computed as
described in Section 14.6. Note a
supported method. It checks that the password message class is never included
in allowed for
the request or indication.

9.1.3. Receiving a Request particular method. If the message class is "Success Response" or Indication

After
"Error Response", the agent has done checks that the basic processing of transaction ID matches a message,
transaction that is still in progress. If the agent
performs FINGERPRINT extension
is being used, the agent checks listed below in order specified:

o If that the message does not contain 1) a MESSAGE-INTEGRITY or a
MESSAGE-INTEGRITY-SHA256 FINGERPRINT attribute is
present and 2) a USERNAME attribute:

* contains the correct value. If any errors are detected,
the message is a request, the server MUST reject silently discarded. In the request case when STUN is being
multiplexed with another protocol, an error response. This response MUST use an error code
of 400 (Bad Request).

* If the message may indicate that this is an indication,
not really a STUN message; in this case, the agent MUST silently
discard the indication.

o If should try to
parse the USERNAME message as a different protocol.

The STUN agent then does not contain any checks that are required by a username value currently valid
within
authentication mechanism that the server:

* If usage has specified (see
Section 9).

Once the authentication checks are done, the STUN agent checks for
unknown attributes and known-but-unexpected attributes in the
message. Unknown comprehension-optional attributes MUST be ignored
by the agent. Known-but-unexpected attributes SHOULD be ignored by
the agent. Unknown comprehension-required attributes cause
processing that depends on the message class and is a request, described below.

At this point, further processing depends on the server MUST reject message class of the
request.

6.3.1. Processing a Request

If the request contains one or more unknown comprehension-required
attributes, the server replies with an error response. This response MUST use with an error
code of 401 (Unauthenticated).

* If the message is 420 (Unknown Attribute) and includes an indication, UNKNOWN-ATTRIBUTES
attribute in the agent MUST silently
discard response that lists the indication.

o If unknown comprehension-
required attributes.

Otherwise, the MESSAGE-INTEGRITY-SHA256 attribute is present compute server then does any additional checking that the
value for
method or the message integrity specific usage requires. If all the checks succeed,
the server formulates a success response as described in Section 14.6,
using below.

When run over UDP or DTLS-over-UDP, a request received by the password associated with server
could be the username. If first request of a transaction or could be a
retransmission. The server MUST respond to retransmissions such that
the MESSAGE-
INTEGRITY-SHA256 attribute following property is not present, then use the same
password to compute preserved: if the value for client receives the message integrity as
described in Section 14.5. If
response to the resulting value does retransmission and not match
the contents of the corresponding attribute (MESSAGE-INTEGRITY-
SHA256 or MESSAGE-INTEGRITY):

* If response that was sent to
the message is a original request, the overall state on the client and server MUST reject is
identical to the case where only the request
with an error response. This response MUST use an error code
of 401 (Unauthenticated).

* If to the message original
retransmission is an indication, the agent MUST silently
discard received or where both responses are received (in
which case the indication.

If these checks pass, client will use the agent continues first). The easiest way to process meet
this requirement is for the request server to remember all transaction IDs
received over UDP or
indication. Any response generated by a DTLS-over-UDP and their corresponding responses
in the last 40 seconds. However, this requires the server to a request hold
state and is inappropriate for any requests that
contains a MESSAGE-INTEGRITY-SHA256 attribute are not
authenticated. Another way is to reprocess the request and recompute
the response. The latter technique MUST include only be applied to requests
that are idempotent (a request is considered idempotent when the same
request can be safely repeated without impacting the overall state of
the
MESSAGE-INTEGRITY-SHA256 attribute, computed using system) and result in the password
utilized to authenticate same success response for the same
request. Any response generated by a
server The Binding method is considered to be idempotent. Note
that there are certain rare network events that could cause the
reflexive transport address value to change, resulting in a different
mapped address in different success responses. Extensions to STUN
MUST discuss the implications of request retransmissions on servers
that contains only do not store transaction state.

6.3.1.1. Forming a MESSAGE-INTEGRITY attribute
MUST include Success or Error Response

When forming the MESSAGE-INTEGRITY attribute, computed using response (success or error), the
password utilized to authenticate server follows the request. This means that only
one
rules of these attributes can appear in a response. Section 6. The method of the response MUST
NOT contain is the USERNAME attribute.

If any same as that
of the checks fail, a request, and the message class is either "Success Response" or
"Error Response".

For an error response, the server MUST NOT include a MESSAGE-
INTEGRITY-SHA256, MESSAGE-INTEGRITY, or USERNAME add an ERROR-CODE attribute in
containing the error response. This is because, code specified in these failure cases, the server
cannot determine the shared secret necessary to compute the MESSAGE-
INTEGRITY-SHA256 or MESSAGE-INTEGRITY attributes.

9.1.4. Receiving a Response processing above. The client looks
reason phrase is not fixed but SHOULD be something suitable for the MESSAGE-INTEGRITY or
error code. For certain errors, additional attributes are added to
the MESSAGE-INTEGRITY-
SHA256 attribute message. These attributes are spelled out in the response. If present and if description
where the client only
sent only one error code is specified. For example, for an error code of MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
420 (Unknown Attribute), the server MUST include an UNKNOWN-
ATTRIBUTES attribute. Certain authentication errors also cause
attributes to be added (see Section 9). Extensions may define other
errors and/or additional attributes to add in error cases.

If the request (because of server authenticated the external indication in
Section 9.1.2, or this being a subsequent request as defined in
Section 9.1.5) using an authentication
mechanism, then the algorithm in server SHOULD add the response has appropriate authentication
attributes to match otherwise the response MUST be discarded. (see Section 9).

The client then computes the message integrity over server also adds any attributes required by the response as
defined in Section 14.5 specific method
or Section 14.6, respectively, using usage. In addition, the same
password it utilized for server SHOULD add a SOFTWARE attribute to
the request. If message.

For the resulting value matches Binding method, no additional checking is required unless the contents of
usage specifies otherwise. When forming the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute, respectively, success response, the response is considered authenticated.
If
server adds an XOR-MAPPED-ADDRESS attribute to the value does not match, or if both MESSAGE-INTEGRITY and
MESSAGE-INTEGRITY-SHA256 were absent, response; this
attribute contains the processing depends on source transport address of the request been sent over a reliable
message. For UDP or an unreliable transport.

If DTLS-over-UDP, this is the request was sent over an unreliable transport, source IP address and
source UDP port of the response
MUST be discarded, as if it was never received. This means that
retransmits, if applicable, will continue. If all request message. For TCP and TLS-over-TCP,
this is the responses
received are discarded then instead source IP address and source TCP port of signaling a timeout after
ending the transaction TCP
connection as seen by the server.

6.3.1.2. Sending the layer MUST signal that Success or Error Response

The response (success or error) is sent over the integrity
protection same transport as
the request was violated. received on. If the request was sent received over a reliable transport, UDP or
DTLS-over-UDP, the response MUST
be discarded destination IP address and port of the layer MUST immediately end response
are the transaction source IP address and
signal that the integrity protection was violated.

9.1.5. Sending Subsequent Requests

A client sending subsequent requests to the same server MUST send
only the MESSAGE-INTEGRITY-SHA256 or the MESSAGE-INTEGRITY attribute
that matches port of the attribute that was received in request message,
and the source IP address and port of the response are equal to the
initial request. Here same server means same
destination IP address and port
number, not just of the same URI received request message. If
the request was received over TCP or SRV lookup result.

9.2. Long-Term Credential Mechanism

The long-term credential mechanism relies on a long-term credential,
in TLS-over-TCP, the form of a username and password that are shared between client
and server. The credential is considered long-term since it is
assumed that it response is provisioned for a user, and remains in effect
until
sent back on the user is no longer a subscriber of same TCP connection as the system, or is
changed. This request was received on.

The server is basically a traditional "log-in" username and
password given to users.

Because these usernames and passwords are expected allowed to be valid for
extended periods of time, replay prevention is provided send responses in a different order than it
received the requests.

6.3.2. Processing an Indication

If the indication contains unknown comprehension-required attributes,
the form
of a digest challenge. In this mechanism, indication is discarded and processing ceases.

Otherwise, the client initially sends
a request, without offering agent then does any credentials additional checking that the
method or any integrity checks.
The server rejects this request, providing the user a realm (used to
guide specific usage requires. If all the checks succeed,
the user or agent in selection of a username and password) and
a nonce. The nonce provides a limited replay protection. It then processes the indication. No response is a
cookie, selected generated
for an indication.

For the Binding method, no additional checking or processing is
required, unless the usage specifies otherwise. The mere receipt of
the message by the server, and encoded agent has refreshed the bindings in such a way as the
intervening NATs.

Since indications are not re-transmitted over UDP or DTLS-over-UDP
(unlike requests), there is no need to
indicate a duration handle re-transmissions of validity or client identity from which it
indications at the sending agent.

6.3.3. Processing a Success Response

If the success response contains unknown comprehension-required
attributes, the response is
valid. Only discarded and the server needs transaction is
considered to know about the internal structure of have failed.

Otherwise, the cookie. The client retries then does any additional checking that the request, this time including its
username and
method or the realm, and echoing specific usage requires. If all the nonce provided by checks succeed,
the server.
The client also includes one of then processes the message-integrity attributes
defined in this document, which provides an HMAC over success response.

For the entire
request, including Binding method, the client checks that the XOR-MAPPED-ADDRESS
attribute is present in the nonce. response. The server validates the nonce and client checks the message integrity. address
family specified. If they match, the request it is
authenticated. If an unsupported address family, the nonce is no longer valid,
attribute SHOULD be ignored. If it is considered
"stale", and the server rejects an unexpected but supported
address family (for example, the request, providing a new nonce.

In subsequent requests to Binding transaction was sent over
IPv4, but the same server, address family specified is IPv6), then the client reuses the
nonce, username, realm, MAY
accept and password it used previously. In this
way, subsequent requests are not rejected until the nonce becomes
invalid by the server, in which case the rejection provides a new
nonce to use the client.

Note that value.

6.3.4. Processing an Error Response

If the long-term credential mechanism cannot be used to
protect indications, since indications cannot be challenged. Usages
utilizing indications must either use a short-term credential error response contains unknown comprehension-required
attributes, or omit
authentication and message integrity for them.

To indicate that it supports this specification, a server MUST
prepend if the NONCE attribute value with error response does not contain an ERROR-CODE
attribute, then the character string composed
of "obMatJos2" concatenated with transaction is simply considered to have failed.

Otherwise, the (4 character) Base64 [RFC4648]
encoding of client then does any processing specified by the 24 bit STUN Security Features as defined in
authentication mechanism (see Section 18.1. 9). This may result in a new
transaction attempt.

The 24 bit Security Feature set is encoded as 3 bytes,
with bit 0 as processing at this point depends on the most significant bit of error code, the first byte method,
and bit 23
as the least significant bit of usage; the third byte. If no security
features following are used, then a byte array with all 24 bits set to zero
MUST be encoded instead. For the remainder of this document default rules:

o If the term
"nonce cookie" will refer to error code is 300 through 399, the complete 13 character string
prepended to client SHOULD consider
the NONCE attribute value.

Since transaction as failed unless the long-term credential mechanism ALTERNATE-SERVER extension
(Section 10) is susceptible to offline
dictionary attacks, deployments SHOULD utilize passwords that are
difficult to guess. In cases where being used.

o If the credentials are not entered
by error code is 400 through 499, the user, but are rather placed on a client device during device
provisioning, declares the password SHOULD have at least 128 bits of
randomness. In cases where
transaction failed; in the credentials are entered by case of 420 (Unknown Attribute), the user,
they
response should follow best current practices around password structure.

9.2.1. Bid Down Attack Prevention

This document introduces two new security features contain a UNKNOWN-ATTRIBUTES attribute that provide gives
additional information.

o If the
ability to choose error code is 500 through 599, the algorithm used for password protection as well
as client MAY resend the ability
request; clients that do so MUST limit the number of times they do
this. Unless a specific error code specifies a different value,
the number of retransmissions SHOULD be limited to use 4.

Any other error code causes the client to consider the transaction
failed.

7. FINGERPRINT Mechanism

This section describes an anonymous username. Both of these
capabilities are optional mechanism for STUN that aids in order to remain backwards compatible
with previous versions
distinguishing STUN messages from packets of other protocols when the STUN protocol.

These new capabilities
two are subject to bid-down attacks whereby an
attacker in multiplexed on the message path can remove these capabilities same transport address. This mechanism is
optional, and force
weaker security properties. To prevent these kinds of attacks from
going undetected, the nonce a STUN Usage must describe if and when it is enhanced with additional information. used. The value of the "nonce cookie" will vary based
FINGERPRINT mechanism is not backwards compatible with RFC 3489 and
cannot be used in environments where such compatibility is required.

In some usages, STUN messages are multiplexed on the specific STUN
Security Features bit values selected. When this document makes
reference same transport
address as other protocols, such as the Real-Time Transport Protocol
(RTP). In order to apply the "nonce cookie" processing described in a section discussing a specific Section 6, STUN Security Feature it is understood that
messages must first be separated from the application packets.

Section 5 describes three fixed fields in the corresponding STUN
Security Feature bit header that can be
used for this purpose. However, in some cases, these three fixed
fields may not be sufficient.

When the "nonce cookie" FINGERPRINT extension is set to 1.

For example, used, an agent includes the
FINGERPRINT attribute in messages it sends to another agent.
Section 9.2.4 discussing 14.7 describes the PASSWORD-ALGORITHMS
security feature, placement and value of this attribute.

When the agent receives what it believes is implied that the "Password algorithms" bit,
as defined a STUN message, then, in Section 18.1, is set
addition to 1 in other basic checks, the "nonce cookie".

9.2.2. HMAC Key

For long-term credentials agent also checks that do not use the
message contains a different algorithm, as
specified by FINGERPRINT attribute and that the PASSWORD-ALGORITHM attribute, attribute
contains the key is 16 bytes:

key = MD5(username ":" OpaqueString(realm)
":" OpaqueString(password))

Where MD5 is defined correct value. Section 6.3 describes when in [RFC1321] and [RFC6151], and the OpaqueString
profile is defined in [RFC8265]. The encoding used is UTF-8
[RFC3629].

The 16-byte key
overall processing of a STUN message the FINGERPRINT check is formed by taking
performed. This additional check helps the MD5 hash agent detect messages of the result
other protocols that might otherwise seem to be STUN messages.

8. DNS Discovery of
concatenating the following five fields: (1) a Server

This section describes an optional procedure for STUN that allows a
client to use DNS to determine the username, with any
quotes IP address and trailing nulls removed, as taken from port of a server.
A STUN Usage must describe if and when this extension is used. To
use this procedure, the USERNAME
attribute (in which case OpaqueString has already been applied); (2) client must know a single colon; (3) STUN URI [RFC7064]; the realm, with any quotes and trailing nulls
removed and after processing using OpaqueString; (4)
usage must also describe how the client obtains this URI. Hard-
coding a single colon;
and (5) STUN URI into software is NOT RECOMMENDED in case the password, with any trailing nulls removed and after
processing using OpaqueString. For example, if domain
name is lost or needs to change for legal or other reasons.

When a client wishes to locate a STUN server on the username was
'user', public Internet
that accepts Binding request/response transactions, the realm was 'realm', and STUN URI
scheme is "stun". When it wishes to locate a STUN server that
accepts Binding request/response transactions over a TLS or DTLS
session, the password was 'pass', then URI scheme is "stuns".

The syntax of the
16-byte HMAC key would be "stun" and "stuns" URIs is defined in Section 3.1
of [RFC7064]. STUN Usages MAY define additional URI schemes.

8.1. STUN URI Scheme Semantics

If the result <host> part of performing a "stun" URI contains an MD5 hash on IP address, then this
IP address is used directly to contact the
string 'user:realm:pass', server. A "stuns" URI
containing an IP address MUST be rejected. A future STUN extension
or usage may relax this requirement, provided it demonstrates how to
authenticate the resulting hash being
0x8493fbc53ba582fb4c044c456bdc40eb.

The structure of STUN server and prevent man-in-the-middle attacks.

If the key when used with long-term credentials
facilitates deployment in systems that also utilize SIP [RFC3261].
Typically, SIP systems utilizing SIP's digest authentication
mechanism do URI does not actually store contain an IP address, the password domain name contained
in the database.
Rather, they store a value called H(A1), which <host> part is equal resolved to a transport address using the key
defined above. For example, this mechanism can be used with the
authentication extensions defined SRV
procedures specified in [RFC5090].

When a PASSWORD-ALGORITHM [RFC2782]. The DNS SRV service name is used, the key length and algorithm to
use are described in Section 18.5.1.

9.2.3. Forming a Request

There are two cases when forming a request. In
content of the <scheme> part. The protocol in the first case, this SRV lookup is the first request from
transport protocol the client will run STUN over: "udp" for UDP and
"tcp" for TCP.

The procedures of RFC 2782 are followed to determine the server (as identified by
hostname, if to
contact. RFC 2782 spells out the DNS procedures details of Section 8 are used, else IP
address how a set of SRV records
is sorted and then tried. However, RFC 2782 only states that the
client should "try to connect to the (protocol, address, service)"
without giving any details on what happens in the event of failure.
When following these procedures, if not). In the second case, STUN transaction times out
without receipt of a response, the client is submitting a
subsequent SHOULD retry the request once to
the next server in the order defined by RFC 2782. Such a previous retry is
only possible for request/response transaction has
completed successfully. Forming a request as a consequence transmissions, since indication
transactions generate no response or timeout.

In addition, instead of a 401 querying either the A or 438 error response is covered in Section 9.2.5 and is not
considered the AAAA resource
records for a "subsequent request" domain name, a dual-stack IPv4/IPv6 client MUST query
both and thus does not utilize try the rules
described requests with all the IP addresses received, as
specified in Section 9.2.3.2. [RFC8305].

The difference between a first request default port for STUN requests is 3478, for both TCP and a subsequent request UDP.
The default port for STUN over TLS and STUN over DTLS requests is
5349. Servers can run STUN over DTLS on the same port as STUN over
UDP if the server software supports determining whether the presence or absence of some attributes, so omitting or including
them initial
message is a MUST.

9.2.3.1. First Request

If DTLS or STUN message. Servers can run STUN over TLS on
the client has not completed a successful request/response
transaction with same port as STUN over TCP if the server, it MUST omit server software supports
determining whether the USERNAME, USERHASH,
MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD-
ALGORITHMS, initial message is a TLS or STUN message.

Administrators of STUN servers SHOULD use these ports in their SRV
records for UDP and PASSWORD-ALGORITHM attributes. TCP. In other words, all cases, the
first request port in DNS MUST reflect
the one on which the server is sent as if there were listening.

If no authentication or message
integrity applied.

9.2.3.2. Subsequent Requests

Once a request/response transaction has completed, SRV records are found, the client will
have been presented a realm performs both an A and nonce by AAAA
record lookup of the server, and selected domain name, as described in [RFC8305]. The
result will be a
username and password with list of IP addresses, each of which it authenticated. The can be
simultaneously contacted at the default port using UDP or TCP,
independent of the STUN Usage. For usages that require TLS, the
client SHOULD
cache connects to the username, password, realm, and nonce for subsequent
communications with IP addresses using the server. When default STUN over TLS
port. For usages that require DTLS, the client sends a subsequent
request, it MUST include either connects to the USERNAME or USERHASH, REALM,
NONCE, IP
addresses using the default STUN over DTLS port.

9. Authentication and Message-Integrity Mechanisms

This section defines two mechanisms for STUN that a client and PASSWORD-ALGORITHM attributes with server
can use to provide authentication and message integrity; these cached values.
It MUST include a MESSAGE-INTEGRITY attribute or a MESSAGE-INTEGRITY-
SHA256 attribute, computed two
mechanisms are known as described in Section 14.5 and
Section 14.6 using the cached password. The choice between short-term credential mechanism and the
long-term credential mechanism. These two
attributes depends mechanisms are optional,
and each usage must specify if and when these mechanisms are used.
Consequently, both clients and servers will know which mechanism (if
any) to follow based on knowledge of which usage applies. For
example, a STUN server on the attribute received public Internet supporting ICE would
have no authentication, whereas the STUN server functionality in an
agent supporting connectivity checks would utilize short-term
credentials. An overview of these two mechanisms is given in
Section 2.

Each mechanism specifies the response additional processing required to use
that mechanism, extending the
first request.

9.2.4. Receiving a Request

After the server has done the basic processing of specified in Section 6. The
additional processing occurs in three different places: when forming
a request, it
performs message, when receiving a message immediately after the basic
checks listed below in have been performed, and when doing the order specified. detailed processing of
error responses.

Note that
it is RECOMMENDED agents MUST ignore all attributes that follow MESSAGE-
INTEGRITY, with the REALM value be the domain name exception of the
provider MESSAGE-INTEGRITY-SHA256 and
FINGERPRINT attributes. Similarly, agents MUST ignore all attributes
that follow the MESSAGE-INTEGRITY-SHA256 attribute if the MESSAGE-
INTEGRITY attribute is not present, with the exception of the
FINGERPRINT attribute.

9.1. Short-Term Credential Mechanism

The short-term credential mechanism assumes that, prior to the STUN server:

o If
transaction, the message does not contain client and server have used some other protocol to
exchange a MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute, credential in the server MUST generate an error
response with an error code form of 401 (Unauthenticated). This
response MUST include a REALM value. username and password. This
credential is time-limited. The response MUST include a
NONCE, selected time limit is defined by the server. The server MUST NOT choose usage.
As an example, in the
same NONCE for two requests unless they have ICE usage [RFC8445], the same source IP
address two endpoints use out-
of-band signaling to agree on a username and password, and this
username and port. The server MAY support alternate password
algorithms, in which case it can list them in preferential order
in a PASSWORD-ALGORITHMS attribute. If are applicable for the server adds a
PASSWORD-ALGORITHMS attribute it MUST set duration of the STUN Security
Feature "Password algorithms" bit set media
session.

This credential is used to 1. The server MAY
support anonymous username, form a message-integrity check in which case it MUST set the STUN
Security Feature "Username anonymity" bit set to 1. The each
request and in many responses. There is no challenge and response
SHOULD NOT contain a USERNAME, USERHASH, MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute.

Note: Reusing a NONCE for different source IP addresses or ports was
not explicitly forbidden as
in [RFC5389].

o If the message contains long-term mechanism; consequently, replay is limited by virtue
of the time-limited nature of the credential.

9.1.1. HMAC Key

For short-term credentials, the Hash-Based Message Authentication
Code (HMAC) key is defined as follow:

key = OpaqueString(password)

where the OpaqueString profile is defined in [RFC8265]. The encoding
used is UTF-8 [RFC3629].

9.1.2. Forming a MESSAGE-INTEGRITY Request or Indication

For a MESSAGE-
INTEGRITY-SHA256 attribute, but is missing either the USERNAME or
USERHASH, REALM, request or NONCE attribute, indication message, the server agent MUST generate an
error response with an error code of 400 (Bad Request). This
response SHOULD NOT include a the
USERNAME, USERHASH, NONCE, or REALM.
The response cannot contain a MESSAGE-INTEGRITY-SHA256, and MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute, as the attributes required to generate
them are missing.

o If the NONCE attribute starts with
in the "nonce cookie" with message unless the
STUN Security Feature "Password algorithms" bit set agent knows from an external mechanism
which message integrity algorithm is supported by both agents. In
this case, either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST
be included in addition to 1, USERNAME. The HMAC for the
server performs these checks MESSAGE-
INTEGRITY attribute is computed as described in Section 14.5, and the order specified:

* If the request contains neither PASSWORD-ALGORITHMS nor
PASSWORD-ALGORITHM, then
HMAC for the request MESSAGE-INTEGRITY-SHA256 attributes is processed computed as though
PASSWORD-ALGORITHM were MD5 (Note
described in Section 14.6. Note that if the PASSWORD-
ALGORITHMS attribute password is present but does not contain MD5, this
will result never included
in the request or indication.

9.1.3. Receiving a 400 Bad Request in a later step below).

* Otherwise, unless (1) PASSWORD-ALGORITHM and PASSWORD-
ALGORITHMS are both present, (2) PASSWORD-ALGORITHMS matches or Indication

After the value sent in agent has done the response that sent this NONCE, and (3)
PASSWORD-ALGORITHM matches one basic processing of a message, the entries agent
performs the checks listed below in PASSWORD-
ALGORITHMS, the server MUST generate an error response with an
error code of 400 (Bad Request). order specified:

o If the NONCE is no longer valid and at the same time the MESSAGE-
INTEGRITY message does not contain 1) a MESSAGE-INTEGRITY or a
MESSAGE-INTEGRITY-SHA256 attribute is invalid, the
server MUST generate an error response with an error code of 401.
This response MUST include NONCE, REALM, and PASSWORD-ALGORITHMS
attributes and SHOULD NOT include the USERNAME or USERHASH
attribute. The NONCE attribute value MUST be valid. The response
MAY include 2) a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute, using the previous NONCE to calculate it.

o USERNAME attribute:

* If the NONCE message is no longer valid, a request, the server MUST generate reject the request
with an error response. This response with MUST use an error code
of 438 (Stale Nonce). This response
MUST include NONCE, REALM, and PASSWORD-ALGORITHMS attributes and
SHOULD NOT include 400 (Bad Request).

* If the USERNAME, USERHASH attribute. The NONCE
attribute value message is an indication, the agent MUST be valid. The response MAY include a
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using silently
discard the
previous NONCE to calculate it. Servers can revoke nonces in
order to provide additional security. See Section 5.4 of
[RFC7616] for guidelines. indication.

o If the USERNAME does not contain a username value of currently valid
within the USERNAME or USERHASH attribute server:

* If the message is not valid, a request, the server MUST generate reject the request
with an error response. This response with MUST use an error code
of 401 (Unauthenticated). This response MUST include a REALM value.
The response MUST include a NONCE, selected by

* If the server. The
response MUST include a PASSWORD-ALGORITHMS attribute. The
response SHOULD NOT contain a USERNAME, USERHASH attribute. The
response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, using message is an indication, the previous key to calculate it. agent MUST silently
discard the indication.

o If the MESSAGE-INTEGRITY-SHA256 attribute is present present, compute the
value for the message integrity as described in Section 14.6,
using the password associated with the username. Else, using If the MESSAGE-
INTEGRITY-SHA256 attribute is not present, then use the same password,
password to compute the value for the message integrity as
described in Section 14.5. If the resulting value does not match
the contents of the MESSAGE-INTEGRITY corresponding attribute (MESSAGE-INTEGRITY-
SHA256 or MESSAGE-INTEGRITY):

* If the MESSAGE-
INTEGRITY-SHA256 attribute, message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 401 (Unauthenticated). It

* If the message is an indication, the agent MUST silently
discard the indication.

If these checks pass, the agent continues to process the request or
indication. Any response generated by a server to a request that
contains a MESSAGE-INTEGRITY-SHA256 attribute MUST include REALM and NONCE the
MESSAGE-INTEGRITY-SHA256 attribute, computed using the password
utilized to authenticate the request. Any response generated by a
server to a request that contains only a MESSAGE-INTEGRITY attribute
MUST include the MESSAGE-INTEGRITY attribute, computed using the
password utilized to authenticate the request. This means that only
one of these attributes
and SHOULD can appear in a response. The response MUST
NOT contain the USERNAME attribute.

If any of the checks fail, a server MUST NOT include a MESSAGE-
INTEGRITY-SHA256, MESSAGE-INTEGRITY, or USERNAME attribute in the
error response. This is because, in these failure cases, the server
cannot determine the shared secret necessary to compute the MESSAGE-
INTEGRITY-SHA256 or MESSAGE-INTEGRITY attributes.

9.1.4. Receiving a Response

The client looks for the MESSAGE-INTEGRITY or the MESSAGE-INTEGRITY-
SHA256 attribute in the response. If present and if the client only
sent one of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attributes in the request (because of the external indication in
Section 9.1.2 or because this is a subsequent request as defined in
Section 9.1.5), the algorithm in the response has to match;
otherwise, the response MUST be discarded.

The client then computes the message integrity over the USERNAME, USERHASH, MESSAGE-INTEGRITY, response as
defined in Section 14.5 for the MESSAGE-INTEGRITY attribute or
Section 14.6 for the MESSAGE-INTEGRITY-SHA256 attribute.

If these checks pass, attribute, using the server continues to process
same password it utilized for the request.
Any response generated by If the server MUST include resulting value
matches the contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, computed using respectively, the username response is considered
authenticated. If the value does not match, or if both MESSAGE-
INTEGRITY and password utilized
to authenticate MESSAGE-INTEGRITY-SHA256 are absent, the request, unless processing
depends on whether the request was processed as
though PASSWORD-ALGORITHM was MD5 (because sent over a reliable or an
unreliable transport.

If the request contained
neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM). In that case was sent over an unreliable transport, the MESSAGE-INTEGRITY attribute response
MUST be used discarded, as if it had never been received. This means that
retransmits, if applicable, will continue. If all the responses
received are discarded, then instead of the MESSAGE-
INTEGRITY-SHA256 attribute. The REALM, NONCE, USERNAME and USERHASH
attributes SHOULD NOT be included.

9.2.5. Receiving signaling a Response

If timeout after
ending the response is an error response with an error code of 401
(Unauthenticated) or 438 (Stale Nonce), transaction, the client layer MUST test if the
NONCE attribute value starts with signal that the "nonce cookie". integrity
protection was violated.

If the test
succeeds request was sent over a reliable transport, the response MUST
be discarded, and the "nonce cookie" has layer MUST immediately end the STUN Security Feature
"Password algorithms" bit set to 1 but no PASSWORD-ALGORITHMS
attribute is present, then transaction and
signal that the integrity protection was violated.

9.1.5. Sending Subsequent Requests

A client sending subsequent requests to the same server MUST NOT retry send
only the request with
a new transaction.

If MESSAGE-INTEGRITY-SHA256 or the MESSAGE-INTEGRITY attribute
that matches the attribute that was received in the response is an error response with an error code of 401
(Unauthenticated), to the client SHOULD retry
initial request. Here, "same server" means same IP address and port
number, not just the request with a new
transaction. This request MUST contain a USERNAME same URI or SRV lookup result.

9.2. Long-Term Credential Mechanism

The long-term credential mechanism relies on a USERHASH,
determined by the client as long-term credential,
in the appropriate form of a username and password that are shared between client
and server. The credential is considered long-term since it is
assumed that it is provisioned for a user and remains in effect until
the REALM
from the error response. If user is no longer a subscriber of the "nonce cookie" was present system or until it is
changed. This is basically a traditional "log-in" username and had
the STUN Security Feature "Username anonymity" bit set
password given to users.

Because these usernames and passwords are expected to 1 then the
USERHASH attribute MUST be used, else valid for
extended periods of time, replay prevention is provided in the USERNAME attribute MUST be
used. form
of a digest challenge. In this mechanism, the client initially sends
a request, without offering any credentials or any integrity checks.
The request MUST contain server rejects this request, providing the REALM, copied from user a realm (used to
guide the error
response. user or agent in selection of a username and password) and
a nonce. The request MUST contain nonce provides a limited replay protection. It is a
cookie, selected by the NONCE, copied server and encoded in such a way as to
indicate a duration of validity or client identity from which it is
valid. Only the error
response. If the response contains a PASSWORD-ALGORITHMS attribute, server needs to know about the request MUST contain internal structure of
the PASSWORD-ALGORITHMS attribute with cookie. The client retries the
same content. If request, this time including its
username and the response contains a PASSWORD-ALGORITHMS
attribute, realm and this attribute contains at least one algorithm that is
supported echoing the nonce provided by the server.
The client then also includes one of the request MUST contain a PASSWORD-
ALGORITHM attribute with message-integrity attributes
defined in this document, which provides an HMAC over the first algorithm supported on entire
request, including the list.
If nonce. The server validates the response contains a PASSWORD-ALGORITHMS attribute, nonce and this
attribute does not contain any algorithm that is supported by the
client, then
checks the client MUST NOT retry message integrity. If they match, the request with a new
transaction. The client MUST NOT perform this retry if is
authenticated. If the nonce is no longer valid, it is not
changing considered
"stale", and the USERNAME or USERHASH or REALM or its associated
password, from server rejects the previous attempt.

If request, providing a new nonce.

In subsequent requests to the response is an error response with an error code of 438 (Stale
Nonce), same server, the client MUST retry reuses the request, using
nonce, username, realm, and password it used previously. In this
way, subsequent requests are not rejected until the new NONCE
attribute supplied nonce becomes
invalid by the server, in which case the rejection provides a new
nonce to the 438 (Stale Nonce) response. This retry
MUST also include either client.

Note that the USERNAME or USERHASH, REALM and long-term credential mechanism cannot be used to
protect indications, since indications cannot be challenged. Usages
utilizing indications must either
the MESSAGE-INTEGRITY use a short-term credential or MESSAGE-INTEGRITY-SHA256 attributes.

For all other responses, if omit
authentication and message integrity for them.

To indicate that it supports this specification, a server MUST
prepend the NONCE attribute starts value with the
"nonce cookie" character string composed
of "obMatJos2" concatenated with the (4-character) base64 [RFC4648]
encoding of the 24-bit STUN Security Features as defined in
Section 18.1. The 24-bit Security Feature "Password algorithms"
bit set to 1 but PASSWORD-ALGORITHMS is not present, the response
MUST be ignored.

If the response is an error response encoded as 3 bytes,
with an error code of 400, and
does not contains either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute then the response MUST be discarded, bit 0 as if it was
never received. This means that retransmits, if applicable, will
continue.

Note: In that case the 400 will never reach the application,
resulting in a timeout.

The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute in the response (either success or failure). If
present, the client computes the message integrity over most significant bit of the response first byte and bit 23
as defined in Section 14.5 or Section 14.6, using the same password
it utilized for least significant bit of the request. third byte. If no security
features are used, then a byte array with all 24 bits set to zero
MUST be encoded instead. For the resulting value matches remainder of this document, the
term "nonce cookie" will refer to the
contents of complete 13-character string
prepended to the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute, NONCE attribute value.

Since the response long-term credential mechanism is considered authenticated. If susceptible to offline
dictionary attacks, deployments SHOULD utilize passwords that are
difficult to guess. In cases where the value
does credentials are not match, or if both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-
SHA256 were absent, entered
by the processing depends user, but are rather placed on the request been sent
over a reliable or an unreliable transport.

If client device during device
provisioning, the request was sent over an unreliable transport, password SHOULD have at least 128 bits of
randomness. In cases where the response
MUST be discarded, as if it was never received. credentials are entered by the user,
they should follow best current practices around password structure.

9.2.1. Bid-Down Attack Prevention

This means document introduces two new security features that
retransmits, if applicable, will continue. If all provide the responses
received
ability to choose the algorithm used for password protection as well
as the ability to use an anonymous username. Both of these
capabilities are discarded then instead optional in order to remain backwards compatible
with previous versions of signaling a timeout after
ending the transaction the layer MUST signal that STUN protocol.

These new capabilities are subject to bid-down attacks whereby an
attacker in the integrity
protection was violated.

If message path can remove these capabilities and force
weaker security properties. To prevent these kinds of attacks from
going undetected, the request was sent over a reliable transport, nonce is enhanced with additional information.

The value of the response MUST
be discarded and "nonce cookie" will vary based on the layer MUST immediately end specific STUN
Security Feature bits selected. When this document makes reference
to the transaction and
signal "nonce cookie" in a section discussing a specific STUN
Security Feature it is understood that the integrity protection was violated.

If corresponding STUN
Security Feature bit in the "nonce cookie" is set to 1.

For example, when the response contains a PASSWORD-ALGORITHMS attribute, all security feature (defined
in Section 9.2.4) is used, the
subsequent requests MUST be authenticated using MESSAGE-INTEGRITY-
SHA256 only.

10. ALTERNATE-SERVER Mechanism

This section describes a mechanism corresponding "Password algorithms"
bit (defined in STUN that allows a server Section 18.1) is set to
redirect 1 in the "nonce cookie".

9.2.2. HMAC Key

For long-term credentials that do not use a client to another server. This extension different algorithm, as
specified by the PASSWORD-ALGORITHM attribute, the key is optional, 16 bytes:

key = MD5(username ":" OpaqueString(realm)
":" OpaqueString(password))

Where MD5 is defined in [RFC1321] and
a usage must define if [RFC6151], and when this extension the OpaqueString
profile is used. defined in [RFC8265]. The
ALTERNATE-SERVER attribute carries an IP address.

A server using this extension redirects a client to another server encoding used is UTF-8
[RFC3629].

The 16-byte key is formed by
replying to a request message with an error response message with an
error code taking the MD5 hash of 300 (Try Alternate). The server MUST include at least
one ALTERNATE-SERVER attribute in the error response, which MUST
contain an IP address result of
concatenating the same family following five fields: (1) the username, with any
quotes and trailing nulls removed, as taken from the USERNAME
attribute (in which case OpaqueString has already been applied); (2)
a single colon; (3) the source IP address of realm, with any quotes and trailing nulls
removed and after processing using OpaqueString; (4) a single colon;
and (5) the request message. The server SHOULD include an additional
ALTERNATE-SERVER attribute, password, with any trailing nulls removed and after
processing using OpaqueString. For example, if the mandatory one, that contains an
IP address of username is
'user', the other family than realm is 'realm', and the source IP address password is 'pass', then the
16-byte HMAC key would be the result of performing an MD5 hash on the
request message.
string 'user:realm:pass', the resulting hash being
0x8493fbc53ba582fb4c044c456bdc40eb.

The error response message MAY be authenticated;
however, there are use cases for ALTERNATE-SERVER where
authentication structure of the response is key when used with long-term credentials
facilitates deployment in systems that also utilize SIP [RFC3261].
Typically, SIP systems utilizing SIP's digest authentication
mechanism do not possible or practical. If actually store the
transaction uses TLS or DTLS and if password in the transaction database.
Rather, they store a value called "H(A1)", which is authenticated
by equal to the key
defined above. For example, this mechanism can be used with the
authentication extensions defined in [RFC5090].

When a MESSAGE-INTEGRITY-SHA256 attribute and if PASSWORD-ALGORITHM is used, the server wants to
redirect key length and algorithm to
use are described in Section 18.5.1.

9.2.3. Forming a server that uses a different certificate, then it MUST
include an ALTERNATE-DOMAIN attribute containing the name inside the
subjectAltName of that certificate. This series of conditions on Request

The first request from the
MESSAGE-INTEGRITY-SHA256 attribute indicates that client to the transaction is
authenticated and that server (as identified by
hostname if the client implements this specification DNS procedures of Section 8 are used and
therefore can process by IP
address if not) is handled according to the rules in Section 9.2.3.1.
When the ALTERNATE-DOMAIN attribute.

A client using this extension handles initiates a 300 (Try Alternate) error
code subsequent request once a previous
request/response transaction has completed successfully, it follows
the rules in Section Section 9.2.3.2. Forming a request as follows. The client looks for an ALTERNATE-SERVER attribute a
consequence of a 401 (Unauthenticated) or 438 (Stale Nonce) error
response is covered in Section 9.2.5 and is not considered a
"subsequent request" and thus does not utilize the error response. rules described in
Section 9.2.3.2. Each of these types of requests have a different
mandatory attributes.

9.2.3.1. First Request

If one is found, then the client considers
the current has not completed a successful request/response
transaction as failed, and reattempts the request with the server specified in the attribute, using server, it MUST omit the same transport
protocol used for USERNAME, USERHASH,
MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD-
ALGORITHMS, and PASSWORD-ALGORITHM attributes. In other words, the previous request. That request,
first request is sent as if
authenticated, MUST utilize the same credentials that there were no authentication or message
integrity applied.

9.2.3.2. Subsequent Requests

Once a request/response transaction has completed, the client
would will
have used in the request to been presented a realm and nonce by the server that performed the
redirection. If the transport protocol uses TLS or DTLS, then the and selected a
username and password with which it authenticated. The client looks for an ALTERNATE-DOMAIN attribute. If the attribute is
found, SHOULD
cache the domain MUST be used to validate username, password, realm, and nonce for subsequent
communications with the certificate using server. When the
recommendations in [RFC6125]. The certificate client sends a subsequent
request, it MUST contain an
identifier of type DNS-ID include either the USERNAME or CN-ID, eventually USERHASH, REALM,
NONCE, and PASSWORD-ALGORITHM attributes with wildcards, but
not of type SRV-ID or URI-ID. If the these cached values.
It MUST include a MESSAGE-INTEGRITY attribute is not found, or a MESSAGE-INTEGRITY-
SHA256 attribute, computed as described in Sections 14.5 and 14.6
using the
same domain that was used for cached password. The choice between the original request MUST be used to
validate two attributes
depends on the certificate. If attribute received in the client has been redirected response to the first
request.

9.2.4. Receiving a Request

After the server to which it has already sent this request within done the last five
minutes, basic processing of a request, it MUST ignore the redirection and consider
performs the transaction
to have failed. This prevents infinite ping-ponging between servers checks listed below in case of redirection loops.

11. Backwards Compatibility with RFC 3489

In addition to the backward compatibility already described in
Section 12 of [RFC5389], DTLS MUST NOT order specified. Note that
it is RECOMMENDED that the REALM value be used with [RFC3489] (also
referred to as "classic STUN"). Any STUN request or indication
without the magic cookie (see Section 6 domain name of [RFC5389]) over DTLS MUST
be considered invalid: all requests MUST generate a "500 Server
Error" error response and indications MUST be ignored.

12. Basic Server Behavior

This section defines the behavior
provider of a basic, stand-alone STUN
server.

Historically, "classic the STUN [RFC3489]" only defined server:

o If the behavior of message does not contain a MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute, the server that was providing clients MUST generate an error
response with server reflexive transport
addresses an error code of 401 (Unauthenticated). This
response MUST include a REALM value. The response MUST include a
NONCE, selected by receiving and replying to STUN Binding requests.
[RFC5389] redefined the protocol as an extensible framework server. The server MUST NOT choose the
same NONCE for two requests unless they have the same source IP
address and port. The server MAY support alternate password
algorithms, in which case it can list them in preferential order
in a PASSWORD-ALGORITHMS attribute. If the server functionality became adds a
PASSWORD-ALGORITHMS attribute, it MUST set the sole STUN Usage defined in that
document. This STUN Usage is also known as Basic STUN Server. Security
Feature "Password algorithms" bit to 1. The STUN server MUST MAY support
anonymous username, in which case it MUST set the Binding method. It STUN Security
Feature "Username anonymity" bit set to 1. The response SHOULD
NOT
utilize contain a USERNAME, USERHASH, MESSAGE-INTEGRITY, or MESSAGE-
INTEGRITY-SHA256 attribute.

Note: Reusing a NONCE for different source IP addresses or ports
was not explicitly forbidden in [RFC5389].

o If the short-term message contains a MESSAGE-INTEGRITY or long-term credential mechanism. This a MESSAGE-
INTEGRITY-SHA256 attribute, but is
because missing either the USERNAME or
USERHASH, REALM, or NONCE attribute, the server MUST generate an
error response with an error code of 400 (Bad Request). This
response SHOULD NOT include a USERNAME, USERHASH, NONCE, or REALM
attribute. The response cannot contain a MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute, as the attributes required to
generate them are missing.

o If the NONCE attribute starts with the "nonce cookie" with the work involved
STUN Security Feature "Password algorithms" bit set to 1, the
server performs these checks in authenticating the order specified:

* If the request is more than contains neither the work in simply processing it. It SHOULD NOT utilize PASSWORD-ALGORITHMS nor the
ALTERNATE-SERVER mechanism for
PASSWORD-ALGORITHM algorithm, then the same reason. It MUST support UDP
and TCP. It MAY support STUN over TCP/TLS or STUN over UDP/DTLS;
however, DTLS request is processed as
though PASSWORD-ALGORITHM were MD5.

* Otherwise, unless (1) PASSWORD-ALGORITHM and TLS provide minimal security benefits PASSWORD-
ALGORITHMS are both present, (2) PASSWORD-ALGORITHMS matches
the value sent in the response that sent this basic
mode NONCE, and (3)
PASSWORD-ALGORITHM matches one of operation. It does not require a keep-alive mechanism
because a TCP or TLS-over-TCP connection is closed after the end entries in PASSWORD-
ALGORITHMS, the server MUST generate an error response with an
error code of 400 (Bad Request).

o If the Binding transaction. It MAY utilize value of the FINGERPRINT mechanism
but MUST NOT require it. Since USERNAME or USERHASH attribute is not valid,
the stand-alone server only runs
STUN, FINGERPRINT provides no benefit. Requiring it would break
compatibility MUST generate an error response with RFC 3489, and such compatibility is desirable in an error code of
401 (Unauthenticated). This response MUST include a
stand-alone REALM value.
The response MUST include a NONCE, selected by the server. Stand-alone STUN servers The
response MUST include a PASSWORD-ALGORITHMS attribute. The
response SHOULD support
backwards compatibility with [RFC3489] clients, NOT contain a USERNAME or USERHASH attribute. The
response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, using the previous key to calculate it.

o If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
value for the message integrity as described in Section 11.

It is RECOMMENDED that administrators of STUN servers provide DNS
entries 14.6,
using the password associated with the username. Otherwise, using
the same password, compute the value for those servers the MESSAGE-INTEGRITY
attribute as described in Section 8. 14.5. If both A and
AAAA Resource Records are returned then the client can simultaneously
send STUN Binding requests to resulting value
does not match the IPv4 and IPv6 addresses (as
specified in [RFC8305]), as contents of the MESSAGE-INTEGRITY attribute or
the MESSAGE-INTEGRITY-SHA256 attribute, the server MUST reject the Binding
request is idempotent. Note
that with an error response. This response MUST use an error
code of 401 (Unauthenticated). It MUST include the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS REALM and
NONCE attributes that are
returned will not necessarily match and SHOULD NOT include the address family of USERNAME, USERHASH,
MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256 attribute.

o If the server
address used.

A basic STUN server NONCE is not a solution for NAT traversal by itself.
However, it can be utilized as part no longer valid, the server MUST generate an error
response with an error code of a solution through STUN
usages. 438 (Stale Nonce). This is discussed further response
MUST include NONCE, REALM, and PASSWORD-ALGORITHMS attributes and
SHOULD NOT include the USERNAME and USERHASH attributes. The
NONCE attribute value MUST be valid. The response MAY include a
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using the
previous NONCE to calculate it. Servers can revoke nonces in
order to provide additional security. See Section 13.

13. STUN Usages

STUN 5.4 of
[RFC7616] for guidelines.

If these checks pass, the server continues to process the request.
Any response generated by itself is not a solution the server MUST include the MESSAGE-
INTEGRITY-SHA256 attribute, computed using the username and password
utilized to authenticate the NAT traversal problem.
Rather, STUN defines a tool request, unless the request was
processed as though PASSWORD-ALGORITHM was MD5 (because the request
contained neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM). In
that can case, the MESSAGE-INTEGRITY attribute MUST be used inside a larger
solution. The term "STUN usage" is used for any solution that uses
STUN as a component.

A STUN usage defines how STUN is actually utilized -- when to send
requests, what to do with instead of
the responses, MESSAGE-INTEGRITY-SHA256 attribute, and which optional
procedures defined here (or in an extension to STUN) are to the REALM, NONCE,
USERNAME, and USERHASH attributes SHOULD NOT be used.
A usage also defines:

o Which STUN methods are used.

o What transports are used. included.

9.2.5. Receiving a Response

If DTLS-over-UDP is used then
implementing the denial-of-service countermeasure described in
Section 4.2.1 of [RFC6347] response is mandatory.

o What authentication an error response with an error code of 401
(Unauthenticated) or 438 (Stale Nonce), the client MUST test if the
NONCE attribute value starts with the "nonce cookie". If so and message-integrity mechanisms are used.

o The considerations around manual vs. automatic key derivation for the integrity mechanism, as discussed in [RFC4107].

o What mechanisms are used to distinguish STUN messages from other
messages. When
"nonce cookie" has the STUN Security Feature "Password algorithms"
bit set to 1 but no PASSWORD-ALGORITHMS attribute is run over TCP or TLS-over-TCP, a framing
mechanism may be required.

o How a STUN present, then
the client determines MUST NOT retry the IP address and port of request with a new transaction.

If the STUN
server.

o How simultaneous use response is an error response with an error code of IPv4 and IPv6 addresses (Happy Eyeballs
[RFC8305]) works 401
(Unauthenticated), the client SHOULD retry the request with non-idempotent transactions when both
address families are found for a new
transaction. This request MUST contain a USERNAME or a USERHASH,
determined by the STUN server.

o Whether backwards compatibility to RFC 3489 is required.

o What optional attributes defined here (such client as FINGERPRINT and
ALTERNATE-SERVER) or in other extensions are required.

o the appropriate username for the REALM
from the error response. If MESSAGE-INTEGRITY-SHA256 truncation the "nonce cookie" is permitted, present and has
the
limits permitted for truncation.

o The keep-alive mechanism if STUN is run over TCP or TLS-over-TCP.

o If Anycast addresses can Security Feature "Username anonymity" bit set to 1, then the
USERHASH attribute MUST be used for used; else, the server in case TCP or
TLS-over-TCP, or authentication are USERNAME attribute MUST be
used.

In addition, any STUN usage must consider The request MUST contain the security implications
of using STUN in that usage. A number of attacks against STUN are
known (see REALM, copied from the Security Considerations section in this document), and
any usage must consider how these attacks can be thwarted or
mitigated.

Finally, a usage must consider whether its usage of STUN is an
example of error
response. The request MUST contain the Unilateral Self-Address Fixing approach to NAT
traversal, and if so, address NONCE, copied from the error
response. If the response contains a PASSWORD-ALGORITHMS attribute,
the questions raised in RFC 3424
[RFC3424].

14. STUN Attributes

After request MUST contain the STUN header are zero or more attributes. Each PASSWORD-ALGORITHMS attribute
MUST be TLV encoded, with the
same content. If the response contains a 16-bit type, 16-bit length, PASSWORD-ALGORITHMS
attribute, and value.
Each STUN attribute MUST end on a 32-bit boundary. As mentioned
above, all fields in an this attribute are transmitted most significant
bit first.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (variable) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Format of STUN Attributes

The value in contains at least one algorithm that is
supported by the length field client, then the request MUST contain a PASSWORD-
ALGORITHM attribute with the length of first algorithm supported on the Value
part of list.
If the response contains a PASSWORD-ALGORITHMS attribute, prior to padding, measured in bytes. Since
STUN aligns attributes on 32-bit boundaries, attributes whose content
is and this
attribute does not a multiple of 4 bytes are padded with 1, 2, or 3 bytes of
padding so contain any algorithm that its value contains is supported by the
client, then the client MUST NOT retry the request with a multiple of 4 bytes. new
transaction. The
padding bits client MUST be set to zero on sending and NOT perform this retry if it is not
changing the USERNAME, USERHASH, REALM, or its associated password
from the previous attempt.

If the response is an error response with an error code of 438 (Stale
Nonce), the client MUST be ignored by retry the receiver.

Any request, using the new NONCE
attribute type MAY appear more than once supplied in a STUN message.
Unless specified otherwise, the order of appearance is significant:
only 438 (Stale Nonce) response. This retry
MUST also include either the first occurrence needs to be processed by a receiver, USERNAME or USERHASH, the REALM, and
any duplicates MAY be ignored by a receiver.

To allow future revisions of this specification to add new attributes
either the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute.

For all other responses, if needed, the NONCE attribute space is divided into two ranges.
Attributes starts with the
"nonce cookie" with type values between 0x0000 and 0x7FFF are
comprehension-required attributes, which means that the STUN agent
cannot successfully process Security Feature "Password algorithms"
bit set to 1 but PASSWORD-ALGORITHMS is not present, the message unless it understands response
MUST be ignored.

If the
attribute. Attributes response is an error response with type values between 0x8000 an error code of 400 (Bad
Request) and 0xFFFF are
comprehension-optional attributes, which means that those attributes
can be ignored by does not contain either the STUN agent MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute, then the response MUST be
discarded, as if it does not understand them.

The set of STUN attribute types is maintained by IANA. The initial
set defined by were never received. This means that
retransmits, if applicable, will continue.

Note: In this specification is found case, the 400 response will never reach the
application, resulting in Section 18.3. a timeout.

The rest of this section describes client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute in the format of response (either success or failure). If
present, the various
attributes client computes the message integrity over the response
as defined in this specification.

14.1. MAPPED-ADDRESS

The MAPPED-ADDRESS attribute indicates a reflexive transport address
of Sections 14.5 or 14.6, using the client. It consists of an 8-bit address family and a 16-bit
port, followed by a fixed-length value representing same password it
utilized for the IP address. request. If the address family resulting value matches the
contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute, the response is IPv4, considered authenticated. If the address MUST be 32 bits. value
does not match, or if both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-
SHA256 are absent, the processing depends on the request being sent
over a reliable or an unreliable transport.

If the
address family is IPv6, request was sent over an unreliable transport, the address response
MUST be 128 bits. All fields
must be in network byte order.

The format of the MAPPED-ADDRESS attribute is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Family | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address (32 bits or 128 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Format of MAPPED-ADDRESS Attribute

The address family can take on discarded, as if it had never been received. This means that
retransmits, if applicable, will continue. If all the following values:

0x01:IPv4
0x02:IPv6

The first 8 bits responses
received are discarded, then instead of signaling a timeout after
ending the MAPPED-ADDRESS transaction, the layer MUST signal that the integrity
protection was violated.

If the request was sent over a reliable transport, the response MUST
be set to 0 discarded, and the layer MUST immediately end the transaction and
signal that the integrity protection was violated.

If the response contains a PASSWORD-ALGORITHMS attribute, all the
subsequent requests MUST be
ignored by receivers. These bits are present for aligning parameters
on natural 32-bit boundaries. authenticated using MESSAGE-INTEGRITY-
SHA256 only.

10. ALTERNATE-SERVER Mechanism

This attribute section describes a mechanism in STUN that allows a server to
redirect a client to another server. This extension is used only optional, and
a usage must define if and when this extension is used. The
ALTERNATE-SERVER attribute carries an IP address.

A server using this extension redirects a client to another server by servers for achieving backwards
compatibility
replying to a request message with [RFC3489] clients.

14.2. XOR-MAPPED-ADDRESS an error response message with an
error code of 300 (Try Alternate). The XOR-MAPPED-ADDRESS server MUST include at least
one ALTERNATE-SERVER attribute is identical to the MAPPED-ADDRESS
attribute, except that in the reflexive transport error response, which MUST
contain an IP address is obfuscated
through the XOR function.

The format of the XOR-MAPPED-ADDRESS is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Family | X-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| X-Address (Variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: Format same address family as the source IP
address of XOR-MAPPED-ADDRESS Attribute the request message. The Family represents server SHOULD include an
additional ALTERNATE-SERVER attribute, after the mandatory one, that
contains an IP address family, and is encoded
identically to of the Family in MAPPED-ADDRESS.

X-Port is computed by XOR'ing address family other than the mapped port with source IP
address of the most
significant 16 bits request message. The error response message MAY be
authenticated; however, there are use cases for ALTERNATE-SERVER
where authentication of the magic cookie. response is not possible or practical.
If the IP address family is
IPv4, X-Address transaction uses TLS or DTLS, if the transaction is computed
authenticated by XOR'ing a MESSAGE-INTEGRITY-SHA256 attribute, and if the mapped IP address with
server wants to redirect to a server that uses a different
certificate, then it MUST include an ALTERNATE-DOMAIN attribute
containing the
magic cookie. If name inside the IP address family subjectAltName of that certificate.
This series of conditions on the MESSAGE-INTEGRITY-SHA256 attribute
indicates that the transaction is IPv6, X-Address authenticated and that the client
implements this specification and therefore can process the
ALTERNATE-DOMAIN attribute.

A client using this extension handles a 300 (Try Alternate) error
code as follows. The client looks for an ALTERNATE-SERVER attribute
in the error response. If one is
computed by XOR'ing found, then the mapped IP address client considers
the current transaction as failed and reattempts the request with the concatenation of
server specified in the attribute, using the same transport protocol
used for the previous request. That request, if authenticated, MUST
utilize the same credentials that the client would have used in the
request to the server that performed the redirection. If the magic cookie and
transport protocol uses TLS or DTLS, then the 96-bit transaction ID. In all cases, client looks for an
ALTERNATE-DOMAIN attribute. If the
XOR operation works on its inputs in network byte order (that is, attribute is found, the
order they will domain
MUST be encoded in used to validate the message).

The rules for encoding and processing certificate using the first 8 bits recommendations in
[RFC6125]. The certificate MUST contain an identifier of the
attribute's value, the rules for handling multiple occurrences type DNS-ID
or CN-ID (eventually with wildcards) but not of type SRV-ID or URI-
ID. If the
attribute, and the rules for processing address families are attribute is not found, the same
as domain that was used for MAPPED-ADDRESS.

Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
encoding of
the transport address. The former encodes original request MUST be used to validate the transport
address by exclusive-or'ing certificate. If
the client has been redirected to a server to which it with has already
sent this request within the magic cookie. The latter
encodes last five minutes, it directly MUST ignore the
redirection and consider the transaction to have failed. This
prevents infinite ping-ponging between servers in binary. case of redirection
loops.

11. Backwards Compatibility with RFC 3489 originally specified

In addition to the backward compatibility already described in
Section 12 of [RFC5389], DTLS MUST NOT be used with [RFC3489]
(referred to as "classic STUN"). Any STUN request or indication
without the magic cookie (see Section 6 of [RFC5389]) over DTLS MUST
be considered invalid: all requests MUST generate a 500 (Server
Error) error response, and indications MUST be ignored.

12. Basic Server Behavior

This section defines the behavior of a basic, stand-alone STUN
server.

Historically, "classic STUN" [RFC3489] only
MAPPED-ADDRESS. However, deployment experience found that some NATs
rewrite defined the 32-bit binary payloads containing behavior of a
server that was providing clients with server reflexive transport
addresses by receiving and replying to STUN Binding requests.
[RFC5389] redefined the NAT's public IP
address, such protocol as STUN's MAPPED-ADDRESS attribute, in an extensible framework, and the well-meaning
but misguided attempt at providing a generic Application Layer
Gateway (ALG) function. Such behavior interferes with
server functionality became the operation
of sole STUN and Usage defined in that
document. This STUN Usage is also causes failure of STUN's message-integrity checking.

14.3. USERNAME known as "Basic STUN Server".

The USERNAME attribute is used for message integrity. STUN server MUST support the Binding method. It identifies SHOULD NOT
utilize the username and password combination used short-term or long-term credential mechanism. This is
because the work involved in authenticating the message-integrity
check.

The value of USERNAME request is a variable-length value containing more than
the
authentication username. work in simply processing it. It MUST contain a UTF-8 [RFC3629] encoded
sequence of less than 509 bytes, and MUST have been processed using SHOULD NOT utilize the UsernameCasePreserved profile [RFC8265]. A compliant
implementation MUST be able to parse UTF-8 encoded sequence of 763 or
less bytes, to be compatible with [RFC5389] that mistakenly assumed
up to 6 bytes per characters encoded.

14.4. USERHASH

The USERHASH attribute is used as a replacement
ALTERNATE-SERVER mechanism for the USERNAME
attribute when username anonymity is supported.

The value same reason. It MUST support UDP
and TCP. It MAY support STUN over TCP/TLS or STUN over UDP/DTLS;
however, DTLS and TLS provide minimal security benefits in this basic
mode of USERHASH has operation. It does not require a fixed length keep-alive mechanism
because a TCP or TLS-over-TCP connection is closed after the end of 32 bytes. The username
MUST have been processed using
the UsernameCasePreserved profile
[RFC8265] and Binding transaction. It MAY utilize the realm FINGERPRINT mechanism
but MUST have been processed using NOT require it. Since the
OpaqueString profile [RFC8265] before hashing.

The following stand-alone server only runs
STUN, FINGERPRINT provides no benefit. Requiring it would break
compatibility with RFC 3489, and such compatibility is the operation desirable in a
stand-alone server. Stand-alone STUN servers SHOULD support
backwards compatibility with clients using [RFC3489], as described in
Section 11.

It is RECOMMENDED that administrators of STUN servers provide DNS
entries for those servers as described in Section 8. If both A and
AAAA resource records are returned, then the client will perform can
simultaneously send STUN Binding requests to hash the username:

userhash = SHA-256(UsernameCasePreserved(username)
":" OpaqueString(realm))

14.5. MESSAGE-INTEGRITY

The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] IPv4 and IPv6
addresses (as specified in [RFC8305]), as the Binding request is
idempotent. Note that the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS
attributes that are returned will not necessarily match the address
family of the server address used.

A basic STUN message. The MESSAGE-INTEGRITY attribute server is not a solution for NAT traversal by itself.
However, it can be present utilized as part of a solution through STUN
Usages. This is discussed further in
any Section 13.

13. STUN message type. Since it uses the SHA-1 hash, Usages

STUN by itself is not a solution to the HMAC will NAT traversal problem.
Rather, STUN defines a tool that can be 20 bytes. used inside a larger
solution. The key term "STUN Usage" is used for any solution that uses
STUN as a component.

A STUN Usage defines how STUN is actually utilized -- when to send
requests, what to do with the HMAC depends on responses, and which credential mechanism optional
procedures defined here (or in an extension to STUN) are to be used.
A usage also defines:

o Which STUN methods are used.

o What transports are used. If DTLS-over-UDP is used, then
implementing the denial-of-service countermeasure described in use.
Section 9.1.1 defines the key for the short-term credential mechanism 4.2.1 of [RFC6347] is mandatory.

o What authentication and Section 9.2.2 defines the key for the long-term credential
mechanism. Other credential message-integrity mechanisms MUST define the are used.

o The considerations around manual vs. automatic key that is
used derivation for
the HMAC.

The text used integrity mechanism, as input discussed in [RFC4107].

o What mechanisms are used to HMAC distinguish STUN messages from other
messages. When STUN is the run over TCP or TLS-over-TCP, a framing
mechanism may be required.

o How a STUN message, up to and
including client determines the attribute preceding IP address and port of the MESSAGE-INTEGRITY attribute.
The length field STUN
server.

o How simultaneous use of IPv4 and IPv6 addresses (Happy Eyeballs
[RFC8305]) works with non-idempotent transactions when both
address families are found for the STUN message header is adjusted to point server.

o Whether backwards compatibility to RFC 3489 is required.

o What optional attributes defined here (such as FINGERPRINT and
ALTERNATE-SERVER) or in other extensions are required.

o If MESSAGE-INTEGRITY-SHA256 truncation is permitted, and the end of the MESSAGE-INTEGRITY attribute.
limits permitted for truncation.

o The value of the
MESSAGE-INTEGRITY attribute keep-alive mechanism if STUN is set to a dummy value.

Once run over TCP or TLS-over-TCP.

o If anycast addresses can be used for the computation server in case 1) TCP or
TLS-over-TCP or 2) authentication is performed, used.

In addition, any STUN Usage must consider the value security implications
of the MESSAGE-INTEGRITY
attribute is filled in, and the value using STUN in that usage. A number of attacks against STUN are
known (see the length Security Considerations section in the this document), and
any usage must consider how these attacks can be thwarted or
mitigated.

Finally, a usage must consider whether its usage of STUN
header is set to its correct value -- the length an
example of the entire
message. Similarly, when validating the MESSAGE-INTEGRITY, Unilateral Self-Address Fixing approach to NAT
traversal and, if so, address the
length field questions raised in RFC 3424
[RFC3424].

14. STUN Attributes

After the STUN header must are zero or more attributes. Each attribute
MUST be adjusted to point to TLV encoded, with a 16-bit type, 16-bit length, and value.
Each STUN attribute MUST end on a 32-bit boundary. As mentioned
above, all fields in an attribute are transmitted most significant
bit first.

Figure 4: Format of STUN Attributes

The value in the Length field MUST contain the end length of the MESSAGE-INTEGRITY attribute Value
part of the attribute, prior to calculating the HMAC over
the padding, measured in bytes. Since
STUN message, up to and including the attribute preceding the
MESSAGE-INTEGRITY attribute. Such adjustment aligns attributes on 32-bit boundaries, attributes whose content
is necessary when
attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256, appear
after MESSAGE-INTEGRITY. See also [RFC5769] for examples not a multiple of such
calculations.

14.6. MESSAGE-INTEGRITY-SHA256

The MESSAGE-INTEGRITY-SHA256 attribute 4 bytes are padded with 1, 2, or 3 bytes of
padding so that its value contains an HMAC-SHA256
[RFC2104] a multiple of the STUN message. 4 bytes. The MESSAGE-INTEGRITY-SHA256
attribute can
padding bits MUST be present set to zero on sending and MUST be ignored by
the receiver.

Any attribute type MAY appear more than once in any a STUN message type. The MESSAGE-
INTEGRITY-SHA256 attribute contains an initial portion of message.
Unless specified otherwise, the HMAC-
SHA-256 [RFC2104] order of appearance is significant:
only the STUN message. The value will be at most 32
bytes, but MUST first occurrence needs to be at least 16 bytes, processed by a receiver, and MUST
any duplicates MAY be ignored by a multiple receiver.

To allow future revisions of 4
bytes. The value must be this specification to add new attributes
if needed, the full 32 bytes attribute space is divided into two ranges.
Attributes with type values between 0x0000 and 0x7FFF are
comprehension-required attributes, which means that the STUN agent
cannot successfully process the message unless it understands the
attribute. Attributes with type values between 0x8000 and 0xFFFF are
comprehension-optional attributes, which means that those attributes
can be ignored by the STUN Usage
explicitly specifies that truncation agent if it does not understand them.

The set of STUN attribute types is allowed. STUN Usages may
specify a minimum length longer than 16 bytes. maintained by IANA. The key for the HMAC depends on which credential mechanism initial
set defined by this specification is found in use. Section 9.1.1 defines the key for 18.3.

The rest of this section describes the short-term credential mechanism
and Section 9.2.2 defines format of the key for various
attributes defined in this specification.

14.1. MAPPED-ADDRESS

The MAPPED-ADDRESS attribute indicates a reflexive transport address
of the long-term credential
mechanism. Other credential mechanism MUST define client. It consists of an 8-bit address family and a 16-bit
port, followed by a fixed-length value representing the key that is
used for IP address.
If the HMAC.

The text used as input to HMAC address family is IPv4, the STUN message, up to and
including address MUST be 32 bits. If the attribute preceding
address family is IPv6, the MESSAGE-INTEGRITY-SHA256
attribute. address MUST be 128 bits. All fields
must be in network byte order.

The length field format of the STUN message header is adjusted
to point to the end MAPPED-ADDRESS attribute is:

Figure 5: Format of MAPPED-ADDRESS Attribute

The address family can take on the MESSAGE-INTEGRITY-SHA256 attribute. following values:

0x01:IPv4
0x02:IPv6

The
value first 8 bits of the MESSAGE-INTEGRITY-SHA256 attribute is MAPPED-ADDRESS MUST be set to a dummy
value.

Once the computation 0 and MUST be
ignored by receivers. These bits are present for aligning parameters
on natural 32-bit boundaries.

This attribute is performed, the value of the MESSAGE-
INTEGRITY-SHA256 used only by servers for achieving backwards
compatibility with [RFC3489] clients.

14.2. XOR-MAPPED-ADDRESS

The XOR-MAPPED-ADDRESS attribute is filled in, and the value of identical to the length
in MAPPED-ADDRESS
attribute, except that the STUN header reflexive transport address is set to its correct value -- obfuscated
through the length XOR function.

The format of the
entire message. Similarly, when validating XOR-MAPPED-ADDRESS is:

Figure 6: Format of XOR-MAPPED-ADDRESS Attribute

The Family field represents the MESSAGE-INTEGRITY-
SHA256, IP address family and is encoded
identically to the length Family field in MAPPED-ADDRESS.

X-Port is computed by XOR'ing the STUN header must be adjusted to point
to mapped port with the end most
significant 16 bits of the MESSAGE-INTEGRITY-SHA256 attribute prior to
calculating the HMAC over the STUN message, up to and including the
attribute preceding magic cookie. If the MESSAGE-INTEGRITY-SHA256 attribute. Such
adjustment is necessary when attributes, such as FINGERPRINT, appear
after MESSAGE-INTEGRITY-SHA256. See also Appendix B.1 for examples
of such calculations.

14.7. FINGERPRINT

The FINGERPRINT attribute MAY be present in all STUN messages.

The value of IP address family is
IPv4, X-Address is computed by XOR'ing the attribute mapped IP address with the
magic cookie. If the IP address family is IPv6, X-Address is
computed as by XOR'ing the CRC-32 mapped IP address with the concatenation of
the STUN
message up to (but excluding) magic cookie and the FINGERPRINT attribute itself,
XOR'ed with 96-bit transaction ID. In all cases, the 32-bit value 0x5354554e. (The
XOR operation ensures
that works on its inputs in network byte order (that is, the FINGERPRINT test
order they will not report a false positive on a
packet containing a CRC-32 generated by an application protocol.) be encoded in the message).

The 32-bit CRC is rules for encoding and processing the one defined in ITU V.42 [ITU.V42.2002], which
has a generator polynomial first 8 bits of x^32 + x^26 + x^23 + x^22 + x^16 + x^12
+ x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1. See the sample
code
attribute's value, the rules for handling multiple occurrences of the CRC-32
attribute, and the rules for processing address families are the same
as for MAPPED-ADDRESS.

Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in Section 8 their
encoding of [RFC1952].

When present, the FINGERPRINT attribute MUST be transport address. The former encodes the last attribute transport
address by XOR'ing it with the magic cookie. The latter encodes it
directly in binary. RFC 3489 originally specified only MAPPED-
ADDRESS. However, deployment experience found that some NATs rewrite
the 32-bit binary payloads containing the NAT's public IP address,
such as STUN's MAPPED-ADDRESS attribute, in the message, and thus will appear after MESSAGE-INTEGRITY well-meaning but
misguided attempt to provide a generic Application Layer Gateway
(ALG) function. Such behavior interferes with the operation of STUN
and
MESSAGE-INTEGRITY-SHA256. also causes failure of STUN's message-integrity checking.

14.3. USERNAME

The FINGERPRINT USERNAME attribute can aid in distinguishing STUN packets from
packets of other protocols. See Section 7.

As with MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, is used for message integrity. It identifies
the CRC username and password combination used in the FINGERPRINT attribute covers the length field from the STUN
message header. Therefore, this message-integrity
check.

The value must be correct and include
the CRC attribute as part of USERNAME is a variable-length value containing the message length, prior to computation
authentication username. It MUST contain a UTF-8-encoded [RFC3629]
sequence of the CRC. When fewer than 509 bytes and MUST have been processed using
the FINGERPRINT attribute in OpaqueString profile [RFC8265]. A compliant implementation MUST
be able to parse a message, the
attribute is first placed into the message UTF-8-encoded sequence of 763 or fewer octets to
be compatible with a dummy value, then [RFC5389].

Note: [RFC5389] mistakenly referenced the CRC is computed, definition of UTF-8 in
[RFC2279]. [RFC2279] assumed up to 6 octets per characters
encoded. [RFC2279] was replaced by [RFC3629], which allows only 4
octets per character encoded, consistent with changes made in
Unicode 2.0 and then ISO/IEC 10646.

Note: This specification uses the value OpaqueString profile instead of
the attribute is updated.
If UsernameCasePreserved profile for username string processing
in order to improve compatibility with deployed password stores.
Many password databases used for HTTP and SIP Digest
authentication store the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute are
also present, then they must MD5 hash of username:realm:password
instead of storing a plain text password. In [RFC3489], STUN
authentication was designed to be present compatible with these existing
databases to the correct message-
integrity value before the CRC is computed, since the CRC is done
over the value extent possible, which like SIP and HTTP
performed no pre-processing of the MESSAGE-INTEGRITY usernames and MESSAGE-INTEGRITY-SHA256
attributes as well.

14.8. ERROR-CODE passwords other than
prohibiting non-space ASCII control characters. The ERROR-CODE attribute is next revision
of the STUN specification, [RFC5389], used in error response messages. It
contains a numeric error code value in the range of 300 SASLprep [RFC4013]
stringprep [RFC3454] profile to 699 plus a
textual reason phrase encoded in UTF-8 [RFC3629], pre-process usernames and is consistent
in its code assignments
passwords. SASLprep uses Unicode Normalization Form KC
(Compatibility Decomposition, followed by Canonical Composition)
[UAX15] and semantics with SIP [RFC3261] prohibits various control, space, and HTTP
[RFC7231]. non-text,
deprecated, or inappropriate codepoints. The reason phrase is meant for diagnostic purposes, PRECIS framework
[RFC8264] obsoletes stringprep. PRECIS handling of usernames and
can be anything appropriate for the error code. Recommended reason
phrases for the defined error codes
passwords [RFC8265] uses Unicode Normalization Form C (Canonical
Decomposition, followed by Canonical Composition). While there
are included in the IANA registry
for error codes. The reason phrase MUST specific cases where different username strings under HTTP
Digest could be mapped to a UTF-8 [RFC3629] encoded
sequence of less than 128 characters (which can be as long as 509
bytes when encoding them or 763 bytes when decoding them).

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, should single STUN username processed with
OpaqueString, these cases are extremely unlikely and easy to
detect and correct. With a UsernameCasePreserved profile, it
would be 0 |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: ERROR-CODE Attribute

To facilitate processing, the class of more likely that valid usernames under HTTP Digest would
not match their processed forms (specifically usernames containing
bidirectional text and compatibility forms). Operators are free
to further restrict the error code (the hundreds
digit) allowed codepoints in usernames to avoid
problematic characters.

14.4. USERHASH

The USERHASH attribute is encoded separately from used as a replacement for the rest USERNAME
attribute when username anonymity is supported.

The value of the code, as shown in
Figure 7. USERHASH has a fixed length of 32 bytes. The Reserved bits SHOULD be 0, username
MUST have been processed using the OpaqueString profile [RFC8265],
and are for alignment on 32-bit
boundaries. Receivers the realm MUST ignore these bits. have been processed using the OpaqueString profile
[RFC8265] before hashing.

The Class represents following is the hundreds digit operation that the client will perform to hash
the username:

userhash = SHA-256(OpaqueString(username) ":" OpaqueString(realm))

14.5. MESSAGE-INTEGRITY

The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of
the error code. STUN message. The value MUST MESSAGE-INTEGRITY attribute can be between 3
and 6. present in
any STUN message type. Since it uses the SHA-1 hash, the HMAC will
be 20 bytes.

The Number represents key for the binary encoding of HMAC depends on which credential mechanism is in use.
Section 9.1.1 defines the error code
modulo 100, key for the short-term credential
mechanism, and its value Section 9.2.2 defines the key for the long-term
credential mechanism. Other credential mechanisms MUST be between 0 define the
key that is used for the HMAC.

The text used as input to HMAC is the STUN message, up to and 99.
including the attribute preceding the MESSAGE-INTEGRITY attribute.
The following error codes, along with their recommended reason
phrases, are defined:

300 Try Alternate: Length field of the STUN message header is adjusted to point to
the end of the MESSAGE-INTEGRITY attribute. The client should contact an alternate server for
this request. This error response MUST only be sent if value of the
request included either a USERNAME or USERHASH
MESSAGE-INTEGRITY attribute and is set to a
valid dummy value.

Once the computation is performed, the value of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute;
otherwise, it MUST NOT be sent
attribute is filled in, and error code 400 (Bad Request) the value of the length in the STUN
header is
suggested. This error response MUST set to its correct value -- the length of the entire
message. Similarly, when validating the MESSAGE-INTEGRITY, the
Length field in the STUN header must be protected with adjusted to point to the
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, and
receivers MUST validate end
of the MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 of this response before redirecting themselves attribute prior to
an alternate server.

Note: Failure calculating the HMAC over
the STUN message, up to generate and validate message integrity including the attribute preceding the
MESSAGE-INTEGRITY attribute. Such adjustment is necessary when
attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256, appear
after MESSAGE-INTEGRITY. See also [RFC5769] for a 300
response allows examples of such
calculations.

14.6. MESSAGE-INTEGRITY-SHA256

The MESSAGE-INTEGRITY-SHA256 attribute contains an on-path attacker to falsify a 300 response thus
causing subsequent HMAC-SHA256
[RFC2104] of the STUN messages to message. The MESSAGE-INTEGRITY-SHA256
attribute can be sent to a victim.

400 Bad Request: present in any STUN message type. The request was malformed. MESSAGE-
INTEGRITY-SHA256 attribute contains an initial portion of the HMAC-
SHA-256 [RFC2104] of the STUN message. The value will be at most 32
bytes, but it MUST be at least 16 bytes and MUST be a multiple of 4
bytes. The client SHOULD NOT
retry value must be the request without modification from full 32 bytes unless the previous attempt.
The server STUN Usage
explicitly specifies that truncation is allowed. STUN Usages may not be able to generate
specify a valid MESSAGE-INTEGRITY
or MESSAGE-INTEGRITY-SHA256 minimum length longer than 16 bytes.

The key for this error, so the client MUST NOT
expect a valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute HMAC depends on this response.

401 Unauthenticated: The request did not contain which credential mechanism is in use.
Section 9.1.1 defines the correct
credentials to proceed. The client should retry key for the request with
proper credentials.

420 Unknown Attribute: The server received a STUN packet containing
a comprehension-required attribute that it did not understand.
The server short-term credential
mechanism, and Section 9.2.2 defines the key for the long-term
credential mechanism. Other credential mechanism MUST put this unknown attribute in define the UNKNOWN-
ATTRIBUTE attribute of its error response.

438 Stale Nonce: The NONCE key
that is used by for the client was no longer valid. HMAC.

The client should retry, using text used as input to HMAC is the NONCE provided in STUN message, up to and
including the response.

500 Server Error: The server has suffered a temporary error. attribute preceding the MESSAGE-INTEGRITY-SHA256
attribute. The
client should try again.

14.9. REALM Length field of the STUN message header is adjusted
to point to the end of the MESSAGE-INTEGRITY-SHA256 attribute. The REALM
value of the MESSAGE-INTEGRITY-SHA256 attribute may be present in requests is set to a dummy
value.

Once the computation is performed, the value of the MESSAGE-
INTEGRITY-SHA256 attribute is filled in, and responses. It
contains text that meets the grammar for "realm-value" as described value of the length
in [RFC3261] but without the double quotes and their surrounding
whitespace. That is, it is an unquoted realm-value (and STUN header is therefore
a sequence of qdtext or quoted-pair). It MUST be a UTF-8 [RFC3629]
encoded sequence set to its correct value -- the length of less than 128 characters (which can be as long as
509 bytes when encoding them and as long as 763 bytes the
entire message. Similarly, when decoding
them), and MUST have been processed using validating the OpaqueString profile
[RFC8265].

Presence MESSAGE-INTEGRITY-
SHA256, the Length field in the STUN header must be adjusted to point
to the end of the REALM MESSAGE-INTEGRITY-SHA256 attribute in a request indicates that long-term
credentials are being used for authentication. Presence in certain
error responses indicates that prior to
calculating the server wishes HMAC over the client STUN message, up to use a
long-term credential in that realm for authentication.

14.10. NONCE

The NONCE attribute may be present in requests and responses. It
contains a sequence of qdtext or quoted-pair, which are defined in
[RFC3261]. Note that this means that including the NONCE
attribute will not
contain preceding the actual surrounding quote characters. MESSAGE-INTEGRITY-SHA256 attribute. Such
adjustment is necessary when attributes, such as FINGERPRINT, appear
after MESSAGE-INTEGRITY-SHA256. See [RFC7616],
Section 5.4, also Appendix B.1 for guidance on selection examples
of nonce values in a server.
It MUST be less than 128 characters (which can be as long as 509
bytes when encoding them and a long as 763 bytes when decoding them).

14.11. PASSWORD-ALGORITHMS such calculations.

14.7. FINGERPRINT

The PASSWORD-ALGORITHMS FINGERPRINT attribute may MAY be present in requests and
responses. It contains the list of algorithms that the server can
use to derive the long-term password. all STUN messages.

The set value of known algorithms is maintained by IANA. The initial set
defined by this specification is found in Section 18.5.

The the attribute contains a list of algorithm numbers and variable
length parameters. The algorithm number is a 16-bit value computed as defined
in Section 18.5. The parameters start with the length (prior to
padding) CRC-32 of the parameters as a 16-bit value, followed by the
parameters that are specific to each algorithm. The parameters are
padded STUN
message up to a (but excluding) the FINGERPRINT attribute itself,
XOR'ed with the 32-bit boundary, in value 0x5354554e. (The XOR operation ensures
that the same manner as FINGERPRINT test will not report a false positive on a
packet containing a CRC-32 generated by an attribute.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm 1 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 | Algorithm 2 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 Parameter (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...

Figure 8: Format of PASSWORD-ALGORITHMS Attribute

14.12. PASSWORD-ALGORITHM application protocol.)
The PASSWORD-ALGORITHM attribute 32-bit CRC is present only in requests. It
contains the algorithms that one defined in ITU V.42 [ITU.V42.2002], which
has a generator polynomial of x^32 + x^26 + x^23 + x^22 + x^16 + x^12
+ x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1. See the server must use to derive a key from sample
code for the long-term password.

The set of known algorithms is maintained by IANA. The initial set
defined by this specification is found CRC-32 in Section 18.5.

The 8 of [RFC1952].

When present, the FINGERPRINT attribute contains an algorithm number MUST be the last attribute in
the message and variable length
parameters. thus will appear after MESSAGE-INTEGRITY and MESSAGE-
INTEGRITY-SHA256.

The algorithm number is a 16-bit value as defined FINGERPRINT attribute can aid in distinguishing STUN packets from
packets of other protocols. See Section 18.5. The parameters starts 7.

As with MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, the length (prior to
padding) of CRC used
in the parameters as a 16-bit value, followed by FINGERPRINT attribute covers the
parameters that are specific to Length field from the algorithm. The parameters are
padded STUN
message header. Therefore, prior to a 32-bit boundary, in the same manner as an attribute.
Similarly, computation of the padding bits MUST CRC, this
value must be set to zero on sending correct and MUST
be ignored by include the receiver.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Algorithm Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: Format of PASSWORD-ALGORITHM Attribute

14.13. UNKNOWN-ATTRIBUTES

The UNKNOWN-ATTRIBUTES CRC attribute is present only in an error response
when as part of the response code
message length. When using the FINGERPRINT attribute in a message,
the ERROR-CODE attribute is 420.

The attribute contains first placed into the message with a list of 16-bit values, each dummy value;
then, the CRC is computed, and the value of which
represents an the attribute type that was not understood by is updated.
If the server.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 1 Type | Attribute 2 Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 3 Type | Attribute 4 Type ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 10: Format of UNKNOWN-ATTRIBUTES Attribute

Note: In [RFC3489], this field was padded to 32 by duplicating MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute is
also present, then it must be present with the
last attribute. In this version of correct message-
integrity value before the specification, CRC is computed, since the normal
padding rules for CRC is done
over the value of the MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256
attributes are used instead.

14.14. SOFTWARE as well.

14.8. ERROR-CODE

The SOFTWARE ERROR-CODE attribute is used in error response messages. It
contains a textual description of the software
being used by the agent sending numeric error code value in the message. It range of 300 to 699 plus a
textual reason phrase encoded in UTF-8 [RFC3629]; it is used by clients also
consistent in its code assignments and servers. Its value SHOULD include manufacturer semantics with SIP [RFC3261]
and version
number. HTTP [RFC7231]. The attribute has no impact on operation of reason phrase is meant for diagnostic
purposes and can be anything appropriate for the error code.
Recommended reason phrases for the protocol,
and serves only as a tool defined error codes are included
in the IANA registry for diagnostic and debugging purposes. error codes. The
value of SOFTWARE is variable length. It reason phrase MUST be a UTF-8
UTF-8-encoded [RFC3629]
encoded sequence of less fewer than 128 characters (which
can be as long as 509 bytes when encoding them and as long as or 763 bytes when
decoding them).

14.15. ALTERNATE-SERVER

The alternate server represents an alternate transport address
identifying a different STUN server that the STUN client

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, should try.

It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a
single server by IP address.

14.16. ALTERNATE-DOMAIN

The alternate domain represents the domain name that is used to
verify the IP address in the ALTERNATE-SERVER attribute when the
transport protocol uses TLS or DTLS.

The value of ALTERNATE-DOMAIN is variable length. It MUST be a valid
DNS name [RFC1123] (including A-labels [RFC5890]) of 255 or less
ASCII characters.

15. Operational Considerations

STUN MAY be used with anycast addresses, but only with UDP and in
STUN Usages where authentication is not used.

16. Security Considerations

Implementations and deployments 0 |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Format of a STUN Usage using TLS or DTLS
MUST follow ERROR-CODE Attribute

To facilitate processing, the recommendations in [BCP195].

Implementations and deployments class of a STUN Usage using the Long-Term
Credential Mechanism (Section 9.2) MUST follow error code (the hundreds
digit) is encoded separately from the recommendations in
Section 5 rest of [RFC7616].

16.1. Attacks against the Protocol

16.1.1. Outside Attacks

An attacker can try to modify STUN messages in transit, in order to
cause a failure code, as shown in STUN operation. These attacks
Figure 7.

The Reserved bits SHOULD be 0 and are detected for
both requests and responses through alignment on 32-bit
boundaries. Receivers MUST ignore these bits. The Class represents
the message-integrity mechanism,
using either a short-term or long-term credential. Of course, once
detected, hundreds digit of the manipulated packets will error code. The value MUST be dropped, causing between 3
and 6. The Number represents the STUN
transaction to effectively fail. This attack is possible only by an
on-path attacker.

An attacker that can observe, but not modify, STUN messages in-
transit (for example, an attacker present on a shared access medium,
such as Wi-Fi), can see a STUN request, binary encoding of the error code
modulo 100, and then immediately send a
STUN response, typically an its value MUST be between 0 and 99.

The following error response, in order to disrupt STUN
processing. This attack is also prevented codes, along with their recommended reason
phrases, are defined:

300 Try Alternate: The client should contact an alternate server for messages that utilize
MESSAGE-INTEGRITY. However, some
this request. This error responses, those related to
authentication in particular, cannot response MUST only be protected by MESSAGE-
INTEGRITY. When STUN itself is run over a secure transport protocol
(e.g., TLS), these attacks are completely mitigated.

Depending on sent if the STUN usage, these attacks may
request included either a USERNAME or USERHASH attribute and a
valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute;
otherwise, it MUST NOT be sent and error code 400 (Bad Request)
is suggested. This error response MUST be protected with the
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, and
receivers MUST validate the MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 of minimal
consequence this response before redirecting themselves
to an alternate server.

Note: Failure to generate and thus do not require validate message integrity for a
300 response allows an on-path attacker to mitigate.
For example, when falsify a 300
response thus causing subsequent STUN is used messages to be sent to a basic STUN
victim.

400 Bad Request: The request was malformed. The client SHOULD NOT
retry the request without modification from the previous
attempt. The server may not be able to discover generate a
server reflexive candidate valid
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 for usage with ICE, authentication and
message integrity are not required since these attacks are detected
during this error, so
the connectivity check phase. client MUST NOT expect a valid MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute on this response.

401 Unauthenticated: The connectivity checks
themselves, however, require protection for proper operation of ICE
overall. As described in Section 13, STUN usages describe when
authentication and message integrity are needed.

Since STUN uses request did not contain the HMAC of a shared secret for authentication and
integrity protection, it is subject correct
credentials to offline dictionary attacks.
When authentication is utilized, it SHOULD be proceed. The client should retry the request
with proper credentials.

420 Unknown Attribute: The server received a strong password
that is not readily subject to offline dictionary attacks.
Protection of the channel itself, using TLS or DTLS, mitigates these
attacks. STUN supports both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256,
which is subject to bid-down attacks by an on-path attacker packet containing
a comprehension-required attribute that
would strip the MESSAGE-INTEGRITY-SHA256 it did not understand.
The server MUST put this unknown attribute leaving only in the
MESSAGE-INTEGRITY UNKNOWN-
ATTRIBUTE attribute and exploiting a potential vulnerability.
Protection of its error response.

438 Stale Nonce: The NONCE used by the channel itself, client was no longer valid.
The client should retry, using TLS or DTLS, mitigates these
attacks. Timely removal of the support of MESSAGE-INTEGRITY NONCE provided in a
future version of STUN is necessary.

Note: the
response.

500 Server Error: The use of SHA-256 for password hashing does not meet modern
standards, which are aimed at slowing down exhaustive password search
by providing server has suffered a relatively slow minimum time to compute the hash.
Although better algorithms such as Argon2 [I-D.irtf-cfrg-argon2] are
available, SHA-256 was chosen for consistency with [RFC7616].

16.1.2. Inside Attacks

A rogue temporary error. The
client may should try to launch a DoS attack against a server by
sending again.

14.9. REALM

The REALM attribute may be present in requests and responses. It
contains text that meets the grammar for "realm-value" as described
in [RFC3261] but without the double quotes and their surrounding
whitespace. That is, it is an unquoted realm-value (and is therefore
a large number sequence of STUN requests. Fortunately, STUN
requests can qdtext or quoted-pair). It MUST be processed statelessly by a server, making such
attacks hard to launch effectively.

A rogue client may use a STUN server UTF-8-encoded
[RFC3629] sequence of fewer than 128 characters (which can be as long
as 509 bytes when encoding them and as a reflector, sending it
requests with a falsified source IP address long as 763 bytes when
decoding them) and port. In such a
case, MUST have been processed using the response would be delivered to that source IP and port.
There is no amplification OpaqueString
profile [RFC8265].

Presence of the number of packets with this attack
(the STUN server sends one packet REALM attribute in a request indicates that long-term
credentials are being used for each packet sent by authentication. Presence in certain
error responses indicates that the
client), though there is server wishes the client to use a small increase
long-term credential in the amount that realm for authentication.

14.10. NONCE

The NONCE attribute may be present in requests and responses. It
contains a sequence of data,
since STUN responses qdtext or quoted-pair, which are typically larger than requests. This attack
is mitigated by ingress source address filtering.

Revealing the specific software version of the agent through defined in
[RFC3261]. Note that this means that the
SOFTWARE NONCE attribute might allow them to become more vulnerable to
attacks against software that is known to will not
contain security holes.
Implementers SHOULD make usage of the SOFTWARE actual surrounding quote characters. The NONCE attribute
MUST be fewer than 128 characters (which can be as long as 509 bytes
when encoding them and a
configurable option.

16.1.3. Bid-Down Attacks

This document adds the possibility long as 763 bytes when decoding them). See
Section 5.4 of selecting different algorithms [RFC7616] for protecting the confidentiality of the passwords stored on the
server side when using the Long-Term Credential Mechanism, while
still ensuring compatibility with MD5, which was the algorithm used guidance on selection of nonce values in
a previous version of this protocol. server.

14.11. PASSWORD-ALGORITHMS

The PASSWORD-ALGORITHMS attribute may be present in requests and
responses. It works by having contains the list of algorithms that the server send back can
use to derive the client the list long-term password.

The set of known algorithms supported is maintained by IANA. The initial set
defined by this specification is found in Section 18.5.

The attribute contains a
PASSWORD-ALGORITHMS attribute, list of algorithm numbers and having the client send back a
PASSWORD-ALGORITHM attribute containing the variable
length parameters. The algorithm selected.

Because the PASSWORD-ALGORITHMS attribute has to be sent in an
unauthenticated response, an on-path attacker wanting to exploit an
eventual vulnerability number is a 16-bit value as defined
in MD5 can just strip the PASSWORD-ALGORITHMS
attribute from Section 18.5. The parameters start with the unprotected response, thus making length (prior to
padding) of the server
subsequently act parameters as if the client was implementing a previous version
of this protocol.

To protect against this attack and other similar bid-down attacks, 16-bit value, followed by the nonce is enriched with
parameters that are specific to each algorithm. The parameters are
padded to a set 32-bit boundary, in the same manner as an attribute.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm 1 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 | Algorithm 2 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...

Figure 8: Format of security bits which indicates
which security features are PASSWORD-ALGORITHMS Attribute

14.12. PASSWORD-ALGORITHM

The PASSWORD-ALGORITHM attribute is present only in use. In the case of the selection of requests. It
contains the password algorithm that the matching bit server must use to derive a key from
the long-term password.

The set of known algorithms is maintained by IANA. The initial set in the nonce returned
defined by the server this specification is found in the same response that Section 18.5.

The attribute contains the PASSWORD-
ALGORITHMS attribute. Because the nonce used in subsequent
authenticated transactions an algorithm number and variable length
parameters. The algorithm number is verified by a 16-bit value as defined in
Section 18.5. The parameters starts with the server to be identical length (prior to what was originally sent, it cannot be modified
padding) of the parameters as a 16-bit value, followed by an on-path
attacker. Additionally, the client is mandated
parameters that are specific to copy the received
PASSWORD-ALGORITHMS attribute in the next authenticated transaction algorithm. The parameters are
padded to that server.

An on-path attack that removes a 32-bit boundary, in the PASSWORD-ALGORITHMS will be
detected because same manner as an attribute.
Similarly, the client will not padding bits MUST be able to send it back set to zero on sending and MUST
be ignored by the
server in the next authenticated transaction. receiver.

Figure 9: Format of PASSWORD-ALGORITHM Attribute

14.13. UNKNOWN-ATTRIBUTES

The client will detect
that attack because the security bit is set, but the matching UNKNOWN-ATTRIBUTES attribute is missing, ending the session. A client using present only in an older
version of this protocol will not send error response
when the PASSWORD-ALGORITHMS back
but can only use MD5 anyway, so response code in the attack ERROR-CODE attribute is inconsequential. 420 (Unknown
Attribute).

The on-path attack may also try to remove the security bit together
with the PASSWORD-ALGORITHMS attribute, but the server will discover
that when the next authenticated transaction attribute contains a list of 16-bit values, each of which
represents an invalid
nonce.

An on-path attack that removes some algorithms from the PASSWORD-
ALGORITHMS attribute will be equally defeated because type that attribute
will be different from was not understood by the original one when server.

Figure 10: Format of UNKNOWN-ATTRIBUTES Attribute

Note: In [RFC3489], this field was padded to 32 by duplicating the server verifies it
in
last attribute. In this version of the subsequent authenticated transaction.

Note that specification, the bid-down protection mechanism introduced in this
document is inherently limited by normal
padding rules for attributes are used instead.

14.14. SOFTWARE

The SOFTWARE attribute contains a textual description of the fact that it is not possible to
detect an attack until software
being used by the server receives agent sending the second request after message. It is used by clients
and servers. Its value SHOULD include manufacturer and version
number. The attribute has no impact on operation of the 401 response.

SHA-256 was chosen protocol and
serves only as the new default for password hashing a tool for its
compatibility with [RFC7616] but because SHA-256 (like MD5) diagnostic and debugging purposes. The
value of SOFTWARE is variable length. It MUST be a
comparatively fast algorithm, it does little to deter brute force
attacks. Specifically, this means that if the user has UTF-8-encoded
[RFC3629] sequence of fewer than 128 characters (which can be as long
as 509 when encoding them and as long as 763 bytes when decoding
them).

14.15. ALTERNATE-SERVER

The alternate server represents an alternate transport address
identifying a weak
password:

o An attacker who captures different STUN server that the server's password file can often
determine STUN client should try.

It is encoded in the user's password same way as MAPPED-ADDRESS and thus impersonate the user to
other servers where they have used that password. Note that such
an attacker can impersonate the user refers to the server itself without
any brute force attack.

o An attacker who captures a
single exchange can brute force the
user's password and thus potentially impersonate server by IP address.

14.16. ALTERNATE-DOMAIN

The alternate domain represents the user domain name that is used to
verify the
server and other servers where they have used IP address in the same password.

A stronger (which ALTERNATE-SERVER attribute when the
transport protocol uses TLS or DTLS.

The value of ALTERNATE-DOMAIN is to say slower) algorithm, like Argon2
[I-D.irtf-cfrg-argon2], would help both variable length. It MUST be a valid
DNS name [RFC1123] (including A-labels [RFC5890]) of these cases, 255 or fewer
ASCII characters.

15. Operational Considerations

STUN MAY be used with anycast addresses, but only with UDP and in the
case
STUN Usages where authentication is not used.

16. Security Considerations

Implementations and deployments of a STUN Usage using TLS or DTLS
MUST follow the first attack, only after until the database entry for
this user is updated to exclusively use that stronger mechanism.

The bid-down defenses recommendations in this protocol prevent an attacker from
forcing the client [BCP195].

Implementations and server to complete deployments of a handshake STUN Usage using weaker
algorithms than they jointly support, but only if the weakest joint
algorithm is strong enough that it cannot be brute-forced. However,
this does not defend long-term
credential mechanism (Section 9.2) MUST follow the recommendations in
Section 5 of [RFC7616].

16.1. Attacks against many attacks on those algorithms;
specifically, an on-path the Protocol

16.1.1. Outside Attacks

An attacker might perform a bid-down attack on can try to modify STUN messages in transit, in order to
cause a client which supported both Argon2 [I-D.irtf-cfrg-argon2] and
SHA-256 failure in STUN operation. These attacks are detected for password hashing
both requests and use that to collect responses through the message-integrity mechanism,
using either a MESSAGE-
INTEGRITY-SHA256 value which it uses for an offline brute-force
attack. This would be detected when short-term or long-term credential. Of course, once
detected, the server receives manipulated packets will be dropped, causing the second
request, but STUN
transaction to effectively fail. This attack is possible only by an
on-path attacker.

An attacker that does can observe, but not prevent the attacker from obtaining the
MESSAGE-INTEGRITY-SHA256 value.

Similarly, modify, STUN messages in-
transit (for example, an attack against the USERHASH mechanism will not succeed
in establishing attacker present on a session shared access medium,
such as the server will detect that the feature
was discarded on-path, but the client would still have been convinced
to Wi-Fi) can see a STUN request and then immediately send its username in clear a
STUN response, typically an error response, in the USERNAME attribute, thus
disclosing it order to the attacker.

Finally, when the bid-down protection mechanism disrupt STUN
processing. This attack is employed also prevented for a
future upgrade of the HMAC algorithm used messages that utilize
MESSAGE-INTEGRITY. However, some error responses, those related to protect message, it will
offer only a limited protection if the current HMAC algorithm
authentication in particular, cannot be protected by MESSAGE-
INTEGRITY. When STUN itself is
already compromised.

16.2. Attacks Affecting run over a secure transport protocol
(e.g., TLS), these attacks are completely mitigated.

Depending on the Usage

This section lists STUN Usage, these attacks that might may be launched against a usage of
STUN. Each minimal
consequence and thus do not require message integrity to mitigate.
For example, when STUN is used to a basic STUN server to discover a
server reflexive candidate for usage must consider whether with ICE, authentication and
message integrity are not required since these attacks are
applicable to it, and if so, discuss counter-measures.

Most of the attacks in this section revolve around an attacker
modifying detected
during the reflexive address learned by a connectivity check phase. The connectivity checks
themselves, however, require protection for proper operation of ICE
overall. As described in Section 13, STUN client through a
Binding request/response transaction. Usages describe when
authentication and message integrity are needed.

Since STUN uses the usage HMAC of the
reflexive address a shared secret for authentication and
integrity protection, it is subject to offline dictionary attacks.

When authentication is utilized, it SHOULD be with a function strong password
that is not readily subject to offline dictionary attacks.
Protection of the usage, the applicability and
remediation of channel itself, using TLS or DTLS, mitigates these
attacks.

STUN supports both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256,
which makes STUN subject to bid-down attacks are usage-specific. In common
situations, modification of the reflexive address by an on-path attacker.
An attacker is easy to do. Consider, for example, the common situation
where STUN is run directly over UDP. In this case, an on-path
attacker can modify the source IP address of the Binding request
before it arrives at could strip the STUN server. The STUN server will then
return this IP address in MESSAGE-INTEGRITY-SHA256 attribute,
leaving only the XOR-MAPPED-ADDRESS MESSAGE-INTEGRITY attribute to the
client, and send the response back to that (falsified) IP address and
port. If the attacker can also intercept this response, it can
direct it back towards the client. Protecting against this attack by
using a message-integrity check is impossible, since thus exploiting a message-
integrity value cannot cover the source IP address, since
potential vulnerability. Protection of the
intervening NAT must be able to modify this value. Instead, one
solution to preventing channel itself, using TLS
or DTLS, mitigates these attacks. Timely removal of the attacks listed below support of
MESSAGE-INTEGRITY in a future version of STUN is necessary.

Note: The use of SHA-256 for the client password hashing does not meet modern
standards, which are aimed at slowing down exhaustive password
searches by providing a relatively slow minimum time to
verify compute the reflexive address learned,
hash. Although better algorithms such as is done in ICE [RFC8445].

Other usages Argon2 [Argon2] are
available, SHA-256 was chosen for consistency with [RFC7616].

16.1.2. Inside Attacks

A rogue client may use other means try to prevent these attacks.

16.2.1. Attack I: Distributed launch a DoS (DDoS) attack against a Target

In this attack, the attacker provides one or more clients with the
same faked reflexive address that points to the intended target.
This will trick the STUN clients into thinking that their reflexive
addresses are equal to that server by
sending it a large number of the target. If the clients hand out
that reflexive address in order STUN requests. Fortunately, STUN
requests can be processed statelessly by a server, making such
attacks hard to receive traffic on launch effectively.

A rogue client may use a STUN server as a reflector, sending it (for
example, in SIP messages),
requests with a falsified source IP address and port. In such a
case, the traffic will instead response would be sent delivered to the
target. This attack can provide substantial amplification,
especially when used with clients that are using STUN to enable
multimedia applications. However, it can only be launched against
targets for which packets from source IP and port.
There is no amplification of the number of packets with this attack
(the STUN server to the target pass
through the attacker, limiting sends one packet for each packet sent by the cases in which it
client), though there is possible.

16.2.2. Attack II: Silencing a Client

In this attack, small increase in the attacker provides a amount of data,
since STUN client with a faked
reflexive address. The reflexive address it provides responses are typically larger than requests. This attack
is a transport mitigated by ingress source address that routes to nowhere. As a result, filtering.

Revealing the client won't
receive any specific software version of the packets it expects to receive when it hands out agent through the reflexive address. This exploitation
SOFTWARE attribute might allow them to become more vulnerable to
attacks against software that is not very interesting for known to contain security holes.
Implementers SHOULD make usage of the attacker. It impacts SOFTWARE attribute a single client, which is frequently not
the desired target. Moreover, any attacker that can mount
configurable option.

16.1.3. Bid-Down Attacks

This document adds the attack
could also deny service possibility of selecting different algorithms
to protect the client by other means, such as
preventing the client from receiving any response from the STUN
server, or even a DHCP server. As with the attack in Section 16.2.1,
this attack is only possible when confidentiality of the attacker is passwords stored on path for packets
sent from the STUN server towards this unused IP address.

16.2.3. Attack III: Assuming the Identity of a Client

This attack is similar to attack II. However,
side when using the faked reflexive
address points to long-term credential mechanism while still
ensuring compatibility with MD5, which was the attacker itself. algorithm used in
[RFC5389]. This allows selection works by having the attacker server send to
receive traffic that was destined for the client.

16.2.4. Attack IV: Eavesdropping

In this attack,
client the attacker forces list of algorithms supported in a PASSWORD-ALGORITHMS
attribute and having the client to use send back a reflexive
address that routes to itself. It then forwards any packets it
receives to PASSWORD-ALGORITHM
attribute containing the client. This attack would allow algorithm selected.

Because the attacker PASSWORD-ALGORITHMS attribute has to
observe all packets be sent to the client. However, in order to launch
the attack, the an
unauthenticated response, an on-path attacker must have already been able to observe
packets from the client wanting to exploit an
eventual vulnerability in MD5 can just strip the STUN server. In most cases (such as
when the attack is launched PASSWORD-ALGORITHMS
attribute from an access network), this means that the attacker could already observe packets sent to unprotected response, thus making the client. This
attack is, server
subsequently act as a result, only useful for observing traffic by
attackers on the path from if the client to the STUN server, but not
generally on was implementing the path version of packets being routed towards the client.

Note that
this protocol defined in [RFC5389].

To protect against this attack can be trivially launched by and other similar bid-down attacks,
the STUN server
itself, so users nonce is enriched with a set of STUN servers should have security bits that indicates
which security features are in use. In the same level case of trust
in them as any other node that can insert themselves into the
communication flow.

16.3. Hash Agility Plan

This specification uses both HMAC-SHA256 for computation selection of
the
message integrity, sometimes password algorithm, the matching bit is set in the nonce returned
by the server in combination with HMAC-SHA1. If, at a
later time, HMAC-SHA256 the same response that contains the PASSWORD-
ALGORITHMS attribute. Because the nonce used in subsequent
authenticated transactions is found verified by the server to be compromised, identical
to what was originally sent, it cannot be modified by an on-path
attacker. Additionally, the following client is mandated to copy the remedy received
PASSWORD-ALGORITHMS attribute in the next authenticated transaction
to that server.

An on-path attack that removes the PASSWORD-ALGORITHMS will be applied:

o Both a new message-integrity attribute and a new STUN Security
Feature bit
detected because the client will not be allocated able to send it back to the
server in a Standard Track document. the next authenticated transaction. The
new message-integrity attribute client will have its value computed using
a new hash. The STUN Security Feature detect
that attack because the security bit is set but the matching
attribute is missing; this will be used to
simultaneously signal to a STUN end the session. A client using the Long Term
Credential Mechanism that an
older version of this protocol will not send the PASSWORD-ALGORITHMS
back but can only use MD5 anyway, so the attack is inconsequential.

The on-path attack may also try to remove the security bit together
with the PASSWORD-ALGORITHMS attribute, but the server supports this new hash
algorithm, and will prevent bid-down attacks on discover
that when the new message-
integrity attribute.

o STUN Clients and Servers using next authenticated transaction contains an invalid
nonce.

An on-path attack that removes some algorithms from the Short Term Credential Mechanism PASSWORD-
ALGORITHMS attribute will need to update be equally defeated because that attribute
will be different from the original one when the external mechanism that they use to signal
what message-integrity attributes are server verifies it
in use.

The the subsequent authenticated transaction.

Note that the bid-down protection mechanism described introduced in this
document is new,
and thus cannot currently protect against a bid-down attack that
lowers the strength of inherently limited by the hash algorithm to HMAC-SHA1. This is why,
after a transition period, a new document updating this document will
assign a new STUN Security Feature bit for deprecating HMAC-SHA1.
When used, this bit will signal fact that HMAC-SHA1 is deprecated and
should no longer be used.

Similarly, if SHA256 it is found to be compromised, a new user-hash
attribute and a new STUN Security Feature bit will be allocated in a
Standards Track document. The new user-hash attribute will have its
value computed using a new hash. The STUN Security Feature bit will
be used to simultaneously signal not possible to a STUN client using
detect an attack until the Long Term
Credential Mechanism that this server supports this new hash
algorithm for the user-hash attribute, and will prevent bid-down
attacks on receives the new user-hash attribute.

17. IAB Considerations

The IAB has studied second request after
the problem of Unilateral Self-Address Fixing
(UNSAF), which is 401 (Unauthenticated) response.

SHA-256 was chosen as the general process by which a client attempts to
determine new default for password hashing for its address in another realm on the other side of a NAT
through
compatibility with [RFC7616], but because SHA-256 (like MD5) is a collaborative protocol reflection mechanism ([RFC3424]).
STUN can be used
comparatively fast algorithm, it does little to perform this function using a Binding request/
response transaction deter brute-force
attacks. Specifically, this means that if one agent is behind a NAT and the other is on
the public side of the NAT.

The IAB user has suggested a weak
password, an attacker that protocols developed for this purpose
document captures a specific set of considerations. Because some STUN usages
provide UNSAF functions (such as ICE [RFC8445] ), single exchange can use a
brute-force attack to learn the user's password and others do not
(such as SIP Outbound [RFC5626]), answers then potentially
impersonate the user to these considerations
need the server and to be addressed by other servers where the usages themselves.

18. IANA Considerations

18.1. STUN Security Features Registry
same password was used. Note that such an attacker can impersonate
the user to the server itself without any brute-force attack.

A STUN Security Feature set defines 24 bit as flags.

IANA stronger (which is requested to create a new registry containing say, slower) algorithm, like Argon2 [Argon2],
would help both of these cases; however, in the STUN
Security Features that are protected by first case, it would
only help after the database entry for this user is updated to
exclusively use that stronger mechanism.

The bid-down attack
prevention mechanism described defenses in section Section 9.2.1.

The initial STUN Security Features are:

Bit 0: Password algorithms
Bit 1: Username anonymity
Bit 2-23: Unassigned

Bits are assigned starting this protocol prevent an attacker from
forcing the most significant side of the bit
set, so Bit 0 is the leftmost bit client and Bit 23 server to complete a handshake using weaker
algorithms than they jointly support, but only if the rightmost bit.

New Security Features are assigned weakest joint
algorithm is strong enough that it cannot be compromised by a Standards Action [RFC8126].

18.2. STUN Methods Registry

IANA is requested to update the name brute-
force attack. However, this does not defend against many attacks on
those algorithms; specifically, an on-path attacker might perform a
bid-down attack on a client that supports both Argon2 [Argon2] and
SHA-256 for method 0x002 password hashing and the
reference from RFC 5389 use that to RFC-to-be collect a MESSAGE-
INTEGRITY-SHA256 value that it can then use for an offline brute-
force attack. This would be detected when the following STUN methods:

0x000: (Reserved)
0x001: Binding
0x002: (Reserved; prior to [RFC5389] this was SharedSecret)

18.3. STUN Attribute Registry

18.3.1. Updated Attributes

IANA is requested to update server receives the names for attributes 0x0002, 0x0004,
0x0005, 0x0007, and 0x000B, and
second request, but that does not prevent the reference attacker from RFC 5389 to RFC-
to-be for the following STUN methods:

Comprehension-required range (0x0000-0x7FFF):
0x0000: (Reserved)
0x0001: MAPPED-ADDRESS
0x0002: (Reserved; prior to [RFC5389] this was RESPONSE-ADDRESS)
0x0004: (Reserved; prior to [RFC5389] this was SOURCE-ADDRESS)
0x0005: (Reserved; prior to [RFC5389] this was CHANGED-ADDRESS)
0x0006: USERNAME
0x0007: (Reserved; prior to [RFC5389] this was PASSWORD)
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x000B: (Reserved; prior to [RFC5389] this was REFLECTED-FROM)
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS

Comprehension-optional range (0x8000-0xFFFF)
0x8022: SOFTWARE
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT

18.3.2. New Attributes

IANA is requested to add the following attribute to obtaining
the STUN
Attribute Registry:

Comprehension-required range (0x0000-0x7FFF):
0xXXXX: MESSAGE-INTEGRITY-SHA256
0xXXXX: PASSWORD-ALGORITHM
0xXXXX: value.

Similarly, an attack against the USERHASH

Comprehension-optional range (0x8000-0xFFFF)
0xXXXX: PASSWORD-ALGORITHMS
0xXXXX: ALTERNATE-DOMAIN

18.4. STUN Error Code Registry

IANA is requested to update mechanism will not succeed
in establishing a session as the reference from RFC 5389 server will detect that the feature
was discarded on path, but the client would still have been convinced
to RFC-to-be
for send its username in the Error Codes given clear in Section 14.8.

IANA is requested the USERNAME attribute, thus
disclosing it to change the name of attacker.

Finally, when the 401 Error Code from
"Unauthorized" to "Unauthenticated".

18.5. STUN Password Algorithm Registry

IANA bid-down protection mechanism is requested to create a new registry employed for Password Algorithm.

A Password Algorithm is a hex number in the range 0x0000 - 0xFFFF.

The initial Password Algorithms are:

0x0000: Reserved
0x0001: MD5
0x0002: SHA-256
0x0003-0xFFFF: Unassigned

Password Algorithms in the first half
future upgrade of the range (0x0000 - 0x7FFF)
are assigned by IETF Review [RFC8126]. Password Algorithms in the
second half of HMAC algorithm used to protect messages, it
will offer only a limited protection if the range (0x8000 - 0xFFFF) are assigned by Designated
Expert [RFC8126].

18.5.1. Password Algorithms

18.5.1.1. MD5

This password current HMAC algorithm is taken from [RFC1321].

The key length is 16 bytes and
already compromised.

16.2. Attacks Affecting the parameters value is empty.

Note: Usage

This algorithm MUST only section lists attacks that might be used for compatibility with legacy
systems.

key = MD5(username ":" OpaqueString(realm)
":" OpaqueString(password))

18.5.1.2. SHA-256

This password algorithm is taken from [RFC7616].

The key length is 32 bytes and the parameters value is empty.

key = SHA-256(username ":" OpaqueString(realm)
":" OpaqueString(password))

18.6. launched against a usage of
STUN. Each STUN UDP and TCP Port Numbers

IANA is requested Usage must consider whether these attacks are
applicable to update it and, if so, discuss countermeasures.

Most of the reference from RFC 5389 to RFC-to-be
for attacks in this section revolve around an attacker
modifying the reflexive address learned by a STUN client through a
Binding request/response transaction. Since the usage of the
reflexive address is a function of the following ports in usage, the Service Name applicability and Transport Protocol
Port Number Registry.

stun 3478/tcp Session Traversal Utilities for NAT (STUN) port
stun 3478/udp Session Traversal Utilities for NAT (STUN) port
stuns 5349/tcp Session Traversal Utilities for NAT (STUN) port

19. Changes Since RFC 5389

This specification obsoletes [RFC5389]. This specification differs
from RFC 5389 in
remediation of these attacks are usage-specific. In common
situations, modification of the following ways:

o Added support reflexive address by an on-path
attacker is easy to do. Consider, for DTLS-over-UDP [RFC6347].

o Made clear that example, the RTO is considered stale if there common situation
where STUN is no
transactions with run directly over UDP. In this case, an on-path
attacker can modify the server.

o Aligned source IP address of the RTO calculation with [RFC6298].

o Updated Binding request
before it arrives at the cipher suites for TLS.

o Added support for STUN URI [RFC7064].

o Added support for SHA256 message integrity.

o Updated server. The STUN server will then
return this IP address in the PRECIS support XOR-MAPPED-ADDRESS attribute to [RFC8265].

o Added protocol the
client and registry send the response back to choose that (falsified) IP address and
port. If the password encryption
algorithm.

o Added support for anonymous username.

o Added protocol attacker can also intercept this response, it can
direct it back towards the client. Protecting against this attack by
using a message-integrity check is impossible, since a message-
integrity value cannot cover the source IP address and registry the
intervening NAT must be able to modify this value. Instead, one
solution to prevent the attacks listed below is for preventing biddown the client to
verify the reflexive address learned, as is done in ICE [RFC8445].

Other usages may use other means to prevent these attacks.

o Sharing

16.2.1. Attack I: Distributed DoS (DDoS) against a NONCE is no longer permitted.

o Added Target

In this attack, the possibility attacker provides one or more clients with the
same faked reflexive address that points to the intended target.
This will trick the STUN clients into thinking that their reflexive
addresses are equal to that of using a domain name in the alternate
server mechanism.

o Added more C snippets.

o Added test vector.

20. References

20.1. Normative References

[ITU.V42.2002]
International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 2002.

[KARN87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates target. If the clients hand out
that reflexive address in Reliable Transport Protocols", August 1987.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for use order to receive traffic on it (for
example, in RFCs SIP messages), the traffic will instead be sent to the
target. This attack can provide substantial amplification,
especially when used with clients that are using STUN to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR enable
multimedia applications. However, it can only be launched against
targets for
specifying which packets from the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates STUN server to the target pass
through the attacker, limiting the cases in which it is possible.

16.2.2. Attack II: Silencing a Client

In this attack, the Context attacker provides a STUN client with a faked
reflexive address. The reflexive address it provides is a transport
address that routes to nowhere. As a result, the client won't
receive any of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations the packets it expects to receive when it hands out
the reflexive address. This exploitation is not very interesting for
the MD5 Message-Digest and attacker. It impacts a single client, which is frequently not
the desired target. Moreover, any attacker that can mount the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<http://www.rfc-editor.org/info/rfc6151>.

[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC7064] Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
Huguenin, "URI Scheme for attack
could also deny service to the Session Traversal Utilities
for NAT (STUN) Protocol", RFC 7064, DOI 10.17487/RFC7064,
November 2013, <https://www.rfc-editor.org/info/rfc7064>.

[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) client by other means, such as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
August 2014, <https://www.rfc-editor.org/info/rfc7350>.

[RFC7616] Shekh-Yusef, R., Ahrens, D., and S. Bremer, "HTTP Digest
Access Authentication", RFC 7616, DOI 10.17487/RFC7616,
September 2015, <https://www.rfc-editor.org/info/rfc7616>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 8200, STD 86,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rf8200>.

[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.

[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.

20.2. Informative References

[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations
preventing the client from receiving any response from the STUN
server, or even a DHCP server. As with the attack described in
Section 16.2.1, this attack is only possible when the attacker is on
path for Secure Use packets sent from the STUN server towards this unused IP
address.

16.2.3. Attack III: Assuming the Identity of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.

[I-D.ietf-tram-stun-pmtud]
Petit-Huguenin, M. and G. Salgueiro, "Path MTU Discovery
Using Session Traversal Utilities a Client

This attack is similar to attack II. However, the faked reflexive
address points to the attacker itself. This allows the attacker to
receive traffic that was destined for NAT (STUN)", draft-
ietf-tram-stun-pmtud-10 (work the client.

16.2.4. Attack IV: Eavesdropping

In this attack, the attacker forces the client to use a reflexive
address that routes to itself. It then forwards any packets it
receives to the client. This attack allows the attacker to observe
all packets sent to the client. However, in progress), September
2018.

[I-D.irtf-cfrg-argon2]
Biryukov, A., Dinu, D., Khovratovich, D., and S.
Josefsson, "The memory-hard Argon2 password hash and
proof-of-work function", draft-irtf-cfrg-argon2-04 (work order to launch the
attack, the attacker must have already been able to observe packets
from the client to the STUN server. In most cases (such as when the
attack is launched from an access network), this means that the
attacker could already observe packets sent to the client. This
attack is, as a result, only useful for observing traffic by
attackers on the path from the client to the STUN server, but not
generally on the path of packets being routed towards the client.

Note that this attack can be trivially launched by the STUN server
itself, so users of STUN servers should have the same level of trust
in progress), November 2018.

[RFC1952] Deutsch, P., "GZIP file format the users of STUN servers as any other node that can insert itself
into the communication flow.

16.3. Hash Agility Plan

This specification version 4.3",
RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations uses HMAC-SHA256 for
UNilateral Self-Address Fixing (UNSAF) Across Network
Address Translation", RFC 3424, DOI 10.17487/RFC3424,
November 2002, <https://www.rfc-editor.org/info/rfc3424>.

[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal computation of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
DOI 10.17487/RFC3489, March 2003,
<https://www.rfc-editor.org/info/rfc3489>.

[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
June 2005, <https://www.rfc-editor.org/info/rfc4107>.

[RFC5090] Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
and W. Beck, "RADIUS Extension for Digest Authentication",
RFC 5090, DOI 10.17487/RFC5090, February 2008,
<http://www.rfc-editor.org/info/rfc5090>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/info/rfc5389>.

[RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
"Managing Client-Initiated Connections in the Session
Initiation Protocol (SIP)", RFC 5626,
DOI 10.17487/RFC5626, October 2009,
<https://www.rfc-editor.org/info/rfc5626>.

[RFC5766] Mahy, R., Matthews, P., message
integrity, sometimes in combination with HMAC-SHA1. If, at a later
time, HMAC-SHA256 is found to be compromised, the following remedy
should be applied:

o Both a new message-integrity attribute and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions a new STUN Security
Feature bit will be allocated in a Standards Track document. The
new message-integrity attribute will have its value computed using
a new hash. The STUN Security Feature bit will be used to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.

[RFC5769] Denis-Courmont, R., "Test Vectors for Session Traversal
Utilities for NAT (STUN)", RFC 5769, DOI 10.17487/RFC5769,
April 2010, <https://www.rfc-editor.org/info/rfc5769>.

[RFC5780] MacDonald, D.
simultaneously 1) signal to a STUN client using the long-term
credential mechanism that this server supports this new hash
algorithm and B. Lowekamp, "NAT Behavior Discovery
Using Session Traversal Utilities for NAT (STUN)",
RFC 5780, DOI 10.17487/RFC5780, May 2010,
<https://www.rfc-editor.org/info/rfc5780>.

[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., 2) prevent bid-down attacks on the new message-
integrity attribute.

o STUN clients and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <https://www.rfc-editor.org/info/rfc6544>.

[RFC7231] Fielding, R. servers using the short-term credential mechanism
will need to update the external mechanism that they use to signal
what message-integrity attributes are in use.

The bid-down protection mechanism described in this document is new
and R. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics thus cannot currently protect against a bid-down attack that
lowers the strength of the hash algorithm to HMAC-SHA1. This is why,
after a transition period, a new document updating this one will
assign a new STUN Security Feature bit for deprecating HMAC-SHA1.
When used, this bit will signal that HMAC-SHA1 is deprecated and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC8126] Cotton, M., Leiba, B.,
should no longer be used.

Similarly, if HMAC-SHA256 is found to be compromised, a new userhash
attribute and T. Narten, "Guidelines a new STUN Security Feature bit will be allocated in a
Standards Track document. The new userhash attribute will have its
value computed using a new hash. The STUN Security Feature bit will
be used to simultaneously 1) signal to a STUN client using the long-
term credential mechanism that this server supports this new hash
algorithm for
Writing an IANA the userhash attribute and 2) prevent bid-down attacks
on the new userhash attribute.

17. IAB Considerations Section

The IAB has studied the problem of Unilateral Self-Address Fixing
(UNSAF), which is the general process by which a client attempts to
determine its address in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, May 2008,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8445] Keranen, A., Holmberg, C., another realm on the other side of a NAT
through a collaborative protocol reflection mechanism [RFC3424].
STUN can be used to perform this function using a Binding request/
response transaction if one agent is behind a NAT and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol the other is on
the public side of the NAT.

The IAB has suggested that protocols developed for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

Appendix A. C Snippet this purpose
document a specific set of considerations. Because some STUN Usages
provide UNSAF functions (such as ICE [RFC8445]) and others do not
(such as SIP Outbound [RFC5626]), answers to Determine these considerations
need to be addressed by the usages themselves.

18. IANA Considerations

18.1. STUN Message Types

Given Security Features Registry

A STUN Security Feature set defines 24 bits as flags.

IANA has created a 16-bit new registry containing the STUN message type value in host byte order Security Features
that are protected by the bid-down attack prevention mechanism
described in msg_type
parameter, below Section 9.2.1.

The initial STUN Security Features are:

Bit 0: Password algorithms
Bit 1: Username anonymity
Bit 2-23: Unassigned

Bits are C macros to determine assigned starting from the most significant side of the bit
set, so Bit 0 is the leftmost bit and Bit 23 is the rightmost bit.

New Security Features are assigned by Standards Action [RFC8126].

18.2. STUN message types:

<CODE BEGINS>
#define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000)
#define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010)
#define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100)
#define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110)
<CODE ENDS> Methods Registry

A function to convert STUN method is a hex number in the range 0x000-0xFFF. The encoding
of a STUN method and class into a STUN message type:

<CODE BEGINS>
int type(int method, int cls) {
return (method & 0x1F80) << 2 | (method & 0x0070) << 1
| (method & 0x000F) | (cls & 0x0002) << 7
| (cls & 0x0001) << 4;
}
<CODE ENDS>

A function is described in Section 5.

STUN methods in the range 0x000-0x07F are assigned by IETF Review
[RFC8126]. STUN methods in the range 0x080-0x0FF are assigned by
Expert Review [RFC8126]. The responsibility of the expert is to extract
verify that the selected codepoint(s) is not in use and that the
request is not for an abnormally large number of codepoints.
Technical review of the extension itself is outside the scope of the
designated expert responsibility.

IANA has updated the name for method 0x002 as described below as well
as updated the reference from RFC 5389 to RFC 8489 for the message type:

<CODE BEGINS>
int method(int type) {
return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
| (type & 0x000F);
}
<CODE ENDS>

A function following
STUN methods:

0x000: Reserved
0x001: Binding
0x002: Reserved; was SharedSecret prior to extract [RFC5389]

18.3. STUN Attributes Registry

A STUN attribute type is a hex number in the class from range 0x0000-0xFFFF.
STUN attribute types in the message type:

<CODE BEGINS>
int cls(int type) {
return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
}
<CODE ENDS>

Appendix B. Test Vectors

This section augments range 0x0000-0x7FFF are considered
comprehension-required; STUN attribute types in the list range
0x8000-0xFFFF are considered comprehension-optional. A STUN agent
handles unknown comprehension-required and comprehension-optional
attributes differently.

STUN attribute types in the first half of test vectors defined the comprehension-required
range (0x0000-0x3FFF) and in [RFC5769]
with MESSAGE-INTEGRITY-SHA256. All the formats first half of the comprehension-
optional range (0x8000-0xBFFF) are assigned by IETF Review [RFC8126].
STUN attribute types in the second half of the comprehension-required
range (0x4000-0x7FFF) and definitions
listed in Section 2 the second half of the comprehension-
optional range (0xC000-0xFFFF) are assigned by Expert Review
[RFC8126]. The responsibility of the expert is to verify that the
selected codepoint(s) are not in use and that the request is not for
an abnormally large number of codepoints. Technical review of the
extension itself is outside the scope of [RFC5769] apply here.

B.1. Sample Request with Long-Term Authentication with MESSAGE-
INTEGRITY-SHA256 the designated expert
responsibility.

18.3.1. Updated Attributes

IANA has updated the names for attributes 0x0002, 0x0004, 0x0005,
0x0007, and USERHASH

This request uses 0x000B as well as updated the reference from RFC 5389 to
RFC 8489 for each the following parameters:

Username: "<U+30DE><U+30C8><U+30EA><U+30C3><U+30AF><U+30B9>" (without
quotes) unaffected by OpaqueString [RFC8265] processing

Password: "The<U+00AD>M<U+00AA>tr<U+2168>" and "TheMatrIX" (without
quotes) respectively before and after OpaqueString processing

Nonce: "obMatJos2QAAAf//499k954d6OL34oL9FSTvy64sA" (without quotes)

Realm: "example.org" (without quotes)
00 01 00 9c Request type and message length
21 12 a4 42 Magic cookie
78 ad 34 33 }
c6 ad 72 c0 } Transaction ID
29 da 41 2e }
XX XX 00 20 USERHASH attribute header
4a 3c f3 8f }
ef 69 92 bd }
a9 52 c6 78 }
04 17 da 0f } Userhash value (32 bytes)
24 81 94 15 }
56 9e 60 b2 }
05 c4 6e 41 }
40 7f 17 04 }
00 15 00 29 NONCE STUN methods.

In addition, [RFC5389] introduced a mistake in the name of attribute header
6f 62 4d 61 }
74 4a 6f 73 }
32 41 41 41 }
43 66 2f 2f }
34 39 39 6b } Nonce value and padding (3 bytes)
39 35 34 64 }
36 4f 4c 33 }
34 6f 4c 39 }
46 53 54 76 }
79 36 34 73 }
41 00 00 00 }
00 14 00 0b
0x0003; [RFC5389] called it CHANGE-ADDRESS when it was actually
previously called CHANGE-REQUEST. Thus, IANA has updated the
description for 0x0003 to read "Reserved; was CHANGED-REQUEST prior
to [RFC5389]".

Comprehension-required range (0x0000-0x7FFF):
0x0000: Reserved
0x0001: MAPPED-ADDRESS
0x0002: Reserved; was RESPONSE-ADDRESS prior to [RFC5389]
0x0003: Reserved; was CHANGED-REQUEST prior to [RFC5389]
0x0004: Reserved; was SOURCE-ADDRESS prior to [RFC5389]
0x0005: Reserved; was CHANGED-ADDRESS prior to [RFC5389]
0x0006: USERNAME
0x0007: Reserved; was PASSWORD prior to [RFC5389]
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x000B: Reserved; was REFLECTED-FROM prior to [RFC5389]
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS

Comprehension-optional range (0x8000-0xFFFF)
0x8022: SOFTWARE
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT

18.3.2. New Attributes

IANA has added the following attribute header
65 78 61 6d }
70 6c 65 2e } Realm value (11 bytes) and padding (1 byte)
6f 72 67 00 }
XX XX 00 20 to the "STUN Attributes"
registry:

Comprehension-required range (0x0000-0x7FFF):
0x001C: MESSAGE-INTEGRITY-SHA256 attribute header
c4 ec a2 b6 }
24 6f 26
0x001D: PASSWORD-ALGORITHM
0x001E: USERHASH

Comprehension-optional range (0x8000-0xFFFF)
0x8002: PASSWORD-ALGORITHMS
0x8003: ALTERNATE-DOMAIN

18.4. STUN Error Codes Registry

A STUN error code is a number in the range 0-699. STUN error codes
are accompanied by a textual reason phrase in UTF-8 [RFC3629] that is
intended only for human consumption and can be }
bc 2f 77 49 }
07 c2 00 a3 } HMAC-SHA256 value
76 c7 c2 8e }
b4 d1 26 60 }
bb fe 9f 28 }
0e 85 71 f2 }

Note: Before publication, anything appropriate;
this document proposes only suggested values.

STUN error codes are consistent in codepoint assignments and
semantics with SIP [RFC3261] and HTTP [RFC7231].

New STUN error codes are assigned based on IETF Review [RFC8126].
The specification must carefully consider how clients that do not
understand this error code will process it before granting the
request. See the rules in Section 6.3.4.

IANA has updated the reference from RFC 5389 to RFC 8489 for the
error codes defined in Section 14.8.

IANA has changed the name of the 401 error code from "Unauthorized"
to "Unauthenticated".

18.5. STUN Password Algorithms Registry

IANA has created a new registry titled "STUN Password Algorithms".

A password algorithm is a hex number in the XX XX placeholder must be replaced range 0x0000-0xFFFF.

The initial contents of the "Password Algorithm" registry are as
follows:

0x0000: Reserved
0x0001: MD5
0x0002: SHA-256
0x0003-0xFFFF: Unassigned

Password algorithms in the first half of the range (0x0000-0x7FFF)
are assigned by IETF Review [RFC8126]. Password algorithms in the value
second half of the range (0x8000-0xFFFF) are assigned to MESSAGE-INTEGRITY-SHA256 and USERHASH by
IANA. Expert
Review [RFC8126].

18.5.1. Password Algorithms

18.5.1.1. MD5

This password algorithm is taken from [RFC1321].

The MESSAGE-INTEGRITY-SHA256 attribute key length is 16 bytes, and the parameters value will need to
be updated after this.

Appendix C. Release notes is empty.

Note: This section must algorithm MUST only be removed before publication as an RFC.

C.1. Modifications between draft-ietf-tram-stunbis-21 and draft-ietf-
tram-stunbis-20

o Final edits to clean up bid down protection text to address Eric
Rescorla's DISCUSS used for compatibility with
legacy systems.

key = MD5(username ":" OpaqueString(realm)
":" OpaqueString(password))

18.5.1.2. SHA-256

This password algorithm is taken from [RFC7616].

The key length is 32 bytes, and comments.

C.2. Modifications between draft-ietf-tram-stunbis-20 the parameters value is empty.

key = SHA-256(username ":" OpaqueString(realm)
":" OpaqueString(password))

18.6. STUN UDP and draft-ietf-
tram-stunbis-19

o Updates TCP Port Numbers

IANA has updated the reference from RFC 5389 to address Eric Rescorla's DISCUSS and comments.

o Addressed nits raised by Noriyuki Torii

C.3. Modifications between draft-ietf-tram-stunbis-19 and draft-ietf-
tram-stunbis-18

o Updates following Adam Roach DISCUSS and comments.

C.4. Modifications between draft-ietf-tram-stunbis-18 and draft-ietf-
tram-stunbis-17

o Nits.

C.5. Modifications between draft-ietf-tram-stunbis-17 and draft-ietf-
tram-stunbis-16

o Modifications RFC 8489 for the
following IESG, GENART and SECDIR reviews.

C.6. Modifications between draft-ietf-tram-stunbis-16 ports in the "Service Name and draft-ietf-
tram-stunbis-15

o Replace "failure response" with "error response".

o Fix wrong section number. Transport Protocol Port
Number Registry".

stun 3478/tcp Session Traversal Utilities for NAT (STUN) port
stun 3478/udp Session Traversal Utilities for NAT (STUN) port
stuns 5349/tcp Session Traversal Utilities for NAT (STUN) port

19. Changes since RFC 5389

This specification obsoletes [RFC5389]. This specification differs
from RFC 5389 in the following ways:

o Use "Username anonymity" everywhere. Added support for DTLS-over-UDP [RFC6347].

o Align Made clear that the RTO is considered stale if there are no
transactions with UTF-8 deprecation.

o Fix MESSAGE-INTEGRITY-256.

o Update references. the server.

o Updates in Aligned the IANA sections. RTO calculation with [RFC6298].

o s/HMAC-SHA-1/HMAC-SHA1/, s/HMAC-SHA-256/HMAC-SHA256/, s/SHA1/SHA-
1/, and s/SHA256/SHA-256/. Updated the ciphersuites for TLS.

o Fixed definitions of Added support for STUN clients/servers. URI [RFC7064].

o Fixed STUN Added support for SHA256 message structure definition. integrity.

o Missing text. Updated the Preparation, Enforcement, and Comparison of
Internationalized Strings (PRECIS) support to [RFC8265].

o Add text explicitly saying that responses do not have Added protocol and registry to be in choose the
same orders than requests. password encryption
algorithm.

o Added support for anonymous username.

o /other application/other protocol/ Added protocol and registry for preventing bid-down attacks.

o Add text explicitly saying Specified that the security feature encoding sharing a NONCE is 4
character.

o Fixed discrepancy in section 9.2.3/9.2.3.1.

o s/invalidate/revoke/. no longer permitted.

o Removed sentences about checking USERHASH Added the possibility of using a domain name in responses, as this
should not happen. the alternate
server mechanism.

o Specify that ALTERNATE-SERVER carries an IP address. Added more C snippets.

o More modifications following review...

C.7. Modifications between draft-ietf-tram-stunbis-15 Added test vector.

20. References

20.1. Normative References

[ITU.V42.2002]
International Telecommunication Union, "Error-correcting
procedures for DCEs using asynchronous-to-synchronous
conversion", ITU-T Recommendation V.42, March 2002.

[KARN87] Karn, P. and draft-ietf-
tram-stunbis-14

o Reverted C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", SIGCOMM '87,
Proceedings of the RFC 2119 boilerplate to what was ACM workshop on Frontiers in computer
communications technology, Pages 2-7,
DOI 10.1145/55483.55484, August 1987.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 5389.

o Reverted the V.42 reference to the 2002 version.

o Updated some references.

C.8. Modifications between draft-ietf-tram-stunbis-14 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.

[RFC2104] Krawczyk, H., Bellare, M., and draft-ietf-
tram-stunbis-13

o Reorder the paragraphs in section 9.1.4.

o The realm is now processed through Opaque in section 9.2.2.

o Make clear in section 9.2.4 that it is an exclusive-xor.

o Removed text that implied that nonce sharing was explicitly
permitted in R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 5389.

o In same section, s/username/value/ 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for USERCASH.

o Modify the IANA requests to explicitly say that the reserved
codepoints were prior use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 5389.

C.9. Modifications between draft-ietf-tram-stunbis-13 and draft-ietf-
tram-stunbis-12

o Update references.

o Fixes some text following Shepherd review.

o Update co-author info.

C.10. Modifications between draft-ietf-tram-stunbis-12 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2782] Gulbrandsen, A., Vixie, P., and draft-ietf-
tram-stunbis-11

o Clarifies the procedure to define a new hash algorithm for
message-integrity.

o Explain the procedure to deprecate SHA1 as message-integrity.

o Added procedure L. Esibov, "A DNS RR for Happy Eyeballs (RFC 6555).

o Added verification that Happy Eyeballs works in
specifying the STUN Usage
checklist.

o Add reference to location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 RFC.

o Changed co-author affiliation.

C.11. Modifications between draft-ietf-tram-stunbis-11 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and draft-ietf-
tram-stunbis-10

o Made clear that the same HMAC than received Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in response the Context of short
term credential must be used Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for subsequent transactions.

o s/URL/URI/

o The "nonce cookie" is now mandatory to signal that SHA256 must be
used in the next transaction.

o s/SHA1/SHA256/

o Changed co-author affiliation.

C.12. Modifications between draft-ietf-tram-stunbis-10 MD5 Message-Digest and draft-ietf-
tram-stunbis-09

o Removed the reserved value in the security registry, as it does
not make sense in a bitset.

o Updated change list.

o Updated the minimum truncation size for M-I-256 to 16 bytes.

o Changed the truncation order to match HMAC-MD5 Algorithms",
RFC 7518.

o Fixed bugs in truncation boundary text.

o Stated that STUN Usages have to explicitly state that they can use
truncation.

o Removed truncation from the MESSAGE-INTEGRITY attribute.

o Add reference to C code in 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.

[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 1952.

o Replaced 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 2818 reference to 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC7064] Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
Huguenin, "URI Scheme for the Session Traversal Utilities
for NAT (STUN) Protocol", RFC 6125.

C.13. Modifications between draft-ietf-tram-stunbis-09 7064, DOI 10.17487/RFC7064,
November 2013, <https://www.rfc-editor.org/info/rfc7064>.

[RFC7350] Petit-Huguenin, M. and draft-ietf-
tram-stunbis-08

o Packets discarded in a reliable or unreliable transaction triggers
an attack error instead of a timeout error. An attack error on a
reliable transport is signaled immediately instead G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
August 2014, <https://www.rfc-editor.org/info/rfc7350>.

[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
<https://www.rfc-editor.org/info/rfc7616>.

[RFC8174] Leiba, B., "Ambiguity of waiting for
the timeout.

o Explicitly state that a received 400 response without
authentication will be dropped until timeout.

o Clarify the SHOULD omit/include rules Uppercase vs Lowercase in LTCM.

o If the nonce RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8200] Deering, S. and the hmac are both invalid, then a 401 is sent
instead of a 438.

o The 401 R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

[RFC8265] Saint-Andre, P. and 438 error response to subsequent requests may use the
previous NONCE/password to authenticate, if they are still
available.

o Change "401 Unauthorized" to "401 Unauthenticated"

o Make clear that in some cases it is impossible to add a MI or MI2
even if the text says SHOULD NOT.

C.14. Modifications between draft-ietf-tram-stunbis-08 A. Melnikov, "Preparation,
Enforcement, and draft-ietf-
tram-stunbis-07

o Updated list Comparison of changes since Internationalized Strings
Representing Usernames and Passwords", RFC 5389.

o More examples are automatically generated.

o Message integrity truncation is fixed at a multiple of 4 bytes,
because the padding will not decrease by more than this.

o USERHASH contains the 32 bytes of the hash, not a character
string.

o Updated the example to use the USERHASH attribute 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.

[RFC8305] Schinazi, D. and the modified
NONCE attribute.

o Updated ICEbis reference.

C.15. Modifications between draft-ietf-tram-stunbis-07 T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.

20.2. Informative References

[Argon2] Biryukov, A., Dinu, D., Khovratovich, D., and draft-ietf-
tram-stunbis-06

o Add USERHASH attribute to carry the hashed version of the
username.

o Add IANA registry S.
Josefsson, "The memory-hard Argon2 password hash and
proof-of-work function", Work in Progress, draft-irtf-
cfrg-argon2-09, November 2019.

[BCP195] Sheffer, Y., Holz, R., and nonce encoding P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security Features that
need to be protected from bid-down attacks.

o Modified MESSAGE-INTEGRITY (TLS) and MESSAGE-INTEGRITY-SHA256 to support
truncation limits (pending cryptographic review),

C.16. Modifications between draft-ietf-tram-stunbis-06 Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015,
<https://www.rfc-editor.org/info/bcp195>.

[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.

[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, DOI 10.17487/RFC2279, January 1998,
<https://www.rfc-editor.org/info/rfc2279>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and draft-ietf-
tram-stunbis-05

o Changed I-D references to E.
Schooler, "SIP: Session Initiation Protocol", RFC references.

o Changed CHANGE-ADDRESS to CHANGE-REQUEST (Errata #4233).

o Added test vector 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations for MESSAGE-INTEGRITY-SHA256.

o
UNilateral Self-Address Fixing (UNSAF) Across Network
Address additional review comments from Jonathan Lennox Translation", RFC 3424, DOI 10.17487/RFC3424,
November 2002, <https://www.rfc-editor.org/info/rfc3424>.

[RFC3454] Hoffman, P. and
Brandon Williams.

C.17. Modifications between draft-ietf-tram-stunbis-05 M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
DOI 10.17487/RFC3454, December 2002,
<https://www.rfc-editor.org/info/rfc3454>.

[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and draft-ietf-
tram-stunbis-04

o R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address review comments from Jonathan Lennox and Brandon Williams.

C.18. Modifications between draft-ietf-tram-stunbis-04 Translators (NATs)", RFC 3489,
DOI 10.17487/RFC3489, March 2003,
<https://www.rfc-editor.org/info/rfc3489>.

[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and draft-ietf-
tram-stunbis-03

o Remove SCTP.

o Remove DANE.

o s/MESSAGE-INTEGRITY2/MESSAGE-INTEGRITY-SHA256/

o Remove Salted SHA256 password hash.

o The RTO delay between transactions is removed.

o Make clear that reusing NONCE will trigger a wasted round trip.

C.19. Modifications between draft-ietf-tram-stunbis-03 Passwords", RFC 4013, DOI 10.17487/RFC4013, February
2005, <https://www.rfc-editor.org/info/rfc4013>.

[RFC4107] Bellovin, S. and draft-ietf-
tram-stunbis-02

o SCTP prefix is now 0b00000101 instead of 0x11.

o Add SCTP at various places it was needed.

o Update the hash agility plan to take in account HMAC-SHA-256.

o Adds the bid-down attack on message-integrity in the security
section.

C.20. Modifications between draft-ietf-tram-stunbis-02 R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
June 2005, <https://www.rfc-editor.org/info/rfc4107>.

[RFC5090] Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
and draft-ietf-
tram-stunbis-01

o STUN hash algorithm agility (currently only SHA-1 is allowed).

o Clarify terminology, text W. Beck, "RADIUS Extension for Digest Authentication",
RFC 5090, DOI 10.17487/RFC5090, February 2008,
<https://www.rfc-editor.org/info/rfc5090>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and guidance D. Wing,
"Session Traversal Utilities for STUN fragmentation.

o Clarify whether it's valid to share nonces across TURN
allocations.

o Prevent the server to allocate the same NONCE to clients with
different IP address and/or different port. This prevent sharing
the nonce between TURN allocations NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/info/rfc5389>.

[RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
"Managing Client-Initiated Connections in TURN.

o Add reference to draft-ietf-uta-tls-bcp

o Add a new attribute ALTERNATE-DOMAIN to verify the certificate of the ALTERNATE-SERVER after a 300 over (D)TLS.

o The RTP delay between transactions applies only to parallel
transactions, not Session
Initiation Protocol (SIP)", RFC 5626,
DOI 10.17487/RFC5626, October 2009,
<https://www.rfc-editor.org/info/rfc5626>.

[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to serial transactions. That prevents a 3RTT
delay between the first transaction Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.

[RFC5769] Denis-Courmont, R., "Test Vectors for Session Traversal
Utilities for NAT (STUN)", RFC 5769, DOI 10.17487/RFC5769,
April 2010, <https://www.rfc-editor.org/info/rfc5769>.

[RFC5780] MacDonald, D. and the second transaction B. Lowekamp, "NAT Behavior Discovery
Using Session Traversal Utilities for NAT (STUN)",
RFC 5780, DOI 10.17487/RFC5780, May 2010,
<https://www.rfc-editor.org/info/rfc5780>.

[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with long term authentication.

o Add text saying ORIGIN can increase a request size beyond the MTU Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <https://www.rfc-editor.org/info/rfc6544>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and so require Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an SCTPoUDP transport.

o Move the Acknowledgments IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8264] Saint-Andre, P. and Contributor sections to the end M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
the document,
Internationalized Strings in accordance with Application Protocols",
RFC 7322 section 4.

C.21. Modifications between draft-ietf-tram-stunbis-01 8264, DOI 10.17487/RFC8264, October 2017,
<https://www.rfc-editor.org/info/rfc8264>.

[RFC8445] Keranen, A., Holmberg, C., and draft-ietf-
tram-stunbis-00

o Add negotiation mechanism J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for new password algorithms.

o Describe the MESSAGE-INTEGRITY/MESSAGE-INTEGRITY2 protocol.

o Add support Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[STUN-PMTUD]
Petit-Huguenin, M., Salgueiro, G., and F. Garrido, "Path
MTU Discovery Using Session Traversal Utilities for SCTP NAT
(STUN)", Work in Progress, draft-ietf-tram-stun-pmtud-14,
November 2019.

[UAX15] Unicode Standard Annex #15, "Unicode Normalization Forms",
edited by Mark Davis and Ken Whistler. An integral part
of The Unicode Standard,
<http://unicode.org/reports/tr15/>.

Appendix A. C Snippet to solve the fragmentation problem.

o Merge RFC 7350:

* Split the "Sending over..." sections Determine STUN Message Types

Given a 16-bit STUN message type value in 3.

* Add DTLS-over-UDP as transport.

* Update host byte order in msg_type
parameter, below are C macros to determine the cipher suites STUN message types:

A function to convert method and cipher/compression restrictions.

* class into a message type:

<CODE BEGINS>
int type(int method, int cls) {
return (method & 0x1F80) << 2 | (method & 0x0070) << 1
| (method & 0x000F) | (cls & 0x0002) << 7
| (cls & 0x0001) << 4;
}
<CODE ENDS>

A stuns uri with an IP address is rejected.

* Replace most of function to extract the RFC 3489 compatibility by a reference method from the message type:

<CODE BEGINS>
int method(int type) {
return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
| (type & 0x000F);
}
<CODE ENDS>

A function to extract the class from the message type:

<CODE BEGINS>
int cls(int type) {
return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
}
<CODE ENDS>

Appendix B. Test Vectors

This section in RFC 5389.

* Update augments the STUN Usages list of test vectors defined in [RFC5769]
with transport applicability.

o Merge RFC 7064:

* DNS discovery is done from the URI.

* Reorganized the text about default ports.

o Add more C snippets.

o Make clear that MESSAGE-INTEGRITY-SHA256. All the cached RTO is discarded only if there is no
new translations for 10 minutes.

C.22. Modifications between draft-salgueiro-tram-stunbis-02 and draft-
ietf-tram-stunbis-00

o Draft adopted as WG item.

C.23. Modifications between draft-salgueiro-tram-stunbis-02 formats and draft-
salgueiro-tram-stunbis-01

o Add definition definitions
listed in Section 2 of MESSAGE-INTEGRITY2.

o Update text [RFC5769] apply here.

B.1. Sample Request with Long-Term Authentication with MESSAGE-
INTEGRITY-SHA256 and reference from RFC 2988 to RFC 6298.

o s/The IAB has mandated/The IAB has suggested/ (Errata #3737).

o Fix the figure for USERHASH

This request uses the UNKNOWN-ATTRIBUTES (Errata #2972).

o Fix section number following parameters:

Username: "<U+30DE><U+30C8><U+30EA><U+30C3><U+30AF><U+30B9>" (without
quotes) unaffected by OpaqueString [RFC8265] processing

Password: "The<U+00AD>M<U+00AA>tr<U+2168>" and make clear that the original domain name is
used for the server certificate verification. This is consistent
with what RFC 5922 (section 4) is doing. (Errata #2010)

o Remove text transitioning from RFC 3489.

o Add definition of MESSAGE-INTEGRITY2.

o Update text "TheMatrIX" (without
quotes) respectively before and reference from RFC 2988 to RFC 6298.

o s/The IAB has mandated/The IAB has suggested/ (Errata #3737).

o Fix the figure for the UNKNOWN-ATTRIBUTES (Errata #2972).

o Fix section number after OpaqueString [RFC8265]
processing

Nonce: "obMatJos2AAACf//499k954d6OL34oL9FSTvy64sA" (without quotes)

Realm: "example.org" (without quotes)
00 01 00 9c Request type and make clear that the original domain name is
used for the server certificate verification. This is consistent
with what RFC 5922 (section 4) is doing. (Errata #2010)

C.24. Modifications between draft-salgueiro-tram-stunbis-01 message length
21 12 a4 42 Magic cookie
78 ad 34 33 }
c6 ad 72 c0 } Transaction ID
29 da 41 2e }
00 1e 00 20 USERHASH attribute header
4a 3c f3 8f }
ef 69 92 bd }
a9 52 c6 78 }
04 17 da 0f } Userhash value (32 bytes)
24 81 94 15 }
56 9e 60 b2 }
05 c4 6e 41 }
40 7f 17 04 }
00 15 00 29 NONCE attribute header
6f 62 4d 61 }
74 4a 6f 73 }
32 41 41 41 }
43 66 2f 2f }
34 39 39 6b } Nonce value and draft-
salgueiro-tram-stunbis-00

o Restore the RFC 5389 text.

o Add list of open issues. padding (3 bytes)
39 35 34 64 }
36 4f 4c 33 }
34 6f 4c 39 }
46 53 54 76 }
79 36 34 73 }
41 00 00 00 }
00 14 00 0b REALM attribute header
65 78 61 6d }
70 6c 65 2e } Realm value (11 bytes) and padding (1 byte)
6f 72 67 00 }
00 1c 00 20 MESSAGE-INTEGRITY-SHA256 attribute header
e4 68 6c 8f }
0e de b5 90 }
13 e0 70 90 }
01 0a 93 ef } HMAC-SHA256 value
cc bc cc 54 }
4c 0a 45 d9 }
f8 30 aa 6d }
6f 73 5a 01 }

Acknowledgements

Thanks to Michael Tuexen, Tirumaleswar Reddy, Oleg Moskalenko, Simon
Perreault, Benjamin Schwartz, Rifaat Shekh-Yusef, Alan Johnston,
Jonathan Lennox, Brandon Williams, Olle Johansson, Martin Thomson,
Mihaly Meszaros, Tolga Asveren, Noriyuki Torii, Spencer Dawkins, Dale
Worley, Matthew Miller, Peter Saint-Andre, Julien Elie, Mirja
Kuehlewind, Eric Rescorla, Ben Campbell, Adam Roach, Alexey Melnikov,
and Benjamin Kaduk for the comments, suggestions, and questions that
helped improve this document.

The authors Acknowledgements section of RFC 5389 appeared as follows:

The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel
Garcia, Bruce Lowekamp, and Chris Sullivan for their comments, and
Baruch Sterman and Alan Hawrylyshen for initial implementations.
Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
Schulzrinne for IESG and IAB input on this work.

Contributors

Christian Huitema and Joel Weinberger were original co-authors coauthors of
RFC 3489.

Authors' Addresses

Marc Petit-Huguenin
Impedance Mismatch

Email: marc@petit-huguenin.org

Gonzalo Salgueiro
Cisco
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
US
United States of America

Email: gsalguei@cisco.com

Jonathan Rosenberg
Five9
Edison, NJ
US
United States of America

Email: jdrosen@jdrosen.net
URI: http://www.jdrosen.net

Dan Wing
Citrix Systems, Inc.
United States of America

Email: dwing-ietf@fuggles.com
Rohan Mahy
Unaffiliated

Email: rohan.ietf@gmail.com

Philip Matthews
Nokia
600 March Road
Ottawa, Ontario K2K 2T6
Canada

Phone: 613-784-3139
Email: philip_matthews@magma.ca