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Versions: (RFC 3489) 00 01 02 03 04 05 06 07
08 09 10 11 12 13 14 15 16 17 18 RFC 5389
BEHAVE Working Group J. Rosenberg
Internet-Draft Cisco
Obsoletes: 3489 (if approved) C. Huitema
Intended status: Standards Track Microsoft
Expires: January 9, 2008 R. Mahy
Plantronics
P. Matthews
Avaya
D. Wing
Cisco
July 8, 2007
Session Traversal Utilities for (NAT) (STUN)
draft-ietf-behave-rfc3489bis-07
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This Internet-Draft will expire on January 9, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
Session Traversal Utilities for NAT (STUN) is a protocol that serves
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as a tool for other protocols in dealing with 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, and as a keep-alive protocol to maintain NAT bindings.
STUN works with many existing 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 is an
important change from the previous version of this specification (RFC
3489), which presented STUN as a complete solution.
This document obsoletes RFC 3489.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Evolution from RFC 3489 . . . . . . . . . . . . . . . . . . . 4
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. STUN Message Structure . . . . . . . . . . . . . . . . . . . . 10
7. Base Protocol Procedures . . . . . . . . . . . . . . . . . . . 12
7.1. Forming a Request or an Indication . . . . . . . . . . . 12
7.2. Sending the Request or Indication . . . . . . . . . . . . 13
7.2.1. Sending over UDP . . . . . . . . . . . . . . . . . . . 13
7.2.2. Sending over TCP or TLS-over-TCP . . . . . . . . . . . 14
7.3. Receiving a STUN Message . . . . . . . . . . . . . . . . 15
7.3.1. Processing a Request . . . . . . . . . . . . . . . . . 16
7.3.1.1. Forming a Success or Error Response . . . . . . . 17
7.3.1.2. Sending the Success or Error Response . . . . . . 17
7.3.2. Processing an Indication . . . . . . . . . . . . . . . 18
7.3.3. Processing a Success Response . . . . . . . . . . . . 18
7.3.4. Processing an Error Response . . . . . . . . . . . . . 18
8. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 19
9. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 19
10. Authentication and Message-Integrity Mechanisms . . . . . . . 20
10.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 21
10.1.1. Forming a Request or Indication . . . . . . . . . . . 21
10.1.2. Receiving a Request or Indication . . . . . . . . . . 21
10.1.3. Receiving a Response . . . . . . . . . . . . . . . . . 22
10.2. Long-term Credential Mechanism . . . . . . . . . . . . . 23
10.2.1. Forming a Request . . . . . . . . . . . . . . . . . . 24
10.2.1.1. First Request . . . . . . . . . . . . . . . . . . 24
10.2.1.2. Subsequent Requests . . . . . . . . . . . . . . . 24
10.2.2. Receiving a Request . . . . . . . . . . . . . . . . . 24
10.2.3. Receiving a Response . . . . . . . . . . . . . . . . . 25
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11. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . . 26
12. Backwards Compatibility with RFC 3489 . . . . . . . . . . . . 26
12.1. Changes to Client Processing . . . . . . . . . . . . . . 27
12.2. Changes to Server Processing . . . . . . . . . . . . . . 27
13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 28
14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 29
14.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 30
14.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 31
14.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 32
14.4. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 32
14.5. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 33
14.6. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 33
14.7. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.8. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.9. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 35
14.10. SERVER . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.11. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 36
15. Security Considerations . . . . . . . . . . . . . . . . . . . 36
15.1. Attacks against the Protocol . . . . . . . . . . . . . . 36
15.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . . 36
15.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . 36
15.2. Attacks Affecting the Usage . . . . . . . . . . . . . . . 36
15.2.1. Attack I: DDoS Against a Target . . . . . . . . . . . 37
15.2.2. Attack II: Silencing a Client . . . . . . . . . . . . 37
15.2.3. Attack III: Assuming the Identity of a Client . . . . 37
15.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 38
15.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . . 38
16. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 38
17. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
17.1. STUN Methods Registry . . . . . . . . . . . . . . . . . . 39
17.2. STUN Attribute Registry . . . . . . . . . . . . . . . . . 39
17.3. STUN Error Code Registry . . . . . . . . . . . . . . . . 40
18. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 40
19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 42
20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
20.1. Normative References . . . . . . . . . . . . . . . . . . 42
20.2. Informational References . . . . . . . . . . . . . . . . 43
Appendix A. C Snippet to Determine STUN Message Types . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44
Intellectual Property and Copyright Statements . . . . . . . . . . 46
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1. Introduction
The protocol defined in this specification, Session Traversal
Utilities for NAT, provides a toolkit of functions 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
[I-D.ietf-mmusic-ice], or to relay packets between two endpoints
[I-D.ietf-behave-turn].
In keeping with its toolkit 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 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) [I-D.ietf-mmusic-ice] is one usage of ICE. SIP
Outbound [I-D.ietf-sip-outbound] is another usage of ICE. 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. Evolution from RFC 3489
STUN was originally defined in RFC 3489 [RFC3489]. That
specification, sometimes referred to as "classic STUN", represented
itself as a complete solution to the NAT traversal problem. In that
solution, a client would discover whether it was behind a NAT,
determine its NAT type, discover its IP address and port on the
public side of the outermost NAT, and then utilize that IP address
and port within the body of protocols, such as the Session Initiation
Protocol (SIP) [RFC3261]. However, experience since the publication
of RFC 3489 has found that classic STUN simply does not work
sufficiently well to be a deployable solution. The address and port
learned through classic STUN are sometimes usable for communications
with a peer, and sometimes not. Classic STUN provided no way to
discover whether it would, in fact, work or not, and it provided no
remedy in cases where it did not. Furthermore, classic STUN's
algorithm for classification of NAT types was found to be faulty, as
many NATs did not fit cleanly into the types defined there. Classic
STUN also had security vulnerabilities which required an extremely
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complicated mechanism to address, and despite the complexity of the
mechanism, were not fully remedied.
For these reasons, this specification obsoletes RFC 3489, and instead
describes STUN as a tool that is utilized as part of a complete NAT
traversal solution. ICE is a complete NAT traversal solutions for
protocols based on the offer/answer [RFC3264] methodology, such as
SIP. SIP Outbound is a complete solution for traversal of SIP
signaling, and it uses STUN in a very different way. Though it is
possible that a protocol may be able to use STUN by itself (classic
STUN) as a traversal solution, such usage is not described here and
is strongly discouraged for the reasons described above.
The on-the-wire protocol described here is changed only slightly from
classic STUN. The protocol now runs over TCP in addition to UDP.
Extensibility was added to the protocol in a more structured way. A
magic-cookie mechanism for demultiplexing STUN with application
protocols was added by stealing 32 bits from the 128 bit transaction
ID defined in RFC 3489, allowing the change to be backwards
compatible. Mapped addresses are encoded using a new exclusive-or
format. There are other, more minor changes. See Section 18 for a
more complete listing.
Due to the change in scope, STUN has also been renamed from "Simple
Traversal of UDP Through NAT" to "Session Traversal Utilities for
NAT". The acronym remains STUN, which is all anyone ever remembers
anyway.
3. Overview of Operation
This section is descriptive only.
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/--------\
// STUN \\
| Agent |
\\ (server) //
\--------/
+----------------+ Public Internet
................| NAT 2 |.......................
+----------------+
+----------------+ Private NET 2
................| NAT 1 |.......................
+----------------+
/--------\
// STUN \\
| Agent |
\\ (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
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 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 a client
sends an indication to the server and the server does not respond.
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, this simply 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
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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 the STUN client. As this packet passes back
through a NAT, the NAT will modify the destination transport address,
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.,
[I-D.ietf-mmusic-ice], [I-D.ietf-sip-outbound]). In these usages,
there must be a way to inspect a packet and determine if it is a STUN
packet or not. STUN provides two 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 has optional mechanisms for providing authentication and message
integrity when required. These mechanisms revolve around the use of
a username, password, and message-integrity value. Two of these
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.
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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
[I-D.ietf-mmusic-ice] the two endpoints use out-of-band signaling to
exchange a username and password. The client then includes the
username and a message-integrity value in the request message, where
the message-integrity value is computed as a cryptographic hash of
the message contents and the password. If the server replies with a
success response, then the response will include a message-integrity
value (computed using the same username and password), but the
username is not included. Error responses are not message-integrity
protected.
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 [RFC2617]. Initially, the client sends a request
message (e.g., a Binding Request) without any username or message-
integrity value included. The server replies with an error response
indicating that the request must be authenticated. This error
response includes a realm value and a nonce value. The client then
uses the realm value to help it select a username and password (for
example, the client might have a number of username and password
combinations stored, each one keyed by a different realm value). The
client then retries the request, this time including the realm, the
username, the nonce, and a message-integrity value in the request,
where the message-integrity value is computed as a cryptographic hash
of the message contents and the password. The nonce is provided by
the server and merely echoed by the client into the request. It is
chosen by the server such that it encodes information about the
client, the time-of-day, or other parameters. In this way, if an
attacker should attempt to replay the request, the server would find
the nonce invalid, and then reject the request. If the server
replies with a success response, then the response will include a
message-integrity value (computed using the same username and
password), but realm, username, and nonce are not included. Error
responses are not message-integrity protected. If the client has
further requests to send, it can try to reuse the same username,
realm, and nonce values. If the server does not accept them, it will
reply with an error response giving a realm and nonce value again.
4. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant STUN
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implementations.
5. Definitions
STUN Agent: An entity that implements the STUN protocol. Agents can
act as STUN clients for some transactions and as STUN servers for
other transactions.
STUN Client: A logical role in the STUN protocol. A STUN client
sends STUN requests or STUN indications, and receives STUN
responses. The term "STUN client" is also used colloquially to
refer to a STUN agent that only acts as a STUN client.
STUN Server: A logical role in the STUN protocol. A STUN server
receives STUN requests or STUN indications and sends STUN
responses. The term "STUN server" is also used colloquially to
refer to a STUN agent that only acts as a STUN server.
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 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
which represent a shared secret between client and server. Short
term credentials are obtained through some kind of protocol
mechanism between the client server, preceding the STUN exchange.
A short term credential has an explicit temporal scope, which may
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be based on a specific amount of time (such as 5 minutes) or on an
event (such as termination of a SIP 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
6. 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 RFC791 [RFC0791]. Unless otherwise noted, numeric constants are
in decimal (base 10).
All STUN messages MUST start with a 20-byte header followed by zero
or more Attributes. The STUN header contains a STUN message type,
magic cookie, transaction ID, and message length.
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 two bits of every STUN message MUST be zeroes.
This can be used to differentiate STUN packets from other protocols
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when STUN is multiplexed with other protocols on the same port.
The message type defines the message class (request, success
response, failure 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 consists 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:
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
|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 four
each method, a request, success response, error response and
indication are defined 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] which 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 RFC 3489 [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 in this revised specification. 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.
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The transaction ID is a 96 bit identifier, used to uniquely identify
STUN transactions. The transaction ID is chosen by the STUN client.
It primarily serves to correlate requests with responses, though it
also plays a small role in helping to prevent certain types of
attacks. As such, the transaction ID MUST be uniformly and randomly
chosen from the interval 0 .. 2**96-1. 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 four bytes, the last two 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 is given in
Section 14.
7. 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.
7.1. Forming a Request or an Indication
When formulating a request or indication message, the client MUST
follow the rules in Section 6 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 client then adds any attributes specified by the method or the
usage. For example, some usages may specify that the client use an
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authentication method (Section 10) or the FINGERPRINT attribute
(Section 8).
For the Binding method with no authentication, no attributes are
required unless the usage specifies otherwise.
7.2. Sending the Request or Indication
The client then sends the request to the server. This document
specifies how to send STUN messages over UDP, TCP, or TLS-over-TCP;
other transport protocols may be added in the future. The STUN usage
must specify which transport protocol is used, and how the client
determines the IP address and port of the server. Section 9
describes a DNS-based method of determining the IP address and port
of a server which 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).
7.2.1. Sending over UDP
When running STUN over UDP 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 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, and
is computed as described in RFC 2988 [RFC2988], with two exceptions.
First, the initial value for RTO SHOULD be configurable (rather than
the 3s recommended in RFC 2988). In fixed- line access links, a
value of 100ms is RECOMMENDED. Secondly, the value of RTO MUST NOT
be rounded up to the nearest second. Rather, a 1ms accuracy MUST be
maintained. As with TCP, the usage of Karn's algorithm is
RECOMMENDED. When applied to STUN, it means that RTT estimates
SHOULD NOT be computed from STUN transactions which result in the
retransmission of a request.
The value for RTO SHOULD be cached by an 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 after
10 minutes.
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Retransmissions continue until a response is received, or until a
total of 7 requests have been sent. If, after the last request, a
duration equal to 16 times the RTO has passed without a response, the
client SHOULD consider the transaction to have failed. A STUN
transaction over UDP is also considered failed if there has been a
transport failure of some sort, such as a fatal ICMP error. For
example, assuming an RTO of 100ms, requests would be sent at times
0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms, and 6300ms. If the client
has not received a response after 7900ms, the client will consider
the transaction to have timed out.
7.2.2. Sending over TCP or TLS-over-TCP
For TCP and TLS-over-TCP, the client opens a TCP connection to the
server.
In some usage 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 ontop 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.
For TLS-over-TCP, the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST
be supported at a minimum. Implementations MAY also support any
other ciphersuite. When it receives the TLS Certificate message, the
client SHOULD verify the certificate and inspect the site identified
by the certificate. If the certificate is invalid, revoked, or if it
does not identify the appropriate party, the client MUST NOT send the
STUN message or otherwise proceed with the STUN transaction. The
client MUST verify the identity of the server. To do that, it
follows the identification procedures defined in Section 3.1 of RFC
2818 [RFC2818]. Those procedures assume the client is dereferencing
a URI. For purposes of usage with this specification, the client
treats the domain name or IP address used in Section 8.1 as the host
portion of the URI that has been dereferenced. If DNS was not used,
the client MUST be configured with a set of authorized domains whose
certificates will be accepted.
Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
itself, and there are no retransmissions at the STUN protocol level.
However, for a request/response transaction, if the client has not
received a response after 7900ms, it considers the transaction to
have timed out. This value has been chosen to equalize the TCP and
UDP timeouts for the default initial RTO.
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In addition, if the client is unable to establish the TCP connection,
or the TCP connection is reset or fails before a response is
received, any request/response transaction in progress is considered
to have failed
The client MAY send multiple transactions over a single TCP (or TLS-
over-TCP) connection, and it MAY send another request before
receiving a response to the previous. 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
[I-D.ietf-behave-turn]) that were learned though STUN requests
sent over that connection, and;
o if multiplexing other application protocols over that port, has
finished using that other application, and;
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., [I-D.ietf-mmusic-ice-tcp]).
At the server end, the server SHOULD keep the connection open, and
let the client close it. If a server becomes overloaded and needs to
close connections to free up resources, it SHOULD close an existing
connection rather than reject new connection requests. The server
SHOULD NOT close a connection if a request was received over that
connection for which a response was not sent. A server MUST NOT ever
open a connection back towards the client in order to send a
response.
7.3. Receiving a STUN Message
This section specifies the processing of a STUN message. The
processing specified here is for STUN messages as defined in this
specification; additional rules for backwards compatibility are
defined in in Section 12. Those additional procedures are optional,
and usages can elect to utilize them. First, a set of processing
operations are applied that are independent of the class. This is
followed by class-specific processing, described in the subsections
which follow.
When a STUN agent receives a STUN message, it first checks that the
message obeys the rules of Section 6. It checks that the first two
bits are 0, that the magic cookie field has the correct value, that
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the message length is sensible, and that the method value is a
supported method. If the message-class is Success Response or Error
Response, the agent checks that the transaction ID matches a
transaction that is still in progress. If the FINGERPRINT extension
is being used, the agent checks that the FINGERPRINT attribute is
present and contains the correct value. If any errors are detected,
the message is silently discarded. In the case when STUN is being
multiplexed with another protocol, an error may indicate that this is
not really a STUN message; in this case, the agent should try to
parse the message as a different protocol.
The STUN agent then does any checks that are required by a
authentication mechanism that the usage has specified (see
Section 10.
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 described below.
At this point, further processing depends on the message class of the
request.
7.3.1. Processing a Request
If the request contains one or more unknown comprehension-required
attributes, the server replies with an error response with an error
code of 420 Unknown attributes, and includes an UNKNOWN-ATTRIBUTES
attribute in the response that lists the unknown comprehension-
required attributes.
The server then does any additional checking that the method or the
specific usage requires. If all the checks succeed, the server
formulates a success response as described below.
If the request uses UDP transport and is a retransmission of a
request for which the server has already generated a success response
within the last 10 seconds, the server MUST retransmit the same
success response. One way for a server to do this is to remember all
transaction IDs received over UDP and their corresponding responses
in the last 10 seconds. Another way is to reprocess the request and
recompute the response. The latter technique MUST only be applied to
requests which are idempotent and result in the same success response
for the same request. The Binding method is considered to idempotent
in this way (even though certain rare network events could cause the
reflexive transport address value to change). Extensions to STUN
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SHOULD state whether their request types have this property or not.
7.3.1.1. Forming a Success or Error Response
When forming the response (success or error), the server follows the
rules of section 6. The method of the response is the same as that
of the request, and the message class is either "Success Response" or
"Error Response".
For an error response, the server MUST add an ERROR-CODE attribute
containing the error code specified in the processing above. The
reason phrase is not fixed, but SHOULD be something suitable for the
error code. For certain errors, additional attributes are added to
the message. These attributes are spelled out in the description
where the error code is specified. For example, for an error code of
420 Unknown Attribute, the server MUST include an UNKNOWN-ATTRIBUTES
attribute. Certain authentication errors also cause attributes to be
added (see Section 10). Extensions may define other errors and/or
additional attributes to add in error cases.
If the server authenticated the request using an authentication
mechanism, then the server SHOULD add the appropriate authentication
attributes to the response (see Section 10).
The server also adds any attributes required by the specific method
or usage. In addition, the server SHOULD add a SERVER attribute to
the message.
For the Binding method, no additional checking is required unless the
usage specifies otherwise. When forming the success response, the
server adds a XOR-MAPPED-ADDRESS attribute to the response, where the
contents of the attribute are the source transport address of the
request message. For UDP, this is the source IP address and source
UDP port of the request message. For TCP and TLS-over-TCP, this is
the source IP address and source TCP port of the TCP connection as
seen by the server.
7.3.1.2. Sending the Success or Error Response
The response (success or error) is sent over the same transport as
the request was received on. If the request was received over UDP,
the destination IP address and port of the response is the source IP
address and port of the received request message, and the source IP
address and port of the response is equal to the destination IP
address and port of the received request message. If the request was
received over TCP or TLS-over-TCP, the response is sent back on the
same TCP connection as the request was received on.
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7.3.2. Processing an Indication
If the indication contains unknown comprehension-required attributes,
the indication is discarded and processing ceases.
The server then does any additional checking that the method or the
specific usage requires. If all the checks succeed, the server then
processes the indication. No response is 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 has refreshed the "bindings" in the
intervening NATs.
Since indications are not re-transmitted over UDP (unlike requests),
there is no need to handle re-transmissions of indications at the
server.
7.3.3. Processing a Success Response
If the success response contains unknown comprehension-required
attributes, the response is discarded and the transaction is
considered to have failed.
The client then does any additional checking that the method or the
specific usage requires. If all the checks succeed, the client then
processes the success response.
For the Binding method, the client checks that the XOR-MAPPED-ADDRESS
attribute is present in the response. The client checks the address
family specified. If it is an unsupported address family, the
attribute SHOULD be ignored. If it is an unexpected but supported
address family (for example, the Binding transaction was sent over
IPv4, but the address family specified is IPv6), then the client MAY
accept and use the value.
7.3.4. Processing an Error Response
If the error response contains unknown comprehension-required
attributes, or if the error response does not contain an ERROR-CODE
attribute, then the transaction is simply considered to have failed.
The client then does any processing specified by the authentication
mechanism (see Section 10). This may result in a new transaction
attempt.
The processing at this point depends on the error-code, the method,
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and the usage; the following are the default rules:
o If the error code is 300 through 399, the client SHOULD consider
the transaction as failed unless the ALTERNATE-SERVER extension is
being used. See Section 11.
o If the error code is 400 through 499, the client declares the
transaction failed; in the case of 420, the response should
contain a UNKNOWN-ATTRIBUTES attribute that gives additional
information.
o If the error code is 500 through 599, the client MAY resend the
request; clients that do so MUST limit the number of times they do
this. Any other error code causes the client to consider the
transaction failed.
8. FINGERPRINT Mechanism
This section describes an optional mechanism for STUN that aids in
distinguishing STUN messages from packets of other protocols when the
two are multiplexed on the same transport address. This mechanism is
optional, and a STUN usage must describe if and when it is used.
In some usages, STUN messages are multiplexed on the same transport
address as other protocols, such as RTP. In order to apply the
processing described in Section 7, STUN messages must first be
separated from the application packets. Section 6 describes three
fixed fields in the STUN header that can be used for this purpose.
However, in some cases, these three fixed fields may not be
sufficient.
When the FINGERPRINT extension is used, an agent includes the
FINGERPRINT attribute in messages it sends to another agent.
Section 14.5 describes the placement and value of this attribute.
When the agent receives what it believes is a STUN message, then, in
addition to other basic checks, the agent also checks that the
message contains a FINGERPRINT attribute and that the attribute
contains the correct value (see Section 7.3. This additional check
helps the agent detect messages of other protocols that might
otherwise seem to be STUN messages.
9. DNS Discovery of a Server
This section describes an optional procedure for STUN that allows a
client to use DNS to determine the IP address and port of a server.
A STUN usage must describe if and when this extension is used. To
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use this procedure, the client must have a domain name and a service
name; the usage must also describe how the client obtains these.
When a client wishes to locate a STUN server in the public Internet
that accepts Binding Request/Response transactions, the SRV service
name is "stun". STUN usages MAY define additional DNS SRV service
names.
The domain name is resolved to a transport address using the SRV
procedures specified in [RFC2782]. The DNS SRV service name is the
service name provided as input to this procedure. The protocol in
the SRV lookup is the transport protocol the client will run STUN
over: "udp" for UDP, "tcp" for TCP, and "tls" for TLS-over-TCP. If,
in the future, additional SRV records are defined for TLS over other
transport protocols, those will need to utilize an SRV transport
token of the form "tls-foo" for transport protocol "foo".
The procedures of RFC 2782 are followed to determine the server to
contact. RFC 2782 spells out the details of how a set of SRV records
are sorted and then tried. However, RFC2782 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 the STUN transaction times out
without receipt of a response, the client SHOULD retry the request to
the next server in the list of servers from the DNS SRV response.
Such a retry is only possible for request/response transmissions,
since indication transactions generate no response or timeout.
The default port for STUN requests is 3478, for both TCP and UDP.
Administrators SHOULD use this port in their SRV records for UDP and
TCP, but MAY use others. There is no default port for STUN over TLS,
however a STUN server SHOULD use a port number for TLS different from
3478 so that the server can determine whether the first message it
will receive after the TCP connection is set up, is a STUN message or
a TLS message.
If no SRV records were found, the client performs an A or AAAA record
lookup of the domain name. The result will be a list of IP
addresses, each of which can be contacted at the default port using
UDP or TCP, independent of the STUN usage. For usages that require
TLS, lack of SRV records is equivalent to a failure of the
transaction, since the request or indication MUST NOT be sent unless
SRV records provided a transport address specifically for TLS.
10. Authentication and Message-Integrity Mechanisms
This section defines two mechanisms for STUN that a client and server
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can use to provide authentication and message-integrity; these two
mechanisms are known as the short-term credential mechanism and the
long-term credential mechanism. These two mechanisms are optional,
and each usage must specify if and when these mechanisms are used.
An overview of these two mechanisms is given in .
Each mechanism specifies the additional processing required to use
that mechanism, extending the processing specified in Section 7. The
additional processing occurs in three different places: when forming
a message; when receiving a message immediately after the the basic
checks have been performed; and when doing the detailed processing of
error responses.
10.1. Short-Term Credential Mechanism
The short-term credential mechanism assumes that, prior to the STUN
transaction, the client and server have used some other protocol to
exchange a credential in the form of a username and password. This
credential is time-limited. The time-limit is defined by the usage.
As an example, in the ICE usage [I-D.ietf-mmusic-ice], the two
endpoints use out-of-band signaling to agree on a username and
password, and this username and password is applicable for the
duration of the media session.
This credential is used to form a message integrity check in each
request and in many responses. There is no challenge and response as
in the long term mechanism; consequently, replay is prevented by
virtue of the time-limited nature of the credential.
10.1.1. Forming a Request or Indication
For a request or indication message, the agent MUST include the
USERNAME and MESSAGE-INTEGRITY attributes in the message. The HMAC
for the MESSAGE-INTEGRITY attribute is computed as described in
Section 14.4. The key for the HMAC is the password. Note that the
password is never included in the request or indication.
10.1.2. Receiving a Request or Indication
After the agent has done the basic processing of a message, the agent
performs the checks listed below in order specified:
o If the message does not contain both a MESSAGE-INTEGRITY and a
USERNAME attribute:
* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 400.
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* If the message is an indication, the server MUST silently
discard the indication.
o If the USERNAME does not contain a username value currently valid
within the server:
* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 401.
* If the message is an indication, the server MUST silently
discard the indication.
o Using the password associated with the username, compute the value
for the message-integrity as described in Section 14.4. If the
resulting value does not match the contents of the MESSAGE-
INTEGRITY attribute:
* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 431.
* If the message is an indication, the server MUST silently
discard the indication.
If these checks pass, the server continues to process the request or
indication. Any response generated by the server MUST include the
MESSAGE-INTEGRITY attribute, computed using the username and password
utilized to authenticate the request.
If any of the checks fail, the server MUST NOT include a MESSAGE-
INTEGRITY or USERNAME attribute in the error response.
10.1.3. Receiving a Response
The processing here takes place prior to the processing in
Section 7.3.3 or Section 7.3.4.
The client looks for the MESSAGE-INTEGRITY attribute in the response.
If present, the client computes the message integrity over the
response as defined in Section 14.4, using the same password it
utilized for the request. If the resulting value matches the
contents of the MESSAGE-INTEGRITY attribute, the response is
considered authenticated. If the value does not match, or if
MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if
it was never received. This means that retransmits, if applicable,
will continue.
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10.2. Long-term Credential Mechanism
The long-term credential mechanism relies on a long term credential,
in 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 is provisioned for a user, and remains in effect
until the user is no longer a subscriber of the system, or is
changed. This is basically a traditional "log-in" username and
password given to users.
Because these usernames and passwords are expected to be valid for
extended periods of time, replay prevention is provided in the form
of a digest challenge. In this mechanism, the client initially sends
a request, without offering any credentials or any integrity checks.
The server rejects this request, providing the user a realm (used to
guide the user or agent in selection of a username and password) and
a nonce. The nonce provides the replay protection. It is a cookie,
selected by the server, and encoded in such a way as to indicate a
duration of validity or client identity from which it is valid. The
client retries the request, this time including its username, the
realm, and echoing the nonce provided by the server. The client also
includes a message-integrity, which provides an HMAC over the entire
request, including the nonce. The server validates the nonce, and
checks the message-integrity. If they match, the request is
authenticated. If the nonce is no longer valid, it is considered
"stale", and the server rejects the request, providing a new nonce.
In subsequent requests to the same server, the client reuses the
nonce, username, realm 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
the client.
Note that 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, or
omit authentication and message integrity for them.
Since the long-term credential mechanism is susceptible to offline
dictionary attacks, deployments SHOULD utilize strong passwords.
For STUN servers used in conjunction with SIP servers, it is
desirable to use the same credentials for authentication to the SIP
server and STUN server. Typically, SIP systems utilizing SIP's
digest authentication mechanism do not actually store the password in
the database. Rather, they store a value called H(A1), which is
computed as:
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H(A1) = MD5(username ":" realm ":" password)
If a system wishes to utilize this credential, the STUN password
would be computed by taking the user-entered username and password,
and using H(A1) as the STUN password. It is RECOMMENDED that clients
utilize this construction for the STUN password.
10.2.1. Forming a Request
There are two cases when forming a request. In the first case, this
is the first request from the client to the server (as identified by
its IP address and port). In the second case, the client is
submitting a subsequent request once a previous request/response
transaction has completed successfully.
10.2.1.1. First Request
If the client has not completed a successful request/response
transaction with the server, it SHOULD omit the USERNAME, MESSAGE-
INTEGRITY, REALM, and NONCE attributes. In other words, the very
first request is sent as if there were no authentication or message
integrity applied.
10.2.1.2. Subsequent Requests
Once a request/response transaction has completed successfully, the
client will have been been presented a realm and nonce by the server,
and selected a username and password with which it authenticated.
The client SHOULD cache the username, password, realm, and nonce for
subsequent communications with the server. When the client sends a
subsequent request, it SHOULD include the USERNAME, REALM, and NONCE
attributes with these cached values. It SHOULD include a MESSAGE-
INTEGRITY attributed, computed as described in Section 14.4 using the
cached password as the key.
10.2.2. Receiving a Request
After the server has done the basic processing of a request, it
performs the checks listed below in the order specified:
o If the message:
* does not contain a MESSAGE-INTEGRITY attribute,
* OR, it contains a USERNAME whose value is not a valid username,
the server MUST generate an error response with an error code of
401. This response MUST include a REALM value. It is RECOMENDED
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that the REALM value by the domain name of the provider of the
STUN server. The response MUST include a NONCE, selected by the
server.
o If the message contains a MESSAGE-INTEGRITY attribute, but is
missing the USERNAME, REALM or NONCE attributes, the server MUST
generate an error response with an error code of 400.
o If the NONCE is no longer valid, the server MUST generate an error
response with an error code of 438 (Stale Nonce). This response
MUST include a NONCE and REALM attribute.
o Using the password associated with the username in the USERNAME
attribute, compute the value for the message-integrity as
described in Section 14.4. If the resulting value does not match
the contents of the MESSAGE-INTEGRITY attribute, the server MUST
reject the request with an error response. This response MUST use
an error code of 401. It MUST include a REALM and NONCE
attribute.
If these checks pass, the server continues to process the request or
indication. Any response generated by the server MUST include the
MESSAGE-INTEGRITY attribute, computed using the username and password
utilized to authenticate the request. The REALM, NONCE, and USERNAME
attributes SHOULD NOT be included.
10.2.3. Receiving a Response
The processing here takes place prior to the processing in
Section 7.3.3 or Section 7.3.4.
If the response is an error response, with an error code of 401
(Unauthorized), the client SHOULD retry the request with a new
transaction. This request MUST contain a USERNAME, determined by the
client as the appropriate username for the REALM from the error
response. The request MUST contain the REALM, copied from the error
response. The request MUST contain the NONCE, copied from the error
response. The request MUST contain the MESSAGE-INTEGRITY attribute,
computed using the password associated with the username in the
USERNAME attribute. The client MUST NOT perform this retry if it is
not changing the USERNAME or REALM or its associated password, from
the previous attempt.
If the response is an error response with an error code of 438, the
client MUST retry the request, using the new NONCE supplied in the
438 response. This retry MUST also include the USERNAME, REALM and
MESSAGE-INTEGRITY.
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The client looks for the MESSAGE-INTEGRITY attribute in the response
(either success or failure). If present, the client computes the
message integrity over the response as defined in Section 14.4, using
the same password it utilized for the request. If the resulting
value matches the contents of the MESSAGE-INTEGRITY attribute, the
response is considered authenticated. If the value does not match,
or if MESSAGE-INTEGRITY was absent, the response MUST be discarded,
as if it was never received. This means that retransmits, if
applicable, will continue.
11. ALTERNATE-SERVER Mechanism
This section describes a mechanism in STUN that allows a server to
redirect a client to another server. This extension is optional, and
a usage must define if and when this extension is used. To prevent
denial-of-service attacks, this extension MUST only be used in
situations where the client and server are using an authentication
and message-integrity mechanism.
A server using this extension redirects a client to another server by
replying to a request message with an error response message with an
error code of 300 (Try Alternate). The server MUST include a
ALTERNATE-SERVER attribute in the error response. The error response
message MUST be authenticated, which in practice means the request
message must have passed the authentication checks.
A client using this extension handles a 300 (Try Alternate) error
code as follows. If the error response has passed the authentication
checks, then the client looks for a ALTERNATE-SERVER attribute in the
error response. If one is found, then the client considers the
current transaction as failed, and re-attempts the request with the
server specified in the attribute. The client SHOULD reuse any
authentication credentials from the old request in the new
transaction.
12. Backwards Compatibility with RFC 3489
This section define procedures that allow a degree of backwards
compatible with the original protocol defined in RFC 3489 [RFC3489].
This mechanism is optional, meant to be utilized only in cases where
a new client can connect to an old server, or vice-a-versa. A usage
must define if and when this procedure is used.
Section 18 lists all the changes between this specification and RFC
3489 [RFC3489]. However, not all of these differences are important,
because "classic STUN" was only used in a few specific ways. For the
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purposes of this extension, the important changes are the following.
In RFC 3489:
o UDP was the only supported transport;
o The field that is now the Magic Cookie field was a part of the
transaction id field, and transaction ids were 128 bits long;
o The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding
method used the MAPPED-ADDRESS attribute instead
o There were two comprehension-required attributes, RESPONSE-ADDRESS
and CHANGE-REQUEST, that have been removed from this
specification.
* These attributes are now part of the NAT Behavior Discovery
usage.
12.1. Changes to Client Processing
A client that wants to interoperate with a [RFC3489] server SHOULD
send a request message that uses the Binding method, contains no
attributes, and uses UDP as the transport protocol to the server. If
successful, the success response received from the server will
contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS
attribute; other than this change, the processing of the response is
identical to the procedures described above.
12.2. Changes to Server Processing
A STUN server can detect when a given Binding Request message was
sent from an RFC 3489 [RFC3489] client by the absence of the correct
value in the Magic Cookie field. When the server detects an RFC 3489
client, it SHOULD copy the value seen in the Magic Cookie field in
the Binding Request to the Magic Cookie field in the Binding Response
message, and insert a MAPPED-ADDRESS attribute instead of an XOR-
MAPPED-ADDRESS attribute.
The client might, in rare situations, include either the RESPONSE-
ADDRESS or CHANGE-REQUEST attributes. In these situations, the
server will view these as unknown comprehension-required attributes
and reply with an error response. Since the mechanisms utilizing
those attributes are no longer supported, this behavior is
acceptable.
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13. STUN Usages
STUN by itself is not a solution to the NAT traversal problem.
Rather, STUN defines a toolkit of functions that can be used inside a
larger solution. The term "STUN Usage" is used for any solution that
uses STUN as a component.
At the time of writing, three STUN usages are defined: Interactive
Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice], Client-
initiated connections for SIP [I-D.ietf-sip-outbound], and NAT
Behavior Discovery [I-D.ietf-behave-nat-behavior-discovery]. Other
STUN usages may be defined in the future.
A STUN usage defines how STUN is actually utilized - when to send
requests, what to do with the responses, and which optional
procedures defined here (or in an extension to STUN) are to be used.
A usage would also define:
o Which STUN methods are used;
o What authentication and message integrity mechanisms are used;
o What mechanisms are used to distinguish STUN messages from other
messages. When STUN is run over TCP, a framing mechanism may be
required;
o How a STUN client determines the IP address and port of the STUN
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.
In addition, any STUN usage must consider the security implications
of using STUN in that usage. A number of attacks against STUN are
known (see 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 the Unilateral Self-Address Fixing approach to NAT
traversal, and if so, address the questions raised in RFC 3424.
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14. STUN Attributes
After the STUN header are zero or more attributes. Each attribute
MUST be 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.
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 5: Format of STUN Attributes
The value in the Length field MUST contain the length of the Value
part of the attribute, prior to padding, measured in bytes. Since
STUN aligns attributes on 32 bit boundaries, attributes whose content
is not a multiple of 4 bytes are padded with 1, 2 or 3 bytes of
padding so that its value contains a multiple of 4 bytes. The
padding bits are ignored, and may be any value.
Any attribute type MAY appear more than once in a STUN message.
Unless specified otherwise, the order of appearance is significant:
only the first occurance needs to be processed by a receiver, and any
duplicates MAY be ignored by a receiver.
To allow future revisions of this specification to add new attributes
if needed, the 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 agent if it does not understand them.
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The STUN Attribute types defined by this specification are:
Comprehension-required range (0x0000-0x7FFF):
0x0000: (Reserved)
0x0001: MAPPED-ADDRESS
0x0006: USERNAME
0x0007: (Reserved; was PASSWORD)
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS
Comprehension-optional range (0x8000-0xFFFF)
0x8022: SERVER
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT
The rest of this section describes the format of the various
attributes defined in this specification.
14.1. MAPPED-ADDRESS
The MAPPED-ADDRESS attribute indicates a reflexive transport address
of the client. It consists of an eight bit address family, and a
sixteen bit port, followed by a fixed length value representing the
IP address. If the address family is IPv4, the address MUST be 32
bits. If the address family is IPv6, the address 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 7: Format of MAPPED-ADDRESS attribute
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The address family can take on the following values:
0x01:IPv4
0x02:IPv6
The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be
ignored by receivers. These bits are present for aligning parameters
on natural 32 bit boundaries.
This attribute is used only by servers for achieving backwards
compatibility with RFC 3489 [RFC3489] clients.
14.2. XOR-MAPPED-ADDRESS
The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS
attribute, except that the reflexive transport 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|x x x x x x x x| Family | X-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| X-Address (Variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Format of XOR-MAPPED-ADDRESS Attribute
The Family represents the IP address family, and is encoded
identically to the Family in MAPPED-ADDRESS.
X-Port is the mapped port, exclusive or'd with most significant 16
bits of the magic cookie. If the IP address family is IPv4,
X-Address is the mapped IP address exclusive or'd with the magic
cookie. If the IP address family is IPv6, the X-Address is the
mapped IP address exclusively or'ed with the magic cookie and the 96-
bit transaction ID.
For example, using the "^" character to indicate exclusive or, if the
IP address is 192.168.1.1 (0xc0a80101) and the port is 5555 (0x15B3),
the X-Port would be 0x15B3 ^ 0x2112 = 0x34A1, and the X-Address would
be 0xc0a80101 ^ 0x2112A442 = 0xe1baa543.
The rules for encoding and processing the first 8 bits of the
attribute's value, the rules for handling multiple occurrences of the
attribute, and the rules for processing addresses families are the
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same as for MAPPED-ADDRESS.
NOTE: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
encoding of the transport address. The former encodes the transport
address by exclusive-or'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 well-meaning
but misguided attempt at providing a generic ALG function. Such
behavior interferes with the operation of STUN and also causes
failure of STUN's message integrity checking.
14.3. USERNAME
The USERNAME attribute is used for message integrity. It identifies
the username and password combination used in the message integrity
check.
The value of USERNAME is a variable length value. It MUST contain a
UTF-8 encoded sequence of less than 128 characters.
14.4. MESSAGE-INTEGRITY
The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of
the STUN message. The MESSAGE-INTEGRITY attribute can be present in
any STUN message type. Since it uses the SHA1 hash, the HMAC will be
20 bytes. The text used as input to HMAC is the STUN message,
including the header, up to and including the attribute preceding the
MESSAGE-INTEGRITY attribute. With the exception of the FINGERPRINT
attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore
all other attributes that follow MESSAGE-INTEGRITY.
The key used as input to HMAC is the password.
Since the hash is computed over the entire STUN message, it includes
the length field from the STUN message header. This length indicates
the length of the entire message, including the MESSAGE-INTEGRITY
attribute itself. Consequently, the MESSAGE-INTEGRITY attribute MUST
be inserted into the message (with dummy content) prior to the
computation of the integrity check. Once the computation is
performed, the value of the attribute can be filled in. This ensures
the length has the correct value when the hash is performed.
Similarly, when validating the MESSAGE-INTEGRITY, the length field
should be adjusted to point to the end of the MESSAGE-INTEGRITY
attribute prior to calculating the HMAC. Such adjustment is
necessary when attributes, such as FINTERPRINT, appear after MESSAGE-
INTEGRITY.
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14.5. FINGERPRINT
The FINGERPRINT attribute may be present in all STUN messages. The
value of the attribute is computed as the CRC-32 of the STUN message
up to (but excluding) the FINGERPRINT attribute itself, xor-d with
the 32 bit value 0x5354554e (the XOR helps in cases where an
application packet is also using CRC-32 in it). The 32 bit CRC is
the one defined in ITU V.42 [ITU.V42.1994], which has a generator
polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.
When present, the FINGERPRINT attribute MUST be the last attribute in
the message, and thus will appear after MESSAGE-INTEGRITY.
The FINGERPRINT attribute can aid in distinguishing STUN packets from
packets of other protocols. See Section 8.
When using the FINGERPRINT attribute in a message, the attribute is
first placed into the message with a dummy value, then the CRC is
computed, and then the value of the attribute is updated. If the
MESSAGE-INTEGRITY attribute is also present, then it must be present
with the correct message-integrity value before the CRC is computed,
since the CRC is done over the value of the MESSAGE-INTEGRITY
attribute as well.
14.6. ERROR-CODE
The ERROR-CODE attribute is used in Error Response messages. It
contains a numeric error code value in the range of 300 to 699 plus a
textual reason phrase encoded in UTF-8, and is consistent in its code
assignments and semantics with SIP [RFC3261] and HTTP [RFC2616]. The
reason phrase is meant for user consumption, and can be anything
appropriate for the error code. Recommended reason phrases for the
defined error codes are presented below. The reason phrase MUST be a
UTF-8 encoded sequence of less than 128 characters.
To facilitate processing, the class of the error code (the hundreds
digit) is encoded separately from the rest of the code.
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 be 0 |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Reserved bits SHOULD be 0, and are for alignment on 32-bit
boundaries. Receivers MUST ignore these bits. The Class represents
the hundreds digit of the error code. The value MUST be between 3
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and 6. The number represents the error code modulo 100, and its
value MUST be between 0 and 99.
The following error codes, along with their recommended reason
phrases (in brackets) are defined:
300 Try Alternate: The client should contact an alternate server for
this request. This error response MUST only be sent if the
request included a USERNAME attribute and a valid MESSAGE-
INTEGRITY attribute; otherwise it MUST NOT be sent and error
code 400 is suggested. This error response MUST be protected
with the MESSAGE-INTEGRITY attribute, and receivers MUST
validate the MESSAGE-INTEGRITY of this response before
redirecting themselves to an alternate server.
Note: failure to generate and validate message-integrity
for a 300 response allows an on-path attacker to falsify a
300 response thus causing subsequent STUN messages to be
sent to a 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 generate a valid
MESSAGE-INTEGRITY for this error, so the client MUST NOT expect
a valid MESSAGE-INTEGRITY attribute on this response.
401 Unauthorized: The request did not contain the expected MESSAGE-
INTEGRITY attribute. The server MAY include the MESSAGE-
INTEGRITY attribute in its error response.
420 Unknown Attribute: The server received STUN packet containing a
comprehension-required attribute which it did not understand.
The server MUST put this unknown attribute in the UNKNOWN-
ATTRIBUTE attribute of its error response.
438 Stale Nonce: The NONCE used by the client was no longer valid.
The client should retry, using the NONCE provided in the
response.
500 Server Error: The server has suffered a temporary error. The
client should try again.
14.7. REALM
The REALM attribute may be present in requests and responses. It
contains text which meets the grammar for "realm-value" as described
in RFC 3261 [RFC3261] but without the double quotes and their
surrounding whitespace. That is, it is an unquoted realm-value. It
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MUST be a UTF-8 encoded sequence of less than 128 characters.
Presence of the REALM attribute in a request indicates that long-term
credentials are being used for authentication. Presence in certain
error responses indicates that the server wishes the client to use a
long-term credential for authentication.
14.8. NONCE
The NONCE attribute may be present in requests and responses. It
contains a sequence of qdtext or quoted-pair, which are defined in
RFC 3261 [RFC3261]. See RFC 2617 [RFC2617], Section 4.3, for
guidance on selection of nonce values in a server. It MUST be less
than 128 characters.
14.9. UNKNOWN-ATTRIBUTES
The UNKNOWN-ATTRIBUTES attribute is present only in an error response
when the response code in the ERROR-CODE attribute is 420.
The attribute contains a list of 16 bit values, each of which
represents an attribute type that was not understood by 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 11: Format of UNKNOWN-ATTRIBUTES attribute
Note: In [RFC3489], this field was padded to 32 by duplicating the
last attribute. In this version of the specification, the normal
padding rules for attributes are used instead.
14.10. SERVER
The server attribute contains a textual description of the software
being used by the server, including manufacturer and version number.
The attribute has no impact on operation of the protocol, and serves
only as a tool for diagnostic and debugging purposes. The value of
SERVER is variable length. It MUST be a UTF-8 encoded sequence of
less than 128 characters
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14.11. ALTERNATE-SERVER
The alternate server represents an alternate transport address
identifying a different STUN server which the STUN client should try.
It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a
single server by IP address. The IP address family MUST be identical
to that of the source IP address of the request.
This attribute MUST only appear in an error response that contains a
MESSAGE-INTEGRITY attribute. This prevents it from being used in
denial-of-service attacks.
15. Security Considerations
15.1. Attacks against the Protocol
15.1.1. Outside Attacks
An attacker can try to modify STUN messages in transit, in order to
cause a failure in STUN operation. These attacks are prevented for
both requests and responses through the message integrity mechanism,
using either a short term or long term credential.
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, and then immediately send a STUN
response, typically an error response, in order to disrupt STUN
processing. This attack is also prevented for messages that utilize
MESSAGE-INTEGRITY. However, some error responses, those related to
authentication in particular, cannot be protected by MESSAGE-
INTEGRITY. When STUN itself is run over a secure transport protocol
(e.g., TLS), these attacks are completely mitigated.
15.1.2. Inside Attacks
A rogue client may try to launch a DoS attack against a server by
sending it a large number of STUN requests. Fortunately, STUN
requests can be processed statelessly by a server, making such
attacks hard to launch.
15.2. Attacks Affecting the Usage
This section lists attacks that might be launched against a usage of
STUN. Each STUN usage must consider whether these attacks are
applicable to it, and if so, discuss counter-measures.
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Most of the attacks in this section revolve around an attacker
modifying the reflexive address learned by a STUN client through a
Binding Request/Binding Response transaction. Since the usage of the
reflexive address is a function of the usage, the applicability and
remediation of these attacks is usage-specific. In common
situations, modification of the reflexive address by an on-path
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 the STUN server. The STUN server will then
return this IP address in the XOR-MAPPED-ADDRESS attribute to 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, since the intervening NAT must be able to modify
this value. Instead, one solution to preventing the attacks listed
below is for the client to verify the reflexive address learned, as
is done in ICE [I-D.ietf-mmusic-ice]. Other usages may use other
means to prevent these attacks.
15.2.1. Attack I: DDoS 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 of the target. If the clients hand out
that reflexive address in order to receive traffic on it (for
example, in 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 enable
multimedia applications.
15.2.2. Attack II: Silencing a Client
In this attack, the 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 the packets it expects to receive when it hands out
the reflexive address. This exploitation is not very interesting for
the attacker. It impacts a single client, which is frequently not
the desired target. Moreover, any attacker that can mount the attack
could also deny service to the client by other means, such as
preventing the client from receiving any response from the STUN
server, or even a DHCP server.
15.2.3. Attack III: Assuming the Identity of a Client
This attack is similar to attack III. However, the faked reflexive
address points to the attacker itself. This allows the attacker to
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receive traffic which was destined for the client.
15.2.4. 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 would allow the attacker to
observe all packets sent to the client. However, in 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.
15.3. Hash Agility Plan
This specification uses SHA-1 for computation of the message
integrity. If, at a later time, SHA-1 is found to be compromised,
the following is the remedy that will be applied.
We will define a STUN extension which introduces a new message
integrity attribute, computed using a new hash. Clients would be
required to include both the new and old message integrity attributes
in their requests or indications. A new server will utilize the new
message integrity attribute, and an old one, the old. After a
transition period where mixed implementations are in deployment, the
old message-integrity attribute will be deprecated by another
specification, and clients will cease including it in requests.
16. IAB Considerations
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 another realm on the other side of a NAT
through a collaborative protocol reflection mechanism (RFC3424
[RFC3424]). STUN can be used to perform this function using a
BindingRequest/BindingResponse transaction if one agent is behind a
NAT and the other is on the public side of the NAT.
The IAB has mandated that protocols developed for this purpose
document a specific set of considerations. Because some STUN usages
provide UNSAF functions (such as ICE [I-D.ietf-mmusic-ice] ), and
others do not (such as SIP Outbound [I-D.ietf-sip-outbound]), answers
to these considerations need to be addressed by the usages
themselves.
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17. IANA Considerations
IANA is hereby requested to create three new registries: a STUN
methods registry, a STUN Attributes registry, and a STUN Error Codes
registry.
17.1. STUN Methods Registry
A STUN method is a hex number in the range 0x000 - 0x3FF. The
encoding of STUN method into a STUN message is described in
Section 6.
The initial STUN methods are:
0x000: (Reserved)
0x001: Binding
0x002: (Reserved; was SharedSecret)
STUN methods in the range 0x000 - 0x1FF are assigned by IETF
Consensus [RFC2434]. STUN methods in the range 0x200 - 0x3FF are
assigned on a First Come First Served basis [RFC2434]
17.2. STUN Attribute Registry
A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF.
STUN attribute types in the range 0x0000 - 0x7FFF are considered
comprehension-required; STUN attribute types in the range 0x8000 -
0xFFFF are considered comprehension-optional. A STUN agent handles
unknown comprehension-required and comprehension-optional attributes
differently.
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The initial STUN Attributes types are:
Comprehension-required range (0x0000-0x7FFF):
0x0000: (Reserved)
0x0001: MAPPED-ADDRESS
0x0006: USERNAME
0x0007: (Reserved; was PASSWORD)
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS
Comprehension-optional range (0x8000-0xFFFF)
0x8022: SERVER
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT
STUN Attribute types in the first half of the comprehension-required
range (0x0000 - 0x3FFF) and in the first half of the comprehension-
optional range (0x8000 - 0xBFFF) are assigned by IETF Consensus
[RFC2434]. STUN Attribute types in the second half of the
comprehension-required range (0x4000 - 0x7FFF) and in the second half
of the comprehension-optional range (0xC000 - 0xFFFF) are assigned on
a First Come First Served basis [RFC2434].
17.3. STUN Error Code 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 which is intended
only for human consumption and can be anything appropriate; this
document proposes only suggested values.
STUN error codes are consistent in codepoint assignments and
semantics with SIP [RFC3261] and HTTP [RFC2616].
The initial values in this registry are given in Section 14.6.
New STUN error codes are assigned on a Specification-Required basis
[RFC2434]. 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 7.3.4.
18. Changes Since RFC 3489
This specification obsoletes RFC3489 [RFC3489]. This specification
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differs from RFC3489 in the following ways:
o Removed the notion that STUN is a complete NAT traversal solution.
STUN is now a toolkit that can be used to produce a NAT traversal
solution. As a consequence, changed the name of the protocol to
Session Traversal Utilities for NAT.
o Introduced the concept of STUN usages, and described what a usage
of STUN must document.
o Removed the usage of STUN for NAT type detection and binding
lifetime discovery. These techniques have proven overly brittle
due to wider variations in the types of NAT devices than described
in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS,
CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes.
o Added a fixed 32-bit magic cookie and reduced length of
transaction ID by 32 bits. The magic cookie begins at the same
offset as the original transaction ID.
o Added the XOR-MAPPED-ADDRESS attribute, which is included in
Binding Responses if the magic cookie is present in the request.
Otherwise the RFC3489 behavior is retained (that is, Binding
Response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED-
ADDRESS regarding this change.
o Introduced formal structure into the Message Type header field,
with an explicit pair of bits for indication of request, response,
error response or indication. Consequently, the message type
field is split into the class (one of the previous four) and
method.
o Explicitly point out that the most significant two bits of STUN
are 0b00, allowing easy differentiation with RTP packets when used
with ICE.
o Added the FINGERPRINT attribute to provide a method of definitely
detecting the difference between STUN and another protocol when
the two protocols are multiplexed together.
o Added support for IPv6. Made it clear that an IPv4 client could
get a v6 mapped address, and vice-a-versa.
o Added long-term credential-based authentication.
o Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes.
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o Removed the SharedSecret method, and thus the PASSWORD attribute.
This method was almost never implemented and is not needed with
current usages.
o Removed recommendation to continue listening for STUN Responses
for 10 seconds in an attempt to recognize an attack.
o Changed transaction timers to be more TCP friendly.
o Removed the STUN example that centered around the separation of
the control and media planes. Instead, provided more information
on using STUN with protocols.
o Defined a generic padding mechanism that changes the
interpretation of the length attribute. This would, in theory,
break backwards compatibility. However, the mechanism in RFC 3489
never worked for the few attributes that weren't aligned naturally
on 32 bit boundaries.
o REALM, USERNAME, SERVER, reason phrases and NONCE limited to 127
characters.
19. Acknowledgements
The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
Jennings, Bob Penfield, Xavier Marjou, 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.
20. References
20.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
Rosenberg, et al. Expires January 9, 2008 [Page 42]
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[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[ITU.V42.1994]
International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 1994.
20.2. Informational References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-16 (work in progress), June 2007.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[I-D.ietf-behave-turn]
Rosenberg, J., "Obtaining Relay Addresses from Simple
Traversal Underneath NAT (STUN)",
draft-ietf-behave-turn-03 (work in progress), March 2007.
[I-D.ietf-sip-outbound]
Jennings, C. and R. Mahy, "Managing Client Initiated
Connections in the Session Initiation Protocol (SIP)",
draft-ietf-sip-outbound-09 (work in progress), June 2007.
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Internet-Draft STUN July 2007
[I-D.ietf-behave-nat-behavior-discovery]
MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
Using STUN", draft-ietf-behave-nat-behavior-discovery-00
(work in progress), February 2007.
[I-D.ietf-mmusic-ice-tcp]
Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE",
draft-ietf-mmusic-ice-tcp-03 (work in progress),
March 2007.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
Appendix A. C Snippet to Determine STUN Message Types
Given an 16-bit STUN message type value in host byte order in
msg_type parameter, below are C macros to determine the STUN message
types:
#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)
Authors' Addresses
Jonathan Rosenberg
Cisco
Edison, NJ
US
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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Christian Huitema
Microsoft
One Microsoft Way
Redmond, WA 98052
US
Email: huitema@microsoft.com
Rohan Mahy
Plantronics
345 Encinal Street
Santa Cruz, CA 95060
US
Email: rohan@ekabal.com
Philip Matthews
Avaya
1135 Innovation Drive
Ottawa, Ontario K2K 3G7
Canada
Phone: +1 613 592 4343 x224
Fax:
Email: philip_matthews@magma.ca
URI:
Dan Wing
Cisco
771 Alder Drive
San Jose, CA 95035
US
Email: dwing@cisco.com
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