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rfc:rfc5389

Network Working Group J. Rosenberg Request for Comments: 5389 Cisco Obsoletes: 3489 R. Mahy Category: Standards Track P. Matthews

                                                          Unaffiliated
                                                               D. Wing
                                                                 Cisco
                                                          October 2008
             Session Traversal Utilities for NAT (STUN)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 Session Traversal Utilities for NAT (STUN) is a protocol that serves
 as a tool for other protocols in dealing with Network Address
 Translator (NAT) 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 ……………………………………………..8 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

Rosenberg, et al. Standards Track [Page 1] RFC 5389 STUN October 2008

      7.2.1. Sending over UDP ...................................13
      7.2.2. Sending over TCP or TLS-over-TCP ...................14
 7.3. Receiving a STUN Message ..................................16
      7.3.1. Processing a Request ...............................17
             7.3.1.1. Forming a Success or Error Response .......18
             7.3.1.2. Sending the Success or Error Response .....19
      7.3.2. Processing an Indication ...........................19
      7.3.3. Processing a Success Response ......................19
      7.3.4. Processing an Error Response .......................20

8. FINGERPRINT Mechanism ……………………………………20 9. DNS Discovery of a Server ………………………………..21 10. Authentication and Message-Integrity Mechanisms ……………22

 10.1. Short-Term Credential Mechanism ..........................22
      10.1.1. Forming a Request or Indication ...................23
      10.1.2. Receiving a Request or Indication .................23
      10.1.3. Receiving a Response ..............................24
 10.2. Long-Term Credential Mechanism ...........................24
      10.2.1. Forming a Request .................................25
             10.2.1.1. First Request ............................25
             10.2.1.2. Subsequent Requests ......................26
      10.2.2. Receiving a Request ...............................26
      10.2.3. Receiving a Response ..............................27

11. ALTERNATE-SERVER Mechanism ………………………………28 12. Backwards Compatibility with RFC 3489 …………………….28

 12.1. Changes to Client Processing .............................29
 12.2. Changes to Server Processing .............................29

13. Basic Server Behavior …………………………………..30 14. STUN Usages ……………………………………………30 15. STUN Attributes ………………………………………..31

 15.1. MAPPED-ADDRESS ...........................................32
 15.2. XOR-MAPPED-ADDRESS .......................................33
 15.3. USERNAME .................................................34
 15.4. MESSAGE-INTEGRITY ........................................34
 15.5. FINGERPRINT ..............................................36
 15.6. ERROR-CODE ...............................................36
 15.7. REALM ....................................................38
 15.8. NONCE ....................................................38
 15.9. UNKNOWN-ATTRIBUTES .......................................38
 15.10. SOFTWARE ................................................39
 15.11. ALTERNATE-SERVER ........................................39

16. Security Considerations …………………………………39

 16.1. Attacks against the Protocol .............................39
      16.1.1. Outside Attacks ...................................39
      16.1.2. Inside Attacks ....................................40
 16.2. Attacks Affecting the Usage ..............................40
      16.2.1. Attack I: Distributed DoS (DDoS) against a
              Target ............................................41
      16.2.2. Attack II: Silencing a Client .....................41

Rosenberg, et al. Standards Track [Page 2] RFC 5389 STUN October 2008

      16.2.3. Attack III: Assuming the Identity of a Client .....42
      16.2.4. Attack IV: Eavesdropping ..........................42
 16.3. Hash Agility Plan ........................................42

17. IAB Considerations ……………………………………..42 18. IANA Considerations …………………………………….43

 18.1. STUN Methods Registry ....................................43
 18.2. STUN Attribute Registry ..................................43
 18.3. STUN Error Code Registry .................................44
 18.4. STUN UDP and TCP Port Numbers ............................45

19. Changes since RFC 3489 ………………………………….45 20. Contributors …………………………………………..47 21. Acknowledgements ……………………………………….47 22. References …………………………………………….47

 22.1. Normative References .....................................47
 22.2. Informative References ...................................48

Appendix A. C Snippet to Determine STUN Message Types ………….50

Rosenberg, et al. Standards Track [Page 3] RFC 5389 STUN October 2008

1. Introduction

 The protocol defined in this specification, Session Traversal
 Utilities for NAT, provides a tool for dealing with NATs.  It
 provides a means for an endpoint to determine the IP address and port
 allocated by a NAT that corresponds to its private IP address and
 port.  It also provides a way for an endpoint to keep a NAT binding
 alive.  With some extensions, the protocol can be used to do
 connectivity checks between two endpoints [MMUSIC-ICE], or to relay
 packets between two endpoints [BEHAVE-TURN].
 In keeping with its tool nature, this specification defines an
 extensible packet format, defines operation over several transport
 protocols, and provides for two forms of authentication.
 STUN is intended to be used in 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) [MMUSIC-ICE] is one usage of STUN.  SIP Outbound
 [SIP-OUTBOUND] is another usage of STUN.  In some cases, a usage will
 require extensions to STUN.  A STUN extension can be in the form of
 new methods, attributes, or error response codes.  More information
 on STUN usages can be found in Section 14.

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.

Rosenberg, et al. Standards Track [Page 4] RFC 5389 STUN October 2008

 Classic STUN also had a security vulnerability -- attackers could
 provide the client with incorrect mapped addresses under certain
 topologies and constraints, and this was fundamentally not solvable
 through any cryptographic means.  Though this problem remains with
 this specification, those attacks are now mitigated through the use
 of more complete solutions that make use of STUN.
 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 [MMUSIC-ICE] is a complete NAT traversal
 solution for protocols based on the offer/answer [RFC3264]
 methodology, such as SIP.  SIP Outbound [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 19 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.

Rosenberg, et al. Standards Track [Page 5] RFC 5389 STUN October 2008

                             /-----\
                           // STUN  \\
                          |   Server  |
                           \\       //
                             \-----/
                        +--------------+             Public Internet
        ................|     NAT 2    |.......................
                        +--------------+
                        +--------------+             Private NET 2
        ................|     NAT 1    |.......................
                        +--------------+
                            /-----\
                          //  STUN \\
                         |    Client |
                          \\       //               Private NET 1
                            \-----/
               Figure 1: One Possible STUN Configuration
 One possible STUN configuration is shown in Figure 1.  In this
 configuration, there are two entities (called STUN agents) that
 implement the STUN protocol.  The lower agent in the figure is the
 client, and is connected to private network 1.  This network connects
 to private network 2 through NAT 1.  Private network 2 connects to
 the public Internet through NAT 2.  The upper agent in the figure is
 the server, and resides on the public Internet.
 STUN is a client-server protocol.  It supports two types of
 transactions.  One is a request/response transaction in which a
 client sends a request to a server, and the server returns a
 response.  The second is an indication transaction in which either
 agent -- client or server -- sends an indication that generates no
 response.  Both types of transactions include a transaction ID, which
 is a randomly selected 96-bit number.  For request/response

Rosenberg, et al. Standards Track [Page 6] RFC 5389 STUN October 2008

 transactions, this transaction ID allows the client to associate the
 response with the request that generated it; for indications, the
 transaction ID serves as a debugging aid.
 All STUN messages start with a fixed header that includes a method, a
 class, and the transaction ID.  The method indicates which of the
 various requests or indications this is; this specification defines
 just one method, Binding, but other methods are expected to be
 defined in other documents.  The class indicates whether this is a
 request, a success response, an error response, or an indication.
 Following the fixed header comes zero or more attributes, which are
 Type-Length-Value extensions that convey additional information for
 the specific message.
 This document defines a single method called Binding.  The Binding
 method can be used either in request/response transactions or in
 indication transactions.  When used in request/response transactions,
 the Binding method can be used to determine the particular "binding"
 a NAT has allocated to a STUN client.  When used in either request/
 response or in indication transactions, the Binding method can also
 be used to keep these "bindings" alive.
 In the Binding request/response transaction, a Binding request is
 sent from a STUN client to a STUN server.  When the Binding request
 arrives at the STUN server, it may have passed through one or more
 NATs between the STUN client and the STUN server (in Figure 1, there
 were two such NATs).  As the Binding request message passes through a
 NAT, the NAT will modify the source transport address (that is, the
 source IP address and the source port) of the packet.  As a result,
 the source transport address of the request received by the server
 will be the public IP address and port created by the NAT closest to
 the server.  This is called a reflexive transport address.  The STUN
 server copies that source transport address into an XOR-MAPPED-
 ADDRESS attribute in the STUN Binding response and sends the Binding
 response back to the STUN client.  As this packet passes back through
 a NAT, the NAT will modify the destination transport address in the
 IP header, but the transport address in the XOR-MAPPED-ADDRESS
 attribute within the body of the STUN response will remain untouched.
 In this way, the client can learn its reflexive transport address
 allocated by the outermost NAT with respect to the STUN server.
 In some usages, STUN must be multiplexed with other protocols (e.g.,
 [MMUSIC-ICE], [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 three fields in the STUN header with fixed values that
 can be used for this purpose.  If this is not sufficient, then STUN
 packets can also contain a FINGERPRINT value, which can further be
 used to distinguish the packets.

Rosenberg, et al. Standards Track [Page 7] RFC 5389 STUN October 2008

 STUN defines a set of optional procedures that a usage can decide to
 use, called mechanisms.  These mechanisms include DNS discovery, a
 redirection technique to an alternate server, a fingerprint attribute
 for demultiplexing, and two authentication and message-integrity
 exchanges.  The authentication mechanisms revolve around the use of a
 username, password, and message-integrity value.  Two authentication
 mechanisms, the long-term credential mechanism and the short-term
 credential mechanism, are defined in this specification.  Each usage
 specifies the mechanisms allowed with that usage.
 In the long-term credential mechanism, the client and server share a
 pre-provisioned username and password and perform a digest challenge/
 response exchange inspired by (but differing in details) to the one
 defined for HTTP [RFC2617].  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 [MMUSIC-ICE] the two endpoints use out-of-band
 signaling to exchange a username and password.  These are used to
 integrity protect and authenticate the request and response.  There
 is no challenge or nonce used.

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
 implementations.

5. Definitions

 STUN Agent:  A STUN agent is an entity that implements the STUN
    protocol.  The entity can be either a STUN client or a STUN
    server.
 STUN Client:  A STUN client is an entity that sends STUN requests and
    receives STUN responses.  A STUN client can also send indications.
    In this specification, the terms STUN client and client are
    synonymous.
 STUN Server:  A STUN server is an entity that receives STUN requests
    and sends STUN responses.  A STUN server can also send
    indications.  In this specification, the terms STUN server and
    server are synonymous.
 Transport Address:  The combination of an IP address and port number
    (such as a UDP or TCP port number).

Rosenberg, et al. Standards Track [Page 8] RFC 5389 STUN October 2008

 Reflexive Transport Address:  A transport address learned by a client
    that identifies that client as seen by another host on an IP
    network, typically a STUN server.  When there is an intervening
    NAT between the client and the other host, the reflexive transport
    address represents the mapped address allocated to the client on
    the public side of the NAT.  Reflexive transport addresses are
    learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
    MAPPED-ADDRESS) in STUN responses.
 Mapped Address:  Same meaning as reflexive address.  This term is
    retained only for historic reasons and due to the naming of the
    MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.
 Long-Term Credential:  A username and associated password that
    represent a shared secret between client and server.  Long-term
    credentials are generally granted to the client when a subscriber
    enrolls in a service and persist until the subscriber leaves the
    service or explicitly changes the credential.
 Long-Term Password:  The password from a long-term credential.
 Short-Term Credential:  A temporary username and associated password
    that represent a shared secret between client and server.  Short-
    term credentials are obtained through some kind of protocol
    mechanism between the client and server, preceding the STUN
    exchange.  A short-term credential has an explicit temporal scope,
    which may be based on a specific amount of time (such as 5
    minutes) or on an event (such as termination of a 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, which defines the initial period of
    time between transmission of a request and the first retransmit of
    that request.

Rosenberg, et al. Standards Track [Page 9] RFC 5389 STUN October 2008

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 RFC 791 [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 2 bits of every STUN message MUST be zeroes.
 This can be used to differentiate STUN packets from other protocols
 when STUN is multiplexed with other protocols on the same port.
 The message type defines the message class (request, success
 response, 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 consist of a
 single indication message).  Response classes are split into error
 and success responses to aid in quickly processing the STUN message.

Rosenberg, et al. Standards Track [Page 10] RFC 5389 STUN October 2008

 The message type field is decomposed further into the following
 structure:
                      0                 1
                      2  3  4 5 6 7 8 9 0 1 2 3 4 5
                     +--+--+-+-+-+-+-+-+-+-+-+-+-+-+
                     |M |M |M|M|M|C|M|M|M|C|M|M|M|M|
                     |11|10|9|8|7|1|6|5|4|0|3|2|1|0|
                     +--+--+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 3: Format of STUN Message Type Field
 Here the bits in the message type field are shown as most significant
 (M11) through least significant (M0).  M11 through M0 represent a 12-
 bit encoding of the method.  C1 and C0 represent a 2-bit encoding of
 the class.  A class of 0b00 is a request, a class of 0b01 is an
 indication, a class of 0b10 is a success response, and a class of
 0b11 is an error response.  This specification defines a single
 method, Binding.  The method and class are orthogonal, so that for
 each method, a request, success response, error response, and
 indication are possible for that method.  Extensions defining new
 methods MUST indicate which classes are permitted for that method.
 For example, a Binding request has class=0b00 (request) and
 method=0b000000000001 (Binding) and is encoded into the first 16 bits
 as 0x0001.  A Binding response has class=0b10 (success response) and
 method=0b000000000001, and is encoded into the first 16 bits as
 0x0101.
    Note: This unfortunate encoding is due to assignment of values in
    [RFC3489] that did not consider encoding Indications, Success, and
    Errors using bit fields.
 The magic cookie field MUST contain the fixed value 0x2112A442 in
 network byte order.  In 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.
 The transaction ID is a 96-bit identifier, used to uniquely identify
 STUN transactions.  For request/response transactions, the
 transaction ID is chosen by the STUN client for the request and
 echoed by the server in the response.  For indications, it is chosen
 by the agent sending the indication.  It primarily serves to
 correlate requests with responses, though it also plays a small role

Rosenberg, et al. Standards Track [Page 11] RFC 5389 STUN October 2008

 in helping to prevent certain types of attacks.  The server also uses
 the transaction ID as a key to identify each transaction uniquely
 across all clients.  As such, the transaction ID MUST be uniformly
 and randomly chosen from the interval 0 .. 2**96-1, and SHOULD be
 cryptographically random.  Resends of the same request reuse the same
 transaction ID, but the client MUST choose a new transaction ID for
 new transactions unless the new request is bit-wise identical to the
 previous request and sent from the same transport address to the same
 IP address.  Success and error responses MUST carry the same
 transaction ID as their corresponding request.  When an agent is
 acting as a STUN server and STUN client on the same port, the
 transaction IDs in requests sent by the agent have no relationship to
 the transaction IDs in requests received by the agent.
 The message length MUST contain the size, in bytes, of the message
 not including the 20-byte STUN header.  Since all STUN attributes are
 padded to a multiple of 4 bytes, the last 2 bits of this field are
 always zero.  This provides another way to distinguish STUN packets
 from packets of other protocols.
 Following the STUN fixed portion of the header are zero or more
 attributes.  Each attribute is TLV (Type-Length-Value) encoded.  The
 details of the encoding, and of the attributes themselves are given
 in Section 15.

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 agent 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 agent then adds any attributes specified by the method or the
 usage.  For example, some usages may specify that the agent use an
 authentication method (Section 10) or the FINGERPRINT attribute
 (Section 8).

Rosenberg, et al. Standards Track [Page 12] RFC 5389 STUN October 2008

 If the agent is sending a request, it SHOULD add a SOFTWARE attribute
 to the request.  Agents MAY include a SOFTWARE attribute in
 indications, depending on the method.  Extensions to STUN should
 discuss whether SOFTWARE is useful in new indications.
 For the Binding method with no authentication, no attributes are
 required unless the usage specifies otherwise.
 All STUN messages sent over UDP SHOULD be less than the path MTU, if
 known.  If the path MTU is unknown, messages SHOULD be the smaller of
 576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
 IPv6 [RFC2460].  This value corresponds to the overall size of the IP
 packet.  Consequently, for IPv4, the actual STUN message would need
 to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
 UDP header, assuming no IP options are used).  STUN provides no
 ability to handle the case where the request is under the MTU but the
 response would be larger than the MTU.  It is not envisioned that
 this limitation will be an issue for STUN.  The MTU limitation is a
 SHOULD, and not a MUST, to account for cases where STUN itself is
 being used to probe for MTU characteristics [BEHAVE-NAT].  Outside of
 this or similar applications, the MTU constraint MUST be followed.

7.2. Sending the Request or Indication

 The agent then sends the request or indication.  This document
 specifies how to send STUN messages over UDP, TCP, 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 agent
 determines the IP address and port of the recipient.  Section 9
 describes a DNS-based method of determining the IP address and port
 of a server that a usage may elect to use.  STUN may be used with
 anycast addresses, but only with UDP and in usages where
 authentication is not used.
 At any time, a client MAY have multiple outstanding STUN requests
 with the same STUN server (that is, multiple transactions in
 progress, with different transaction IDs).  Absent other limits to
 the rate of new transactions (such as those specified by ICE for
 connectivity checks or when STUN is run over TCP), a client SHOULD
 space new transactions to a server by RTO and SHOULD limit itself to
 ten outstanding transactions to the same server.

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

Rosenberg, et al. Standards Track [Page 13] RFC 5389 STUN October 2008

 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 (RTT),
 and is computed as described in RFC 2988 [RFC2988], with two
 exceptions.  First, the initial value for RTO SHOULD be configurable
 (rather than the 3 s recommended in RFC 2988) and SHOULD be greater
 than 500 ms.  The exception cases for this "SHOULD" are when other
 mechanisms are used to derive congestion thresholds (such as the ones
 defined in ICE for fixed rate streams), or when STUN is used in non-
 Internet environments with known network capacities.  In fixed-line
 access links, a value of 500 ms is RECOMMENDED.  Second, the value of
 RTO SHOULD NOT be rounded up to the nearest second.  Rather, a 1 ms
 accuracy SHOULD be maintained.  As with TCP, the usage of Karn's
 algorithm is RECOMMENDED [KARN87].  When applied to STUN, it means
 that RTT estimates SHOULD NOT be computed from STUN transactions that
 result in the retransmission of a request.
 The value for RTO SHOULD be cached by a client after the completion
 of the transaction, and used as the starting value for RTO for the
 next transaction to the same server (based on equality of IP
 address).  The value SHOULD be considered stale and discarded after
 10 minutes.
 Retransmissions continue until a response is received, or until a
 total of Rc requests have been sent.  Rc SHOULD be configurable and
 SHOULD have a default of 7.  If, after the last request, a duration
 equal to Rm times the RTO has passed without a response (providing
 ample time to get a response if only this final request actually
 succeeds), the client SHOULD consider the transaction to have failed.
 Rm SHOULD be configurable and SHOULD have a default of 16.  A STUN
 transaction over UDP is also considered failed if there has been a
 hard ICMP error [RFC1122].  For example, assuming an RTO of 500 ms,
 requests would be sent at times 0 ms, 500 ms, 1500 ms, 3500 ms, 7500
 ms, 15500 ms, and 31500 ms.  If the client has not received a
 response after 39500 ms, 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.

Rosenberg, et al. Standards Track [Page 14] RFC 5389 STUN October 2008

 In some usages of STUN, STUN is sent as the only protocol over the
 TCP connection.  In this case, it can be sent without the aid of any
 additional framing or demultiplexing.  In other usages, or with other
 extensions, it may be multiplexed with other data over a TCP
 connection.  In that case, STUN MUST be run on top of some kind of
 framing protocol, specified by the usage or extension, which allows
 for the agent to extract complete STUN messages and complete
 application layer messages.  The STUN service running on the well-
 known port or ports discovered through the DNS procedures in
 Section 9 is for STUN alone, and not for STUN multiplexed with other
 data.  Consequently, no framing protocols are used in connections to
 those servers.  When additional framing is utilized, the usage will
 specify how the client knows to apply it and what port to connect to.
 For example, in the case of ICE connectivity checks, this information
 is learned through out-of-band negotiation between client and server.
 When STUN is run by itself over TLS-over-TCP, the
 TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST be implemented 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 or 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.  Alternatively, a
 client MAY be configured with a set of domains or IP addresses that
 are trusted; if a certificate is received that identifies one of
 those domains or IP addresses, the client considers the identity of
 the server to be verified.
 When STUN is run multiplexed with other protocols over a TLS-over-TCP
 connection, the mandatory ciphersuites and TLS handling procedures
 operate as defined by those protocols.
 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 by Ti seconds after it sent the SYN to establish
 the connection, it considers the transaction to have timed out.  Ti
 SHOULD be configurable and SHOULD have a default of 39.5s.  This
 value has been chosen to equalize the TCP and UDP timeouts for the
 default initial RTO.

Rosenberg, et al. Standards Track [Page 15] RFC 5389 STUN October 2008

 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
    [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., [MMUSIC-ICE-TCP]).
 At the server end, the server SHOULD keep the connection open, and
 let the client close it, unless the server has determined that the
 connection has timed out (for example, due to the client
 disconnecting from the network).  Bindings learned by the client will
 remain valid in intervening NATs only while the connection remains
 open.  Only the client knows how long it needs the binding.  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.  Servers SHOULD follow best practices regarding connection
 management in cases of overload.

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 Section 12.  Those additional procedures are optional, and
 usages can elect to utilize them.  First, a set of processing
 operations is applied that is independent of the class.  This is
 followed by class-specific processing, described in the subsections
 that follow.

Rosenberg, et al. Standards Track [Page 16] RFC 5389 STUN October 2008

 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
 the message length is sensible, and that the method value is a
 supported method.  It checks that the message class is allowed for
 the particular 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 Attribute), 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.
 When run over UDP, a request received by the server could be the
 first request of a transaction, or a retransmission.  The server MUST
 respond to retransmissions such that the following property is
 preserved: if the client receives the response to the retransmission
 and not the response that was sent to the original request, the
 overall state on the client and server is identical to the case where
 only the response to the original retransmission is received, or

Rosenberg, et al. Standards Track [Page 17] RFC 5389 STUN October 2008

 where both responses are received (in which case the client will use
 the first).  The easiest way to meet this requirement is for the
 server to remember all transaction IDs received over UDP and their
 corresponding responses in the last 40 seconds.  However, this
 requires the server to hold state, and will be inappropriate for any
 requests which are not authenticated.  Another way is to reprocess
 the request and recompute the response.  The latter technique MUST
 only be applied to requests that are idempotent (a request is
 considered idempotent when the same request can be safely repeated
 without impacting the overall state of the system) and result in the
 same success response for the same request.  The Binding method is
 considered to be idempotent.  Note that there are certain rare
 network events that could cause the reflexive transport address value
 to change, resulting in a different mapped address in different
 success responses.  Extensions to STUN MUST discuss the implications
 of request retransmissions on servers that do not store transaction
 state.

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 SOFTWARE 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

Rosenberg, et al. Standards Track [Page 18] RFC 5389 STUN October 2008

 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 are the source IP
 address and port of the received request message, and the source IP
 address and port of the response are 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.

7.3.2. Processing an Indication

 If the indication contains unknown comprehension-required attributes,
 the indication is discarded and processing ceases.
 The agent then does any additional checking that the method or the
 specific usage requires.  If all the checks succeed, the agent 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 agent 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
 sending agent.

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

Rosenberg, et al. Standards Track [Page 19] RFC 5389 STUN October 2008

 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,
 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 (Unknown Attribute), 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.  The
 FINGERPRINT mechanism is not backwards compatible with RFC 3489, and
 cannot be used in environments where such compatibility is required.
 In some usages, STUN messages are multiplexed on the same transport
 address as other protocols, such as the Real Time Transport Protocol
 (RTP).  In order to apply the processing described in Section 7, STUN
 messages must first be separated from the application packets.

Rosenberg, et al. Standards Track [Page 20] RFC 5389 STUN October 2008

 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 15.5 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.  Section 7.3 describes when in the
 overall processing of a STUN message the FINGERPRINT check is
 performed.  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
 use this procedure, the client must know a server's domain name and a
 service name; the usage must also describe how the client obtains
 these.  Hard-coding the domain name of the server into software is
 NOT RECOMMENDED in case the domain name is lost or needs to change
 for legal or other reasons.
 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".  When it wishes to locate a STUN server that accepts
 Binding request/response transactions over a TLS session, the SRV
 service name is "stuns".  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 and "tcp" for TCP.  Note that only "tcp" is
 defined with "stuns" at this time.
 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
 is sorted and then tried.  However, RFC 2782 only states that the
 client should "try to connect to the (protocol, address, service)"
 without giving any details on what happens in the event of failure.
 When following these procedures, if the STUN transaction times out
 without receipt of a response, the client SHOULD retry the request to

Rosenberg, et al. Standards Track [Page 21] RFC 5389 STUN October 2008

 the next server in the ordered defined by RFC 2782.  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 of STUN servers SHOULD use this port in their SRV
 records for UDP and TCP.  In all cases, the port in DNS MUST reflect
 the one on which the server is listening.  The default port for STUN
 over TLS is 5349.  Servers can run STUN over TLS on the same port as
 STUN over TCP if the server software supports determining whether the
 initial message is a TLS or STUN 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, the client connects to one of the IP addresses using the default
 STUN over TLS port.

10. Authentication and Message-Integrity Mechanisms

 This section defines two mechanisms for STUN that a client and server
 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.
 Consequently, both clients and servers will know which mechanism (if
 any) to follow based on knowledge of which usage applies.  For
 example, a STUN server on the public Internet supporting ICE would
 have no authentication, whereas the STUN server functionality in an
 agent supporting connectivity checks would utilize short-term
 credentials.  An overview of these two mechanisms is given in
 Section 3.
 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 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.

Rosenberg, et al. Standards Track [Page 22] RFC 5389 STUN October 2008

 As an example, in the ICE usage [MMUSIC-ICE], the two endpoints use
 out-of-band signaling to agree on a username and password, and this
 username and password are 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 15.4.  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 (Bad Request).
  • If the message is an indication, the agent 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 (Unauthorized).
  • If the message is an indication, the agent MUST silently

discard the indication.

 o  Using the password associated with the username, compute the value
    for the message integrity as described in Section 15.4.  If the
    resulting value does not match the contents of the MESSAGE-
    INTEGRITY attribute:

Rosenberg, et al. Standards Track [Page 23] RFC 5389 STUN October 2008

  • 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 (Unauthorized).
  • If the message is an indication, the agent MUST silently

discard the indication.

 If these checks pass, the agent continues to process the request or
 indication.  Any response generated by a server MUST include the
 MESSAGE-INTEGRITY attribute, computed using the password utilized to
 authenticate the request.  The response MUST NOT contain the USERNAME
 attribute.
 If any of the checks fail, a server MUST NOT include a MESSAGE-
 INTEGRITY or USERNAME attribute in the error response.  This is
 because, in these failure cases, the server cannot determine the
 shared secret necessary to compute MESSAGE-INTEGRITY.

10.1.3. Receiving a Response

 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 15.4, 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.

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

Rosenberg, et al. Standards Track [Page 24] RFC 5389 STUN October 2008

 duration of validity or client identity from which it is valid.  The
 client retries the request, this time including its username and 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 passwords that are
 difficult to guess.  In cases where the credentials are not entered
 by the user, but are rather placed on a client device during device
 provisioning, the password SHOULD have at least 128 bits of
 randomness.  In cases where the credentials are entered by the user,
 they should follow best current practices around password structure.

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.  Forming a request as a
 consequence of a 401 or 438 error response is covered in
 Section 10.2.3 and is not considered a "subsequent request" and thus
 does not utilize the rules described in Section 10.2.1.2.

10.2.1.1. First Request

 If the client has not completed a successful request/response
 transaction with the server (as identified by hostname, if the DNS
 procedures of Section 9 are used, else IP address if not), 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.

Rosenberg, et al. Standards Track [Page 25] RFC 5389 STUN October 2008

10.2.1.2. Subsequent Requests

 Once a request/response transaction has completed successfully, the
 client will have 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 attribute, computed as described in Section 15.4 using the
 cached password.

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, the
    server MUST generate an error response with an error code of 401
    (Unauthorized).  This response MUST include a REALM value.  It is
    RECOMMENDED that the REALM value be the domain name of the
    provider of the STUN server.  The response MUST include a NONCE,
    selected by the server.  The response SHOULD NOT contain a
    USERNAME or MESSAGE-INTEGRITY attribute.
 o  If the message contains a MESSAGE-INTEGRITY attribute, but is
    missing the USERNAME, REALM, or NONCE attribute, the server MUST
    generate an error response with an error code of 400 (Bad
    Request).  This response SHOULD NOT include a USERNAME, NONCE,
    REALM, or MESSAGE-INTEGRITY attribute.
 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 NONCE and REALM attributes and SHOULD NOT include the
    USERNAME or MESSAGE-INTEGRITY attribute.  Servers can invalidate
    nonces in order to provide additional security.  See Section 4.3
    of [RFC2617] for guidelines.
 o  If the username in the USERNAME attribute is not valid, the server
    MUST generate an error response with an error code of 401
    (Unauthorized).  This response MUST include a REALM value.  It is
    RECOMMENDED that the REALM value be the domain name of the
    provider of the STUN server.  The response MUST include a NONCE,
    selected by the server.  The response SHOULD NOT contain a
    USERNAME or MESSAGE-INTEGRITY attribute.

Rosenberg, et al. Standards Track [Page 26] RFC 5389 STUN October 2008

 o  Using the password associated with the username in the USERNAME
    attribute, compute the value for the message integrity as
    described in Section 15.4.  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 (Unauthorized).  It MUST include REALM and
    NONCE attributes and SHOULD NOT include the USERNAME or MESSAGE-
    INTEGRITY attribute.
 If these checks pass, the server continues to process the request.
 Any response generated by the server (excepting the cases described
 above) 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

 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 (Stale
 Nonce), the client MUST retry the request, using the new NONCE
 supplied in the 438 (Stale Nonce) response.  This retry MUST also
 include the USERNAME, REALM, and MESSAGE-INTEGRITY.
 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 15.4, 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.

Rosenberg, et al. Standards Track [Page 27] RFC 5389 STUN October 2008

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.
 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 an
 ALTERNATE-SERVER attribute in the error response.  The error response
 message MAY be authenticated; however, there are uses cases for
 ALTERNATE-SERVER where authentication of the response is not possible
 or practical.
 A client using this extension handles a 300 (Try Alternate) error
 code as follows.  The client looks for an ALTERNATE-SERVER attribute
 in the error response.  If one is found, then the client considers
 the current transaction as failed, and reattempts the request with
 the server specified in the attribute, using the same transport
 protocol used for the previous request.  That request, if
 authenticated, MUST utilize the same credentials that the client
 would have used in the request to the server that performed the
 redirection.  If the client has been redirected to a server on which
 it has already tried this request within the last five minutes, it
 MUST ignore the redirection and consider the transaction to have
 failed.  This prevents infinite ping-ponging between servers in case
 of redirection loops.

12. Backwards Compatibility with RFC 3489

 This section defines procedures that allow a degree of backwards
 compatibility 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 versa.
 A usage must define if and when this procedure is used.
 Section 19 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
 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.

Rosenberg, et al. Standards Track [Page 28] RFC 5389 STUN October 2008

 o  The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding
    method used the MAPPED-ADDRESS attribute instead.
 o  There were three comprehension-required attributes, RESPONSE-
    ADDRESS, CHANGE-REQUEST, and CHANGED-ADDRESS, that have been
    removed from this specification.
  • CHANGE-REQUEST and CHANGED-ADDRESS are now part of the NAT

Behavior Discovery usage [BEHAVE-NAT], and the other is

       deprecated.

12.1. Changes to Client Processing

 A client that wants to interoperate with an [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.  A client seeking to interoperate with an older server
 MUST be prepared to receive either.  Furthermore, the client MUST
 ignore any Reserved comprehension-required attributes that might
 appear in the response.  Of the Reserved attributes in Section 18.2,
 0x0002, 0x0004, 0x0005, and 0x000B may appear in Binding responses
 from a server compliant to RFC 3489.  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.
 The RFC 3489 version of STUN lacks both the magic cookie and the
 FINGERPRINT attribute that allows for a very high probability of
 correctly identifying STUN messages when multiplexed with other
 protocols.  Therefore, STUN implementations that are backwards

Rosenberg, et al. Standards Track [Page 29] RFC 5389 STUN October 2008

 compatible with RFC 3489 SHOULD NOT be used in cases where STUN will
 be multiplexed with another protocol.  However, that should not be an
 issue as such multiplexing was not available in RFC 3489.

13. Basic Server Behavior

 This section defines the behavior of a basic, stand-alone STUN
 server.  A basic STUN server provides clients with server reflexive
 transport addresses by receiving and replying to STUN Binding
 requests.
 The STUN server MUST support the Binding method.  It SHOULD NOT
 utilize the short-term or long-term credential mechanism.  This is
 because the work involved in authenticating the request is more than
 the work in simply processing it.  It SHOULD NOT utilize the
 ALTERNATE-SERVER mechanism for the same reason.  It MUST support UDP
 and TCP.  It MAY support STUN over TCP/TLS; however, TLS provides
 minimal security benefits in this basic mode of operation.  It MAY
 utilize the FINGERPRINT mechanism but MUST NOT require it.  Since the
 stand-alone server only runs STUN, FINGERPRINT provides no benefit.
 Requiring it would break compatibility with RFC 3489, and such
 compatibility is desirable in a stand-alone server.  Stand-alone STUN
 servers SHOULD support backwards compatibility with [RFC3489]
 clients, as described in Section 12.
 It is RECOMMENDED that administrators of STUN servers provide DNS
 entries for those servers as described in Section 9.
 A basic STUN server is not a solution for NAT traversal by itself.
 However, it can be utilized as part of a solution through STUN
 usages.  This is discussed further in Section 14.

14. STUN Usages

 STUN by itself is not a solution to the NAT traversal problem.
 Rather, STUN defines a tool 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) [MMUSIC-ICE], Client-initiated
 connections for SIP [SIP-OUTBOUND], and NAT Behavior Discovery
 [BEHAVE-NAT].  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:

Rosenberg, et al. Standards Track [Page 30] RFC 5389 STUN October 2008

 o  Which STUN methods are used.
 o  What authentication and message-integrity mechanisms are used.
 o  The considerations around manual vs. automatic key derivation for
    the integrity mechanism, as discussed in [RFC4107].
 o  What mechanisms are used to distinguish STUN messages from other
    messages.  When 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
 [RFC3424].

15. 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 4: Format of STUN Attributes

Rosenberg, et al. Standards Track [Page 31] RFC 5389 STUN October 2008

 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 occurrence 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.
 The set of STUN attribute types is maintained by IANA.  The initial
 set defined by this specification is found in Section 18.2.
 The rest of this section describes the format of the various
 attributes defined in this specification.

15.1. MAPPED-ADDRESS

 The MAPPED-ADDRESS attribute indicates a reflexive transport address
 of the client.  It consists of an 8-bit address family and a 16-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.

Rosenberg, et al. Standards Track [Page 32] RFC 5389 STUN October 2008

 The format of the MAPPED-ADDRESS attribute is:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0 0 0 0 0|    Family     |           Port                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                 Address (32 bits or 128 bits)                 |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 5: Format of MAPPED-ADDRESS Attribute
 The address family can take on 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.

15.2. 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 6: Format of XOR-MAPPED-ADDRESS Attribute
 The Family represents the IP address family, and is encoded
 identically to the Family in MAPPED-ADDRESS.

Rosenberg, et al. Standards Track [Page 33] RFC 5389 STUN October 2008

 X-Port is computed by taking the mapped port in host byte order,
 XOR'ing it with the most significant 16 bits of the magic cookie, and
 then the converting the result to network byte order.  If the IP
 address family is IPv4, X-Address is computed by taking the mapped IP
 address in host byte order, XOR'ing it with the magic cookie, and
 converting the result to network byte order.  If the IP address
 family is IPv6, X-Address is computed by taking the mapped IP address
 in host byte order, XOR'ing it with the concatenation of the magic
 cookie and the 96-bit transaction ID, and converting the result to
 network byte order.
 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 address families are the 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.

15.3. 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 [RFC3629] encoded sequence of less than 513 bytes, and MUST
 have been processed using SASLprep [RFC4013].

15.4. 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.

Rosenberg, et al. Standards Track [Page 34] RFC 5389 STUN October 2008

 The key for the HMAC depends on whether long-term or short-term
 credentials are in use.  For long-term credentials, the key is 16
 bytes:
          key = MD5(username ":" realm ":" SASLprep(password))
 That is, the 16-byte key is formed by taking the MD5 hash of the
 result of concatenating the following five fields: (1) the username,
 with any quotes and trailing nulls removed, as taken from the
 USERNAME attribute (in which case SASLprep has already been applied);
 (2) a single colon; (3) the realm, with any quotes and trailing nulls
 removed; (4) a single colon; and (5) the password, with any trailing
 nulls removed and after processing using SASLprep.  For example, if
 the username was 'user', the realm was 'realm', and the password was
 'pass', then the 16-byte HMAC key would be the result of performing
 an MD5 hash on the string 'user:realm:pass', the resulting hash being
 0x8493fbc53ba582fb4c044c456bdc40eb.
 For short-term credentials:
                        key = SASLprep(password)
 where MD5 is defined in RFC 1321 [RFC1321] and SASLprep() is defined
 in RFC 4013 [RFC4013].
 The structure of the key when used with long-term credentials
 facilitates deployment in systems that also utilize SIP.  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 equal to the key defined above.
 Based on the rules above, the hash used to construct MESSAGE-
 INTEGRITY includes the length field from the STUN message header.
 Prior to performing the hash, the MESSAGE-INTEGRITY attribute MUST be
 inserted into the message (with dummy content).  The length MUST then
 be set to point to the length of the message up to, and including,
 the MESSAGE-INTEGRITY attribute itself, but excluding any attributes
 after it.  Once the computation is performed, the value of the
 MESSAGE-INTEGRITY attribute can be filled in, and the value of the
 length in the STUN header can be set to its correct value -- the
 length of the entire message.  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
 FINGERPRINT, appear after MESSAGE-INTEGRITY.

Rosenberg, et al. Standards Track [Page 35] RFC 5389 STUN October 2008

15.5. 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'ed 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.V42.2002], 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.
 As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute
 covers the length field from the STUN message header.  Therefore,
 this value must be correct and include the CRC attribute as part of
 the message length, prior to computation of the CRC.  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.

15.6. 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 [RFC3629], 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 included in the IANA registry
 for error codes.  The reason phrase MUST be a UTF-8 [RFC3629] encoded
 sequence of less than 128 characters (which can be as long as 763
 bytes).
     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)                                ..
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 7: ERROR-CODE Attribute

Rosenberg, et al. Standards Track [Page 36] RFC 5389 STUN October 2008

 To facilitate processing, the class of the error code (the hundreds
 digit) is encoded separately from the rest of the code, as shown in
 Figure 7.
 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
 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, 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 (Bad Request) 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 correct
      credentials to proceed.  The client should retry the request
      with proper credentials.
 420  Unknown Attribute: The server received a STUN packet containing
      a comprehension-required attribute that it did not understand.
      The server 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.

Rosenberg, et al. Standards Track [Page 37] RFC 5389 STUN October 2008

15.7. REALM

 The REALM attribute may be present in requests and responses.  It
 contains text that 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 (and
 is therefore a sequence of qdtext or quoted-pair).  It MUST be a
 UTF-8 [RFC3629] encoded sequence of less than 128 characters (which
 can be as long as 763 bytes), and MUST have been processed using
 SASLprep [RFC4013].
 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.

15.8. 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].  Note that this means that the NONCE attribute
 will not contain actual quote characters.  See RFC 2617 [RFC2617],
 Section 4.3, for guidance on selection of nonce values in a server.
 It MUST be less than 128 characters (which can be as long as 763
 bytes).

15.9. 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 8: Format of UNKNOWN-ATTRIBUTES Attribute

Rosenberg, et al. Standards Track [Page 38] RFC 5389 STUN October 2008

    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.

15.10. SOFTWARE

 The SOFTWARE attribute contains a textual description of the software
 being used by the agent sending the message.  It is used by clients
 and servers.  Its value SHOULD include manufacturer and version
 number.  The attribute has no impact on operation of the protocol,
 and serves only as a tool for diagnostic and debugging purposes.  The
 value of SOFTWARE is variable length.  It MUST be a UTF-8 [RFC3629]
 encoded sequence of less than 128 characters (which can be as long as
 763 bytes).

15.11. ALTERNATE-SERVER

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

16. Security Considerations

16.1. Attacks against the Protocol

16.1.1. Outside Attacks

 An attacker can try to modify STUN messages in transit, in order to
 cause a failure in STUN operation.  These attacks are detected for
 both requests and responses through the message-integrity mechanism,
 using either a short-term or long-term credential.  Of course, once
 detected, the manipulated packets will be dropped, causing the STUN
 transaction to effectively fail.  This attack is possible only by an
 on-path attacker.
 An attacker that can observe, but not modify, STUN messages in-
 transit (for example, an attacker present on a shared access medium,
 such as Wi-Fi), can see a STUN request, 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.

Rosenberg, et al. Standards Track [Page 39] RFC 5389 STUN October 2008

 Depending on the STUN usage, these attacks may be of minimal
 consequence and thus do not require message integrity to mitigate.
 For example, when STUN is used to a basic STUN server to discover a
 server reflexive candidate for usage with ICE, authentication and
 message integrity are not required since these attacks are detected
 during the connectivity check phase.  The connectivity checks
 themselves, however, require protection for proper operation of ICE
 overall.  As described in Section 14, STUN usages describe when
 authentication and message integrity are needed.
 Since STUN uses the HMAC of a shared secret for authentication and
 integrity protection, it is subject to offline dictionary attacks.
 When authentication is utilized, it SHOULD be with a strong password
 that is not readily subject to offline dictionary attacks.
 Protection of the channel itself, using TLS, mitigates these attacks.
 However, STUN is most often run over UDP and in those cases, strong
 passwords are the only way to protect against these attacks.

16.1.2. 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.
 A rogue client may use a STUN server as a reflector, sending it
 requests with a falsified source IP address and port.  In such a
 case, the response would be delivered to that source IP and port.
 There is no amplification of the number of packets with this attack
 (the STUN server sends one packet for each packet sent by the
 client), though there is a small increase in the amount of data,
 since STUN responses are typically larger than requests.  This attack
 is mitigated by ingress source address filtering.
 Revealing the specific software version of the agent through the
 SOFTWARE attribute might allow them to become more vulnerable to
 attacks against software that is known to contain security holes.
 Implementers SHOULD make usage of the SOFTWARE attribute a
 configurable option.

16.2. 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.
 Most of the attacks in this section revolve around an attacker
 modifying the reflexive address learned by a STUN client through a

Rosenberg, et al. Standards Track [Page 40] RFC 5389 STUN October 2008

 Binding request/response transaction.  Since the usage of the
 reflexive address is a function of the usage, the applicability and
 remediation of these attacks are 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, and send the response back to that (falsified) IP address and
 port.  If the attacker can also intercept this response, it can
 direct it back towards the client.  Protecting against this attack by
 using a message-integrity check is impossible, since 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 [MMUSIC-ICE].
 Other usages may use other means to prevent these attacks.

16.2.1. Attack I: Distributed DoS (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.  However, it can only be launched against
 targets for which packets from the STUN server to the target pass
 through the attacker, limiting the cases in which it is possible.

16.2.2. Attack II: Silencing a Client

 In this attack, the 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.  As with the attack in Section 16.2.1,
 this attack is only possible when the attacker is on path for packets
 sent from the STUN server towards this unused IP address.

Rosenberg, et al. Standards Track [Page 41] RFC 5389 STUN October 2008

16.2.3. Attack III: Assuming the Identity of a Client

 This attack is similar to attack II.  However, the faked reflexive
 address points to the attacker itself.  This allows the attacker to
 receive traffic that was destined for the client.

16.2.4. Attack IV: Eavesdropping

 In this attack, the attacker forces the client to use a reflexive
 address that routes to itself.  It then forwards any packets it
 receives to the client.  This attack 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.

16.3. Hash Agility Plan

 This specification uses HMAC-SHA-1 for computation of the message
 integrity.  If, at a later time, HMAC-SHA-1 is found to be
 compromised, the following is the remedy that will be applied.
 We will define a STUN extension that 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.
 It is also important to note that the HMAC is done using a key that
 is itself computed using an MD5 of the user's password.  The choice
 of the MD5 hash was made because of the existence of legacy databases
 that store passwords in that form.  If future work finds that an HMAC
 of an MD5 input is not secure, and a different hash is needed, it can
 also be changed using this plan.  However, this would require
 administrators to repopulate their databases.

17. 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

Rosenberg, et al. Standards Track [Page 42] RFC 5389 STUN October 2008

 through a collaborative protocol reflection mechanism (RFC3424
 [RFC3424]).  STUN can be used to perform this function using a
 Binding request/response 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 [MMUSIC-ICE] ), and others do
 not (such as SIP Outbound [SIP-OUTBOUND]), answers to these
 considerations need to be addressed by the usages themselves.

18. IANA Considerations

 IANA has created three new registries: a "STUN Methods Registry", a
 "STUN Attributes Registry", and a "STUN Error Codes Registry".  IANA
 has also changed the name of the assigned IANA port for STUN from
 "nat-stun-port" to "stun".

18.1. STUN Methods Registry

 A STUN method is a hex number in the range 0x000 - 0xFFF.  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 - 0x7FF are assigned by IETF Review
 [RFC5226].  STUN methods in the range 0x800 - 0xFFF are assigned by
 Designated Expert [RFC5226].  The responsibility of the expert is to
 verify that the selected codepoint(s) are not in use and that the
 request is not for an abnormally large number of codepoints.
 Technical review of the extension itself is outside the scope of the
 designated expert responsibility.

18.2. 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.
 The initial STUN Attributes types are:

Rosenberg, et al. Standards Track [Page 43] RFC 5389 STUN October 2008

 Comprehension-required range (0x0000-0x7FFF):
   0x0000: (Reserved)
   0x0001: MAPPED-ADDRESS
   0x0002: (Reserved; was RESPONSE-ADDRESS)
   0x0003: (Reserved; was CHANGE-ADDRESS)
   0x0004: (Reserved; was SOURCE-ADDRESS)
   0x0005: (Reserved; was CHANGED-ADDRESS)
   0x0006: USERNAME
   0x0007: (Reserved; was PASSWORD)
   0x0008: MESSAGE-INTEGRITY
   0x0009: ERROR-CODE
   0x000A: UNKNOWN-ATTRIBUTES
   0x000B: (Reserved; was REFLECTED-FROM)
   0x0014: REALM
   0x0015: NONCE
   0x0020: XOR-MAPPED-ADDRESS
 Comprehension-optional range (0x8000-0xFFFF)
   0x8022: SOFTWARE
   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 Review
 [RFC5226].  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 by
 Designated Expert [RFC5226].  The responsibility of the expert is to
 verify that the selected codepoint(s) are not in use, and that the
 request is not for an abnormally large number of codepoints.
 Technical review of the extension itself is outside the scope of the
 designated expert responsibility.

18.3. 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 [RFC3629] that 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 15.6.

Rosenberg, et al. Standards Track [Page 44] RFC 5389 STUN October 2008

 New STUN error codes are assigned based on IETF Review [RFC5226].
 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.

18.4. STUN UDP and TCP Port Numbers

 IANA has previously assigned port 3478 for STUN.  This port appears
 in the IANA registry under the moniker "nat-stun-port".  In order to
 align the DNS SRV procedures with the registered protocol service,
 IANA is requested to change the name of protocol assigned to port
 3478 from "nat-stun-port" to "stun", and the textual name from
 "Simple Traversal of UDP Through NAT (STUN)" to "Session Traversal
 Utilities for NAT", so that the IANA port registry would read:
 stun   3478/tcp   Session Traversal Utilities for NAT (STUN) port
 stun   3478/udp   Session Traversal Utilities for NAT (STUN) port
 In addition, IANA has assigned port number 5349 for the "stuns"
 service, defined over TCP and UDP.  The UDP port is not currently
 defined; however, it is reserved for future use.

19. Changes since RFC 3489

 This specification obsoletes RFC 3489 [RFC3489].  This specification
 differs from RFC 3489 in the following ways:
 o  Removed the notion that STUN is a complete NAT traversal solution.
    STUN is now a tool 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.

Rosenberg, et al. Standards Track [Page 45] RFC 5389 STUN October 2008

 o  Added the XOR-MAPPED-ADDRESS attribute, which is included in
    Binding responses if the magic cookie is present in the request.
    Otherwise, the RFC 3489 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 2 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 versa.
 o  Added long-term-credential-based authentication.
 o  Added the SOFTWARE, REALM, NONCE, and ALTERNATE-SERVER attributes.
 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, SERVER, reason phrases, and NONCE limited to 127
    characters.  USERNAME to 513 bytes.

Rosenberg, et al. Standards Track [Page 46] RFC 5389 STUN October 2008

 o  Changed the DNS SRV procedures for TCP and TLS.  UDP remains the
    same as before.

20. Contributors

 Christian Huitema and Joel Weinberger were original co-authors of RFC
 3489.

21. Acknowledgements

 The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
 Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel
 Garcia, Bruce Lowekamp, and Chris Sullivan for their comments, and
 Baruch Sterman and Alan Hawrylyshen for initial implementations.
 Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
 Schulzrinne for IESG and IAB input on this work.

22. References

22.1. Normative References

 [ITU.V42.2002]    International Telecommunications Union, "Error-
                   correcting Procedures for DCEs Using Asynchronous-
                   to-Synchronous Conversion", ITU-T Recommendation
                   V.42, March 2002.
 [RFC0791]         Postel, J., "Internet Protocol", STD 5, RFC 791,
                   September 1981.
 [RFC1122]         Braden, R., "Requirements for Internet Hosts -
                   Communication Layers", STD 3, RFC 1122,
                   October 1989.
 [RFC1321]         Rivest, R., "The MD5 Message-Digest Algorithm",
                   RFC 1321, April 1992.
 [RFC2104]         Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                   Keyed-Hashing for Message Authentication",
                   RFC 2104, February 1997.
 [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                   Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2460]         Deering, S. and R. Hinden, "Internet Protocol,
                   Version 6 (IPv6) Specification", RFC 2460,
                   December 1998.

Rosenberg, et al. Standards Track [Page 47] RFC 5389 STUN October 2008

 [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.
 [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.
 [RFC2988]         Paxson, V. and M. Allman, "Computing TCP's
                   Retransmission Timer", RFC 2988, November 2000.
 [RFC3629]         Yergeau, F., "UTF-8, a transformation format of ISO
                   10646", STD 63, RFC 3629, November 2003.
 [RFC4013]         Zeilenga, K., "SASLprep: Stringprep Profile for
                   User Names and Passwords", RFC 4013, February 2005.

22.2. Informative References

 [BEHAVE-NAT]      MacDonald, D. and B. Lowekamp, "NAT Behavior
                   Discovery Using STUN", Work in Progress, July 2008.
 [BEHAVE-TURN]     Rosenberg, J., Mahy, R., and P. Matthews,
                   "Traversal Using Relays around NAT (TURN): Relay
                   Extensions to Session  Traversal Utilities for NAT
                   (STUN)", Work in Progress, July 2008.
 [KARN87]          Karn, P. and C. Partridge, "Improving Round-Trip
                   Time Estimates in Reliable Transport Protocols",
                   SIGCOMM 1987, August 1987.
 [MMUSIC-ICE]      Rosenberg, J., "Interactive Connectivity
                   Establishment (ICE): A Protocol for Network Address
                   Translator (NAT) Traversal for Offer/Answer
                   Protocols", Work in Progress, October 2007.
 [MMUSIC-ICE-TCP]  Rosenberg, J., "TCP Candidates with Interactive
                   Connectivity Establishment (ICE)", Work
                   in Progress, July 2008.
 [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.

Rosenberg, et al. Standards Track [Page 48] RFC 5389 STUN October 2008

 [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.
 [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.
 [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.
 [RFC4107]         Bellovin, S. and R. Housley, "Guidelines for
                   Cryptographic Key Management", BCP 107, RFC 4107,
                   June 2005.
 [RFC5226]         Narten, T. and H. Alvestrand, "Guidelines for
                   Writing an IANA Considerations Section in RFCs",
                   BCP 26, RFC 5226, May 2008.
 [SIP-OUTBOUND]    Jennings, C. and R. Mahy, "Managing Client
                   Initiated Connections in the Session Initiation
                   Protocol  (SIP)", Work in Progress, June 2008.

Rosenberg, et al. Standards Track [Page 49] RFC 5389 STUN October 2008

Appendix A. C Snippet to Determine STUN Message Types

 Given a 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
 Rohan Mahy
 Unaffiliated
 EMail: rohan@ekabal.com
 Philip Matthews
 Unaffiliated
 EMail: philip_matthews@magma.ca
 Dan Wing
 Cisco
 771 Alder Drive
 San Jose, CA  95035
 US
 EMail: dwing@cisco.com

Rosenberg, et al. Standards Track [Page 50] RFC 5389 STUN October 2008

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Rosenberg, et al. Standards Track [Page 51]

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