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Versions: (RFC 3489) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 5389

BEHAVE Working Group                                        J. Rosenberg
Internet-Draft                                                     Cisco
Obsoletes: 3489 (if approved)                                 C. Huitema
Intended status: Standards Track                               Microsoft
Expires: January 28, 2008                                        R. Mahy
                                                             Plantronics
                                                             P. Matthews
                                                                   Avaya
                                                                 D. Wing
                                                                   Cisco
                                                           July 27, 2007


              Session Traversal Utilities for (NAT) (STUN)
                    draft-ietf-behave-rfc3489bis-08

Status of this Memo

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   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on January 28, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Session Traversal Utilities for NAT (STUN) is a protocol that serves



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

   STUN is not a NAT traversal solution by itself.  Rather, it is a tool
   to be used in the context of a NAT traversal solution.  This is an
   important change from the previous version of this specification (RFC
   3489), which presented STUN as a complete solution.

   This document obsoletes RFC 3489.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Evolution from RFC 3489  . . . . . . . . . . . . . . . . . . .  4
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  STUN Message Structure . . . . . . . . . . . . . . . . . . . .  9
   7.  Base Protocol Procedures . . . . . . . . . . . . . . . . . . . 12
     7.1.   Forming a Request or an Indication  . . . . . . . . . . . 12
     7.2.   Sending the Request or Indication . . . . . . . . . . . . 12
       7.2.1.  Sending over UDP . . . . . . . . . . . . . . . . . . . 12
       7.2.2.  Sending over TCP or TLS-over-TCP . . . . . . . . . . . 13
     7.3.   Receiving a STUN Message  . . . . . . . . . . . . . . . . 15
       7.3.1.  Processing a Request . . . . . . . . . . . . . . . . . 16
         7.3.1.1.  Forming a Success or Error Response  . . . . . . . 16
         7.3.1.2.  Sending the Success or Error Response  . . . . . . 17
       7.3.2.  Processing an Indication . . . . . . . . . . . . . . . 17
       7.3.3.  Processing a Success Response  . . . . . . . . . . . . 17
       7.3.4.  Processing an Error Response . . . . . . . . . . . . . 18
   8.  FINGERPRINT Mechanism  . . . . . . . . . . . . . . . . . . . . 18
   9.  DNS Discovery of a Server  . . . . . . . . . . . . . . . . . . 19
   10. Authentication and Message-Integrity Mechanisms  . . . . . . . 20
     10.1.  Short-Term Credential Mechanism . . . . . . . . . . . . . 20
       10.1.1. Forming a Request or Indication  . . . . . . . . . . . 21
       10.1.2. Receiving a Request or Indication  . . . . . . . . . . 21
       10.1.3. Receiving a Response . . . . . . . . . . . . . . . . . 22
     10.2.  Long-term Credential Mechanism  . . . . . . . . . . . . . 22
       10.2.1. Forming a Request  . . . . . . . . . . . . . . . . . . 23
         10.2.1.1. First Request  . . . . . . . . . . . . . . . . . . 24
         10.2.1.2. Subsequent Requests  . . . . . . . . . . . . . . . 24
       10.2.2. Receiving a Request  . . . . . . . . . . . . . . . . . 24
       10.2.3. Receiving a Response . . . . . . . . . . . . . . . . . 25



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   11. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . . 25
   12. Backwards Compatibility with RFC 3489  . . . . . . . . . . . . 26
     12.1.  Changes to Client Processing  . . . . . . . . . . . . . . 27
     12.2.  Changes to Server Processing  . . . . . . . . . . . . . . 27
   13. STUN Usages  . . . . . . . . . . . . . . . . . . . . . . . . . 27
   14. STUN Attributes  . . . . . . . . . . . . . . . . . . . . . . . 28
     14.1.  MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . . . 29
     14.2.  XOR-MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . 30
     14.3.  USERNAME  . . . . . . . . . . . . . . . . . . . . . . . . 31
     14.4.  MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 31
     14.5.  FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 32
     14.6.  ERROR-CODE  . . . . . . . . . . . . . . . . . . . . . . . 32
     14.7.  REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     14.8.  NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     14.9.  UNKNOWN-ATTRIBUTES  . . . . . . . . . . . . . . . . . . . 34
     14.10. SERVER  . . . . . . . . . . . . . . . . . . . . . . . . . 35
     14.11. ALTERNATE-SERVER  . . . . . . . . . . . . . . . . . . . . 35
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
     15.1.  Attacks against the Protocol  . . . . . . . . . . . . . . 35
       15.1.1. Outside Attacks  . . . . . . . . . . . . . . . . . . . 36
       15.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . . 36
     15.2.  Attacks Affecting the Usage . . . . . . . . . . . . . . . 36
       15.2.1. Attack I: DDoS Against a Target  . . . . . . . . . . . 37
       15.2.2. Attack II: Silencing a Client  . . . . . . . . . . . . 37
       15.2.3. Attack III: Assuming the Identity of a Client  . . . . 37
       15.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 37
     15.3.  Hash Agility Plan . . . . . . . . . . . . . . . . . . . . 38
   16. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 38
   17. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38
     17.1.  STUN Methods Registry . . . . . . . . . . . . . . . . . . 39
     17.2.  STUN Attribute Registry . . . . . . . . . . . . . . . . . 39
     17.3.  STUN Error Code Registry  . . . . . . . . . . . . . . . . 40
   18. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 40
   19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 42
   20. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     20.1.  Normative References  . . . . . . . . . . . . . . . . . . 42
     20.2.  Informational References  . . . . . . . . . . . . . . . . 42
   Appendix A.  C Snippet to Determine STUN Message Types . . . . . . 44
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44
   Intellectual Property and Copyright Statements . . . . . . . . . . 46











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1.  Introduction

   The protocol defined in this specification, Session Traversal
   Utilities for NAT, provides a 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 [I-D.ietf-mmusic-ice], or
   to relay packets between two endpoints [I-D.ietf-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) [I-D.ietf-mmusic-ice] is one usage of ICE.  SIP
   Outbound [I-D.ietf-sip-outbound] is another usage of ICE.  In some
   cases, a usage will require extensions to STUN.  A STUN extension can
   be in the form of new methods, attributes, or error response codes.
   More information on STUN usages can be found in Section 13.


2.  Evolution from RFC 3489

   STUN was originally defined in RFC 3489 [RFC3489].  That
   specification, sometimes referred to as "classic STUN", represented
   itself as a complete solution to the NAT traversal problem.  In that
   solution, a client would discover whether it was behind a NAT,
   determine its NAT type, discover its IP address and port on the
   public side of the outermost NAT, and then utilize that IP address
   and port within the body of protocols, such as the Session Initiation
   Protocol (SIP) [RFC3261].  However, experience since the publication
   of RFC 3489 has found that classic STUN simply does not work
   sufficiently well to be a deployable solution.  The address and port
   learned through classic STUN are sometimes usable for communications
   with a peer, and sometimes not.  Classic STUN provided no way to
   discover whether it would, in fact, work or not, and it provided no
   remedy in cases where it did not.  Furthermore, classic STUN's
   algorithm for classification of NAT types was found to be faulty, as
   many NATs did not fit cleanly into the types defined there.  Classic
   STUN also had security vulnerabilities which required an extremely
   complicated mechanism to address, and despite the complexity of the



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   mechanism, were not fully remedied.

   For these reasons, this specification obsoletes RFC 3489, and instead
   describes STUN as a tool that is utilized as part of a complete NAT
   traversal solution.  ICE is a complete NAT traversal solution for
   protocols based on the offer/answer [RFC3264] methodology, such as
   SIP.  SIP Outbound is a complete solution for traversal of SIP
   signaling, and it uses STUN in a very different way.  Though it is
   possible that a protocol may be able to use STUN by itself (classic
   STUN) as a traversal solution, such usage is not described here and
   is strongly discouraged for the reasons described above.

   The on-the-wire protocol described here is changed only slightly from
   classic STUN.  The protocol now runs over TCP in addition to UDP.
   Extensibility was added to the protocol in a more structured way.  A
   magic-cookie mechanism for demultiplexing STUN with application
   protocols was added by stealing 32 bits from the 128 bit transaction
   ID defined in RFC 3489, allowing the change to be backwards
   compatible.  Mapped addresses are encoded using a new exclusive-or
   format.  There are other, more minor changes.  See Section 18 for a
   more complete listing.

   Due to the change in scope, STUN has also been renamed from "Simple
   Traversal of UDP Through NAT" to "Session Traversal Utilities for
   NAT".  The acronym remains STUN, which is all anyone ever remembers
   anyway.


3.  Overview of Operation

   This section is descriptive only.




















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                                /--------\
                              //  STUN    \\
                             |    Agent    |
                              \\ (server) //
                                \--------/


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


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

                                /--------\
                              //  STUN    \\
                             |    Agent    |
                              \\ (client) //             Private NET 1
                                \--------/


                 Figure 1: One possible STUN Configuration

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

   STUN is a client-server protocol.  It supports two types of
   transactions.  One is a request/response transaction in which a
   client sends a request to a server, and the server returns a
   response.  The second is an indication transaction in which a client
   sends an indication to the server and the server does not respond.
   Both types of transactions include a transaction ID, which is a
   randomly selected 96-bit number.  For request/response transactions,
   this transaction ID allows the client to associate the response with
   the request that generated it; for indications, this simply serves as
   a debugging aid.

   All STUN messages start with a fixed header that includes a method, a
   class, and the transaction ID.  The method indicates which of the
   various requests or indications this is; this specification defines
   just one method, Binding, but other methods are expected to be



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   defined in other documents.  The class indicates whether this is a
   request, a success response, an error response, or an indication.
   Following the fixed header comes zero or more attributes, which are
   type-length-value extensions that convey additional information for
   the specific message.

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

   In the Binding request/response transaction, a Binding Request is
   sent from a STUN client to a STUN server.  When the Binding Request
   arrives at the STUN server, it may have passed through one or more
   NATs between the STUN client and the STUN server (in Figure 1, there
   were two such NATs).  As the Binding Request message passes through a
   NAT, the NAT will modify the source transport address (that is, the
   source IP address and the source port) of the packet.  As a result,
   the source transport address of the request received by the server
   will be the public IP address and port created by the NAT closest to
   the server.  This is called a reflexive transport address.  The STUN
   server copies that source transport address into an XOR-MAPPED-
   ADDRESS attribute in the STUN Binding Response and sends the Binding
   Response back to the the STUN client.  As this packet passes back
   through a NAT, the NAT will modify the destination transport address
   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.,
   [I-D.ietf-mmusic-ice], [I-D.ietf-sip-outbound]).  In these usages,
   there must be a way to inspect a packet and determine if it is a STUN
   packet or not.  STUN provides three fields in the STUN header with
   fixed values that can be used for this purpose.  If this is not
   sufficient, then STUN packets can also contain a FINGERPRINT value
   which can further be used to distinguish the packets.

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



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   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 [I-D.ietf-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:  An entity that implements the STUN protocol.  Agents can
      act as STUN clients for some transactions and as STUN servers for
      other transactions.

   STUN Client:  A logical role in the STUN protocol.  A STUN client
      sends STUN requests or STUN indications, and receives STUN
      responses.  The term "STUN client" is also used colloquially to
      refer to a STUN agent that only acts as a STUN client.

   STUN Server:  A logical role in the STUN protocol.  A STUN server
      receives STUN requests or STUN indications and sends STUN
      responses.  The term "STUN server" is also used colloquially to
      refer to a STUN agent that only acts as a STUN server.

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

   Reflexive Transport Address:  A transport address learned by a client
      that identifies that client as seen by another host on an IP
      network, typically a STUN server.  When there is an intervening
      NAT between the client and the other host, the reflexive transport
      address represents the mapped address allocated to the client on



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      the public side of the NAT.  Reflexive transport addresses are
      learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
      MAPPED-ADDRESS) in STUN responses.

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

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

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

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

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

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

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

   RTO:  Retransmission TimeOut


6.  STUN Message Structure

   STUN messages are encoded in binary using network-oriented format
   (most significant byte or octet first, also commonly known as big-
   endian).  The transmission order is described in detail in Appendix B
   of RFC791 [RFC0791].  Unless otherwise noted, numeric constants are
   in decimal (base 10).



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

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

                  Figure 2: Format of STUN Message Header

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

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

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

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

                Figure 3: Format of STUN Message Type Field

   Here the bits in the message type field are shown as most-significant
   (M11) through least-significant (M0).  M11 through M0 represent a 12-
   bit encoding of the method.  C1 and C0 represent a 2 bit encoding of
   the class.  A class of 0b00 is a Request, a class of 0b01 is an
   indication, a class of 0b10 is a success response, and a class of
   0b11 is an error response.  This specification defines a single
   method, Binding.  The method and class are orthogonal, so that four



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   each method, a request, success response, error response and
   indication are defined for that method.

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

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

   The magic cookie field MUST contain the fixed value 0x2112A442 in
   network byte order.  In RFC 3489 [RFC3489], this field was part of
   the transaction ID; placing the magic cookie in this location allows
   a server to detect if the client will understand certain attributes
   that were added in this revised specification.  In addition, it aids
   in distinguishing STUN packets from packets of other protocols when
   STUN is multiplexed with those other protocols on the same port.

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

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

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




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7.  Base Protocol Procedures

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

7.1.  Forming a Request or an Indication

   When formulating a request or indication message, the client MUST
   follow the rules in Section 6 when creating the header.  In addition,
   the message class MUST be either "Request" or "Indication" (as
   appropriate), and the method must be either Binding or some method
   defined in another document.

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

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

7.2.  Sending the Request or Indication

   The client then sends the request to the server.  This document
   specifies how to send STUN messages over UDP, TCP, or TLS-over-TCP;
   other transport protocols may be added in the future.  The STUN usage
   must specify which transport protocol is used, and how the client
   determines the IP address and port of the server.  Section 9
   describes a DNS-based method of determining the IP address and port
   of a server which a usage may elect to use.  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).

7.2.1.  Sending over UDP

   When running STUN over UDP it is possible that the STUN message might
   be dropped by the network.  Reliability of STUN request/response
   transactions is accomplished through retransmissions of the request
   message by the client application itself.  STUN indications are not



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   retransmitted; thus indication transactions over UDP are not
   reliable.

   A client SHOULD retransmit a STUN request message starting with an
   interval of RTO ("Retransmission TimeOut"), doubling after each
   retransmission.  The RTO is an estimate of the round-trip-time, and
   is computed as described in RFC 2988 [RFC2988], with two exceptions.
   First, the initial value for RTO SHOULD be configurable (rather than
   the 3s recommended in RFC 2988) and SHOULD be greater than 100ms.  In
   fixed- line access links, a value of 100ms is RECOMMENDED.  Secondly,
   the value of RTO MUST NOT be rounded up to the nearest second.
   Rather, a 1ms accuracy MUST be maintained.  As with TCP, the usage of
   Karn's algorithm is RECOMMENDED.  When applied to STUN, it means that
   RTT estimates SHOULD NOT be computed from STUN transactions which
   result in the retransmission of a request.

   The value for RTO SHOULD be cached by an client after the completion
   of the transaction, and used as the starting value for RTO for the
   next transaction to the same server (based on equality of IP
   address).  The value SHOULD be considered stale and discarded after
   10 minutes.

   Retransmissions continue until a response is received, or until a
   total of 7 requests have been sent.  If, after the last request, a
   duration equal to 16 times the RTO has passed without a response, the
   client SHOULD consider the transaction to have failed.  A STUN
   transaction over UDP is also considered failed if there has been a
   transport failure of some sort, such as a fatal ICMP error.  For
   example, assuming an RTO of 100ms, requests would be sent at times
   0ms, 100ms, 300ms, 700ms, 1500ms, 3100ms, and 6300ms.  If the client
   has not received a response after 7900ms, the client will consider
   the transaction to have timed out.

7.2.2.  Sending over TCP or TLS-over-TCP

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

   In some usage of STUN, STUN is sent as the only protocol over the TCP
   connection.  In this case, it can be sent without the aid of any
   additional framing or demultiplexing.  In other usages, or with other
   extensions, it may be multiplexed with other data over a TCP
   connection.  In that case, STUN MUST be run 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.

   For TLS-over-TCP, the TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST



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   be supported at a minimum.  Implementations MAY also support any
   other ciphersuite.  When it receives the TLS Certificate message, the
   client SHOULD verify the certificate and inspect the site identified
   by the certificate.  If the certificate is invalid, revoked, or if it
   does not identify the appropriate party, the client MUST NOT send the
   STUN message or otherwise proceed with the STUN transaction.  The
   client MUST verify the identity of the server.  To do that, it
   follows the identification procedures defined in Section 3.1 of RFC
   2818 [RFC2818].  Those procedures assume the client is dereferencing
   a URI.  For purposes of usage with this specification, the client
   treats the domain name or IP address used in Section 8.1 as the host
   portion of the URI that has been dereferenced.  If DNS was not used,
   the client MUST be configured with a set of authorized domains whose
   certificates will be accepted.

   Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
   itself, and there are no retransmissions at the STUN protocol level.
   However, for a request/response transaction, if the client has not
   received a response 7900ms after it sent the SYN to establish the
   connection, it considers the transaction to have timed out.  This
   value has been chosen to equalize the TCP and UDP timeouts for the
   default initial RTO.

   In addition, if the client is unable to establish the TCP connection,
   or the TCP connection is reset or fails before a response is
   received, any request/response transaction in progress is considered
   to have failed

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

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

   o  has no plans to use any resources (such as a mapped address
      (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
      [I-D.ietf-behave-turn]) that were learned though STUN requests
      sent over that connection, and;

   o  if multiplexing other application protocols over that port, has
      finished using that other application, and;

   o  if using that learned port with a remote peer, has established
      communications with that remote peer, as is required by some TCP
      NAT traversal techniques (e.g., [I-D.ietf-mmusic-ice-tcp]).




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   At the server end, the server SHOULD keep the connection open, and
   let the client close it.  If a server becomes overloaded and needs to
   close connections to free up resources, it SHOULD close an existing
   connection rather than reject new connection requests.  The server
   SHOULD NOT close a connection if a request was received over that
   connection for which a response was not sent.  A server MUST NOT ever
   open a connection back towards the client in order to send a
   response.

7.3.  Receiving a STUN Message

   This section specifies the processing of a STUN message.  The
   processing specified here is for STUN messages as defined in this
   specification; additional rules for backwards compatibility are
   defined in in Section 12.  Those additional procedures are optional,
   and usages can elect to utilize them.  First, a set of processing
   operations are applied that are independent of the class.  This is
   followed by class-specific processing, described in the subsections
   which follow.

   When a STUN agent receives a STUN message, it first checks that the
   message obeys the rules of Section 6.  It checks that the first two
   bits are 0, that the magic cookie field has the correct value, that
   the message length is sensible, and that the method value is a
   supported method.  If the message-class is Success Response or Error
   Response, the agent checks that the transaction ID matches a
   transaction that is still in progress.  If the FINGERPRINT extension
   is being used, the agent checks that the FINGERPRINT attribute is
   present and contains the correct value.  If any errors are detected,
   the message is silently discarded.  In the case when STUN is being
   multiplexed with another protocol, an error may indicate that this is
   not really a STUN message; in this case, the agent should try to
   parse the message as a different protocol.

   The STUN agent then does any checks that are required by a
   authentication mechanism that the usage has specified (see
   Section 10.

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

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




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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.

   If the request uses UDP transport and is a retransmission of a
   request for which the server has already generated a success response
   within the last 10 seconds, the server MUST retransmit the same
   success response.  One way for a server to do this is to remember all
   transaction IDs received over UDP and their corresponding responses
   in the last 10 seconds.  Another way is to reprocess the request and
   recompute the response.  The latter technique MUST only be applied to
   requests which are idempotent and result in the same success response
   for the same request.  The Binding method is considered to idempotent
   in this way (even though certain rare network events could cause the
   reflexive transport address value to change).  Extensions to STUN
   SHOULD state whether their request types have this property or not.

7.3.1.1.  Forming a Success or Error Response

   When forming the response (success or error), the server follows the
   rules of section 6.  The method of the response is the same as that
   of the request, and the message class is either "Success Response" or
   "Error Response".

   For an error response, the server MUST add an ERROR-CODE attribute
   containing the error code specified in the processing above.  The
   reason phrase is not fixed, but SHOULD be something suitable for the
   error code.  For certain errors, additional attributes are added to
   the message.  These attributes are spelled out in the description
   where the error code is specified.  For example, for an error code of
   420 (Unknown Attribute), the server MUST include an UNKNOWN-
   ATTRIBUTES attribute.  Certain authentication errors also cause
   attributes to be added (see Section 10).  Extensions may define other
   errors and/or additional attributes to add in error cases.

   If the server authenticated the request using an authentication
   mechanism, then the server SHOULD add the appropriate authentication
   attributes to the response (see Section 10).

   The server also adds any attributes required by the specific method



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   or usage.  In addition, the server SHOULD add a SERVER attribute to
   the message.

   For the Binding method, no additional checking is required unless the
   usage specifies otherwise.  When forming the success response, the
   server adds a XOR-MAPPED-ADDRESS attribute to the response, where the
   contents of the attribute are the source transport address of the
   request message.  For UDP, this is the source IP address and source
   UDP port of the request message.  For TCP and TLS-over-TCP, this is
   the source IP address and source TCP port of the TCP connection as
   seen by the server.

7.3.1.2.  Sending the Success or Error Response

   The response (success or error) is sent over the same transport as
   the request was received on.  If the request was received over UDP,
   the destination IP address and port of the response is the source IP
   address and port of the received request message, and the source IP
   address and port of the response is equal to the destination IP
   address and port of the received request message.  If the request was
   received over TCP or TLS-over-TCP, the response is sent back on the
   same TCP connection as the request was received on.

7.3.2.  Processing an Indication

   If the indication contains unknown comprehension-required attributes,
   the indication is discarded and processing ceases.

   The server then does any additional checking that the method or the
   specific usage requires.  If all the checks succeed, the server then
   processes the indication.  No response is generated for an
   indication.

   For the Binding method, no additional checking or processing is
   required, unless the usage specifies otherwise.  The mere receipt of
   the message by the server has refreshed the "bindings" in the
   intervening NATs.

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

7.3.3.  Processing a Success Response

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




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   The client then does any additional checking that the method or the
   specific usage requires.  If all the checks succeed, the client then
   processes the success response.

   For the Binding method, the client checks that the XOR-MAPPED-ADDRESS
   attribute is present in the response.  The client checks the address
   family specified.  If it is an unsupported address family, the
   attribute SHOULD be ignored.  If it is an unexpected but supported
   address family (for example, the Binding transaction was sent over
   IPv4, but the address family specified is IPv6), then the client MAY
   accept and use the value.

7.3.4.  Processing an Error Response

   If the error response contains unknown comprehension-required
   attributes, or if the error response does not contain an ERROR-CODE
   attribute, then the transaction is simply considered to have failed.

   The client then does any processing specified by the authentication
   mechanism (see Section 10).  This may result in a new transaction
   attempt.

   The processing at this point depends on the error-code, the method,
   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.



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   In some usages, STUN messages are multiplexed on the same transport
   address as other protocols, such as RTP.  In order to apply the
   processing described in Section 7, STUN messages must first be
   separated from the application packets.  Section 6 describes three
   fixed fields in the STUN header that can be used for this purpose.
   However, in some cases, these three fixed fields may not be
   sufficient.

   When the FINGERPRINT extension is used, an agent includes the
   FINGERPRINT attribute in messages it sends to another agent.
   Section 14.5 describes the placement and value of this attribute.
   When the agent receives what it believes is a STUN message, then, in
   addition to other basic checks, the agent also checks that the
   message contains a FINGERPRINT attribute and that the attribute
   contains the correct value (see Section 7.3.  This additional check
   helps the agent detect messages of other protocols that might
   otherwise seem to be STUN messages.


9.  DNS Discovery of a Server

   This section describes an optional procedure for STUN that allows a
   client to use DNS to determine the IP address and port of a server.
   A STUN usage must describe if and when this extension is used.  To
   use this procedure, the client must have a domain name and a service
   name; the usage must also describe how the client obtains these.

   When a client wishes to locate a STUN server in the public Internet
   that accepts Binding Request/Response transactions, the SRV service
   name is "stun".  STUN usages MAY define additional DNS SRV service
   names.

   The domain name is resolved to a transport address using the SRV
   procedures specified in [RFC2782].  The DNS SRV service name is the
   service name provided as input to this procedure.  The protocol in
   the SRV lookup is the transport protocol the client will run STUN
   over: "udp" for UDP, "tcp" for TCP, and "tls" for TLS-over-TCP.  If,
   in the future, additional SRV records are defined for TLS over other
   transport protocols, those will need to utilize an SRV transport
   token of the form "tls-foo" for transport protocol "foo".

   The procedures of RFC 2782 are followed to determine the server to
   contact.  RFC 2782 spells out the details of how a set of SRV records
   are sorted and then tried.  However, RFC2782 only states that the
   client should "try to connect to the (protocol, address, service)"
   without giving any details on what happens in the event of failure.
   When following these procedures, if the STUN transaction times out
   without receipt of a response, the client SHOULD retry the request to



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   the next server in the list of servers from the DNS SRV response.
   Such a retry is only possible for request/response transmissions,
   since indication transactions generate no response or timeout.

   The default port for STUN requests is 3478, for both TCP and UDP.
   Administrators SHOULD use this port in their SRV records for UDP and
   TCP, but MAY use others.  There is no default port for STUN over TLS,
   however a STUN server SHOULD use a port number for TLS different from
   3478 so that the server can determine whether the first message it
   will receive after the TCP connection is set up, is a STUN message or
   a TLS message.

   If no SRV records were found, the client performs an A or AAAA record
   lookup of the domain name.  The result will be a list of IP
   addresses, each of which can be contacted at the default port using
   UDP or TCP, independent of the STUN usage.  For usages that require
   TLS, lack of SRV records is equivalent to a failure of the
   transaction, since the request or indication MUST NOT be sent unless
   SRV records provided a transport address specifically for TLS.


10.  Authentication and Message-Integrity Mechanisms

   This section defines two mechanisms for STUN that a client and server
   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 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



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   credential is time-limited.  The time-limit is defined by the usage.
   As an example, in the ICE usage [I-D.ietf-mmusic-ice], the two
   endpoints use out-of-band signaling to agree on a username and
   password, and this username and password is applicable for the
   duration of the media session.

   This credential is used to form a message integrity check in each
   request and in many responses.  There is no challenge and response as
   in the long term mechanism; consequently, replay is prevented by
   virtue of the time-limited nature of the credential.

10.1.1.  Forming a Request or Indication

   For a request or indication message, the agent MUST include the
   USERNAME and MESSAGE-INTEGRITY attributes in the message.  The HMAC
   for the MESSAGE-INTEGRITY attribute is computed as described in
   Section 14.4.  The key for the HMAC is the password.  Note that the
   password is never included in the request or indication.

10.1.2.  Receiving a Request or Indication

   After the agent has done the basic processing of a message, the agent
   performs the checks listed below in order specified:

   o  If the message does not contain both a MESSAGE-INTEGRITY and a
      USERNAME attribute:

      *  If the message is a request, the server MUST reject the request
         with an error response.  This response MUST use an error code
         of 400 (Bad Request).

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

   o  If the USERNAME does not contain a username value currently valid
      within the server:

      *  If the message is a request, the server MUST reject the request
         with an error response.  This response MUST use an error code
         of 401 (Unauthorized).

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

   o  Using the password associated with the username, compute the value
      for the message-integrity as described in Section 14.4.  If the
      resulting value does not match the contents of the MESSAGE-
      INTEGRITY attribute:



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      *  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 server MUST silently
         discard the indication.

   If these checks pass, the server continues to process the request or
   indication.  Any response generated by the server MUST include the
   MESSAGE-INTEGRITY attribute, computed using the password utilized to
   authenticate the request.  The response MUST NOT contain the USERNAME
   attribute.

   If any of the checks fail, the 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 14.4, using the same password it
   utilized for the request.  If the resulting value matches the
   contents of the MESSAGE-INTEGRITY attribute, the response is
   considered authenticated.  If the value does not match, or if
   MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if
   it was never received.  This means that retransmits, if applicable,
   will continue.

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



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   duration of validity or client identity from which it is valid.  The
   client retries the request, this time including its username, the
   realm, and echoing the nonce provided by the server.  The client also
   includes a message-integrity, which provides an HMAC over the entire
   request, including the nonce.  The server validates the nonce, and
   checks the message-integrity.  If they match, the request is
   authenticated.  If the nonce is no longer valid, it is considered
   "stale", and the server rejects the request, providing a new nonce.

   In subsequent requests to the same server, the client reuses the
   nonce, username, realm and password it used previously.  In this way,
   subsequent requests are not rejected until the nonce becomes invalid
   by the server, in which case the rejection provides a new nonce to
   the client.

   Note that the long-term credential mechanism cannot be used to
   protect indications, since indications cannot be challenged.  Usages
   utilizing indications must either use a short-term credential, or
   omit authentication and message integrity for them.

   Since the long-term credential mechanism is susceptible to offline
   dictionary attacks, deployments SHOULD utilize strong passwords.

   For STUN servers used in conjunction with SIP servers, it is
   desirable to use the same credentials for authentication to the SIP
   server and STUN server.  Typically, SIP systems utilizing SIP's
   digest authentication mechanism do not actually store the password in
   the database.  Rather, they store a value called H(A1), which is
   computed as:

                H(A1) = MD5(username ":" realm ":" password)

   If a system wishes to utilize this credential, the STUN password
   would be computed by taking the user-entered username and password,
   and using H(A1) as the STUN password.  It is RECOMMENDED that clients
   utilize this construction for the STUN password.

10.2.1.  Forming a Request

   There are two cases when forming a request.  In the first case, this
   is the first request from the client to the server (as identified by
   its IP address and port).  In the second case, the client is
   submitting a subsequent request once a previous request/response
   transaction has completed successfully.







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10.2.1.1.  First Request

   If the client has not completed a successful request/response
   transaction with the server, it SHOULD omit the USERNAME, MESSAGE-
   INTEGRITY, REALM, and NONCE attributes.  In other words, the very
   first request is sent as if there were no authentication or message
   integrity applied.

10.2.1.2.  Subsequent Requests

   Once a request/response transaction has completed successfully, the
   client will have been been presented a realm and nonce by the server,
   and selected a username and password with which it authenticated.
   The client SHOULD cache the username, password, realm, and nonce for
   subsequent communications with the server.  When the client sends a
   subsequent request, it SHOULD include the USERNAME, REALM, and NONCE
   attributes with these cached values.  It SHOULD include a MESSAGE-
   INTEGRITY attributed, computed as described in Section 14.4 using the
   cached password as the key.

10.2.2.  Receiving a Request

   After the server has done the basic processing of a request, it
   performs the checks listed below in the order specified:

   o  If the message:

      *  does not contain a MESSAGE-INTEGRITY attribute,

      *  OR, it contains a USERNAME whose value is not a valid username,

      the server MUST generate an error response with an error code of
      401 (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.

   o  If the message contains a MESSAGE-INTEGRITY attribute, but is
      missing the USERNAME, REALM or NONCE attributes, the server MUST
      generate an error response with an error code of 400 (Bad
      Request).

   o  If the NONCE is no longer valid, the server MUST generate an error
      response with an error code of 438 (Stale Nonce).  This response
      MUST include a NONCE and REALM attribute.

   o  Using the password associated with the username in the USERNAME
      attribute, compute the value for the message-integrity as



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      described in Section 14.4.  If the resulting value does not match
      the contents of the MESSAGE-INTEGRITY attribute, the server MUST
      reject the request with an error response.  This response MUST use
      an error code of 401 (Unauthorized).  It MUST include a REALM and
      NONCE attribute.

   If these checks pass, the server continues to process the request or
   indication.  Any response generated by the server MUST include the
   MESSAGE-INTEGRITY attribute, computed using the username and password
   utilized to authenticate the request.  The REALM, NONCE, and USERNAME
   attributes SHOULD NOT be included.

10.2.3.  Receiving a Response

   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 14.4, using
   the same password it utilized for the request.  If the resulting
   value matches the contents of the MESSAGE-INTEGRITY attribute, the
   response is considered authenticated.  If the value does not match,
   or if MESSAGE-INTEGRITY was absent, the response MUST be discarded,
   as if it was never received.  This means that retransmits, if
   applicable, will continue.


11.  ALTERNATE-SERVER Mechanism

   This section describes a mechanism in STUN that allows a server to
   redirect a client to another server.  This extension is optional, and
   a usage must define if and when this extension is used.  To prevent
   denial-of-service attacks, this extension MUST only be used in



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   situations where the client and server are using an authentication
   and message-integrity mechanism.

   A server using this extension redirects a client to another server by
   replying to a request message with an error response message with an
   error code of 300 (Try Alternate).  The server MUST include a
   ALTERNATE-SERVER attribute in the error response.  The error response
   message MUST be authenticated, which in practice means the request
   message must have passed the authentication checks.

   A client using this extension handles a 300 (Try Alternate) error
   code as follows.  If the error response has passed the authentication
   checks, then the client looks for a ALTERNATE-SERVER attribute in the
   error response.  If one is found, then the client considers the
   current transaction as failed, and re-attempts the request with the
   server specified in the attribute.  The client SHOULD reuse any
   authentication credentials from the old request in the new
   transaction.


12.  Backwards Compatibility with RFC 3489

   This section define procedures that allow a degree of backwards
   compatible with the original protocol defined in RFC 3489 [RFC3489].
   This mechanism is optional, meant to be utilized only in cases where
   a new client can connect to an old server, or vice-a-versa.  A usage
   must define if and when this procedure is used.

   Section 18 lists all the changes between this specification and RFC
   3489 [RFC3489].  However, not all of these differences are important,
   because "classic STUN" was only used in a few specific ways.  For the
   purposes of this extension, the important changes are the following.
   In RFC 3489:

   o  UDP was the only supported transport;

   o  The field that is now the Magic Cookie field was a part of the
      transaction id field, and transaction ids were 128 bits long;

   o  The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding
      method used the MAPPED-ADDRESS attribute instead;

   o  There were two comprehension-required attributes, RESPONSE-ADDRESS
      and CHANGE-REQUEST, that have been removed from this
      specification;

      *  These attributes are now part of the NAT Behavior Discovery
         usage.



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12.1.  Changes to Client Processing

   A client that wants to interoperate with a [RFC3489] server SHOULD
   send a request message that uses the Binding method, contains no
   attributes, and uses UDP as the transport protocol to the server.  If
   successful, the success response received from the server will
   contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS
   attribute; other than this change, the processing of the response is
   identical to the procedures described above.

12.2.  Changes to Server Processing

   A STUN server can detect when a given Binding Request message was
   sent from an RFC 3489 [RFC3489] client by the absence of the correct
   value in the Magic Cookie field.  When the server detects an RFC 3489
   client, it SHOULD copy the value seen in the Magic Cookie field in
   the Binding Request to the Magic Cookie field in the Binding Response
   message, and insert a MAPPED-ADDRESS attribute instead of an XOR-
   MAPPED-ADDRESS attribute.

   The client might, in rare situations, include either the RESPONSE-
   ADDRESS or CHANGE-REQUEST attributes.  In these situations, the
   server will view these as unknown comprehension-required attributes
   and reply with an error response.  Since the mechanisms utilizing
   those attributes are no longer supported, this behavior is
   acceptable.


13.  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) [I-D.ietf-mmusic-ice], Client-
   initiated connections for SIP [I-D.ietf-sip-outbound], and NAT
   Behavior Discovery [I-D.ietf-behave-nat-behavior-discovery].  Other
   STUN usages may be defined in the future.

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

   o  Which STUN methods are used;




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   o  What authentication and message integrity mechanisms are used;

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

   o  How a STUN client determines the IP address and port of the STUN
      server;

   o  Whether backwards compatibility to RFC 3489 is required;

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

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

   Finally, a usage must consider whether its usage of STUN is an
   example of the Unilateral Self-Address Fixing approach to NAT
   traversal, and if so, address the questions raised in RFC 3424.


14.  STUN Attributes

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

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

                    Figure 5: Format of STUN Attributes

   The value in the Length field MUST contain the length of the Value
   part of the attribute, prior to padding, measured in bytes.  Since
   STUN aligns attributes on 32 bit boundaries, attributes whose content
   is not a multiple of 4 bytes are padded with 1, 2 or 3 bytes of
   padding so that its value contains a multiple of 4 bytes.  The



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   padding bits are ignored, and may be any value.

   Any attribute type MAY appear more than once in a STUN message.
   Unless specified otherwise, the order of appearance is significant:
   only the first occurance needs to be processed by a receiver, and any
   duplicates MAY be ignored by a receiver.

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

   The STUN Attribute types defined by this specification are:

     Comprehension-required range (0x0000-0x7FFF):
       0x0000: (Reserved)
       0x0001: MAPPED-ADDRESS
       0x0006: USERNAME
       0x0007: (Reserved; was PASSWORD)
       0x0008: MESSAGE-INTEGRITY
       0x0009: ERROR-CODE
       0x000A: UNKNOWN-ATTRIBUTES
       0x0014: REALM
       0x0015: NONCE
       0x0020: XOR-MAPPED-ADDRESS

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

   The rest of this section describes the format of the various
   attributes defined in this specification.

14.1.  MAPPED-ADDRESS

   The MAPPED-ADDRESS attribute indicates a reflexive transport address
   of the client.  It consists of an eight bit address family, and a
   sixteen bit port, followed by a fixed length value representing the
   IP address.  If the address family is IPv4, the address MUST be 32
   bits.  If the address family is IPv6, the address MUST be 128 bits.
   All fields must be in network byte order.





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   The format of the MAPPED-ADDRESS attribute is:

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

               Figure 7: Format of MAPPED-ADDRESS attribute

   The address family can take on the following values:

     0x01:IPv4
     0x02:IPv6

   The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be
   ignored by receivers.  These bits are present for aligning parameters
   on natural 32 bit boundaries.

   This attribute is used only by servers for achieving backwards
   compatibility with RFC 3489 [RFC3489] clients.

14.2.  XOR-MAPPED-ADDRESS

   The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS
   attribute, except that the reflexive transport address is obfuscated
   through the XOR function.

   The format of the XOR-MAPPED-ADDRESS is:

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

             Figure 9: Format of XOR-MAPPED-ADDRESS Attribute

   The Family represents the IP address family, and is encoded
   identically to the Family in MAPPED-ADDRESS.

   X-Port is the mapped port, exclusive or'd with most significant 16



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   bits of the magic cookie.  If the IP address family is IPv4,
   X-Address is the mapped IP address exclusive or'd with the magic
   cookie.  If the IP address family is IPv6, the X-Address is the
   mapped IP address exclusively or'ed with the magic cookie and the 96-
   bit transaction ID.

   For example, using the "^" character to indicate exclusive or, if the
   IP address is 192.168.1.1 (0xc0a80101) and the port is 5555 (0x15B3),
   the X-Port would be 0x15B3 ^ 0x2112 = 0x34A1, and the X-Address would
   be 0xc0a80101 ^ 0x2112A442 = 0xe1baa543.

   The rules for encoding and processing the first 8 bits of the
   attribute's value, the rules for handling multiple occurrences of the
   attribute, and the rules for processing addresses families are the
   same as for MAPPED-ADDRESS.

   NOTE: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
   encoding of the transport address.  The former encodes the transport
   address by exclusive-or'ing it with the magic cookie.  The latter
   encodes it directly in binary.  RFC 3489 originally specified only
   MAPPED-ADDRESS.  However, deployment experience found that some NATs
   rewrite the 32-bit binary payloads containing the NAT's public IP
   address, such as STUN's MAPPED-ADDRESS attribute, in the well-meaning
   but misguided attempt at providing a generic ALG function.  Such
   behavior interferes with the operation of STUN and also causes
   failure of STUN's message integrity checking.

14.3.  USERNAME

   The USERNAME attribute is used for message integrity.  It identifies
   the username and password combination used in the message integrity
   check.

   The value of USERNAME is a variable length value.  It MUST contain a
   UTF-8 encoded sequence of less than 128 characters (which can be as
   long as 763 bytes).

14.4.  MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of
   the STUN message.  The MESSAGE-INTEGRITY attribute can be present in
   any STUN message type.  Since it uses the SHA1 hash, the HMAC will be
   20 bytes.  The text used as input to HMAC is the STUN message,
   including the header, up to and including the attribute preceding the
   MESSAGE-INTEGRITY attribute.  With the exception of the FINGERPRINT
   attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore
   all other attributes that follow MESSAGE-INTEGRITY.




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   The key used as input to HMAC is the password.

   Based on the rules above, the hash includes the length field from the
   STUN message header.  This length indicates the length of the entire
   message, including the MESSAGE-INTEGRITY attribute itself.
   Consequently, the MESSAGE-INTEGRITY attribute MUST be inserted into
   the message (with dummy content) prior to the computation of the
   integrity check.  Once the computation is performed, the value of the
   attribute can be filled in.  This ensures the length has the correct
   value when the hash is performed.  Similarly, when validating the
   MESSAGE-INTEGRITY, the length field should be adjusted to point to
   the end of the MESSAGE-INTEGRITY attribute prior to calculating the
   HMAC.  Such adjustment is necessary when attributes, such as
   FINTERPRINT, appear after MESSAGE-INTEGRITY.

14.5.  FINGERPRINT

   The FINGERPRINT attribute may be present in all STUN messages.  The
   value of the attribute is computed as the CRC-32 of the STUN message
   up to (but excluding) the FINGERPRINT attribute itself, xor-d with
   the 32 bit value 0x5354554e (the XOR helps in cases where an
   application packet is also using CRC-32 in it).  The 32 bit CRC is
   the one defined in ITU V.42 [ITU.V42.1994], which has a generator
   polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.
   When present, the FINGERPRINT attribute MUST be the last attribute in
   the message, and thus will appear after MESSAGE-INTEGRITY.

   The FINGERPRINT attribute can aid in distinguishing STUN packets from
   packets of other protocols.  See Section 8.

   When using the FINGERPRINT attribute in a message, the attribute is
   first placed into the message with a dummy value, then the CRC is
   computed, and then the value of the attribute is updated.  If the
   MESSAGE-INTEGRITY attribute is also present, then it must be present
   with the correct message-integrity value before the CRC is computed,
   since the CRC is done over the value of the MESSAGE-INTEGRITY
   attribute as well.

14.6.  ERROR-CODE

   The ERROR-CODE attribute is used in Error Response messages.  It
   contains a numeric error code value in the range of 300 to 699 plus a
   textual reason phrase encoded in UTF-8, and is consistent in its code
   assignments and semantics with SIP [RFC3261] and HTTP [RFC2616].  The
   reason phrase is meant for user consumption, and can be anything
   appropriate for the error code.  Recommended reason phrases for the
   defined error codes are presented below.  The reason phrase MUST be a
   UTF-8 encoded sequence of less than 128 characters (which can be as



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   long as 763 bytes).

   To facilitate processing, the class of the error code (the hundreds
   digit) is encoded separately from the rest of the code.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Reserved, should be 0         |Class|     Number    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Reason Phrase (variable)                                ..
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Reserved bits SHOULD be 0, and are for alignment on 32-bit
   boundaries.  Receivers MUST ignore these bits.  The Class represents
   the hundreds digit of the error code.  The value MUST be between 3
   and 6.  The number represents the error code modulo 100, and its
   value MUST be between 0 and 99.

   The following error codes, along with their recommended reason
   phrases (in brackets) are defined:

   300  Try Alternate: The client should contact an alternate server for
        this request.  This error response MUST only be sent if the
        request included a USERNAME attribute and a valid MESSAGE-
        INTEGRITY attribute; otherwise it MUST NOT be sent and error
        code 400 (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 expected MESSAGE-
        INTEGRITY attribute.  The server MAY include the MESSAGE-
        INTEGRITY attribute in its error response.






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   420  Unknown Attribute: The server received STUN packet containing a
        comprehension-required attribute which it did not understand.
        The server MUST put this unknown attribute in the UNKNOWN-
        ATTRIBUTE attribute of its error response.

   438  Stale Nonce: The NONCE used by the client was no longer valid.
        The client should retry, using the NONCE provided in the
        response.

   500  Server Error: The server has suffered a temporary error.  The
        client should try again.

14.7.  REALM

   The REALM attribute may be present in requests and responses.  It
   contains text which meets the grammar for "realm-value" as described
   in RFC 3261 [RFC3261] but without the double quotes and their
   surrounding whitespace.  That is, it is an unquoted realm-value.  It
   MUST be a UTF-8 encoded sequence of less than 128 characters (which
   can be as long as 763 bytes).

   Presence of the REALM attribute in a request indicates that long-term
   credentials are being used for authentication.  Presence in certain
   error responses indicates that the server wishes the client to use a
   long-term credential for authentication.

14.8.  NONCE

   The NONCE attribute may be present in requests and responses.  It
   contains a sequence of qdtext or quoted-pair, which are defined in
   RFC 3261 [RFC3261].  See RFC 2617 [RFC2617], Section 4.3, for
   guidance on selection of nonce values in a server.  It MUST be less
   than 128 characters (which can be as long as 763 bytes).

14.9.  UNKNOWN-ATTRIBUTES

   The UNKNOWN-ATTRIBUTES attribute is present only in an error response
   when the response code in the ERROR-CODE attribute is 420.













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   The attribute contains a list of 16 bit values, each of which
   represents an attribute type that was not understood by the server.

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


             Figure 11: Format of UNKNOWN-ATTRIBUTES attribute

      Note: In [RFC3489], this field was padded to 32 by duplicating the
      last attribute.  In this version of the specification, the normal
      padding rules for attributes are used instead.

14.10.  SERVER

   The server attribute contains a textual description of the software
   being used by the server, including manufacturer and version number.
   The attribute has no impact on operation of the protocol, and serves
   only as a tool for diagnostic and debugging purposes.  The value of
   SERVER is variable length.  It MUST be a UTF-8 encoded sequence of
   less than 128 characters (which can be as long as 763 bytes).

14.11.  ALTERNATE-SERVER

   The alternate server represents an alternate transport address
   identifying a different STUN server which the STUN client should try.

   It is encoded in the same way as MAPPED-ADDRESS, and thus refers to a
   single server by IP address.  The IP address family MUST be identical
   to that of the source IP address of the request.

   This attribute MUST only appear in an error response that contains a
   MESSAGE-INTEGRITY attribute.  This prevents it from being used in
   denial-of-service attacks.


15.  Security Considerations

15.1.  Attacks against the Protocol







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15.1.1.  Outside Attacks

   An attacker can try to modify STUN messages in transit, in order to
   cause a failure in STUN operation.  These attacks are 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.

15.1.2.  Inside Attacks

   A rogue client may try to launch a DoS attack against a server by
   sending it a large number of STUN requests.  Fortunately, STUN
   requests can be processed statelessly by a server, making such
   attacks hard to launch.

   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 with this attack, and it is mitigated by
   ingress source address filtering.

15.2.  Attacks Affecting the Usage

   This section lists attacks that might be launched against a usage of
   STUN.  Each STUN usage must consider whether these attacks are
   applicable to it, and if so, discuss counter-measures.

   Most of the attacks in this section revolve around an attacker
   modifying the reflexive address learned by a STUN client through a
   Binding Request/Binding Response transaction.  Since the usage of the
   reflexive address is a function of the usage, the applicability and
   remediation of these attacks is usage-specific.  In common
   situations, modification of the reflexive address by an on-path
   attacker is easy to do.  Consider, for example, the common situation
   where STUN is run directly over UDP.  In this case, an on-path
   attacker can modify the source IP address of the Binding Request



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   before it arrives at the STUN server.  The STUN server will then
   return this IP address in the XOR-MAPPED-ADDRESS attribute to the
   client.  Protecting against this attack by using a message-integrity
   check is impossible, since a message-integrity value cannot cover the
   source IP address, since the intervening NAT must be able to modify
   this value.  Instead, one solution to preventing the attacks listed
   below is for the client to verify the reflexive address learned, as
   is done in ICE [I-D.ietf-mmusic-ice].  Other usages may use other
   means to prevent these attacks.

15.2.1.  Attack I: DDoS Against a Target

   In this attack, the attacker provides one or more clients with the
   same faked reflexive address that points to the intended target.
   This will trick the STUN clients into thinking that their reflexive
   addresses are equal to that of the target.  If the clients hand out
   that reflexive address in order to receive traffic on it (for
   example, in SIP messages), the traffic will instead be sent to the
   target.  This attack can provide substantial amplification,
   especially when used with clients that are using STUN to enable
   multimedia applications.

15.2.2.  Attack II: Silencing a Client

   In this attack, the attacker provides a STUN client with a faked
   reflexive address.  The reflexive address it provides is a transport
   address that routes to nowhere.  As a result, the client won't
   receive any of the packets it expects to receive when it hands out
   the reflexive address.  This exploitation is not very interesting for
   the attacker.  It impacts a single client, which is frequently not
   the desired target.  Moreover, any attacker that can mount the attack
   could also deny service to the client by other means, such as
   preventing the client from receiving any response from the STUN
   server, or even a DHCP server.

15.2.3.  Attack III: Assuming the Identity of a Client

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

15.2.4.  Attack IV: Eavesdropping

   In this attack, the attacker forces the client to use a reflexive
   address that routes to itself.  It then forwards any packets it
   receives to the client.  This attack would allow the attacker to
   observe all packets sent to the client.  However, in order to launch
   the attack, the attacker must have already been able to observe



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   packets from the client to the STUN server.  In most cases (such as
   when the attack is launched from an access network), this means that
   the attacker could already observe packets sent to the client.  This
   attack is, as a result, only useful for observing traffic by
   attackers on the path from the client to the STUN server, but not
   generally on the path of packets being routed towards the client.

15.3.  Hash Agility Plan

   This specification uses SHA-1 for computation of the message
   integrity.  If, at a later time, SHA-1 is found to be compromised,
   the following is the remedy that will be applied.

   We will define a STUN extension which introduces a new message
   integrity attribute, computed using a new hash.  Clients would be
   required to include both the new and old message integrity attributes
   in their requests or indications.  A new server will utilize the new
   message integrity attribute, and an old one, the old.  After a
   transition period where mixed implementations are in deployment, the
   old message-integrity attribute will be deprecated by another
   specification, and clients will cease including it in requests.


16.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing"
   (UNSAF), which is the general process by which a client attempts to
   determine its address in another realm on the other side of a NAT
   through a collaborative protocol reflection mechanism (RFC3424
   [RFC3424]).  STUN can be used to perform this function using a
   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 [I-D.ietf-mmusic-ice] ), and
   others do not (such as SIP Outbound [I-D.ietf-sip-outbound]), answers
   to these considerations need to be addressed by the usages
   themselves.


17.  IANA Considerations

   IANA is hereby requested to create three new registries: a STUN
   methods registry, a STUN Attributes registry, and a STUN Error Codes
   registry.





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17.1.  STUN Methods Registry

   A STUN method is a hex number in the range 0x000 - 0x3FF.  The
   encoding of STUN method into a STUN message is described in
   Section 6.

   The initial STUN methods are:

     0x000: (Reserved)
     0x001: Binding
     0x002: (Reserved; was SharedSecret)

   STUN methods in the range 0x000 - 0x1FF are assigned by IETF
   Consensus [RFC2434].  STUN methods in the range 0x200 - 0x3FF are
   assigned on a First Come First Served basis [RFC2434]

17.2.  STUN Attribute Registry

   A STUN Attribute type is a hex number in the range 0x0000 - 0xFFFF.
   STUN attribute types in the range 0x0000 - 0x7FFF are considered
   comprehension-required; STUN attribute types in the range 0x8000 -
   0xFFFF are considered comprehension-optional.  A STUN agent handles
   unknown comprehension-required and comprehension-optional attributes
   differently.

   The initial STUN Attributes types are:

     Comprehension-required range (0x0000-0x7FFF):
       0x0000: (Reserved)
       0x0001: MAPPED-ADDRESS
       0x0006: USERNAME
       0x0007: (Reserved; was PASSWORD)
       0x0008: MESSAGE-INTEGRITY
       0x0009: ERROR-CODE
       0x000A: UNKNOWN-ATTRIBUTES
       0x0014: REALM
       0x0015: NONCE
       0x0020: XOR-MAPPED-ADDRESS

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

   STUN Attribute types in the first half of the comprehension-required
   range (0x0000 - 0x3FFF) and in the first half of the comprehension-
   optional range (0x8000 - 0xBFFF) are assigned by IETF Consensus
   [RFC2434].  STUN Attribute types in the second half of the



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   comprehension-required range (0x4000 - 0x7FFF) and in the second half
   of the comprehension-optional range (0xC000 - 0xFFFF) are assigned on
   a First Come First Served basis [RFC2434].

17.3.  STUN Error Code Registry

   A STUN Error code is a number in the range 0 - 699.  STUN error codes
   are accompanied by a textual reason phrase in UTF-8 which is intended
   only for human consumption and can be anything appropriate; this
   document proposes only suggested values.

   STUN error codes are consistent in codepoint assignments and
   semantics with SIP [RFC3261] and HTTP [RFC2616].

   The initial values in this registry are given in Section 14.6.

   New STUN error codes are assigned on a Specification-Required basis
   [RFC2434].  The specification must carefully consider how clients
   that do not understand this error code will process it before
   granting the request.  See the rules in Section 7.3.4.


18.  Changes Since RFC 3489

   This specification obsoletes RFC3489 [RFC3489].  This specification
   differs from RFC3489 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.

   o  Added the XOR-MAPPED-ADDRESS attribute, which is included in
      Binding Responses if the magic cookie is present in the request.
      Otherwise the RFC3489 behavior is retained (that is, Binding



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      Response includes MAPPED-ADDRESS).  See discussion in XOR-MAPPED-
      ADDRESS regarding this change.

   o  Introduced formal structure into the Message Type header field,
      with an explicit pair of bits for indication of request, response,
      error response or indication.  Consequently, the message type
      field is split into the class (one of the previous four) and
      method.

   o  Explicitly point out that the most significant two bits of STUN
      are 0b00, allowing easy differentiation with RTP packets when used
      with ICE.

   o  Added the FINGERPRINT attribute to provide a method of definitely
      detecting the difference between STUN and another protocol when
      the two protocols are multiplexed together.

   o  Added support for IPv6.  Made it clear that an IPv4 client could
      get a v6 mapped address, and vice-a-versa.

   o  Added long-term credential-based authentication.

   o  Added the SERVER, REALM, NONCE, and ALTERNATE-SERVER attributes.

   o  Removed the SharedSecret method, and thus the PASSWORD attribute.
      This method was almost never implemented and is not needed with
      current usages.

   o  Removed recommendation to continue listening for STUN Responses
      for 10 seconds in an attempt to recognize an attack.

   o  Changed transaction timers to be more TCP friendly.

   o  Removed the STUN example that centered around the separation of
      the control and media planes.  Instead, provided more information
      on using STUN with protocols.

   o  Defined a generic padding mechanism that changes the
      interpretation of the length attribute.  This would, in theory,
      break backwards compatibility.  However, the mechanism in RFC 3489
      never worked for the few attributes that weren't aligned naturally
      on 32 bit boundaries.

   o  REALM, USERNAME, SERVER, reason phrases and NONCE limited to 127
      characters.






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19.  Acknowledgements

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


20.  References

20.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2988]  Paxson, V. and M. Allman, "Computing TCP's Retransmission
              Timer", RFC 2988, November 2000.

   [ITU.V42.1994]
              International Telecommunications Union, "Error-correcting
              Procedures for DCEs Using Asynchronous-to-Synchronous
              Conversion", ITU-T Recommendation V.42, 1994.

20.2.  Informational References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,



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              June 2002.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-16 (work in progress), June 2007.

   [RFC3489]  Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
              "STUN - Simple Traversal of User Datagram Protocol (UDP)
              Through Network Address Translators (NATs)", RFC 3489,
              March 2003.

   [I-D.ietf-behave-turn]
              Rosenberg, J., "Obtaining Relay Addresses from Simple
              Traversal Underneath NAT (STUN)",
              draft-ietf-behave-turn-03 (work in progress), March 2007.

   [I-D.ietf-sip-outbound]
              Jennings, C. and R. Mahy, "Managing Client Initiated
              Connections in the Session Initiation Protocol  (SIP)",
              draft-ietf-sip-outbound-09 (work in progress), June 2007.

   [I-D.ietf-behave-nat-behavior-discovery]
              MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using STUN", draft-ietf-behave-nat-behavior-discovery-00
              (work in progress), February 2007.

   [I-D.ietf-mmusic-ice-tcp]
              Rosenberg, J., "TCP Candidates with Interactive
              Connectivity Establishment (ICE",
              draft-ietf-mmusic-ice-tcp-03 (work in progress),
              March 2007.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,



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              October 1998.


Appendix A.  C Snippet to Determine STUN Message Types

   Given an 16-bit STUN message type value in host byte order in
   msg_type parameter, below are C macros to determine the STUN message
   types:

   #define IS_REQUEST(msg_type)       (((msg_type) & 0x0110) == 0x0000)
   #define IS_INDICATION(msg_type)    (((msg_type) & 0x0110) == 0x0010)
   #define IS_SUCCESS_RESP(msg_type)  (((msg_type) & 0x0110) == 0x0100)
   #define IS_ERR_RESP(msg_type)      (((msg_type) & 0x0110) == 0x0110)



Authors' Addresses

   Jonathan Rosenberg
   Cisco
   Edison, NJ
   US

   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net


   Christian Huitema
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: huitema@microsoft.com


   Rohan Mahy
   Plantronics
   345 Encinal Street
   Santa Cruz, CA  95060
   US

   Email: rohan@ekabal.com








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   Philip Matthews
   Avaya
   1135 Innovation Drive
   Ottawa, Ontario  K2K 3G7
   Canada

   Phone: +1 613 592 4343 x224
   Fax:
   Email: philip_matthews@magma.ca
   URI:


   Dan Wing
   Cisco
   771 Alder Drive
   San Jose, CA  95035
   US

   Email: dwing@cisco.com
































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Full Copyright Statement

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