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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                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires:  August 5, 2006                                      C. Huitema
                                                               Microsoft
                                                                 R. Mahy
                                                             Plantronics
                                                                 D. Wing
                                                           Cisco Systems
                                                           February 2006


Simple Traversal of UDP Through Network Address Translators (NAT) (STUN)
                    draft-ietf-behave-rfc3489bis-03

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on August 5, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Simple Traversal of UDP Through NATs (STUN) is a lightweight protocol
   that provides the ability for applications to determine the public IP
   addresses and ports allocated to them by the NAT and to keep NAT



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   bindings open.  These addresses and ports can be placed into protocol
   payloads where a client needs to provide a publically routable IP
   address.  STUN works with many existing NATs, and does not require
   any special behavior from them.  As a result, it allows a wide
   variety of applications to work through existing NAT infrastructure.


Table of Contents

   1.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  5
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  7
   6.  STUN Message Structure . . . . . . . . . . . . . . . . . . . .  9
   7.  STUN Transactions  . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.   Request Transaction Reliability . . . . . . . . . . . . . 11
   8.  General Client Behavior  . . . . . . . . . . . . . . . . . . . 12
     8.1.   Request Message Types . . . . . . . . . . . . . . . . . . 12
       8.1.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . 12
       8.1.2.  Obtaining a Shared Secret  . . . . . . . . . . . . . . 13
       8.1.3.  Formulating the Request Message  . . . . . . . . . . . 14
       8.1.4.  Processing Responses . . . . . . . . . . . . . . . . . 14
       8.1.5.  Using the Mapped Address . . . . . . . . . . . . . . . 15
     8.2.   Indication Message Types  . . . . . . . . . . . . . . . . 17
       8.2.1.  Formulating the Indication Message . . . . . . . . . . 17
   9.  General Server Behavior  . . . . . . . . . . . . . . . . . . . 17
     9.1.   Request Message Types . . . . . . . . . . . . . . . . . . 17
       9.1.1.  Receive Request Message  . . . . . . . . . . . . . . . 17
       9.1.2.  Constructing the Response  . . . . . . . . . . . . . . 19
       9.1.3.  Sending the Response . . . . . . . . . . . . . . . . . 19
     9.2.   Indication Message Types  . . . . . . . . . . . . . . . . 19
   10. Short-Term Passwords . . . . . . . . . . . . . . . . . . . . . 19
   11. STUN Attributes  . . . . . . . . . . . . . . . . . . . . . . . 20
     11.1.  MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . . . 21
     11.2.  RESPONSE-ADDRESS  . . . . . . . . . . . . . . . . . . . . 21
     11.3.  CHANGED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 22
     11.4.  CHANGE-REQUEST  . . . . . . . . . . . . . . . . . . . . . 22
     11.5.  SOURCE-ADDRESS  . . . . . . . . . . . . . . . . . . . . . 23
     11.6.  USERNAME  . . . . . . . . . . . . . . . . . . . . . . . . 23
     11.7.  PASSWORD  . . . . . . . . . . . . . . . . . . . . . . . . 23
     11.8.  MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 23
     11.9.  ERROR-CODE  . . . . . . . . . . . . . . . . . . . . . . . 24
     11.10. REFLECTED-FROM  . . . . . . . . . . . . . . . . . . . . . 26
     11.11. ALTERNATE-SERVER  . . . . . . . . . . . . . . . . . . . . 26
     11.12. REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     11.13. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     11.14. UNKNOWN-ATTRIBUTES  . . . . . . . . . . . . . . . . . . . 27



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     11.15. XOR-MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . 27
     11.16. SERVER  . . . . . . . . . . . . . . . . . . . . . . . . . 28
     11.17. ALTERNATE-SERVER  . . . . . . . . . . . . . . . . . . . . 28
     11.18. BINDING-LIFETIME  . . . . . . . . . . . . . . . . . . . . 29
   12. STUN Usages  . . . . . . . . . . . . . . . . . . . . . . . . . 29
     12.1.  Defined STUN Usages . . . . . . . . . . . . . . . . . . . 29
     12.2.  Binding Discovery . . . . . . . . . . . . . . . . . . . . 29
       12.2.1. Applicability  . . . . . . . . . . . . . . . . . . . . 29
       12.2.2. Client Discovery of Server . . . . . . . . . . . . . . 30
       12.2.3. Server Determination of Usage  . . . . . . . . . . . . 30
       12.2.4. New Requests or Indications  . . . . . . . . . . . . . 30
       12.2.5. New Attributes . . . . . . . . . . . . . . . . . . . . 30
       12.2.6. New Error Response Codes . . . . . . . . . . . . . . . 30
       12.2.7. Client Procedures  . . . . . . . . . . . . . . . . . . 30
       12.2.8. Server Procedures  . . . . . . . . . . . . . . . . . . 30
       12.2.9. Security Considerations for Binding Discovery  . . . . 30
     12.3.  Connectivity Check  . . . . . . . . . . . . . . . . . . . 31
       12.3.1. Applicability  . . . . . . . . . . . . . . . . . . . . 31
       12.3.2. Client Discovery of Server . . . . . . . . . . . . . . 31
       12.3.3. Server Determination of Usage  . . . . . . . . . . . . 31
       12.3.4. New Requests or Indications  . . . . . . . . . . . . . 31
       12.3.5. New Attributes . . . . . . . . . . . . . . . . . . . . 31
       12.3.6. New Error Response Codes . . . . . . . . . . . . . . . 31
       12.3.7. Client Procedures  . . . . . . . . . . . . . . . . . . 31
       12.3.8. Server Procedures  . . . . . . . . . . . . . . . . . . 32
       12.3.9. Security Considerations for Connectivity Check . . . . 32
     12.4.  NAT Keepalives  . . . . . . . . . . . . . . . . . . . . . 32
       12.4.1. Applicability  . . . . . . . . . . . . . . . . . . . . 32
       12.4.2. Client Discovery of Server . . . . . . . . . . . . . . 32
       12.4.3. Server Determination of Usage  . . . . . . . . . . . . 32
       12.4.4. New Requests or Indications  . . . . . . . . . . . . . 33
       12.4.5. New Attributes . . . . . . . . . . . . . . . . . . . . 33
       12.4.6. New Error Response Codes . . . . . . . . . . . . . . . 33
       12.4.7. Client Procedures  . . . . . . . . . . . . . . . . . . 33
       12.4.8. Server Procedures  . . . . . . . . . . . . . . . . . . 33
       12.4.9. Security Considerations for NAT Keepalives . . . . . . 33
     12.5.  Short-Term Password . . . . . . . . . . . . . . . . . . . 33
       12.5.1. Applicability  . . . . . . . . . . . . . . . . . . . . 33
       12.5.2. Client Discovery of Server . . . . . . . . . . . . . . 34
       12.5.3. Server Determination of Usage  . . . . . . . . . . . . 34
       12.5.4. New Requests or Indications  . . . . . . . . . . . . . 34
       12.5.5. New Attributes . . . . . . . . . . . . . . . . . . . . 35
       12.5.6. New Error Response Codes . . . . . . . . . . . . . . . 35
       12.5.7. Client Procedures  . . . . . . . . . . . . . . . . . . 35
       12.5.8. Server Procedures  . . . . . . . . . . . . . . . . . . 35
       12.5.9. Security Considerations for Short-Term Password  . . . 35
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 36
     13.1.  Attacks on STUN . . . . . . . . . . . . . . . . . . . . . 36



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       13.1.1. Attack I: DDoS Against a Target  . . . . . . . . . . . 36
       13.1.2. Attack II: Silencing a Client  . . . . . . . . . . . . 36
       13.1.3. Attack III: Assuming the Identity of a Client  . . . . 37
       13.1.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 37
     13.2.  Launching the Attacks . . . . . . . . . . . . . . . . . . 37
       13.2.1. Approach I: Compromise a Legitimate STUN Server  . . . 38
       13.2.2. Approach II: DNS Attacks . . . . . . . . . . . . . . . 38
       13.2.3. Approach III: Rogue Router or NAT  . . . . . . . . . . 38
       13.2.4. Approach IV: Man in the Middle . . . . . . . . . . . . 39
       13.2.5. Approach V: Response Injection Plus DoS  . . . . . . . 39
       13.2.6. Approach VI: Duplication . . . . . . . . . . . . . . . 39
     13.3.  Countermeasures . . . . . . . . . . . . . . . . . . . . . 40
     13.4.  Residual Threats  . . . . . . . . . . . . . . . . . . . . 42
   14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 42
     14.1.  Problem Definition  . . . . . . . . . . . . . . . . . . . 42
     14.2.  Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 43
     14.3.  Brittleness Introduced by STUN  . . . . . . . . . . . . . 43
     14.4.  Requirements for a Long Term Solution . . . . . . . . . . 45
     14.5.  Issues with Existing NAPT Boxes . . . . . . . . . . . . . 46
     14.6.  In Closing  . . . . . . . . . . . . . . . . . . . . . . . 46
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 47
     15.1.  STUN Message Type Registry  . . . . . . . . . . . . . . . 47
     15.2.  STUN Attribute Registry . . . . . . . . . . . . . . . . . 47
   16. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 48
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 49
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49
     18.1.  Normative References  . . . . . . . . . . . . . . . . . . 49
     18.2.  Informational References  . . . . . . . . . . . . . . . . 50
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52
   Intellectual Property and Copyright Statements . . . . . . . . . . 53





















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1.  Applicability Statement

   This protocol is not a cure-all for the problems associated with NAT.
   It does not enable incoming TCP connections through NAT.  It allows
   incoming UDP packets through NAT, but only through a subset of
   existing NAT types.  In particular, STUN does not enable incoming UDP
   packets through "symmetric NATs", which is

      a NAT where all requests from the same internal IP address and
      port, to a specific destination IP address and port, are mapped to
      the same external IP address and port.  If the same host sends a
      packet with the same source address and port, but to a different
      destination, a different mapping is used.  Furthermore, only the
      external host that receives a packet can send a UDP packet back to
      the internal host.

   This type of NAT is common in large enterprises.  STUN does not work
   when it is used to obtain an address to communicate with a peer which
   happens to be behind the same NAT.  STUN does not work when the STUN
   server is not in a common shared address realm.

   In order to work with such a NAT, a media relay such as TURN [3] is
   required.  All other types of NATs work without a media relay.

   For a more complete discussion of the limitations of STUN, see
   Section 14.


2.  Introduction

   Network Address Translators (NATs), while providing many benefits,
   also come with many drawbacks.  The most troublesome of those
   drawbacks is the fact that they break many existing IP applications,
   and make it difficult to deploy new ones.  Guidelines have been
   developed [17] that describe how to build "NAT friendly" protocols,
   but many protocols simply cannot be constructed according to those
   guidelines.  Examples of such protocols include almost all peer-to-
   peer protocols, such as multimedia communications, file sharing and
   games.

   To combat this problem, Application Layer Gateways (ALGs) have been
   embedded in NATs.  ALGs perform the application layer functions
   required for a particular protocol to traverse a NAT.  Typically,
   this involves rewriting application layer messages to contain
   translated addresses, rather than the ones inserted by the sender of
   the message.  ALGs have serious limitations, including scalability,
   reliability, and speed of deploying new applications.




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   Many existing proprietary protocols, such as those for online games
   (such as the games described in RFC3027 [18]) and Voice over IP, have
   developed tricks that allow them to operate through NATs without
   changing those NATs and without relying on ALG behavior in the NATs.
   This document takes some of those ideas and codifies them into an
   interoperable protocol that can meet the needs of many applications.

   The protocol described here, Simple Traversal of UDP Through NAT
   (STUN), provides a toolkit of functions.  These functions allow
   entities behind a NAT to learn the address bindings allocated by the
   NAT, to keep those bindings open, and communicate with other STUN-
   aware to validate connecivity.  STUN requires no changes to NATs, and
   works with an arbitrary number of NATs in tandem between the
   application entity and the public Internet.


3.  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
   [1] and indicate requirement levels for compliant STUN
   implementations.


4.  Definitions

   STUN Client
      A STUN client (also just referred to as a client) is an entity
      that generates STUN requests.

   STUN Server
      A STUN Server (also just referred to as a server) is an entity
      that receives STUN requests, and sends STUN responses.

   Transport Address
      The combination of an IP address and (UDP or TCP) port.

   Reflexive Transport Address
      A transport address learned by a client which 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 address represents the binding
      allocated to the client on the public side of the NAT.  Reflexive
      transport addresses are learned from the mapped address attribute
      (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) in STUN responses.





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   Mapped Address
      The source IP address and port of the STUN Binding Request packet
      received by the STUN server and inserted into the mapped address
      attribute (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) of the Binding
      Response message.


5.  Overview of Operation

   This section is descriptive only.  Normative behavior is described in
   Section 8 and Section .

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

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

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

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

   Figure 1: Typical STUN Server Configuration

   The typical STUN configuration is shown in Figure 1.  A STUN client
   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 STUN server resides on the public
   Internet.

   STUN is a simple client-server protocol.  Two types of messages are
   available -- request/response in which client sends a request to a
   server, and the server returns a response; and indications which can
   be initiated by the client or by the server and which do not elicit a
   response.  There are two types of requests defined in this
   specification - Binding Requests, sent over UDP, and Shared Secret
   Requests, sent over TLS [6] over TCP.  Shared Secret Requests ask the
   server to return a temporary username and password.  This username



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   and password are used in a subsequent Binding Request and Binding
   Response, for the purposes of authentication and message integrity.

   Binding requests are used to determine the bindings allocated by
   NATs.  The client sends a Binding Request to the server, over UDP.
   The server examines the source IP address and port of the request,
   and copies them into a response that is sent back to the client --
   this is the 'mapped address'.  There are attributes for providing
   message integrity and authentication.

   The STUN client is typically embedded in an application which needs
   to obtain a public IP address and port that can be used to receive
   data.  For example, it might need to obtain an IP address and port to
   receive Real Time Transport Protocol (RTP [14]) traffic.  When the
   application starts, the STUN client within the application sends a
   STUN Shared Secret Request to its server, obtains a username and
   password, and then sends it a Binding Request.  STUN servers can be
   discovered through DNS SRV records [4], and it is generally assumed
   that the client is configured with the domain to use to find the STUN
   server.  Generally, this will be the domain of the provider of the
   service the application is using (such a provider is incented to
   deploy STUN servers in order to allow its customers to use its
   application through NAT).  Of course, a client can determine the
   address or domain name of a STUN server through other means.  A STUN
   server can even be embedded within an end system.

   The STUN Binding Request is used to discover the public IP address
   and port mappings generated by the NAT.  Binding Requests are sent to
   the STUN server using UDP.  When a Binding Request arrives at the
   STUN server, it may have passed through one or more NATs between the
   STUN client and the STUN server.  As a result, the source address of
   the request received by the server will be the mapped address created
   by the NAT closest to the server.  The STUN server copies that source
   IP address and port into a STUN Binding Response, and sends it back
   to the source IP address and port of the STUN request.  Every type of
   NAT will route that response so that it arrives at the STUN client.

   When the STUN client receives the STUN Binding Response, it compares
   the IP address and port in the packet with the local IP address and
   port it bound to when the request was sent.  If these do not match,
   the STUN client knows is behind one or more NATs.  If the STUN server
   is publicly routable the IP address and port in the STUN Binding
   Response are also publicly routable, and can be used by any host on
   the public Internet to send packets to the application that sent the
   STUN request.  An application need only listen on the IP address and
   port from which the STUN request was sent.  Packets sent by a host on
   the public Internet to the public address and port learned by STUN
   will be received by the application, so long as conditions permit.



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   The conditions in which these packets will not be received by the
   client are described in Section 1.

   It should be noted that the configuration in Figure 1 is not the only
   permissible configuration.  The STUN server can be located anywhere,
   including within another client.  The only requirement is that the
   STUN server is reachable by the client, and if the client is trying
   to obtain a publicly routable address, that the server reside on the
   public Internet.


6.  STUN Message Structure

   STUN messages are TLV (type-length-value) encoded using big endian
   (network ordered) binary.  STUN messages are encoded using binary
   fields.  All integer fields are carried in network byte order, that
   is, most significant byte (octet) first.  This byte order is commonly
   known as big-endian.  The transmission order is described in detail
   in Appendix B of RFC791 [2].  Unless otherwise noted, numeric
   constants are in decimal (base 10).  All STUN messages start with a
   single STUN header followed by a STUN payload.  The payload is a
   series of STUN attributes, the set of which depends on the message
   type.  The STUN header contains a STUN message type, transaction ID,
   and length.  The length indicates the total length of the STUN
   payload, not including the 20-byte header.

   There are two categories of STUN message types:  Requests and
   Indications.

   Upon receiving a STUN request, a STUN server will send a STUN success
   response or a STUN error response.  All STUN success responses MUST
   have a type whose value is 0x100 higher than their associated
   request, and all STUN error responses MUST have a type whose value is
   0x110 higher than their associated request.  Any newly defined STUN
   message types MUST use message type values 0x100 and 0x110 higher for
   their success and error responses, respectively.  STUN Requests are
   sent reliably (Section 7.1).  The transaction ID is used to correlate
   requests and responses.

   An indication message can be sent from the client to the server, or
   from the server to the client.  Indication messages are not sent
   reliably do not have an associated success response message type or
   associated error response message type.  Indication messages can be
   sent by the STUN client to the server, or from the STUN server to the
   client.  The transaction ID is used to distinguish indication
   messages.





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   All STUN messages consist of a 20 byte header:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2: Format of STUN Message Header

   The most significant two bits of every STUN message are 0b00.  This,
   combined with the magic cookie, aids in differentiating STUN packets
   from other protocols when STUN is multiplexed with other protocols on
   the same port.

   The STUN message types Binding Request, Response, and Error Response
   are defined in Section 8 and Section 9.1.  The Shared Secret Request,
   Response, and Error Response are described in Section 12.5.  Their
   values are enumerated in Section 15.

   The message length is the size, in bytes, of the message not
   including the 20 byte STUN header.

   The magic cookie is a fixed value, 0x2112A442.  In the previous
   version of this specification [13] this field was part of the
   transaction ID.  This fixed value affords easy identification of a
   STUN message when STUN is multiplexed with other protocols on the
   same port, as is done for example in [12] and [15].  The magic cookie
   additionally indicates the STUN client is compliant with this
   specification.  The magic cookie is present in all STUN messages --
   requests, success responses and error responses.

   The transaction ID is a 96 bit identifier.  STUN transactions are
   identified by their unique 96-bit transaction ID.  This transaction
   ID is chosen by the STUN client and MUST be unique for each new STUN
   transaction by that STUN client.  Any two requests that are not bit-
   wise identical, and not sent to the same server from the same IP
   address and port, MUST have a different transaction ID.  The
   transaction ID MUST be uniformly and randomly distributed between 0
   and 2**96 - 1.  The large range is needed because the transaction ID



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   serves as a form of randomization, helping to prevent replays of
   previously signed responses from the server.

   After the STUN header are zero or more attributes.  Each attribute is
   TLV encoded, with a 16 bit type, 16 bit length, and variable value:

      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                 ....        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: Format of STUN Attributes

   The attribute types defined in this specification are in Section 11 .


7.  STUN Transactions

   STUN clients are allowed to pipeline STUN requests.  That is, a STUN
   client MAY have multiple outstanding STUN requests with different
   transaction IDs and not wait for completion of a STUN request/
   response exchange before sending another STUN request.

7.1.  Request Transaction Reliability

   When running STUN over UDP it is possible that the STUN request or
   its response might be dropped by the network.  Reliability of STUN
   request message types is is accomplished through client
   retransmissions.  Clients SHOULD retransmit the request starting with
   an interval of 100ms, doubling every retransmit until the interval
   reaches 1.6 seconds.  Retransmissions continue with intervals of 1.6
   seconds until a response is received, or a total of 9 requests have
   been sent.  If no response is received by 1.6 seconds after the last
   request has been sent, the client SHOULD consider the transaction to
   have failed.  In other words, requests would be sent at times 0ms,
   100ms, 300ms, 700ms, 1500ms, 3100ms, 4700ms, 6300ms, and 7900ms.  At
   9500ms, the client considers the transaction to have failed if no
   response has been received.

   When running STUN over TCP, TCP is responsible for ensuring delivery.
   The STUN application SHOULD NOT retransmit STUN requests when running
   over TCP.

   For STUN requests, failure occurs if there is a transport failure of
   some sort (generally, due to fatal ICMP errors in UDP or connection



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   failures in TCP) or if retransmissions of the same STUN Request
   doesn't elicit a Response.  If a failure occurs and the SRV query
   indicated other STUN servers are available, the client SHOULD create
   a new request, which is identical to the previous, but has a
   different transaction ID and MESSAGE INTEGRITY attribute (the HMAC
   will change because the transaction ID has changed).  That request is
   sent to the next element in the list as specified by RFC2782.

   The Indication message types are not sent reliably.


8.  General Client Behavior

   There are two classes of client behavior -- one for the request
   message types and another for the indication message types.

8.1.  Request Message Types

   This section applies to client behavior for the Request message types
   -- Binding Request and Shared Secret Request.  For Request message
   types, the client must discover the STUN server's address and port,
   obtain a shared secret, formulate the Request, transmit the request
   reliability, process the Binding Response, and use the information in
   the Response.

8.1.1.  Discovery

   Unless stated otherwise by a STUN usage, DNS is used to discover the
   STUN server following these procedures.

   The client will be configured with a domain name of the provider of
   the STUN servers.  This domain name is resolved to an IP address and
   port using the SRV procedures specified in RFC2782 [4].  The
   mechanism for configuring the STUN client with the domain name to
   look up is not in scope of this document.

   The DNS SRV service name is "stun".  The protocol is "udp" for
   sending Binding Requests, or "tcp" for sending Shared Secret
   Requests.  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; those details for STUN are described in Section 8.1.3.

   The default port for STUN requests is 3478, for both TCP and UDP.
   Administrators SHOULD use this port in their SRV records, but MAY use
   others.  If no SRV records were found, the client performs an A or



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   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.

8.1.2.  Obtaining a Shared Secret

   As discussed in Section 13, there are several attacks possible on
   STUN systems.  Many of these attacks are prevented through integrity
   protection of requests and responses.  To provide that integrity,
   STUN makes use of a shared secret between client and server which is
   used as the keying material for the MESSAGE-INTEGRITY attribute in
   STUN messages.  STUN allows for the shared secret to be obtained in
   any way (for example Kerberos [16] or ICE [12]).  The shared secret
   MUST have at least 128 bits of randomness.

   When a client is needs to send a Request or an Indication, it can do
   one of three things:

   1.  send the message without MESSAGE-INTEGRITY, if permitted by the
       STUN usage.

   2.  use a short term credential, as determined by the STUN usage.  In
       this case, the STUN Request or STUN Indication would contain the
       USERNAME and MESSAGE-INTEGRITY attributes.  The message would not
       contain the NONCE attribute.  The key for MESSAGE-INTEGRITY is
       the password.

   3.  use long term credential, as determined by STUN usage.  In this
       case, the STUN request contains the USERNAME, REALM, and MESSAGE-
       INTEGRITY attributes.  The request does not contain the NONCE
       attribute.  The key for MESSAGE-INTEGRITY is MD5(unq(USERNAME-
       value) ":" unq(REALM-value) ":" password).

   Based on the STUN usage, the server does one of four things:

   1.  The server processes the request and generates a response.  If
       the request included the MESSAGE-INTEGRITY attribute, the server
       would also include MESSAGE-INTEGRITY in its response.

   2.  The server generates an error response indicating that MESSAGE-
       INTEGRITY with short-term or with long-term credentials are
       required (error 401).  To indicate that short-term credentials
       are required, the REALM attribute MUST NOT be present in the
       error response.  To indicate short-term credentials are required,
       the REALM attribute MOST be present in the error response.

   3.  The server generates an error response indicating that a NONCE
       attribute is required (error 435) or that the supplied NONCE
       attribute's value is stale (error 437).



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   4.  The server generates an error response indicating that the short-
       term credentials are no longer valid (error 430).  The client
       will have to obtain new short-term credentials appropriate to its
       STUN usage.

   In all of the above error responses, the NONCE attribute MAY
   optionally be included in the error response, in which case the
   client MUST include that NONCE in the subsequent STUN transaction.
   The NONCE value is not stored by the STUN client; it is only valid
   for the subsequent STUN transaction and that transactions
   retransmissions.

   STUN messages generated in order to obtain the shared secret are
   formulated like other messages by following Section 8.1.3.

8.1.3.  Formulating the Request Message

   The client follows the syntax rules defined in Section 6 and the
   transmission rules of Section 7.  The message type of the MUST be a
   request type; "Binding Request" or "Shared Secret Request" are the
   two defined by this document.

   The client creates a STUN message following the STUN message
   structure described in Section 6.  The client SHOULD add a MESSAGE-
   INTEGRITY and USERNAME attribute to the Request message.

   Once formulated, the client sends the Binding Request.  Reliability
   is accomplished through client retransmissions, following the
   procedure in Section 7.1.

   The client MAY send multiple requests on the connection, and it may
   pipeline requests (that is, it can have multiple requests outstanding
   at the same time).  When using TCP the client SHOULD close the
   connection as soon as it has received the STUN Response.

8.1.4.  Processing Responses

   All responses, whether success responses or error responses, MUST
   first be authenticated by the client.  Authentication is performed by
   first comparing the Transaction ID of the response to an oustanding
   request.  If there is no match, the client MUST discard the response.
   Then the client SHOULD check the response for a MESSAGE-INTEGRITY
   attribute.  If not present, and the client placed a MESSAGE-INTEGRITY
   attribute into the associated request, it MUST discard the response.
   If MESSAGE-INTEGRITY is present, the client computes the HMAC over
   the response as described in Section 11.8.  The key to use depends on
   the shared secret mechanism.  If the STUN Shared Secret Request was
   used, the key MUST be same as used to compute the MESSAGE-INTEGRITY



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   attribute in the request.

   If the computed HMAC matches the one from the response, processing
   continues.  The response can either be a Binding Response or Binding
   Error Response.

   If the response is an Error Response, the client checks the response
   code from the ERROR-CODE attribute of the response.  For a 400
   response code, the client SHOULD display the reason phrase to the
   user.  For a 420 response code, the client SHOULD retry the request,
   this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
   attribute of the response.  For a 430 response code, the client
   SHOULD obtain a new one-time username and password, and retry the
   Allocate Request with a new transaction.  For 401 and 432 response
   codes, if the client had omitted the USERNAME or MESSAGE-INTEGRITY
   attribute as indicated by the error, it SHOULD try again with those
   attributes.  A new one-time username and password is needed in that
   case.  For a 431 response code, the client SHOULD alert the user, and
   MAY try the request again after obtaining a new username and
   password.  For a 300 response code, the client SHOULD attempt a new
   transaction to the server indicated in the ALTERNATE-SERVER
   attribute.  For a 500 response code, the client MAY wait several
   seconds and then retry the request with a new username and password.
   For a 600 response code, client MUST NOT retry the request and SHOULD
   display the reason phrase to the user.  Unknown response codes
   between 400 and 499 are treated like a 400, unknown response codes
   between 500 and 599 are treated like a 500, and unknown response
   codes between 600 and 699 are treated like a 600.  Any response
   between 100 and 299 MUST result in the cessation of request
   retransmissions, but otherwise is discarded.

   Binding Responses containing unknown optional attributes (greater
   than 0x7FFF) MUST be ignored by the STUN client.  Binding Responses
   containing unknown mandatory attributions (less than or equal to
   0x7FFF) MUST be discarded and considered immediately as a failed
   transaction.

   It is also possible for an IPv4 host to receive a XOR-MAPPED-ADDRESS
   or MAPPED-ADDRESS containing an IPv6 address, or for an IPv6 host to
   receive a XOR-MAPPED-ADDRESS or MAPPED-ADDRESS containing an IPv4
   address.  Clients MUST be prepared for this case.

8.1.5.  Using the Mapped Address

   This section applies to the Binding Response message type.  The
   Binding Response message type always includes either the MAPPED-
   ADDRESS attribute or the XOR-MAPPED-ADDRESS attribute, depending on
   the presence of the magic cookie in the corresponding Binding



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   Request.

   The mapped address present in the binding response can be used by
   clients to facilitate traversal of NATs for many applications.  NAT
   traversal is problematic for applications which require a client to
   insert an IP address and port into a message, to which subsequent
   messages will be delivered by other entities in a network.  Normally,
   the client would insert the IP address and port from a local
   interface into the message.  However, if the client is behind a NAT,
   this local interface will be a private address.  Clients within other
   address realms will not be able to send messages to that address.

   An example of a such an application is SIP, which requires a client
   to include IP address and port information in several places,
   including the Session Description Protocol (SDP [19]) body carried by
   SIP.  The IP address and port present in the SDP is used for receipt
   of media.

   To use STUN as a technique for traversal of SIP and other protocols,
   when the client wishes to send a protocol message, it figures out the
   places in the protocol data unit where it is supposed to insert its
   own IP address along with a port.  Instead of directly using a port
   allocated from a local interface, the client allocates a port from
   the local interface, and from that port, initiates the STUN
   procedures described above.  The mapped address in the Binding
   Response (XOR-MAPPED-ADDRESS or MAPPED- ADDRESS) provides the client
   with an alternative IP address and port which it can then include in
   the protocol payload.  This IP address and port may be within a
   different address family than the local interfaces used by the
   client.  This is not an error condition.  In such a case, the client
   would use the learned IP address and port as if the client was a host
   with an interface within that address family.

   In the case of SIP, to populate the SDP appropriately, a client would
   generate two STUN Binding Request messages at the time a call is
   initiated or answered.  One is used to obtain the IP address and port
   for RTP, and the other, for the Real Time Control Protocol
   (RTCP)[14].  The client might also need to use STUN to obtain IP
   addresses and ports for usage in other parts of the SIP message.  The
   detailed usage of STUN to facilitate SIP NAT traversal is outside the
   scope of this specification.

   As discussed above, the addresses learned by STUN may not be usable
   with all entities with whom a client might wish to communicate.  The
   way in which this problem is handled depends on the application
   protocol.  The ideal solution is for a protocol to allow a client to
   include a multiplicity of addresses and ports in the PDU.  One of
   those can be the address and port determined from STUN, and the



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   others can include addresses and ports learned from other techniques.
   The application protocol would then provide a means for dynamically
   detecting which one works.  An example of such an an approach is
   Interactive Connectivity Establishment (ICE [12]).

8.2.  Indication Message Types

   This section applies to client behavior for the Indication message
   types.

8.2.1.  Formulating the Indication Message

   The client follows the syntax rules defined in Section 6 and the
   transmission rules of Section 7.  The message type MUST be one of the
   Indication message types; none are defined by this document.

   The client creates a STUN message following the STUN message
   structure described in Section 6.  The client SHOULD add a MESSAGE-
   INTEGRITY and USERNAME attribute to the Request message.

   Once formulated, the client sends the Indication message.  Indication
   message types are not sent reliably, do not elicit a response from
   the server, and are not retransmitted.

   The client MAY send multiple indications on the connection, and it
   may pipeline indication messages.  When using TCP the client SHOULD
   close the TCP connection as soon as it has transmitted the indication
   message.


9.  General Server Behavior

9.1.  Request Message Types

   The server behavior for receiving request message types is described
   in this section.

9.1.1.  Receive Request Message

   A STUN server MUST be prepared to receive Request and Indication
   messages on the IP address and UDP or TCP port that will be
   discovered by the STUN client when the STUN client follows its
   discovery procedures described in Section 8.1.1.  Depending on the
   usage, the STUN server will listen for incoming UDP STUN messages,
   incoming TCP STUN messages, or incoming TLS exchanges followed by TCP
   STUN messages.  The usages describe how the STUN server determines
   the usage.




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   The server checks the request for a MESSAGE-INTEGRITY attribute.  If
   not present, the server generates an error response with an ERROR-
   CODE attribute and a response code of 401.  That error response MUST
   include a NONCE attribute, containing a nonce that the server wishes
   the client to reflect back in a subsequent request (and therefore
   include in the message integrity computation).  The error response
   MUST include a REALM attribute, containing a realm from which the
   username and password are scoped [8].

   If the MESSAGE-INTEGRITY attribute was present, the server checks for
   the existence of the REALM attribute.  If the attribute is not
   present, the server MUST generate an error response.  That error
   response MUST include an ERROR-CODE attribute with response code of
   434.  That error response MUST also include a NONCE and a REALM
   attribute.

   If the REALM attribute was present, the server checks for the
   existence of the NONCE attribute.  If the NONCE attribute is not
   present, the server MUST generate an error response.  That error
   response MUST include an ERROR-CODE attribute with a response code of
   435.  That error response MUST include a NONCE attribute and a REALM
   attribute.

   If the NONCE attribute was present, the server checks for the
   existence of the USERNAME attribute.  If it was not present, the
   server MUST generate an error response.  The error response MUST
   include an ERROR-CODE attribute with a response code of 432.  It MUST
   include a NONCE attribute and a REALM attribute.

   If the USERNAME attribute was present, the server computes the HMAC
   over the request as described in Section 11.8.  The key is computed
   as MD5(unq(USERNAME-value) ":" unq(REALM-value) ":" password), where
   the password is the password associated with the username and realm
   provided in the request.  If the server does not have a record for
   that username within that realm, the server generates an error
   response.  That error response MUST include an ERROR-CODE attribute
   with a response code of 436.  That error response MUST include a
   NONCE attribute and a REALM attribute.

      This format for the key was chosen so as to enable a common
      authentication database for SIP and STUN, as it is expected that
      credentials are usually stored in their hashed forms.

   If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
   attribute in the request, the server MUST generate an error response
   with an ERROR-CODE attribute with a response code of 431.  This
   response MUST include a NONCE attribute and a REALM attribute.




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   If the computed HMAC doesn't differ from the one in the request, but
   the nonce is stale, the server MUST generate an error response.  That
   error response MUST include an ERROR-CODE attribute with response
   code 430.  That error response MUST include a NONCE attribute and a
   REALM attribute.

   The server MUST check for any mantadory attributes in the request
   (values less than or equal to 0x7fff) which it does not understand.
   If it encounters any, the server MUST generate a Binding Error
   Response, and it MUST include an ERROR-CODE attribute with a 420
   response code.  Any attributes that are known, but are not supposed
   to be present in a message (MAPPED-ADDRESS in a request, for example)
   MUST be ignored.

9.1.2.  Constructing the Response

   To construct the STUN Response the STUN server follows the message
   structure described in Section 6.  The server then copies the
   Transaction ID from the Request to the Response.  If the STUN
   response is a success response, the STUN server adds 0x100 to the
   Message Type; if a failure response the STUN server adds 0x110 to the
   Message Type.

   Depending in the Request message type and the message attributes of
   the request, the response is constructed; see Figure 4.

9.1.3.  Sending the Response

   All Response messages are sent to the IP address and port the
   associated Binding Request came from, and sent from the IP address
   and port the Binding Request was sent to.

9.2.  Indication Message Types

   Indication messages cause the server to change its state.  Indication
   message types to not cause the server to send a response message.

   Indication message types are defined in other documents, for example
   in [3].


10.  Short-Term Passwords

   Short-term passwords are useful to provide authentication and
   integrity protection to STUN Request and STUN Response messages.
   Short-term passwords are useful when there is no long-term
   relationship with a STUN server and thus no long-term password is
   shared between the STUN client and STUN server.  Even if there is a



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   long-term password, the issuance of a short-term password is useful
   to prevent dictionary attacks.

   Short-term passwords can be used multiple times for as long as a
   usage allows the same short-term password to be used.  The duration
   of validity is determined by usage.


11.  STUN Attributes

   To allow future revisions of this specification to add new attributes
   if needed, the attribute space is divided into optional and mandatory
   ones.  Attributes with values greater than 0x7fff are optional, which
   means that the message can be processed by the client or server even
   though the attribute is not understood.  Attributes with values less
   than or equal to 0x7fff are mandatory to understand, which means that
   the client or server cannot successfully process the message unless
   it understands the attribute.

   In order to align attributes on word boundaries, the length of the
   all message attributes values MUST be 0 or a multiple of 4 bytes.
   Extensions to this specification MUST also follow this requirement.

   The values of the message attributes are enumerated in Section 15.

   The following figure indicates which attributes are present in which
   messages.  An M indicates that inclusion of the attribute in the
   message is mandatory, O means its optional, C means it's conditional
   based on some other aspect of the message, and - means that the
   attribute is not applicable to that message type.

                                                     Error
               Attribute         Request  Response Response
               ______________________________________________
               MAPPED-ADDRESS       -        C         -
               USERNAME             O        -         -
               PASSWORD             -        -         -
               MESSAGE-INTEGRITY    O        O         O
               ERROR-CODE           -        -         M
               ALTERNATE-SERVER     -        -         C
               REALM                C        C         C
               NONCE                C        -         C
               UNKNOWN-ATTRIBUTES   -        -         C
               XOR-MAPPED-ADDRESS   -        M         -
               XOR-ONLY             O        -         -
               SERVER               -        O         O
               BINDING-LIFETIME     -        O         -




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   Figure 4: Mandatory Attributes and Message Types

11.1.  MAPPED-ADDRESS

   The MAPPED-ADDRESS attribute indicates the mapped IP address and
   port.  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 is 32 bits.  If the
   address family is IPv6, the address is 128 bits.

   For backwards compatibility with RFC3489-compliant STUN clients, if
   the magic cookie was not present in the associated Binding Request,
   this attribute MUST be present in the associated response.

      Discussion:  Some NATs rewrite the 32-bit binary payloads
      containing the NAT's public IP address, such as STUN's MAPPED-
      ADDRESS attribute.  Such behavior interferes with the operation of
      STUN and also causes failure of STUN's message integrity checking.

      Presence of the magic cookie in the STUN Request indicates the
      client is compatible with this specification and is capable of
      processing XOR-MAPPED-ADDRESS.

   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |x x x x x x x x|    Family     |           Port                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Address  (variable)
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: Format of MAPPED-ADDRESS attribute

   The address family can take on the following values:
       0x01:  IPv4
       0x02:  IPv6

   The port is a network byte ordered representation of the port the
   Binding Request arrived from.

   The first 8 bits of the MAPPED-ADDRESS are ignored for the purposes
   of aligning parameters on natural boundaries.

11.2.  RESPONSE-ADDRESS

   The RESPONSE-ADDRESS attribute indicates where the response to a



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   Binding Request should be sent.  Its syntax is identical to MAPPED-
   ADDRESS.

   This attribute is not used by any STUN usages defined in this
   document except for backwards compatibility with RFC3489 clients when
   using the Binding Discovery usage (Section 12.2).  Section 12.2.8
   describes when this attribute must be included in a binding response.

      Issue:  should this attribute be made specific to Binding
      Discovery or moved to another document entirely.

11.3.  CHANGED-ADDRESS

   The CHANGED-ADDRESS attribute indicates the IP address and port where
   responses would have been sent from if the "change IP" and "change
   port" flags had been set in the CHANGE-REQUEST attribute of the
   Binding Request.  Its syntax is identical to MAPPED-ADDRESS.

   This attribute is not used by any STUN usages defined in this
   document except for backwards compatibility with RFC3489 clients when
   using the Binding Discovery usage (Section 12.2).  Section 12.2.8
   describes when this attribute must be included in a binding response.

      Issue:  should this attribute be made specific to Binding
      Discovery or moved to another document entirely.

11.4.  CHANGE-REQUEST

   The CHANGE-REQUEST attribute is used by the client to request that
   the server use a different address and/or port when sending the
   response.  The attribute is 32 bits long, although only two bits (A
   and B) are used:

    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 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 A B 0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The meaning of the flags are:

   A: This is the "change IP" flag.  If true, it requests the server to
      send the Binding Response with a different IP address than the one
      the Binding Request was received on.







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   B: This is the "change port" flag.  If true, it requests the server
      to send the Binding Response with a different port than the one
      the Binding Request was received on.

   This attribute is not used by any STUN usages defined in this
   document.

      Issue:  should this attribute be made specific to Binding
      Discovery or moved to another document entirely.

11.5.  SOURCE-ADDRESS

   The SOURCE-ADDRESS attribute is present in Binding Responses.  It
   indicates the source IP address and port that the server is sending
   the response from.  Its syntax is identical to that of MAPPED-
   ADDRESS.

   This attribute is not used by any STUN usages defined in this
   document except for backwards compatibility with RFC3489 clients when
   using the Binding Discovery usage (Section 12.2).  Section 12.2.8
   describes when this attribute must be included in a binding response.

      Issue:  should this attribute be made specific to Binding
      Discovery or moved to another document entirely.

11.6.  USERNAME

   The USERNAME attribute is used for message integrity.  It identifies
   the shared secret used in the message integrity check.  The USERNAME
   is always present in a Shared Secret Response, along with the
   PASSWORD.  When message integrity is used with Binding Request
   messages, the USERNAME attribute MUST be included.

   The value of USERNAME is a variable length opaque value.

11.7.  PASSWORD

   If the message type is Shared Secret Response it MUST include the
   PASSWORD attribute.

   The value of PASSWORD is a variable length opaque value.  The
   password returned in the Shared Secret Response is used as the HMAC
   in the MESSAGE-INTEGRITY attribute of a subsequent STUN transaction.

11.8.  MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [9] of the STUN
   message.  The MESSAGE-INTEGRITY attribute can be present in any STUN



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   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.  That text is then padded with zeroes so as to
   be a multiple of 64 bytes.  As a result, the MESSAGE-INTEGRITY
   attribute is the last attribute in any STUN message.  However, STUN
   clients MUST be able to successfully parse and process STUN messages
   which have additional attributes after the MESSAGE-INTEGRITY
   attribute.  STUN clients that are compliant with this specification
   SHOULD ignore attributes that are after the MESSAGE-INTEGRITY
   attribute.

   The key used as input to HMAC depends on the STUN usage and the
   shared secret mechanism.

11.9.  ERROR-CODE

   The ERROR-CODE attribute is present in the Binding Error Response and
   Shared Secret Error Response.  It is a numeric value in the range of
   100 to 699 plus a textual reason phrase encoded in UTF-8, and is
   consistent in its code assignments and semantics with SIP [10] and
   HTTP [11].  The reason phrase is meant for user consumption, and can
   be anything appropriate for the response code.  The length of the
   reason phrase MUST be a multiple of 4 (measured in bytes),
   accomplished by added spaces to the end of the text, if necessary.
   Recommended reason phrases for the defined response codes are
   presented below.

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

   The class represents the hundreds digit of the response code.  The
   value MUST be between 1 and 6.  The number represents the response
   code modulo 100, and its value MUST be between 0 and 99.

   The following response codes, along with their recommended reason
   phrases (in brackets) are defined at this time:






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   300  (Try Alternate):  The client should contact an alternate server
        for this request.

   400  (Bad Request):  The request was malformed.  The client should
        not retry the request without modification from the previous
        attempt.

   401  (Unauthorized):  The Binding Request did not contain a MESSAGE-
        INTEGRITY attribute.

   420  (Unknown Attribute):  The server did not understand a mandatory
        attribute in the request.

   430  (Stale Credentials):  The Binding Request did contain a MESSAGE-
        INTEGRITY attribute, but it used a shared secret that has
        expired.  The client should obtain a new shared secret and try
        again.

   431  (Integrity Check Failure):  The Binding Request contained a
        MESSAGE-INTEGRITY attribute, but the HMAC failed verification.
        This could be a sign of a potential attack, or client
        implementation error.

   432  (Missing Username):  The Binding Request contained a MESSAGE-
        INTEGRITY attribute, but not a USERNAME attribute.  Both
        USERNAME and MESSAGE-INTEGRITY must be present for integrity
        checks.

   433  (Use TLS):  The Shared Secret request has to be sent over TLS,
        but was not received over TLS.

   434  (Missing Realm):  The REALM attribute was not present in the
        request.

   435  (Missing Nonce):  The NONCE attribute was not present in the
        request.

   436  (Unknown Username):  The USERNAME supplied in the Request is not
        known or is not known in the given REALM.

   437  (Stale Nonce):  The NONCE attribute was present in the request
        but wasn't valid.

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






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   600  (Global Failure):  The server is refusing to fulfill the
        request.  The client should not retry.

      Issue:  Do 300/500/600 mean that other STUN servers returned in
      the same SRV lookup should be retried / not retried?  With same
      SRV Priority?

11.10.  REFLECTED-FROM

   The REFLECTED-FROM attribute is present only in Binding Responses,
   when the Binding Request contained a RESPONSE-ADDRESS attribute.  The
   attribute contains the identity (in terms of IP address) of the
   source where the request came from.  Its purpose is to provide
   traceability, so that a STUN server cannot be used as a reflector for
   denial-of-service attacks.  Its syntax is identical to the MAPPED-
   ADDRESS attribute.

   This attribute is not used by any STUN usages defined in this
   document.

      Issue:  should this attribute be made specific to Binding
      Discovery or moved to another document entirely.

11.11.  ALTERNATE-SERVER

   The alternate server represents an alternate IP address and port for
   a different TURN server to try.  It is encoded in the same way as
   MAPPED-ADDRESS.

11.12.  REALM

   The REALM attribute is present in Requests and Responses.  It
   contains text which meets the grammar for "realm" as described in
   RFC3261 [10], and will thus contain a quoted string (including the
   quotes).

   Presence of the REALM attribute indicates that long-term credentials
   are used for the values of the USERNAME, PASSWORD, and MESSAGE-
   INTEGRITY attributes.

11.13.  NONCE

   The NONCE attribute is present in Requests and in Error responses.
   It contains a sequence of qdtext or quoted-pair, which are defined in
   RFC3261 [10].






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11.14.  UNKNOWN-ATTRIBUTES

   The UNKNOWN-ATTRIBUTES attribute is present only in a Binding Error
   Response or Shared Secret Error Response when the response code in
   the ERROR-CODE attribute is 420.

   The attribute contains a list of 16 bit values, each of which
   represents an attribute type that was not understood by the server.
   If the number of unknown attributes is an odd number, one of the
   attributes MUST be repeated in the list, so that the total length of
   the list is a multiple of 4 bytes.

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

   Figure 9: Format of UNKNOWN-ATTRIBUTES attribute

11.15.  XOR-MAPPED-ADDRESS

   The XOR-MAPPED-ADDRESS attribute is only present in Binding
   Responses.  It provides the same information that would present in
   the MAPPED-ADDRESS attribute but because the NAT's public IP address
   is obfuscated through the XOR function, STUN messages are able to
   pass through NATs which would otherwise interfere with STUN.  See the
   discussion in Section 11.1.

   This attribute MUST always be present in a Binding Response.

      Note:  Version -02 of this Internet Draft used 0x8020 for this
      attribute, which was in the Optional range of attributes.  This
      attribute has been moved back to 0x0020 as a Mandatory attribute.
      [This paragraph should be removed prior to publication as an RFC.]














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   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 10: Format of XOR-MAPPED-ADDRESS Attribute

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

   X-Port is the mapped port, exclusive or'd with most significant 16
   bits of the magic cookie.  If the IP address family is IPv4,
   X-Address is 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.

      Issue:  The motivation for XORing the IP address is clear.  Is
      there a motivation for XORing the port?

   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.

11.16.  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 length of
   the server attribute MUST be a multiple of 4 (measured in bytes),
   accomplished by added spaces to the end of the text, if necessary.
   The value of SERVER is variable length.

11.17.  ALTERNATE-SERVER

   The alternate server represents an alternate IP address and port for
   a different STUN server to try.  It is encoded in the same way as
   MAPPED-ADDRESS.

   This attribute is MUST only appear in an Error Response.  This
   attribute MUST only appear when using the TURN usage.



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11.18.  BINDING-LIFETIME

   The binding lifetime indicates the number of seconds the NAT binding
   will be valid.  This attribute MUST only be present in Response
   messages.  This attribute MUST NOT be present unless the STUN server
   is aware of the minimum binding lifetime of all NATs on the path
   between the STUN client and the STUN server.


12.  STUN Usages

   STUN is a simple request/response protocol that provides a useful
   capability in several situations.  In this section, different usages
   of STUN are described.  Each usages may differ in how STUN servers
   are discovered, the message types, and the message attributes that
   are supported.

   This specification defines the STUN usages for binding discovery
   (Section 12.2), connectivity check (Section 12.3), NAT keepalives
   (Section 12.4) and short-term password (Section 12.5).

   New STUN usages may be defined by other standards-track documents.
   New STUN usages MUST describe their applicability, client discovery
   of the STUN server, how the server determines the usage, new message
   types (requests or indications), new message attributes, new error
   response codes, and new client and server procedures.

12.1.  Defined STUN Usages

12.2.  Binding Discovery

   The previous version of this specification, RFC3489 [13], described
   only this binding discovery usage.

12.2.1.  Applicability

   Binding discovery is useful to learn reflexive addresses from servers
   on the network.  That is, it is used to determine your dynamically-
   bound 'public' IP address and UDP port that is assigned by a NAT
   between a STUN client and a STUN server.  This usage is used with ICE
   [12].

   When short-term passwords are used with binding discovery, the
   username and password are valid for subsequent transactions for nine
   (9) minutes.






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12.2.2.  Client Discovery of Server

   The general client discovery of server behavior is sufficient for
   this usage.

12.2.3.  Server Determination of Usage

   The general binding server behavior is sufficient for this usage.

12.2.4.  New Requests or Indications

   This usage does not define any new message types.

12.2.5.  New Attributes

   This usage does not define any new message attributes.

12.2.6.  New Error Response Codes

   This usage does not define any new error response codes.

12.2.7.  Client Procedures

   This usage does not define any new client procedures.

12.2.8.  Server Procedures

   In this usage, the short-term password is valid for 30 seconds after
   its initial assignment.

   For backwards compatibility with RFC3489-compliant STUN servers, if
   the STUN server receives a Binding request without the magic cookie,
   the STUN server MUST include the following attributes in the Binding
   response; otherwise these attribute MUST NOT be included:
     MAPPED-ADDRESS
     SOURCE-ADDRESS

   Likewise if the STUN server receives a Binding Request containing the
   CHANGE-REQUEST attribute without the magic cookie, the STUN server
   MUST include the CHANGED-ADDRESS attribute in its Binding Response.

12.2.9.  Security Considerations for Binding Discovery

      Issue:  Currently, the security considerations applies to all the
      various usages.  Split it up to talk about each one?  Create
      subsections talking about each usage?





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12.3.  Connectivity Check

12.3.1.  Applicability

   This STUN usage primarily provides a connectivity check to a peer
   discovered through rendezvous protocols and additionally allows
   learning reflexive address discovery to the peer.

   The username and password exchanged in the rendezvous protocol is
   valid for the duration of the connection being checked.

12.3.2.  Client Discovery of Server

   The client does not follow the general procedure in Section 8.1.1.
   Instead, the client discovers the STUN server's IP address and port
   through a rendezvous protocol such as Session Description Protocol
   (SDP [19]).  An example of such a discovery technique is ICE [12].

12.3.3.  Server Determination of Usage

   The server is aware of this usage because it signalsed this port
   through the rendezvous protocol.

   When operating in this usage, the STUN server is listening on an
   ephemeral port rather than the IANA-assigned STUN port.  The server
   is typically multiplexing two protocols on this port, one protocol is
   STUN and the other protocol is the peer-to-peer protocol using that
   same port.  When used with ICE, the two protocols multiplexed on the
   same port are STUN and RTP [14].

12.3.4.  New Requests or Indications

   This usage does not define any new message types.

12.3.5.  New Attributes

   This usage does not define any new message attributes.

12.3.6.  New Error Response Codes

   This usage does not define any new error response codes.

12.3.7.  Client Procedures

   This usage does not define any new client procedures.






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12.3.8.  Server Procedures

   In this usage, the short-term password is valid as long as the UDP
   port is listening for STUN packets.  For example when used with ICE,
   the short-term password would be valid as long as the RTP session
   (which multiplexes STUN and RTP) is active.

12.3.9.  Security Considerations for Connectivity Check

   The username and password, which are used for STUN's message
   integrity, are exchanged in the rendezvous protocol.  Failure to
   encrypt and integrity protect the rendezvous protocol is equivalent
   in risk to using STUN without message integrity.

12.4.  NAT Keepalives

12.4.1.  Applicability

   This usage is useful in two cases:  keeping a NAT binding open in a
   client connection to a server and detecting server failure and NAT
   reboots.

   The username and password used for STUN integrity can be used for 24
   hours.

      Issue:  do we need message integrity for keepalives when doing
      STUN and SIP on the same port?  Do we need message integrity for
      keepalives when doing STUN and RTP on the same port (recvonly,
      inactive)

      If yes, do we continue using same STUN username/password forever
      (days?)

12.4.2.  Client Discovery of Server

   In this usage, the STUN server and the application protocol are using
   the same fixed port.  While the multiplexing of two applications on
   the same port is similar to the connectivity check (Section 12.3)
   usage, this usage is differs as the server's port is fixed and the
   server's port isn't communicated using a rendezvous protocol.

12.4.3.  Server Determination of Usage

   The server multiplexes both STUN and its application protocol on the
   same port.  The server knows it is has this usage because the URI
   that gets resolved to this port indicates the server supports this
   multiplexing.




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12.4.4.  New Requests or Indications

   This usage does not define any new message types.

12.4.5.  New Attributes

   This usage does not define any new message attributes.

12.4.6.  New Error Response Codes

   This usage does not define any new error response codes.

12.4.7.  Client Procedures

   If the STUN Response indicates the client's mapped address has
   changed from the client's expected mapped address, the client SHOULD
   inform other applications of its new mapped address.  For example, a
   SIP client should send a new registration message indicating the new
   mapped address.

12.4.8.  Server Procedures

   In this usage no authentication is used so there is no duration of
   the short-term password.

12.4.9.  Security Considerations for NAT Keepalives

      Issue:  Currently, the security considerations applies to all the
      various usages.  Split it up to talk about each one?  Create
      subsections talking about each usage?

12.5.  Short-Term Password

   In order to ensure interoperability, this usage describes a TLS-based
   mechanism to obtain a short-term username and short-term password.

12.5.1.  Applicability

   To thwart some on-path attacks described in Section 13, it is
   necessary for the STUN client and STUN server to integrity protect
   the information they exchange over UDP.  In the absence of a long-
   term secret (password) that is shared between them, a short-term
   password can be obtained using the usage described in this section.

   The username and password returned in the STUN Shared Secret Response
   are valid for use in subsequent STUN transactions for nine (9)
   minutes with any hosts that have the same SRV Priority value as
   discovered via Section 12.5.2.  The username and password obtained



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   with this usage are used as the USERNAME and as the HMAC for the
   MESSAGE-ID in a subsequent STUN message, respectively.

   The duration of validity of the username and password obtained via
   this usage depends on the usage of the subsequent STUN messages that
   are protected with that username and password.

12.5.2.  Client Discovery of Server

   The client follows the procedures in Section 8.1.1, except the SRV
   protocol is TCP rather than UDP and the service name "stun-tls".

   For example a client would look up "_stun-tls._tcp.example.com" in
   DNS.

12.5.3.  Server Determination of Usage

   The server advertises this port in the DNS as capable of receiving
   TLS-protected STUN messages for this usage.  The server MAY also
   advertise this same port in DNS for other TCP usages if the server is
   capable of multiplexing those different usages.  For example, the
   server could advertise

12.5.4.  New Requests or Indications

   The message type Shared Secret Request and its associated Shared
   Secret Response and Shared Secret Error Response are defined in this
   section.  Their values are enumerated in Section 15.

   The following figure indicates which attributes are present in the
   Shared Secret Request, Response, and Error Response.  An M indicates
   that inclusion of the attribute in the message is mandatory, O means
   its optional, C means it's conditional based on some other aspect of
   the message, and N/A means that the attribute is not applicable to
   that message type.  Attributes not listed are not applicable to
   Shared Secret Request, Response, or Error Response.

                       Shared   Shared    Shared
                       Secret   Secret    Secret
    Attribute          Request  Response  Error
                                          Response
    ____________________________________________________________________
    USERNAME             -         M         -
    PASSWORD             -         M         -
    ERROR-CODE           -         -         M
    UNKNOWN-ATTRIBUTES   -         -         C
    SERVER               -         O         O
    REALM                C         -         C



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   Note:  As this usage requires running over TLS, MESSAGE-INTEGRITY
   isn't necessary.

12.5.5.  New Attributes

   No new attributes are defined by this usage.

12.5.6.  New Error Response Codes

   This usage does not define any new error response codes.

12.5.7.  Client Procedures

   The client opens up the connection to that address and port, and
   immediately begins TLS negotiation[6].  The client MUST verify the
   identity of the server.  To do that, it follows the identification
   procedures defined in Section 3.1 of RFC2818 [5].  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 9.1 as the host portion of the URI that has been
   dereferenced.  Once the connection is opened, the client sends a
   Shared Secret request.  This request has no attributes, just the
   header.  The transaction ID in the header MUST meet the requirements
   outlined for the transaction ID in a binding request, described in
   Section 9.3 below.

   If the response was a Shared Secret Error Response, the client checks
   the response code in the ERROR-CODE attribute.  If the response was a
   Shared Secret Response, it will contain a short lived username and
   password, encoded in the USERNAME and PASSWORD attributes,
   respectively.

12.5.8.  Server Procedures

   After a client has established a TLS session, the server should
   expect a STUN message containing a Shared Secret Request.  The server
   will generates a response, which can either be a Shared Secret
   Response or a Shared Secret Error Response.

12.5.9.  Security Considerations for Short-Term Password

      Issue:  Currently, the security considerations applies to all the
      various usages.  Split it up to talk about each one?  Create
      subsections talking about each usage?







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13.  Security Considerations

   Issue:  This section has not been revised to properly consider the
   attacks on each of STUN's different usages.  This needs to be done.

13.1.  Attacks on STUN

   Generally speaking, attacks on STUN can be classified into denial of
   service attacks and eavesdropping attacks.  Denial of service attacks
   can be launched against a STUN server itself, or against other
   elements using the STUN protocol.  STUN servers create state through
   the Shared Secret Request mechanism.  To prevent being swamped with
   traffic, a STUN server SHOULD limit the number of simultaneous TLS
   connections it will hold open by dropping an existing connection when
   a new connection request arrives (based on an Least Recently Used
   (LRU) policy, for example).  Similarly, if the server is storing
   short-term passwords it SHOULD limit the number of shared secrets it
   will store.  The attacks of greater interest are those in which the
   STUN server and client are used to launch denial of service (DoS)
   attacks against other entities, including the client itself.  Many of
   the attacks require the attacker to generate a response to a
   legitimate STUN request, in order to provide the client with a faked
   XOR-MAPPED-ADDRESS or MAPPED-ADDRESS.  In the sections below, we
   refer to either the XOR-MAPPED-ADDRESS or MAPPED-ADDRESS as just the
   mapped address (note the lower case).  The attacks that can be
   launched using such a technique include:

13.1.1.  Attack I: DDoS Against a Target

   In this case, the attacker provides a large number of clients with
   the same faked mapped address that points to the intended target.
   This will trick all the STUN clients into thinking that their
   addresses are equal to that of the target.  The clients then hand out
   that address in order to receive traffic on it (for example, in SIP
   or H.323 messages).  However, all of that traffic becomes focused at
   the intended target.  The attack can provide substantial
   amplification, especially when used with clients that are using STUN
   to enable multimedia applications.

13.1.2.  Attack II: Silencing a Client

   In this attack, the attacker seeks to deny a client access to
   services enabled by STUN (for example, a client using STUN to enable
   SIP-based multimedia traffic).  To do that, the attacker provides
   that client with a faked mapped address.  The mapped address it
   provides is an IP 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 mapped address.  This exploitation is not very



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   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.

13.1.3.  Attack III: Assuming the Identity of a Client

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

13.1.4.  Attack IV: Eavesdropping

   In this attack, the attacker forces the client to use a mapped
   address that routes to itself.  It then forwards any packets it
   receives to the client.  This attack would allow the attacker to
   observe all packets sent to the client.  However, in order to launch
   the attack, the attacker must have already been able to observe
   packets from the client to the STUN server.  In most cases (such as
   when the attack is launched from an access network), this means that
   the attacker could already observe packets sent to the client.  This
   attack is, as a result, only useful for observing traffic by
   attackers on the path from the client to the STUN server, but not
   generally on the path of packets being routed towards the client.

13.2.  Launching the Attacks

   It is important to note that attacks of this nature (injecting
   responses with fake mapped addresses) require that the attacker be
   capable of eavesdropping requests sent from the client to the server
   (or to act as a man in the middle for such attacks).  This is because
   STUN requests contain a transaction identifier, selected by the
   client, which is random with 96 bits of entropy.  The server echoes
   this value in the response, and the client ignores any responses that
   don't have a matching transaction ID.  Therefore, in order for an
   attacker to provide a faked response that is accepted by the client,
   the attacker needs to know the transaction ID of the request.  The
   large amount of randomness, combined with the need to know when the
   client sends a request and the IP address and UDP ports used for that
   request, precludes attacks that involve guessing the transaction ID.

   Since all of the above attacks rely on this one primitive - injecting
   a response with a faked mapped address - preventing the attacks is
   accomplished by preventing this one operation.  To prevent it, we
   need to consider the various ways in which it can be accomplished.
   There are several:




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13.2.1.  Approach I: Compromise a Legitimate STUN Server

   In this attack, the attacker compromises a legitimate STUN server
   through a virus or Trojan horse.  Presumably, this would allow the
   attacker to take over the STUN server, and control the types of
   responses it generates.  Compromise of a STUN server can also lead to
   discovery of open ports.  Knowledge of an open port creates an
   opportunity for DoS attacks on those ports (or DDoS attacks if the
   traversed NAT is a full cone NAT).  Discovering open ports is already
   fairly trivial using port probing, so this does not represent a major
   threat.

13.2.2.  Approach II: DNS Attacks

   STUN servers are discovered using DNS SRV records.  If an attacker
   can compromise the DNS, it can inject fake records which map a domain
   name to the IP address of a STUN server run by the attacker.  This
   will allow it to inject fake responses to launch any of the attacks
   above.

13.2.3.  Approach III: Rogue Router or NAT

   Rather than compromise the STUN server, an attacker can cause a STUN
   server to generate responses with the wrong mapped address by
   compromising a router or NAT on the path from the client to the STUN
   server.  When the STUN request passes through the rogue router or
   NAT, it rewrites the source address of the packet to be that of the
   desired mapped address.  This address cannot be arbitrary.  If the
   attacker is on the public Internet (that is, there are no NATs
   between it and the STUN server), and the attacker doesn't modify the
   STUN request, the address has to have the property that packets sent
   from the STUN server to that address would route through the
   compromised router.  This is because the STUN server will send the
   responses back to the source address of the request.  With a modified
   source address, the only way they can reach the client is if the
   compromised router directs them there.  If the attacker is on the
   public Internet, but they can modify the STUN request, they can
   insert a RESPONSE-ADDRESS attribute into the request, containing the
   actual source address of the STUN request.  This will cause the
   server to send the response to the client, independent of the source
   address the STUN server sees.  This gives the attacker the ability to
   forge an arbitrary source address when it forwards the STUN request.

      Todo:  RESPONSE-ADDRESS has been removed from this version of the
      specification.  Reword or remove above paragraph accordingly.

   If the attacker is on a private network (that is, there are NATs
   between it and the STUN server), the attacker will not be able to



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   force the server to generate arbitrary mapped addresses in responses.
   They will only be able force the STUN server to generate mapped
   addresses which route to the private network.  This is because the
   NAT between the attacker and the STUN server will rewrite the source
   address of the STUN request, mapping it to a public address that
   routes to the private network.  Because of this, the attacker can
   only force the server to generate faked mapped addresses that route
   to the private network.  Unfortunately, it is possible that a low
   quality NAT would be willing to map an allocated public address to
   another public address (as opposed to an internal private address),
   in which case the attacker could forge the source address in a STUN
   request to be an arbitrary public address.  This kind of behavior
   from NATs does appear to be rare.

13.2.4.  Approach IV: Man in the Middle

   As an alternative to approach III (Section 13.2.3), if the attacker
   can place an element on the path from the client to the server, the
   element can act as a man-in-the-middle.  In that case, it can
   intercept a STUN request, and generate a STUN response directly with
   any desired value of the mapped address field.  Alternatively, it can
   forward the STUN request to the server (after potential
   modification), receive the response, and forward it to the client.
   When forwarding the request and response, this attack is subject to
   the same limitations on the mapped address described in Approach III
   (Section 13.2.3).

13.2.5.  Approach V: Response Injection Plus DoS

   In this approach, the attacker does not need to be a MitM (as in
   approaches III and IV).  Rather, it only needs to be able to
   eavesdrop onto a network segment that carries STUN requests.  This is
   easily done in multiple access networks such as ethernet or
   unprotected 802.11.  To inject the fake response, the attacker
   listens on the network for a STUN request.  When it sees one, it
   simultaneously launches a DoS attack on the STUN server, and
   generates its own STUN response with the desired mapped address
   value.  The STUN response generated by the attacker will reach the
   client, and the DoS attack against the server is aimed at preventing
   the legitimate response from the server from reaching the client.
   Arguably, the attacker can do without the DoS attack on the server,
   so long as the faked response beats the real response back to the
   client, and the client uses the first response, and ignores the
   second (even though it's different).

13.2.6.  Approach VI: Duplication

   This approach is similar to approach V (Section 13.2.5).  The



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   attacker listens on the network for a STUN request.  When it sees it,
   it generates its own STUN request towards the server.  This STUN
   request is identical to the one it saw, but with a spoofed source IP
   address.  The spoofed address is equal to the one that the attacker
   desires to have placed in the mapped address of the STUN response.
   In fact, the attacker generates a flood of such packets.  The STUN
   server will receive the one original request, plus a flood of
   duplicate fake ones.  It generates responses to all of them.  If the
   flood is sufficiently large for the responses to congest routers or
   some other equipment, there is a reasonable probability that the one
   real response is lost (along with many of the faked ones), but the
   net result is that only the faked responses are received by the STUN
   client.  These responses are all identical and all contain the mapped
   address that the attacker wanted the client to use.

   The flood of duplicate packets is not needed (that is, only one faked
   request is sent), so long as the faked response beats the real
   response back to the client, and the client uses the first response,
   and ignores the second (even though it's different).

   Note that, in this approach, launching a DoS attack against the STUN
   server or the IP network, to prevent the valid response from being
   sent or received, is problematic.  The attacker needs the STUN server
   to be available to handle its own request.  Due to the periodic
   retransmissions of the request from the client, this leaves a very
   tiny window of opportunity.  The attacker must start the DoS attack
   immediately after the actual request from the client, causing the
   correct response to be discarded, and then cease the DoS attack in
   order to send its own request, all before the next retransmission
   from the client.  Due to the close spacing of the retransmits (100ms
   to a few seconds), this is very difficult to do.

   Besides DoS attacks, there may be other ways to prevent the actual
   request from the client from reaching the server.  Layer 2
   manipulations, for example, might be able to accomplish it.

   Fortunately, this approach is subject to the same limitations
   documented in Approach III (Section 13.2.3), which limit the range of
   mapped addresses the attacker can cause the STUN server to generate.

13.3.  Countermeasures

   STUN provides mechanisms to counter the approaches described above,
   and additional, non-STUN techniques can be used as well.

   First off, it is RECOMMENDED that networks with STUN clients
   implement ingress source filtering [7].  This is particularly
   important for the NATs themselves.  As Section 13.2.3 explains, NATs



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   which do not perform this check can be used as "reflectors" in DDoS
   attacks.  Most NATs do perform this check as a default mode of
   operation.  We strongly advise people that purchase NATs to ensure
   that this capability is present and enabled.

   Secondly, it is RECOMMENDED that STUN servers be run on hosts
   dedicated to STUN, with all UDP and TCP ports disabled except for the
   STUN ports.  This is to prevent viruses and Trojan horses from
   infecting STUN servers, in order to prevent their compromise.  This
   helps mitigate Approach I (Section 13.2.1).

   Thirdly, to prevent the DNS attack of Section 13.2.2, Section 8.1.2
   recommends that the client verify the credentials provided by the
   server with the name used in the DNS lookup.

   Finally, all of the attacks above rely on the client taking the
   mapped address it learned from STUN, and using it in application
   layer protocols.  If encryption and message integrity are provided
   within those protocols, the eavesdropping and identity assumption
   attacks can be prevented.  As such, applications that make use of
   STUN addresses in application protocols SHOULD use integrity and
   encryption, even if a SHOULD level strength is not specified for that
   protocol.  For example, multimedia applications using STUN addresses
   to receive RTP traffic would use secure RTP [20].

   The above three techniques are non-STUN mechanisms.  STUN itself
   provides several countermeasures.

   Approaches IV (Section 13.2.4), when generating the response locally,
   and V (Section 13.2.5) require an attacker to generate a faked
   response.  A faked response must match the 96-bit transaction ID of
   the request.  The attack further prevented by using the message
   integrity mechanism provided in STUN, described in Section 11.8.

   Approaches III (Section 13.2.3), IV (Section 13.2.4), when using the
   relaying technique, and VI (Section 13.2.6), however, are not
   preventable through server signatures.  All three approaches are most
   potent when the attacker can modify the request, inserting a
   RESPONSE-ADDRESS that routes to the client.  Fortunately, such
   modifications are preventable using the message integrity techniques
   described in Section 11.8.  However, these three approaches are still
   functional when the attacker modifies nothing but the source address
   of the STUN request.  Sadly, this is the one thing that cannot be
   protected through cryptographic means, as this is the change that
   STUN itself is seeking to detect and report.  It is therefore an
   inherent weakness in NAT, and not fixable in STUN.





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13.4.  Residual Threats

   None of the countermeasures listed above can prevent the attacks
   described in Section 13.2.3 if the attacker is in the appropriate
   network paths.  Specifically, consider the case in which the attacker
   wishes to convince client C that it has address V. The attacker needs
   to have a network element on the path between A and the server (in
   order to modify the request) and on the path between the server and V
   so that it can forward the response to C. Furthermore, if there is a
   NAT between the attacker and the server, V must also be behind the
   same NAT.  In such a situation, the attacker can either gain access
   to all the application-layer traffic or mount the DDOS attack
   described in Section 13.1.1.  Note that any host which exists in the
   correct topological relationship can be DDOSed.  It need not be using
   STUN.


14.  IAB Considerations

      Todo:  The diagnostic usages have been removed from this document,
      which reduces the brittleness of STUN.  This section should be
      updated accordingly.



   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 [21]).
   STUN is an example of a protocol that performs this type of function.
   The IAB has mandated that any protocols developed for this purpose
   document a specific set of considerations.  This section meets those
   requirements.

14.1.  Problem Definition

   From RFC3424 [21], any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why "short term fixes
      usually aren't".

   The specific problem being solved by STUN is to provide a means for a
   client to obtain an address on the public Internet from a non-
   symmetric NAT, for the express purpose of receiving incoming UDP
   traffic from another host, targeted to that address.  STUN does not
   address traversal of NATs using TCP, either incoming or outgoing, and



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   does not address outgoing UDP communications.

14.2.  Exit Strategy

   From RFC3424 [21], any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   STUN by itself does not provide an exit strategy.  This is provided
   by techniques, such as Interactive Connectivity Establishment (ICE
   [12]), which allow a client to determine whether addresses learned
   from STUN are needed, or whether other addresses, such as the one on
   the local interface, will work when communicating with another host.
   With such a detection technique, as a client finds that the addresses
   provided by STUN are never used, STUN queries can cease to be made,
   thus allowing them to phase out.

   STUN can also help facilitate the introduction of other NAT traversal
   techniques such as MIDCOM [22].  As midcom-capable NATs are deployed,
   applications will, instead of using STUN (which also resides at the
   application layer), first allocate an address binding using midcom.
   However, it is a well-known limitation of MIDCOM that it only works
   when the agent knows the middleboxes through which its traffic will
   flow.  Once bindings have been allocated from those middleboxes, a
   STUN detection procedure can validate that there are no additional
   middleboxes on the path from the public Internet to the client.  If
   this is the case, the application can continue operation using the
   address bindings allocated from MIDCOM.  If it is not the case, STUN
   provides a mechanism for self-address fixing through the remaining
   MIDCOM-unaware middleboxes.  Thus, STUN provides a way to help
   transition to full MIDCOM-aware networks.

14.3.  Brittleness Introduced by STUN

   From RFC3424 [21], any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   STUN introduces brittleness into the system in several ways:

   o  The binding acquisition usage is dependant on NAT's behavior when
      forwarding UDP packets from arbitrary hosts on the public side of
      the NAT.  Application specific processing will generally be



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      needed.  For symmetric NATs, the binding acquisition will not
      yield a usable address.  The tight dependency on the specific type
      of NAT makes the protocol brittle.

   o  STUN assumes that the server exists on the public Internet.  If
      the server is located in another private address realm, the user
      may or may not be able to use its discovered address to
      communicate with other users.  There is no way to detect such a
      condition.

   o  The bindings allocated from the NAT need to be continuously
      refreshed.  Since the timeouts for these bindings is very
      implementation specific, the refresh interval cannot easily be
      determined.  When the binding is not being actively used to
      receive traffic, but to wait for an incoming message, the binding
      refresh will needlessly consume network bandwidth.

   o  The use of the STUN server as an additional network element
      introduces another point of potential security attack.  These
      attacks are largely prevented by the security measures provided by
      STUN, but not entirely.

   o  The use of the STUN server as an additional network element
      introduces another point of failure.  If the client cannot locate
      a STUN server, or if the server should be unavailable due to
      failure, the application cannot function.

   o  The use of STUN to discover address bindings will result in an
      increase in latency for applications.  For example, a Voice over
      IP application will see an increase of call setup delays equal to
      at least one RTT to the STUN server.

   o  STUN imposes some restrictions on the network topologies for
      proper operation.  If client A obtains an address from STUN server
      X, and sends it to client B, B may not be able to send to A using
      that IP address.  The address will not work if any of the
      following is true:

      *  The STUN server is not in an address realm that is a common
         ancestor (topologically) of both clients A and B. For example,
         consider client A and B, both of which have residential NAT
         devices.  Both devices connect them to their cable operators,
         but both clients have different providers.  Each provider has a
         NAT in front of their entire network, connecting it to the
         public Internet.  If the STUN server used by A is in A's cable
         operator's network, an address obtained by it will not be
         usable by B. The STUN server must be in the network which is a
         common ancestor to both - in this case, the public Internet.



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      *  The STUN server is in an address realm that is a common
         ancestor to both clients, but both clients are behind the same
         NAT connecting to that address realm.  For example, if the two
         clients in the previous example had the same cable operator,
         that cable operator had a single NAT connecting their network
         to the public Internet, and the STUN server was on the public
         Internet, the address obtained by A would not be usable by B.
         That is because some NATs will not accept an internal packet
         sent to a public IP address which is mapped back to an internal
         address.  To deal with this, additional protocol mechanisms or
         configuration parameters need to be introduced which detect
         this case.

   o  Most significantly, STUN introduces potential security threats
      which cannot be eliminated.  This specification describes
      heuristics that can be used to mitigate the problem, but it is
      provably unsolvable given what STUN is trying to accomplish.
      These security problems are described fully in Section 13.

14.4.  Requirements for a Long Term Solution

   From RFC3424 [21], any UNSAF proposal must provide:

      Identify requirements for longer term, sound technical solutions
      -- contribute to the process of finding the right longer term
      solution.

   Our experience with STUN has led to the following requirements for a
   long term solution to the NAT problem:

   o  Requests for bindings and control of other resources in a NAT need
      to be explicit.  Much of the brittleness in STUN derives from its
      guessing at the parameters of the NAT, rather than telling the NAT
      what parameters to use.

   o  Control needs to be in-band.  There are far too many scenarios in
      which the client will not know about the location of middleboxes
      ahead of time.  Instead, control of such boxes needs to occur in-
      band, traveling along the same path as the data will itself
      travel.  This guarantees that the right set of middleboxes are
      controlled.  This is only true for first-party controls; third-
      party controls are best handled using the MIDCOM framework.

   o  Control needs to be limited.  Users will need to communicate
      through NATs which are outside of their administrative control.
      In order for providers to be willing to deploy NATs which can be
      controlled by users in different domains, the scope of such
      controls needs to be extremely limited - typically, allocating a



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      binding to reach the address where the control packets are coming
      from.

   o  Simplicity is Paramount.  The control protocol will need to be
      implement in very simple clients.  The servers will need to
      support extremely high loads.  The protocol will need to be
      extremely robust, being the precursor to a host of application
      protocols.  As such, simplicity is key.

14.5.  Issues with Existing NAPT Boxes

   From RFC3424 [21], any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   Several of the practical issues with STUN involve future proofing -
   breaking the protocol when new NAT types get deployed.  Fortunately,
   this is not an issue at the current time, since most of the deployed
   NATs are of the types assumed by STUN.  The primary usage STUN has
   found is in the area of VoIP, to facilitate allocation of addresses
   for receiving RTP [14] traffic.  In that application, the periodic
   keepalives are usually (but not always) provided by the RTP traffic
   itself.  However, several practical problems arise for RTP.  First,
   in the absence of [23], RTP assumes that RTCP traffic is on a port
   one higher than the RTP traffic.  This pairing property cannot be
   guaranteed through NATs that are not directly controllable.  As a
   result, RTCP traffic may not be properly received. [23] mitigates
   this by allowing the client to signal a different port for RTCP but
   there will be interoperability problems for some time.

   For VoIP, silence suppression can cause a gap in the transmission of
   RTP packets.  If that silence period exceeds the NAT binding timeout,
   this could result in the loss of a NAT binding in the middle of a
   call.  This can be mitigated by sending occasional packets to keep
   the binding alive.  However, the result is additional brittleness.

14.6.  In Closing

   The problems with STUN are not design flaws in STUN.  The problems in
   STUN have to do with the lack of standardized behaviors and controls
   in NATs.  The result of this lack of standardization has been a
   proliferation of devices whose behavior is highly unpredictable,
   extremely variable, and uncontrollable.  STUN does the best it can in
   such a hostile environment.  Ultimately, the solution is to make the
   environment less hostile, and to introduce controls and standardized
   behaviors into NAT.  However, until such time as that happens, STUN
   provides a good short term solution given the terrible conditions



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   under which it is forced to operate.


15.  IANA Considerations

   IANA is hereby requsted to create two new registries STUN Message
   Types and STUN Attributes.  IANA must assign the following values to
   both registeries before publication of this document as an RFC.  New
   values for both STUN Message Type and STUN Attributes are assigned
   through the IETF consensus process via RFCs approved by the IESG.

15.1.  STUN Message Type Registry

   For STUN Message Types that are request message types, they MUST be
   registered including associated Response message types and Error
   Response message types, and those responses must have values that are
   0x100 and 0x110 higher than their respective Request values.

   For STUN Message Types that are Indication message types, no
   associated restriction applies.  As the message type field is only 14
   bits the range of valid values is 0x001 through 0x3FFF.

   The initial STUN Message Types are:

        0x0001  :  Binding Request
        0x0101  :  Binding Response
        0x0111  :  Binding Error Response
        0x0002  :  Shared Secret Request
        0x0102  :  Shared Secret Response
        0x0112  :  Shared Secret Error Response
        0x0002  :  Shared Secret Request
        0x0102  :  Shared Secret Responsed
        0x0112  :  Shared Secret Error Response

15.2.  STUN Attribute Registry

   STUN attributes values above 0x7FFF are considered optional
   attributes; attributes equal to 0x7FFF or below are considered
   mandatory attributes.  The STUN client and STUN server process
   optional and mandatory attributes differently.  IANA should assign
   values based on the RFC consensus process.










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   The initial STUN Attributes are:

        0x0001: MAPPED-ADDRESS
        0x0002: RESPONSE-ADDRESS
        0x0003: CHANGE-REQUEST
        0x0004: SOURCE-ADDRESS
        0X0005: CHANGED-ADDRESS
        0x0006: USERNAME
        0x0007: PASSWORD
        0x0008: MESSAGE-INTEGRITY
        0x0009: ERROR-CODE
        0x000A: UNKNOWN-ATTRIBUTES
        0x000B: REFLECTED-FROM
        0x000E: ALTERNATE-SERVER
        0x0014: REALM
        0x0015: NONCE
        0x0020: XOR-MAPPED-ADDRESS
        0x8022: SERVER
        0x8023: ALTERNATE-SERVER
        0x8024: BINDING-LIFETIME


16.  Changes Since RFC 3489

   This specification updates RFC3489 [13].  This specification differs
   from RFC3489 in the following ways:

   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  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  Added a fixed 32-bit magic cookie and reduced length of
      transaction ID by 32 bits.  The magic cookie begins at the same
      offset as the original transaction ID.

   o  Added the XOR-MAPPED-ADDRESS attribute, which is included in
      Binding Responses if the magic cookie is present in the request.
      Otherwise the RFC3489 behavior is retained (that is, Binding
      Response includes MAPPED-ADDRESS).  See discussion in XOR-MAPPED-
      ADDRESS regarding this change.





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

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

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


17.  Acknowledgements

   The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
   Jennings, Bob Penfield 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.


18.  References

18.1.  Normative References

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

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

   [3]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
        draft-rosenberg-midcom-turn-08 (work in progress),
        September 2005.

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

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

   [6]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
        RFC 2246, January 1999.

   [7]  Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
        Denial of Service Attacks which employ IP Source Address
        Spoofing", BCP 38, RFC 2827, May 2000.




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   [8]  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.

18.2.  Informational References

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

   [10]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [11]  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.

   [12]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-06 (work in
         progress), October 2005.

   [13]  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.

   [14]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [15]  Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-01 (work in progress), October 2005.

   [16]  Kohl, J. and B. Neuman, "The Kerberos Network Authentication
         Service (V5)", RFC 1510, September 1993.

   [17]  Senie, D., "Network Address Translator (NAT)-Friendly
         Application Design Guidelines", RFC 3235, January 2002.

   [18]  Holdrege, M. and P. Srisuresh, "Protocol Complications with the
         IP Network Address Translator", RFC 3027, January 2001.

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

   [20]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)",



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         RFC 3711, March 2004.

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

   [22]  Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A.
         Rayhan, "Middlebox communication architecture and framework",
         RFC 3303, August 2002.

   [23]  Huitema, C., "Real Time Control Protocol (RTCP) attribute in
         Session Description Protocol (SDP)", RFC 3605, October 2003.







































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Authors' Addresses

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone:  +1 973 952-5000
   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


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

   Email:  dwing@cisco.com













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