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Versions: (draft-rosenberg-midcom-turn) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 5766

Behave                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                                 R. Mahy
Expires: September 5, 2007                                   Plantronics
                                                              C. Huitema
                                                               Microsoft
                                                           March 4, 2007


 Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)
                       draft-ietf-behave-turn-03

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   This Internet-Draft will expire on September 5, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This specification defines a usage of the Simple Traversal Underneath
   NAT (STUN) Protocol for asking the STUN server to relay packets
   towards a client.  This usage is useful for elements behind NATs
   whose mapping behavior is address and port dependent.  The extension
   purposefully restricts the ways in which the relayed address can be



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   used.  In particular, it prevents users from running general purpose
   servers from ports obtained from the STUN server.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Transports . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Tuple Terminology  . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Keepalives . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  New Framing Mechanism for Stream-Oriented Transports . . . . . 10
   6.  New STUN Requests and Indications  . . . . . . . . . . . . . . 10
     6.1.  Allocate Request . . . . . . . . . . . . . . . . . . . . . 11
       6.1.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . 11
       6.1.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . 13
     6.2.  Procedures for all other Requests and Indications  . . . . 17
     6.3.  Set Active Destination Request . . . . . . . . . . . . . . 18
       6.3.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . 18
       6.3.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . 19
     6.4.  Connect Request  . . . . . . . . . . . . . . . . . . . . . 19
       6.4.1.  Server Behavior  . . . . . . . . . . . . . . . . . . . 19
     6.5.  Connection Status Indication . . . . . . . . . . . . . . . 20
     6.6.  Send Indication  . . . . . . . . . . . . . . . . . . . . . 20
       6.6.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . 20
       6.6.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . 21
     6.7.  Data Indication  . . . . . . . . . . . . . . . . . . . . . 21
       6.7.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . 21
       6.7.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . 22
   7.  New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.1.  LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.2.  BANDWIDTH  . . . . . . . . . . . . . . . . . . . . . . . . 23
     7.3.  REMOTE-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 23
     7.4.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     7.5.  RELAY-ADDRESS  . . . . . . . . . . . . . . . . . . . . . . 23
     7.6.  REQUESTED-PORT-PROPS . . . . . . . . . . . . . . . . . . . 23
     7.7.  REQUESTED-TRANSPORT  . . . . . . . . . . . . . . . . . . . 24
     7.8.  REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 25
     7.9.  CONNECT_STAT . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  New Error Response Codes . . . . . . . . . . . . . . . . . . . 25
   9.  Client Procedures  . . . . . . . . . . . . . . . . . . . . . . 26
     9.1.  Receiving and Sending Unencapsulated Data  . . . . . . . . 26
       9.1.1.  Datagram Protocols . . . . . . . . . . . . . . . . . . 26
       9.1.2.  Stream Transport Protocols . . . . . . . . . . . . . . 27
   10. Server Procedures  . . . . . . . . . . . . . . . . . . . . . . 27
     10.1. Receiving Data on Allocated Transport Addresses  . . . . . 27



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       10.1.1. TCP Processing . . . . . . . . . . . . . . . . . . . . 27
       10.1.2. UDP Processing . . . . . . . . . . . . . . . . . . . . 28
     10.2. Receiving Data on Internal Local Transport Addresses . . . 28
     10.3. Lifetime Expiration  . . . . . . . . . . . . . . . . . . . 29
   11. Formal Definition of STUN Usage  . . . . . . . . . . . . . . . 29
     11.1. Applicability Statement  . . . . . . . . . . . . . . . . . 29
     11.2. Client Discovery of Server . . . . . . . . . . . . . . . . 30
     11.3. Server Determination of Usage  . . . . . . . . . . . . . . 31
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
     13.1. New STUN Requests, Responses, and Indications  . . . . . . 33
     13.2. New STUN Attributes  . . . . . . . . . . . . . . . . . . . 33
     13.3. New STUN response codes  . . . . . . . . . . . . . . . . . 34
   14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 34
     14.1. Problem Definition . . . . . . . . . . . . . . . . . . . . 34
     14.2. Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 35
     14.3. Brittleness Introduced by STUN relays  . . . . . . . . . . 35
     14.4. Requirements for a Long Term Solution  . . . . . . . . . . 36
     14.5. Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 36
   15. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 41
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     17.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42
   Intellectual Property and Copyright Statements . . . . . . . . . . 44

























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1.  Introduction

   The Simple Traversal Underneath NAT (STUN) [1] provides a suite of
   tools for facilitating the traversal of NAT.  Specifically, it
   defines the Binding Request, which is used by a client to determine
   its reflexive transport address towards the STUN server.  The
   reflexive transport address can be used by the client for receiving
   packets from peers, but only when the client is behind "good" NATs.
   In particular, if a client is behind a NAT whose mapping behavior
   [10] is address or address and port dependent (sometimes called "bad"
   NATs), the reflexive transport address will not be usable for
   communicating with a peer.

   The only way to obtain a transport address that can be used for
   corresponding with a peer through such a NAT is to make use of a
   relay.  The relay sits on the public side of the NAT, and allocates
   transport addresses to clients reaching it from behind the private
   side of the NAT.  These allocated addresses are from interfaces on
   the relay.  When the relay receives a packet on one of these
   allocated addresses, the relay forwards it toward the client.

   This specification defines a usage of STUN, called the relay usage,
   that allows a client to request an address on the STUN server itself,
   so that the STUN server acts as a relay.  To accomplish that, this
   usage defines a handful of new STUN requests and indications.  The
   Allocate request is the most fundamental component of this usage.  It
   is used to provide the client with a transport address that is
   relayed through the STUN server.  A transport address which relays
   through an intermediary is called a relayed transport address.

   Though a relayed address is highly likely to work when corresponding
   with a peer, it comes at high cost to the provider of the relay
   service.  As a consequence, relayed transport addresses should only
   be used as a last resort.  Protocols using relayed transport
   addresses should make use of mechanisms to dynamically determine
   whether such an address is actually needed.  One such mechanism,
   defined for multimedia session establishment protocols, based on the
   offer/answer protocol in RFC 3264 [5], is Interactive Connectivity
   Establishment (ICE) [9].

   The mechanism defined here was previously a standalone protocol
   called Traversal Using Relay NAT (TURN), and is now defined as a
   usage of STUN.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].


3.  Definitions

   Relayed Transport Address:  A transport address that terminates on a
      server, and is forwarded towards the client.  The STUN Allocate
      Request can be used to obtain a relayed transport address, for
      example.

   STUN relay client:  A STUN client that implements this specification.
      It obtains a relayed transport address that it provides to a small
      number of peers (usually one).

   STUN relay server:  A STUN server that implements this specification.
      It relays data between a STUN relay client and its peer.

   5-tuple:  A combination of the source IP address and port,
      destination IP address and port, and transport protocol (UDP, TCP,
      or TLS over TCP).  It uniquely identifies a TCP connection, TLS
      channel, or bi-directional flow of UDP datagrams.

   permission:  TBD


4.  Overview of Operation

   In a typical configuration, a STUN relay client is connected to a
   private network and through one or more NATs to the public Internet.
   On the public Internet is a STUN relay server.  The STUN Relay usage
   defines several new messages and a new framing mechanism that add the
   ability for a STUN server to act as a packet relay.  The text in this
   section explains the typical usage of this relay extension.

   First the client sends an Allocate request to the server, which the
   server authenticates.  The server generates an Allocate response with
   the allocated address, port, and target transport.  All other STUN
   messages defined by the STUN relay usage happen in the context of an
   allocation.

   A successful Allocate Request just reserves an address on the STUN
   relay server.  Data does not flow through an allocated port until the
   STUN relay client asks the STUN relay server to open a permission.
   It can do this by sending data to the far end with a Send Indication
   for UDP allocations, by sending a ConnectRequest for TCP allocations,
   or by setting the default destination for either transport.  While
   the client can request more than one permission per allocation, it



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   needs to request each permission explicitly and one at a time.  This
   insures that a client can't use a STUN relay server to run a
   traditional server, and partially protects the client from DoS
   attacks.

   Once a permission is open, the client can then receive data flowing
   back from its peer.  Initially this data is wrapped in a STUN Data
   Indication.  Since multiple permissions can be open simultaneously,
   the Data Indication contains the Remote Address attribute so the STUN
   relay client knows which peer sent the data.  The client can send
   data to any of its peers with the Send Indication.

   Once the client wants to primarily receive from one peer, it can send
   a SetActiveDestination request.  All subsequent data received from
   the active peer is forwarded directly to the client and vice versa,
   except that it is wrapped or framed according to the protocol used
   between the STUN relay client and STUN relay server.  The client can
   send subsequent SetActiveDestination requests to change or remove the
   active destination.

   When the STUN relay client to server communication is over a datagram
   protocol (UDP), any datagram received from the active peer that has
   the STUN magic cookie is wrapped in a Data Indication.  Likewise any
   datagram sent by the client that has the STUN magic cookie and is
   intended for the active peer is wrapped in a Send Indication.  This
   wrapping prevents the STUN relay server from inappropriately
   interpreting end-to-end data.

   Over stream-based transports (TCP and TLS over TCP), the STUN relay
   client and server always use some additional framing (defined in
   Section 5) so that end-to-end data is distinguishable from STUN
   control messages.  This additional framing just has a type and a
   length field.  The value of the type field was chosen so it is always
   distinguishable from an unframed STUN request or response.

   The SetActiveDestination Request does not close other bindings.  Data
   to and from other peers is still wrapped in Send and Data indications
   respectively.

   Allocations can also request specific attributes such as the desired
   Lifetime of the allocation, and the maximum Bandwidth.  Clients can
   also request specific port assignment behavior, for example, a
   specific port number, odd or even port numbers, or pairs of
   sequential port numbers.







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4.1.  Transports

   STUN relay clients can communicate with a STUN relay server using
   UDP, TCP, or TLS over TCP.  A STUN relay can even relay traffic
   between two different transports with certain restrictions.  A STUN
   relay can never relay from an unreliable transport (client to server)
   to a reliable transport to the peer.  Note that a STUN relay server
   never has a TLS relationship with a client's peer, since the STUN
   relay server does not interpret data above the TCP layer.  When
   relaying data sent from a stream-based protocol to a UDP peer, the
   STUN relay server emits datagrams which are the same length as the
   length field in the STUN TCP framing or the length field in a Send
   Indication.  Likewise, when a UDP datagram is relayed from a peer
   over a stream-based transport, the length of the datagram is the
   length of the TCP framing or Data Indication.

               +----------------------+--------------------+
               | client to STUN relay | STUN relay to peer |
               +----------------------+--------------------+
               | UDP                  | UDP                |
               | TCP                  | TCP                |
               | TCP                  | UDP                |
               | TLS                  | TCP                |
               | TLS                  | UDP                |
               +----------------------+--------------------+

   For STUN relay clients, using TLS over TCP provides two benefits.
   When using TLS, the client can be assured that the address of the
   client's peers are not visible to an attacker except by traffic
   analysis downstream of the STUN relay server.  Second, the client may
   be able to communicate with STUN relay servers using TLS that it
   would not be able to communicate with using TCP or UDP due to the
   configuration of a firewall between the STUN relay client and its
   server.  TLS between the client and STUN relay server in this case
   just facilitates traversal.

   For TCP connections, the Connection Request allows the client to ask
   the server to open a connection to the peer.  This also adds a
   permission to accept an incoming TCP connection from the remote
   address of the peer.  When the server and the peer try to open a TCP
   connection at the same time, this is called TCP simultaneous open.

   When the STUN relay-to-peer leg is TCP, the STUN relay client needs
   to be aware of the status of these TCP connections.  The STUN relay
   extension defines application states for a TCP connection as follows:
   LISTEN, ESTABLISHED, and CLOSED.  Consequently, the STUN relay server
   sends a ConnectionState Indication for a binding whenever the relay
   connection status changes for one of the client's bindings, except



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   when the status changes due to a STUN relay client request (ex: an
   explicit binding deallocation).

4.2.  Tuple Terminology

   To relay data to and from the correct location, the STUN relay server
   maintains an association between an internal address (called a
   5-tuple) and one or more external 5-tuples, as shown in Figure 1.
   The internal 5-tuple identifies the path between the STUN relay
   client and the STUN relay server.  It consists of the protocol (UDP,
   TCP, or TLS over TCP), the internal local IP address and port number
   and the source IP address and port number of the STUN client, as seen
   by the relay server.  For example, for UDP, the internal 5-tuple is
   the combination of the IP address and port from which the STUN client
   sent its Allocate Request, with the IP address and port from which
   the corresponding Allocate Response was sent.

   The external local transport address is the IP address and port
   allocated to the STUN relay client (the allocated transport address).
   The external 5-tuple is the combination of the external local
   transport address and the IP address and port of an external client
   that the STUN client is communicating with through the STUN server.
   Initially, there aren't any external 5-tuples, since the STUN client
   hasn't communicated with any other hosts yet.  As packets are
   received on or sent from the allocated transport address, external
   5-tuples are created.

      While the terminology used in this document refers to 5-tuples,
      the STUN relay server can store whatever identifier it likes that
      yields identical results.  Specifically, many implementations may
      use a file-descriptor in place of a 5-tuple to represent a TCP
      connection.



















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                                                             +---------+
                                                             |         |
                                                             | External|
                                                           / | Client  |
                                                         //  |         |
                                                        /    |         |
                                                      //     +---------+
                                                     /
                                                   //
                   +-+                            /
                   | |                           /
                   | |                         //
    +---------+    | |          +---------+   /              +---------+
    |         |    |N|          |         | //               |         |
    | STUN    |    | |          |         |/                 | External|
    | Client  |----|A|----------|   STUN  |------------------| Client  |
    |         |    | |^        ^|  Server |^                ^|         |
    |         |    |T||        ||         ||                ||         |
    +---------+    | ||        |+---------+|                |+---------+
       ^           | ||        |           |                |
       |           | ||        |           |                |
       |           +-+|        |           |                |
       |              |        |           |                |
       |
                    Internal     Internal    External         External
     Client         Remote       Local       Local            Remote
     Performing     Transport    Transport   Transport        Transport
     Allocations    Address      Address     Address          Address

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

                          Internal                  External
                          5-Tuple                   5-tuple

                                 Figure 1

4.3.  Keepalives

   Since the main purpose of STUN and the relay extension are to
   traverse NATs, it is natural to consider which elements are
   responsible for generating sufficient periodic traffic to insure that
   NAT bindings stay alive.  Relay clients need to send data frequently
   enough to keep both NAT bindings and the STUN relay server internal
   permissions fresh.  Like NAT bindings, the STUN relay server bindings
   are refreshed by ordinary data traffic relayed to and from the peer.



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   Unlike permissions, allocations on the STUN relay server have an
   explicit expiration time and need to be refreshed explicitly by the
   client.  When an allocation expires, all permissions associated with
   that allocation are automatically deleted.


5.  New Framing Mechanism for Stream-Oriented Transports

   Over stream-based transports, the STUN relay client and server need
   to use additional framing so that end-to-end data is distinguishable
   from STUN control messages, and so that the relay server can perform
   conversion from streams to datagrams and vice versa.  This additional
   framing has a one octet type, one reserved octet, and a 2 octet
   length field.  The first octet of this framing is 0x02 to indicate
   STUN messages or 0x03 to indicate end-to-end data to or from the
   active destination.  Note that the first octet is always
   distinguishable from an unframed STUN request or response (which is
   always 0x00 or 0x01).  The second octet is reserved and MUST be set
   to zero.  The length field counts the number of octets immediately
   after the length field itself.

    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      |  Reserved = 0 |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Use of this framing mechanism is discussed in Section 9 and
   Section 10.


6.  New STUN Requests and Indications

   This usage defines three new requests (along with their success and
   error responses) and three indications.  It also defines processing
   rules for the STUN server and client on receipt of non-STUN messages.
   See Section 9 and Section 10

   The new messages are:












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   Request/Response Transactions
   0x003  :  Allocate
   0x004  :  Set Active Destination
   0x005  :  Connect

   Indications
   0x001  :  Send
   0x002  :  Data
   0x003  :  Connect Status

   In addition to STUN Messages (Requests, Responses, and Indications),
   STUN relay clients and servers send and receive non-STUN packets on
   the same ports used for STUN messages.  How these entities
   distinguish STUN and non-STUN traffic is discussed in Section 9 and
   Section 10.

6.1.  Allocate Request

6.1.1.  Client Behavior

   Client behavior for Allocate requests depends on whether the request
   is an initial one, for the purposes of obtaining a new relayed
   transport address, or a subsequent one, used for refreshing an
   existing allocation.

6.1.1.1.  Initial Requests

   When a client wishes to obtain a transport address, it sends an
   Allocate Request to the server.  This request is constructed and sent
   using the general procedures defined in [1].  The server will
   challenge the request for credentials.  The client MAY either provide
   its credentials to the server directly, or it MAY obtain a short-term
   set of credentials using the Shared Secret request and then use those
   as the credentials in the Allocate request.

   The client SHOULD include a BANDWIDTH attribute, which indicates the
   maximum bandwidth that will be used with this binding.  If the
   maximum is unknown, the attribute is not included in the request.

   The client MAY request a particular lifetime for the allocation by
   including it in the LIFETIME attribute in the request.  The default
   lifetime is 10 minutes.

   The client MAY include a REQUESTED-PORT-PROPS, REQUESTED-TRANSPORT,
   or REQUESTED-IP attribute in the request to obtain specific types of
   transport addresses.  Whether these are needed depends on the
   application using the relay usage.  As an example, the Real Time
   Transport Protocol (RTP) [3] requires that RTP and RTCP ports be an



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   adajacent pair, even and odd respectively, for compatibility with a
   previous version of that specification.  The REQUESTED-PORT-PROPS
   attribute allows the client to ask the relay for those properties.
   The client MUST NOT request the TCP transport in an Allocate request
   sent to the STUN relay server over UDP.

   Processing of the response follows the general procedures of [1].  A
   successful response will include both a RELAY-ADDRESS and an XOR-
   MAPPED-ADDRESS attribute, providing both a relayed transport address
   and a reflexive transport address, respectively, to the client.  The
   server will expire the allocation after LIFETIME seconds have passed
   if not refreshed by another Allocate request.  The server will allow
   the user to send and receive at least the amount of data indicated in
   the BANDWIDTH attribute per allocation.  (At its discretion the
   server can optionally discard data above this threshold.)

   If the response is an error response and contains a 442, 443 or 444
   error code, the client knows that its requested properties could not
   be met.  The client MAY retry with different properties, with the
   same properties (in a hope that something has changed on the server),
   or give up, depending on the needs of the application.  However, if
   the client retries, it SHOULD wait 500ms, and if the request fails
   again, wait 1 second, then 2 seconds, and so on, exponentially
   backing off.

6.1.1.2.  Subsequent Requests

   Before 3/4 of the lifetime of the allocation has passed (the lifetime
   of the allocation is conveyed in the LIFETIME attribute of the
   Allocate Response), the client SHOULD refresh the allocation with
   another Allocate Request if it wishes to keep the allocation.

   To perform a refresh, the client generates an Allocate Request as
   described in Section 6.1.1.1.  If the initial request was
   authenticated with a shared secret P that the client holds with the
   server, or using a short term password derived from P through a
   Shared Secret request, the client MUST use shared secret P, or a
   short-term password derived from it, in the subsequent request.

   In a successful response, the RELAY-ADDRESS contains the same
   transport address as previously obtained, indicating that the binding
   has been refreshed.  The LIFETIME attribute indicates the amount of
   additional time the binding will live without being refreshed.  Note
   that an error response does not imply that the binding has been
   expired, just that the refresh has failed.

   If a client no longer needs an allocation, it SHOULD perform an
   explict deallocation.  If the client wishes to explicitly remove the



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   allocation because it no longer needs it, it generates a subsequent
   Allocate request, but sets the LIFETIME attribute to zero.  This will
   cause the server to remove the allocation, and all associated
   bindings.  For connection-oriented transports such as TCP, the client
   can also remove the allocation (and all associated bindings) by
   closing the relevant connection with the STUN relay server.

6.1.2.  Server Behavior

   The server first processes the request according to the general
   request processing rules in [1].  This includes performing
   authentication, and checking for mandatory unknown attributes.  Due
   to the fact that the STUN server is allocating resources for
   processing the request, Allocate requests MUST be authenticated, and
   furthermore, MUST be authenticated using either a shared secret known
   between the client and server, or a short term password derived from
   it.

      Note that Allocate requests, like most other STUN requests, can be
      sent to the STUN server over UDP, TCP, or TCP/TLS.

   The behavior of the server when receiving an Allocate Request depends
   on whether the request is an initial one, or a subsequent one.  An
   initial request is one whose source and destination transport address
   do not match the internal remote and local transport addresses of an
   existing internal 5-tuple.  A subsequent request is one whose source
   and destination transport address matches the internal remote and
   local transport address of an existing internal 5-tuple.

6.1.2.1.  Initial Requests

   The server attempts to allocate transport addresses.  It first looks
   for the BANDWIDTH attribute for the request.  If present, the server
   determines whether or not it has sufficient capacity to handle a
   binding that will generate the requested bandwidth.

   If it does, the server attempts to allocate a transport address for
   the client.  The Allocate request can contain several additional
   attributes that allow the client to request specific characteristics
   of the transport address.  First, the server checks for the
   REQUESTED-TRANSPORT attribute.  This indicates the transport protocol
   requested by the client.  This specification defines values for UDP
   and TCP.

      As a consequence of the REQUESTED-TRANSPORT attribute, it is
      possible for a client to connect to the server over TCP or TLS
      over TCP and request a UDP transport address.  In this case, the
      server will relay data between the transports.



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   If the requested transport is supported, the server allocates a port
   using the requested transport protocol.  If the REQUESTED-TRANSPORT
   attribute contains a value of the transport protocol unknown to the
   server, or known to the server but not supported by the server in the
   context of this request, the server MUST reject the request and
   include a 442 (Unsupported Transport Protocol) in the response, or
   redirect the request.  If the request did not contain a REQUESTED-
   TRANSPORT attribute, the server MUST use the same transport protocol
   as the request arrived on.

   Next, the server checks for the REQUESTED-IP attribute.  If present,
   it indicates a specific interface from which the client would like
   its transport address allocated.  If this interface is not a valid
   one for allocations on the server, the server MUST reject the request
   and include a 443 (Invalid IP Address) error code in the response, or
   else redirect the request to a server that is known to support this
   IP address.  If the IP address is one that is valid for allocations
   (presumably, the server is configured to know the set of IP addresses
   from which it performs allocations), the server MUST provide an
   allocation from that IP address.  If the attribute is not present,
   the selection of an IP address is at the discretion of the server.

   Finally, the server checks for the REQUESTED-PORT-PROPS attribute.
   If present, it indicates specific port properties desired by the
   client.  This attribute is split into two portions: one portion for
   port behavior and the other for requested port alignment (whether the
   allocated port is odd, even, reserved as a pair, or at the discretion
   of the server).

   If the port behavior requested is for a Specific Port, the server
   MUST attempt to allocate that specific port for the client.  If the
   port is allocated to a different internal 5-tuple associated with the
   same STUN long-term credentials, the client is requesting a move.
   The server SHOULD replace the old internal 5-tuple with the new tuple
   over which this Allocate request arrived.  The server MUST reject the
   move request if any of the attributes other than LIFETIME have
   changed (BANDWIDTH, REQUESTED-TRANSPORT, etc.).

   If the specific port is not available (in use or reserved), the
   server MUST reject the request with a 444 (Invalid Port) response or
   redirect to an alternate server.  For example, the STUN server could
   reject a request for a Specific Port because the port is temporarily
   reserved as part of an adjacent pair of ports, or because the
   requested port is a well-known port (1-1023).

   If the client requests "even" port alignment, the server MUST attempt
   to allocate an even port for the client.  If an even port cannot be
   obtained, the server MUST reject the request with a 444 (Invalid



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   Port) response or redirect to an alternate server.  If the client
   requests odd port alignment, the server MUST attempt to allocate an
   odd port for the client.  If an odd port cannot be obtained, the
   server MUST reject the request with a 444 (Invalid Port) response or
   redirect to an alternate server.  Finally, the "Even port with hold
   of the next higher port" alignment is similar to requesting an even
   port.  It is a request for an even port, and MUST be rejected by the
   server if an even port cannot be provided, or redirected to an
   alternate server.  However, it is also a hint from the client that
   the client will request the next higher port with a separate Allocate
   request.  As such, it is a request for the server to allocate an even
   port whose next higher port is also available, and furthermore, a
   request for the server to not allocate that one higher port to any
   other request except for one that asks for that port explicitly.  The
   server can honor this request for adjacency at its discretion.  The
   only constraint is that the allocated port has to be even.

      Port alignment requests exist for compatibility with
      implementations of RTP which pre-date RFC 3550.  These
      implementations use the port numbering conventions in (now
      obsolete) RFC 1889.

   If any of the requested or desired constraints cannot be met, whether
   it be bandwidth, transport protocol, IP address or port, instead of
   rejecting the request, the server can alternately redirect the client
   to a different server that may be able to fulfill the request.  This
   is accomplished using the 300 error response and ALTERNATE-SERVER
   attribute.  If the server does not redirect and cannot service the
   request because the server has reached capacity, it sends a 507
   (Insufficient Capacity) response.  The server can also reject the
   request with a 486 (Allocation Quota Reached) if the user or client
   is not authorized to request additional allocations.

   The server SHOULD only allocate ports in the range 1024-65535.  This
   is one of several ways to prohibit relayed transport addresses from
   being used to attempt to run standard services.  These guidelines are
   meant to be consistent with [10], since the relay is effectively a
   NAT.

   Once the port is allocated, the server associates it with the
   internal 5-tuple and fills in that 5-tuple.  The internal remote
   transport address of the internal 5-tuple is set to the source
   transport address of the Allocate Request.  The internal local
   transport address of the internal 5-tuple is set to the destination
   transport address of the Allocate Request.  For TCP, this amounts to
   associating the TCP connection from the STUN relay client with the
   allocated transport address.




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   If the Allocate request was authenticated using a shared secret
   between the client and server, this credential MUST be associated
   with the allocation.  If the request was authenticated using a short
   term password derived from a shared secret, that shared secret MUST
   be associated with the allocation.  This is used in all subsequent
   requests and indications to ensure that only the same client can use
   or modify the allocation it was given.

   The allocation created by the Allocate request is also associated
   with a transport address, called the active destination.  This
   transport address is used for forwarding data through the STUN relay
   server, and is described in more detail later.  It is initially set
   to null when the allocation is created.  In addition, the allocation
   created by the server is associated with a set of permissions.  Each
   permission is a specific IP address identifying an external client.
   Initially, this list is null.

   If the LIFETIME attribute was present in the request, and the value
   is larger than the maximum duration the server is willing to use for
   the lifetime of the allocation, the server MAY lower it to that
   maximum.  However, the server MUST NOT increase the duration
   requested in the LIFETIME attribute.  If there was no LIFETIME
   attribute, the server may choose a default duration at its
   discretion.  In either case, the resulting duration is added to the
   current time, and a timer, called the allocation expiration timer, is
   set to fire at or after that time.  Section 10.3 discusses behavior
   when the timer fires.  Note that the LIFETIME attribute in the
   request can be zero.  This typically happens for subsequent
   Allocations, and provides a mechanism to delete the allocation.  It
   will force the immediate deletion of the allocation.

   Once the port has been obtained and the activity timer started for
   the port binding, the server generates an Allocate Response using the
   general procedures defined in [1].  The transport address allocated
   to the client MUST be included in the RELAY-ADDRESS attribute in the
   response.  In addition, this response MUST contain the XOR-MAPPED-
   ADDRESS attribute.  This allows the client to determine its reflexive
   transport address in addition to a relayed transport address, from
   the same Allocate request.

   The server MUST add a LIFETIME attribute to the Allocate Response.
   This attribute contains the duration, in seconds, of the allocation
   expiration timer associated with this allocation.

   The server MUST add a BANDWIDTH attribute to the Allocate Response.
   This MUST be equal to the attribute from the request, if one was
   present.  Otherwise, it indicates a per-binding cap that the server
   is placing on the bandwidth usage on each binding.  Such caps are



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   needed to prevent against denial-of-service attacks (See Section 12).

   The server MUST add, as the final attribute of the request, a
   MESSAGE-INTEGRITY attribute.  The key used in the HMAC MUST be the
   same as that used to validate the request.

6.1.2.2.  Subsequent Requests

   A subsequent Allocate request is one received whose source and
   destination IP address and ports match the internal 5-tuple of an
   existing allocation.  The request is processed using the general
   server procedures in [1] and is processed identically to
   Section 6.1.2.1, with a few important exceptions.

   First, the request MUST be authenticated using the same shared secret
   as the one associated with the allocation, or be authenticated using
   a short term password derived from that shared secret.  If the
   request was authenticated but not with such a matching credential,
   the server MUST generate an Allocate Error Response with an
   appropriate error response code.

   Secondly, if the allocated transport address given out previously to
   the client still matches the constraints in the request (in terms of
   request ports, IP addresses and transport protocols), the same
   allocation granted previously MUST be returned.  However, if one of
   the constraints is not met any longer, because the client changed
   some aspect of the request, the server MUST free the previous
   allocation and allocate a new request to the client.

   Finally, a subsequent Allocate request will set a new allocation
   expiration timer for the allocation, effectively canceling the
   previous lifetime expiration timer.

6.2.  Procedures for all other Requests and Indications

   Other than initial Allocate Requests, all requests and indications
   defined by the relay usage need to be sent in the context of a valid
   allocation.  The source and destination IP address and ports for
   these STUN messages MUST match the internal 5-tuple of an existing
   allocation.  These processed using the general server procedures in
   [1] with a few important additions.  For requests, if there is no
   matching allocation, the server MUST generate a 437 (No Binding) Send
   Error Response.  For indications, if there is no matching allocation,
   the indication is silently discarded.

   All requests and indications MUST be authenticated using the same
   shared secret as the one associated with the allocation, or be
   authenticated using a short term password derived from that shared



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   secret.  If the request was authenticated but not with such a
   matching credential, the server MUST generate an Allocate Error
   Response with an appropriate error response code, such as a 431
   (Integrity Failure) or 436 (Unknown User).

6.3.  Set Active Destination Request

6.3.1.  Client Behavior

   The Set Active Destination request allows the client to create an
   optimized relay function between the client and the server.  When the
   server receives packets from a particular preferred external peer,
   the server will forward those packets towards the client without
   encapsulating them in a Data Indication.  Similarly, the client can
   send non-STUN packets to the server without encapsulation, and these
   are forwarded to the external peer.  Sending and receiving data in
   unencapsulated form is critical for efficiency purposes.  One of the
   primary use cases for the STUN relay usage is in support of Voice
   over IP (VoIP), which uses very small UDP packets to begin with.  The
   extra overhead of an additional layer of encapsulation is considered
   unacceptable.

   The Set Active Destination request is used by the client to provide
   the identity of this preferred external peer.  The Set Active
   Destination address MAY contain a REMOTE-ADDRESS attribute.  This
   attribute, when present, provides the address of the preferred
   external peer to the server.  When absent, it clears the value of the
   preferred external peer.  As a convenience, if the client sets the
   REMOTE-ADDRESS attribute to a peer without a permission, the server
   will add the corresponding permission.

   The client MUST NOT send a Set Active Destination request with a
   REMOTE-ADDRESS attribute over an unreliable link (ex: UDP) if an
   active destination is already set for that allocation.  If the client
   wishes to set a new active destination, it MUST wait until 5 seconds
   after a successful response is received to a Set Destination Request
   removing the active destination.  Failure to wait could cause the
   client to receive and attribute late data forwarded by the STUN relay
   server to the wrong peer.

      Consider the case where the active destination is set, and the
      server is relaying packets towards the client.  The client knows
      the IP address and port where the packets came from - the current
      value of the active destination.  The client issues a Set Active
      Destination Request to change the active destination, and receives
      a response.  A moment later, a data packet is received, not
      encapsulated in a STUN Data Indication.  What is the source if
      this packet?  Is it the active destination that existed prior to



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      the Set Active Destination request, or the one after?  If the
      transport between the client and the STUN server is not reliable,
      there is no way to know.

6.3.2.  Server Behavior

   The Set Active Destination Request is used by a client to set the
   forwarding destination of all data that is not encapsulated in STUN
   Send Indications.  In addition, when a matching permission is
   present, all data received from that external peer will be forwarded
   to the STUN client without being encapsulated in a Data Indication.

   If the Set Active Destination request does not contain a REMOTE-
   ADDRESS attribute, the value of the active destination is cleared.
   If the Set Active Destination request contains a REMOTE-ADDRESS
   attribute, and the active destination is not set, the active
   destination is set to that IP address and port.  If an active
   destination is already set, and the request was received over a
   reliable transport, the active destination is changed to the new
   value.  If the active destination is already set and the request was
   received over UDP, the Set Active Destination request is rejected
   with a 439 Active Destination Already Set error response.  This
   prevents the race condition described in the previous section.

   If the server sets the active destination and there is no permission
   associated with the REMOTE-ADDRESS, the server adds the corresponding
   permission.  Note that if the permission associated with the active
   destination becomes invalid, the server does not reset the active
   destination.  The client is expected to do this explicitly.

6.4.  Connect Request

   The Connect Request is used by a client when it has obtained an
   allocated transport address that is TCP.  The client can use the
   Connect Request to ask the server to open a TCP connection to a
   specified destination address included in the request.

6.4.1.  Server Behavior

   If the allocation is for a UDP port, the server MUST reject the
   request with a 445 (Operation for TCP Only) response.  If the request
   does not contain a REMOTE-ADDRESS attribute, the server sends a 400
   (Bad Request) Connect error response,.

   If the request contains a REMOTE-ADDRESS attribute, the IP address
   contained within it is added to the permissions for this allocation,
   if it was not already present.  This happens regardless of whether
   the subsequent TCP connection attempt succeeds or not.



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   If a connection already exists for this address and port, the server
   returns a 446 (Connection Already Exists) Connect error response.
   Otherwise the server tries to establish the corresponding TCP
   connection and returns a Connect Success Response.  This just
   indicates that the server added the permission and is attempting to
   establish a TCP connection.  The server does not wait for the
   connection attempt to succeed or fail.  The status of the connection
   attempt is returned via the Connect Status Indication.

      Note that the server needs to use the same source connection
      address for all connections/permissions associated with an
      allocation.  For servers written using Berkeley sockets, the
      SO_REUSEADDR flag is typically used to use the same local address
      with multiple sockets.

6.5.  Connection Status Indication

   TODO: Expand this text.

   When the STUN relay to peer leg is TCP, the STUN relay client needs
   to be aware of the status of these TCP connections.  The STUN relay
   extension defines application states for a TCP connection as follows:
   LISTEN, ESTABLISHED, CLOSED.  Consequently, the STUN relay server
   sends a Connection State Indication for a TCP permission whenever the
   relay connection status changes for one of the client's permissions
   except when the status changes due to a STUN relay client request
   (ex: an explicit binding close or deallocation).

      A STUN relay can only relay to a peer over TCP if the client
      communicates with the server over TCP or TLS over TCP.  Because of
      this, the server can be assured that Connection Status Indications
      are received reliably.

6.6.  Send Indication

6.6.1.  Client Behavior

   The Send Indication is used to ask the relay to forward data to a
   peer.  It is typically used to send to a peer other than the active
   destination.  For TCP allocated transport addresses, the client needs
   to wait for the peer to open a connection to the STUN relay server
   before it can send data.  Data sent with a Send request prior to the
   opening of a TCP connection is discarded silently by the server.

   The Send Indication MUST contain a REMOTE-ADDRESS attribute, which
   contains the IP address and port that the data is being sent to.  The
   DATA attribute MAY be present, and contains the data that is to be
   sent towards REMOTE-ADDRESS.  If absent, the server will send an



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   empty UDP packet in the case of UDP.  In the case of TCP, the server
   will do nothing.

   Since Send is an Indication, it generates no response.  The client
   must rely on application layer mechanisms to determine if the data
   was received by the peer.

      Note that Send Indications are not authenticated and do not
      contain a MESSAGE-INTEGRITY attribute.  Just like non-relayed data
      sent over UDP or TCP, the authenticity and integrity of this data
      can only be assured using security mechanisms at higher layers.

6.6.2.  Server Behavior

   A Send Indication is sent by a client after it has completed its
   Allocate transaction, in order to create permissions in the server
   and send data to an external client.

   If a Send Indication contains no REMOTE-ADDRESS, the indication is
   discarded.  If there is no DATA attribute, and the corresponding
   allocation is for TCP, the indication is discarded.

   If the allocation is a UDP allocation, the server creates a UDP
   packet whose payload equals that content.  The server sets the source
   IP address of the packet equal to the allocated transport address.
   The destination transport address is set to the contents of the
   REMOTE-ADDRESS attribute.  If a permission does not exist for this
   destination the server creates one for this allocation.  The server
   then sends the UDP packet.  Note that any retransmissions of this
   packet which might be needed are not handled by the server.  It is
   the responsibility of the client to generate another Send indication
   if needed.

   If the allocation is a TCP allocation, the server checks if it has an
   existing TCP connection open from the allocated transport address to
   the address in the REMOTE-ADDRESS attribute.  If so, the server
   extracts the content of the DATA attribute and sends it over the
   matching TCP connection.  If the server doesn't have an existing TCP
   connection to the destination, it adds the REMOTE-ADDRESS to the
   permission list and discards the data.

6.7.  Data Indication

6.7.1.  Client Behavior

   Once a client has obtained an allocation and created permissions for
   a particular external client, the server can begin to relay packets
   from that external client towards the client.  If the external client



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   is not the active destination, this data is relayed towards the
   client in encapsulated form using the Data Indication.

   The Data Indication contains two attributes - DATA and REMOTE-
   ADDRESS.  The REMOTE-ADDRESS attribute indicates the source transport
   address that the request came from, and it will equal the external
   remote transport address of the external peer.  When processing this
   data, a client MUST treat the data as if it came from this address,
   rather than the stun server itself.  The DATA attribute contains the
   data from the UDP packet or TCP segment that was received.  Note that
   the STUN relay server will not retransmit this indication over UDP.

      Note that Data Indications are not authenticated and do not
      contain a MESSAGE-INTEGRITY attribute.  Just like non-relayed data
      sent over UDP or TCP, the authenticity and integrity of this data
      can only be assured using security mechanisms at higher layers.

6.7.2.  Server Behavior

   A server MUST send data packets towards the client using a Data
   Indication under the conditions described in Section 10.1.  Data
   Indications MUST contain a DATA attribute containing the data to
   send, and MUST contain a REMOTE-ADDRESS attribute indicating where
   the data came from.


7.  New Attributes

   The STUN relay usage defines the following new attributes:


   0x000D: LIFETIME
   0x0010: BANDWIDTH
   0x0012: REMOTE-ADDRESS
   0x0013: DATA
   0x0016: RELAY-ADDRESS
   0x0018: REQUESTED-PORT-PROPS
   0x0019: REQUESTED-TRANSPORT
   0x0022: REQUESTED-IP
   0x0021: TIMER-VAL
   0x0023: CONNECT_STAT

7.1.  LIFETIME

   The lifetime attribute represents the duration for which the server
   will maintain an allocation in the absence of data traffic either
   from or to the client.  It is a 32 bit value representing the number
   of seconds remaining until expiration.



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Lifetime                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.2.  BANDWIDTH

   The bandwidth attribute represents the peak bandwidth, measured in
   kbits per second, that the client expects to use on the binding in
   each direction.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bandwidth                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.3.  REMOTE-ADDRESS

   The REMOTE-ADDRESS specifies the address and port of the peer as seen
   from the STUN relay server.  It is encoded in the same way as MAPPED-
   ADDRESS.

7.4.  DATA

   The DATA attribute is present in Send Indications and Data
   Indications.  It contains raw payload data that is to be sent (in the
   case of a Send Request) or was received (in the case of a Data
   Indication).  It is padded with zeros if its length is not divisible
   evenly by 4 octets

7.5.  RELAY-ADDRESS

   The RELAY-ADDRESS is present in Allocate responses.  It specifies the
   address and port that the server allocated to the client.  It is
   encoded in the same way as MAPPED-ADDRESS.

7.6.  REQUESTED-PORT-PROPS

   This attribute allows the client to request certain properties for
   the port that is allocated by the server.  The attribute can be used
   with any transport protocol that has the notion of a 16 bit port
   space (including TCP and UDP).  The attribute is 32 bits long.  Its
   format is:









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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Reserved = 0        | A |    Specific Port Number       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The two bits labeled A in the diagram above are for requested port
   alignment and have the following meaning:

   00 no specific port alignment
   01 odd port number
   10 even port number
   11 even port number; reserve next higher port

   If the Specific Port Number field is zero, this means that no
   specific port is requested.  If a specific port number is requested
   the value will be in the two low order octets.  All other bits in
   this attribute are reserved and MUST be set to zero.

   Even Port is a request to the server to allocate a port with even
   parity.  The port filter is not used with this property.  Odd Port is
   a request to the server to allocate a port with odd parity.  The port
   filter is not used with this property.  Even port, with a hold on the
   next higher port, is a request to the server to allocate an even
   port.  Furthermore, the client indicates that it will want the next
   higher port as well.  As such, the client requests that the server,
   if it can, not allocate the next higher port to anyone unless that
   port is explicitly requested, which the client will itself do.  The
   port filter is not used with this property.  Finally, the Specific
   Port property is a request for a specific port.  The port that is
   requested is contained in the Port filter.

7.7.  REQUESTED-TRANSPORT

   This attribute is used by the client to request a specific transport
   protocol for the allocated transport address.  It is a 32 bit
   unsigned integer.  Its values are:


   0x0000 0000: UDP
   0x0000 0001: TCP

   If an Allocate request is sent over TCP and requests a UDP
   allocation, or an Allocate request is sent over TLS over TCP and
   requests a UDP or TCP allocation, the server will relay data between
   the two transports.

   Extensions to the relay usage can define additional transport



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   protocols in an IETF-consensus RFC.

7.8.  REQUESTED-IP

   The REQUESTED-IP attribute is used by the client to request that a
   specific IP address be allocated to it.  This attribute is needed
   since it is anticipated that STUN relays will be multi-homed so as to
   be able to allocate more than 64k transport addresses.  As a
   consequence, a client needing a second transport address on the same
   interface as a previous one can make that request.

   The format of this attribute is identical to MAPPED-ADDRESS.
   However, the port component of the attribute is ignored by the
   server.  If a client wishes to request a specific IP address and
   port, it uses both the REQUESTED-IP and REQUESTED-PORT-PROPS
   attributes.

7.9.  CONNECT_STAT

   This attribute us used by the server to convey the status of server-
   to-peer connections.  It is a 32 bit unsigned integer.  Its values
   are:


   0x0000 0000: LISTEN
   0x0000 0001: ESTABLISHED
   0x0000 0002: CLOSED


8.  New Error Response Codes

   The STUN relay usage defines the following new Error response codes:

      437 (No Binding): A request was received by the server that
      requires an allocation to be in place.  However, there is none yet
      in place.

      439 (Active Destination Already Set): A Set Active Destination
      request was received by the server over UDP.  However, the active
      destination is already set to another value.  The client should
      reset the active destination, wait for 5 seconds and set the
      active destination to the new value.

      442 (Unsupported Transport Protocol): The Allocate request asked
      for a transport protocol to be allocated that is not supported by
      the server.





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      443 (Invalid IP Address): The Allocate request asked for a
      transport address to be allocated from a specific IP address that
      is not valid on the server.

      444 (Invalid Port): The Allocate request asked for a port to be
      allocated that is not available on the server.

      445 (Operation for TCP Only): The client tried to send a request
      to perform a TCP-only operation on an allocation, and allocation
      is UDP.

      446 (Connection Already Exists): The client tried to open a
      connection to a peer, but a connection to that peer already
      exists.

      486 (Allocation Quota Reached): The user or client is not
      authorized to request additional allocations.

      507 (Insufficient Capacity): The server cannot allocate a new port
      for this client as it has exhausted its relay capacity.


9.  Client Procedures

9.1.  Receiving and Sending Unencapsulated Data

   Once the active destination has been set, a client will receive both
   STUN and non-STUN data on the socket on which the Allocate Request
   was sent.  The encapsulation behavior depends on the transport
   protocol used between the STUN client and the STUN relay server.

9.1.1.  Datagram Protocols

   If the allocation was over UDP, datagrams which contain the STUN
   magic cookie are treated as STUN requests.  All other data is non-
   STUN data, which MUST be processed as if it had a source IP address
   and port equal to the value of the active destination.

   If the client wants to send data to the peer which contains the magic
   cookie in the same location as a STUN request, it MUST send that data
   encapsulated in a Send Indication, even if the active destination is
   set.

   In addition, once the active destination has been set, the client can
   send data to the active destination by sending the data
   unencapsulated on that same socket.  Unencapsulated data MUST NOT be
   sent if no active destination is set.  Of course, even if the active
   destination is set, the client can send data to that destination at



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   any time by using the Send Indication.

9.1.2.  Stream Transport Protocols

   If the allocation was over TCP or TLS over TCP, the client will
   receive data framed as described in Section 5.  The client MUST treat
   data encapsulated as data with this framing as if it originated from
   the active destination.

   For the STUN relay usage, the client always sends data encapsulated
   using this framing scheme.  It SHOULD frame data to the active
   destination as data or it MAY place the data inside a Send
   Indications and frame this as STUN traffic.


10.  Server Procedures

   Besides the processing of the request and indications described
   above, this specification defines rules for processing of data
   packets received by the STUN server.  There are two cases - receipt
   of any packets on an allocated address, and receipt of non-STUN data
   on its internal local transport address.

10.1.  Receiving Data on Allocated Transport Addresses

10.1.1.  TCP Processing

   If a server receives a TCP connection request on an allocated TCP
   transport address, it checks the permissions associated with that
   allocation.  If the source IP address of the TCP SYN packet matches
   one of the permissions, the TCP connection is accepted.  Otherwise,
   it is rejected.  When a TCP connection is accepted, the server sends
   the corresponding client a Connect Status Indication with the
   CONNECT_STAT attribute set to ESTABLISHED.  No information is passed
   to the client if the server rejects the connection because there is
   no corresponding permission.

   If a server receives data on a TCP connection that terminates on the
   allocated TCP transport address, the server checks the value of the
   active destination.  If it equals the source IP address and port of
   the data packet (in other words, if the active destination identifies
   the other side of the TCP connection), the data is taken from the TCP
   connection and sent towards the client in unencapsulated form.
   Otherwise, the data is sent towards the client in a Data Indication,
   also known as encapsulated form.  In this form, the server MUST add a
   REMOTE-ADDRESS which corresponds to the external remote transport
   address of the TCP connection, and MUST add a DATA attribute
   containing the data received on the TCP connection.



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      Note that, because data is forwarded blindly across TCP bindings,
      TLS will successfully operate over a STUN relay allocated TCP port
      if the linkage to the client is also TCP.

10.1.2.  UDP Processing

   If a server receives a UDP packet on an allocated UDP transport
   address, it checks the permissions associated with that allocation.
   If the source IP address of the UDP packet matches one of the
   permissions, the UDP packet is accepted.  Otherwise, it is discarded.
   If the packet is accepted, it is forwarded to the client.  It will be
   forwarded in either encapsulated or unencapsulated form.

   If the client to server communication is via UDP, the server looks
   for the existence of the STUN magic cookie in the data received from
   the peer.  If the data contains the magic cookie, the server
   encapsulates the data in a Data Indication, sets the REMOTE_ADDRESS
   attribute, and forwards the indication to the client.  If the magic
   cookie is not present, the server checks if the peer is the active
   destination.  If so the data is forwarded unencapsulated, directly to
   the client.  Otherwise the server encapsulates the data in a Data
   Indication, sets the REMOTE_ADDRESS and forwards to the client.

   If the client to server communication is via TCP or TLS, the server
   checks if the peer is the active destination.  If so, the data from
   the peer is framed as Data and sent to the client over the client to
   server connection.  Otherwise, the server encapsulates the data in a
   Data Indication, sets the REMOTE_ADDRESS attribute, frames the
   indication as STUN traffic, and sends the indication over the
   connection to the client.  If the TCP connection generates an error
   (because, for example, the incoming UDP packet rate exceeds the
   throughput of the TCP connection), the data is discarded silently by
   the server.

10.2.  Receiving Data on Internal Local Transport Addresses

   If a server receives non-STUN UDP data from the client on its
   internal local transport address, and it is coming from an internal
   remote transport address associated with an existing allocation, it
   represents UDP data that the client wishes to forward.  If the active
   destination is not set, the server MUST discard the packet.  If the
   active destination is set, the server places the data from the client
   in a UDP payload, and sets the destination address and port to the
   active destination.  The UDP packet is then sent with a source IP
   address and port equal to the allocated transport address.  Note that
   the server will not retransmit the UDP datagram.

   If a server receives framed data on a TCP connection from a client,



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   the server retrieves the allocation bound to that connection.  If the
   active destination for the allocation is not set, the server MUST
   discard the data and close the connection.  If the active destination
   is set, and the allocated transport protocol is TCP, the server
   forwards the data over the connection to the active destination.  The
   data is then sent over that connection.  If the connection is not
   established or if the transmission fails due to a TCP error, the data
   is discarded silently by the server.  If the active destination is
   set, and the allocated transport protocol is UDP, the server places
   the data from the client in a UDP payload, and sets the destination
   address and port to the active destination.  The UDP packet is then
   sent with a source IP address and port equal to the allocated
   transport address.  Note that the server will not retransmit the UDP
   datagram.

   If a TCP connection from a client is closed, the associated
   allocation is destroyed.  This involves terminating any TCP
   connections from the allocated transport address to external peer
   (applicable only when the allocated transport address was TCP), and
   then freeing the allocated transport address (and all associated
   state maintained by the server) for use by other clients.

10.3.  Lifetime Expiration

   When the allocation expiration timer for a binding fires, the server
   MUST destroy the allocation.  This involves terminating any TCP
   connections from the allocated transport address to external peers
   (applicable only when the allocated transport address was TCP), and
   then freeing the allocated transport address (and all associated
   state maintained by the server) for use by other clients.  A
   suggested value for the allocation expiration timer is 10 minutes.

   The server is also expected to run a permission inactivity timer for
   each permission associated with an Allocation.  If no traffic from
   the client is received, the permission inactivity timer will
   eventually expire and the server MUST delete the permission.  A
   suggested value for the permission inactivity timer is 60 seconds.


11.  Formal Definition of STUN Usage

11.1.  Applicability Statement

   STUN requires all usages to define the applicability of the usage
   [1].  This section contains that information for the relay usage.

   The relayed transport address obtained from the Allocate request has
   specific properties which limit its applicability.  The transport



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   address will only be useful for applications that require a client to
   place a transport address into a protocol message, with the
   expectation that the client will be able to receive packets from a
   small number of hosts (typically one).  Data from the peer is only
   relayed to the client after the client sends packets towards the
   peer.  Because of this limitation, relayed transport addresses
   obtained from an Allocate request are only useful when combined with
   rendezvous protocols of some sort, which allow the client to discover
   the addresses of the hosts it will be corresponding with.  Examples
   of such protocols include the Session Initiation Protocol (SIP) [4].

   This limitation is purposeful.  Relayed transport addresses obtained
   from the Allocate request can not be used to run general purpose
   servers, such as a web or email server.  This means that the relay
   usage can be safely permitted to pass through NATs and firewalls
   without fear of compromising the purpose of having them there in the
   first place.  Indeed, a relayed transport address obtained from a
   STUN relay has many of the properties of a transport address obtained
   from a NAT whose filtering policies are address dependent, but whose
   mapping properties are endpoint independent [10], and thus "good"
   NATs.  Indeed, to some degree, the relay turns a bad NAT into a good
   NAT by, quite ironically, adding another NAT function - the relay
   itself.

11.2.  Client Discovery of Server

   STUN requires all usages to define the mechanism by which a client
   discovers the server [1].  This section contains that information for
   the relay usage.

   The relay usage differs from the other usages defined in [1] in that
   it demands substantial resources from the STUN server.  In addition,
   it seems likely that administrators might want to block connections
   from clients to the STUN server for relaying separately from
   connections for the purposes of binding discovery.  As a consequence,
   the relay usage is expected to typically run on a separate port from
   other usages.  The client discovers the address and port of the STUN
   server for the relay usage using the same DNS procedures defined in
   [1], but using an SRV service name of "stun-relay" instead of just
   "stun".

   For example, to find STUN relay servers in the example.com domain,
   the STUN relay client performs a lookup for '_stun-
   relay._udp.example.com', '_stun-relay._tcp.example.com', and '_stun-
   relay._tls.example.com' if the STUN client wants to communicate with
   the STUN relay server using UDP, TCP, or TLS over TCP, respectively.
   The client assumes that all permissable transport protocols are
   supported from the STUN relay server to the peer for the client to



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   server protocol selected.

11.3.  Server Determination of Usage

   STUN requires all usages to define the mechanism by which the server
   determines the usage [1].  This section contains that information for
   the relay usage.

   The STUN server is designed so the relay usage can run on a separate
   source port from non-relay usages.  Since the client looks up the
   port number for the relay usage separately, servers can be configured
   to rely on this property.  The STUN server MAY accept both relay and
   non-relay usages on the same port number, in which case it uses
   framing hints and choice of STUN messages to detect the STUN usage in
   use by a specific client.

   The relay usage is defined by a specific set of requests and
   indications.  As a consequence, the server knows that this usage is
   being used because those request and indications were used.  Over
   UDP, once an active destination has been set, the server also needs
   to check the source address and port of a datagram to determine if
   that source tuple is allocated for the relay usage.  For stream-based
   protocols, the server can recognize STUN relay traffic from other
   usages, since STUN relay traffic on these transports always uses the
   framing described in the next section (Section 5).


12.  Security Considerations

   TODO: Need to spend more time on this.

   STUN servers implementing this relay usage allocate bandwidth and
   port resources to clients, in contrast to the usages defined in [1].
   Therefore, a STUN server providing the relay usage requires
   authentication and authorization of STUN requests.  This
   authentication is provided by mechanisms defined in the STUN
   specification itself.  In particular, digest authentication and the
   usage of short-term passwords, obtained through a digest exchange
   over TLS, are available.  The usage of short-tem passwords ensures
   that the Allocate Requests, which often do not run over TLS, are not
   susceptible to offline dictionary attacks that can be used to guess
   the long lived shared secret between the client and the server.

   Because STUN servers implementing the relay usage allocate resources,
   they can be susceptible to denial-of-service attacks.  All Allocate
   Requests are authenticated, so that an unknown attacker cannot launch
   an attack.  An authenticated attacker can generate multiple Allocate
   Requests, however.  To prevent a single malicious user from



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   allocating all of the resources on the server, it is RECOMMENDED that
   a server implement a modest per user cap on the amount of bandwidth
   that can be allocated.  Such a mechanism does not prevent a large
   number of malicious users from each requesting a small number of
   allocations.  Attacks as these are possible using botnets, and are
   difficult to detect and prevent.  Implementors of the STUN relay
   usage should keep up with best practices around detection of
   anomalous botnet attacks.

   A client will use the transport address learned from the RELAY-
   ADDRESS attribute of the Allocate Response to tell other users how to
   reach them.  Therefore, a client needs to be certain that this
   address is valid, and will actually route to them.  Such validation
   occurs through the message integrity checks provided in the Allocate
   response.  They can guarantee the authenticity and integrity of the
   allocated addresses.  Note that the STUN relay usage is not
   susceptible to the attacks described in Section 12.2.3, 12.2.4,
   12.2.5 or 12.2.6 of RFC 3489 [[TODO: Update references once 3489bis
   is more stable]].  These attacks are based on the fact that a STUN
   server mirrors the source IP address, which cannot be authenticated.
   STUN does not use the source address of the Allocate Request in
   providing the RELAY-ADDRESS, and therefore, those attacks do not
   apply.

   The relay usage cannot be used by clients for subverting firewall
   policies.  The relay usage has fairly limited applicability,
   requiring a user to send a packet to a peer before being able to
   receive a packet from that peer.  This applies to both TCP and UDP.
   Thus, it does not provide a general technique for externalizing TCP
   and UDP sockets.  Rather, it has similar security properties to the
   placement of an address-restricted NAT in the network, allowing
   messaging in from a peer only if the internal client has sent a
   packet out towards the IP address of that peer.  This limitation
   means that the relay usage cannot be used to run web servers, email
   servers, SIP servers, or other network servers that service a large
   number of clients.  Rather, it facilitates rendezvous of NATted
   clients that use some other protocol, such as SIP, to communicate IP
   addresses and ports for communications.

   Confidentiality of the transport addresses learned through Allocate
   requests does not appear to be that important, and therefore, this
   capability is not provided.

   Relay servers are useful even for users not behind a NAT.  They can
   provide a way for truly anonymous communications.  A user can cause a
   call to have its media routed through a STUN server, so that the
   user's IP addresses are never revealed.




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   TCP transport addresses allocated by Allocate requests will properly
   work with TLS and SSL.  However, any relay addresses learned through
   an Allcoate will not operate properly with IPSec Authentication
   Header (AH) [6] in transport mode.  IPSec ESP [7] and any tunnel-mode
   ESP or AH should still operate.


13.  IANA Considerations

   This specification defines several new STUN messages, STUN
   attributes, and STUN response codes.  This section directs IANA to
   add these new protocol elements to the IANA registry of STUN protocol
   elements.

13.1.  New STUN Requests, Responses, and Indications


   Request/Response Transactions
   0x003  :  Allocate
   0x004  :  Set Active Destination
   0x005  :  Connect

   Indications
   0x001  :  Send
   0x002  :  Data
   0x003  :  Connect Status

13.2.  New STUN Attributes


   0x000D: LIFETIME
   0x0010: BANDWIDTH
   0x0012: REMOTE-ADDRESS
   0x0013: DATA
   0x0016: RELAY-ADDRESS
   0x0018: REQUESTED-PORT-PROPS
   0x0019: REQUESTED-TRANSPORT
   0x0022: REQUESTED-IP
   0x0021: TIMER-VAL
   0x0023: CONNECT_STAT











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13.3.  New STUN response codes

   437    No Binding
   439    Active Destination Already Set
   441    -- Wrong User --
   442    Unsupported Transport Protocol
   443    Invalid IP Address
   444    Invalid Port
   445    Operation for TCP Only
   446    Connection Already Exists
   447    --
   486    Allocation Quota Reached
   507    Insufficient Capacity





14.  IAB Considerations

   The IAB has studied the problem of ``Unilateral Self Address
   Fixing'', 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 RFC 3424 [8].
   The STUN relay extension 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 RFC 3424 [8], 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 the STUN relay extension is for
   a client, which may be located behind a NAT of any type, to obtain an
   IP address and port on the public Internet, useful for applications
   that require a client to place a transport address into a protocol
   message, with the expectation that the client will be able to receive
   packets from a single host that will send to this address.  Both UDP
   and TCP are addressed.  It is also possible to send packets so that
   the recipient sees a source address equal to the allocated address.
   STUN relays, by design, does not allow a client to run a server (such
   as a web or SMTP server) using a STUN relay address.  STUN relays are



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   useful even when NAT is not present, to provide anonymity services.

14.2.  Exit Strategy

   From [8], 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.

   It is expected that STUN relays will be useful indefinitely, to
   provide anonymity services.  When used to facilitate NAT traversal,
   STUN relay does not itself provide an exit strategy.  That is
   provided by the Interactive Connectivity Establishment (ICE) [9]
   mechanism.  ICE allows two cooperating clients to interactively
   determine the best addresses to use when communicating.  ICE uses
   STUN-allocated relay addresses as a last resort, only when no other
   means of connectivity exists.  As a result, as NATs phase out, and as
   IPv6 is deployed, ICE will increasingly use other addresses (host
   local addresses).  Therefore, clients will allocate STUN relay
   addresses, but not use them, and therefore, de-allocate them.
   Servers will see a decrease in usage.  Once a provider sees that its
   STUN relay servers are not being used at all (that is, no media flows
   through them), they can simply remove them.  ICE will operate without
   STUN-allocated relay addresses.

14.3.  Brittleness Introduced by STUN relays

   From [8], 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.

   The STUN relay extension introduces brittleness in a few ways.
   First, it adds another server element to any system, which adds
   another point of failure.  It requires clients to demultiplex STUN
   relay packets and data based on hunting for a MAGIC-COOKIE in the
   STUN messages.  It is possible (with extremely small probabilities)
   that this cookie could appear within a data stream, resulting in mis-
   classification.  That might introduce errors into the data stream
   (they would appear as lost packets), and also result in loss of a
   binding.  STUN relay relies on any NAT bindings existing for the
   duration of the bindings held by the STUN relay server.  Neither the
   client nor the STUN relay server have a way of reliably determining
   this lifetime (STUN can provide a means, but it is heuristic in
   nature and not reliable).  Therefore, if there is no activity on an



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   address learned from STUN for some period, the address might become
   useless spontaneously.

   STUN relays will result in potentially significant increases in
   packet latencies, and also increases in packet loss probabilities.
   That is because it introduces an intermediary on the path of a packet
   from point A to B, whose location is determined by application-layer
   processing, not underlying routing topologies.  Therefore, a packet
   sent from one user on a LAN to another on the same LAN may do a trip
   around the world before arriving.  When combined with ICE, some of
   the most problematic cases are avoided (such as this example) by
   avoiding the usage of STUN relay addresses.  However, when used, this
   problem will exist.

   Note that STUN relay does not suffer from many of the points of
   brittleness introduced by the STUN binding or discovery usages.  STUN
   relay will work with all existing NAT types known at the time of
   writing, and for the forseeable future.  STUN relay does not
   introduce any topological constraints.  STUN relay does not rely on
   any heuristics for NAT type classification.

14.4.  Requirements for a Long Term Solution

   >From [8]}, 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 relay continues to validate our belief in
   the requirements outlined in Section 14.4 of STUN.

14.5.  Issues with Existing NAPT Boxes

   >From [8], any UNSAF proposal must provide:

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

   A number of NAT boxes are now being deployed into the market which
   try and provide "generic" ALG functionality.  These generic ALGs hunt
   for IP addresses, either in text or binary form within a packet, and
   rewrite them if they match a binding.  This usage avoids that problem
   by using the XOR-MAPPED-ADDRESS attribute instead of the MAPPED-
   ADDRESS






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15.  Example

   In this example, a client is behind a NAT.  The client has a private
   address of 10.0.1.1.  The STUN server is on the public side of the
   NAT, and is listening for STUN relay requests on 192.0.2.3:8776.  The
   public side of the NAT has an IP address of 192.0.2.1.  The client is
   attempting to send a SIP INVITE to a peer, and wishes to allocate an
   IP address and port for inclusion in the SDP of the INVITE.
   Normally, STUN relays would be used in conjunction with ICE when
   applied to SIP.  For simplicities sake, STUN relay is showed without
   ICE.

   The client communicates with a SIP user agent on the public network.
   This user agent uses a 192.0.2.17:12734 for receipt of its RTP
   packets.


        Client                NAT             STUN Server           Peer
           |                   |                   |                   |
           |(1) Allocate       |                   |                   |
           |S=10.0.1.1:4334    |                   |                   |
           |D=192.0.2.3:8776   |                   |                   |
           |------------------>|                   |                   |
           |                   |                   |                   |
           |                   |(2) Allocate       |                   |
           |                   |S=192.0.2.1:63346  |                   |
           |                   |D=192.0.2.3:8776   |                   |
           |                   |------------------>|                   |
           |                   |                   |                   |
           |                   |(3) Error          |                   |
           |                   |S=192.0.2.3:8776   |                   |
           |                   |D=192.0.2.1:63346  |                   |
           |                   |<------------------|                   |
           |                   |                   |                   |
           |(4) Error          |                   |                   |
           |S=192.0.2.3:8776   |                   |                   |
           |D=10.0.1.1:4334    |                   |                   |
           |<------------------|                   |                   |
           |                   |                   |                   |
           |(5) Allocate       |                   |                   |
           |S=10.0.1.1:4334    |                   |                   |
           |D=192.0.2.3:8776   |                   |                   |
           |------------------>|                   |                   |
           |                   |                   |                   |
           |                   |(6) Allocate       |                   |
           |                   |S=192.0.2.1:63346  |                   |
           |                   |D=192.0.2.3:8776   |                   |
           |                   |------------------>|                   |



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           |                   |                   |                   |
           |                   |(7) Response       |                   |
           |                   |RA=192.0.2.3:32766 |                   |
           |                   |MA=192.0.2.1:63346 |                   |
           |                   |S=192.0.2.3:8776   |                   |
           |                   |D=192.0.2.1:63346  |                   |
           |                   |<------------------|                   |
           |(8) Response       |                   |                   |
           |RA=192.0.2.3:32766 |                   |                   |
           |MA=192.0.2.1:63346 |                   |                   |
           |S=192.0.2.3:8776   |                   |                   |
           |D=10.0.1.1:4334    |                   |                   |
           |<------------------|                   |                   |
           |                   |                   |                   |
           |                   |                   |                   |
           |(9) INVITE         |                   |                   |
           |SDP=192.0.2.3:32766|                   |                   |
           |---------------------------------------------------------->|
           |                   |                   |                   |
           |                   |                   |                   |
           |(10) 200 OK        |                   |                   |
           |SDP=192.0.2.17:12734                   |                   |
           |<----------------------------------------------------------|
           |                   |                   |                   |
           |                   |                   |                   |
           |                   |                   |                   |
           |(11) ACK           |                   |                   |
           |---------------------------------------------------------->|
           |                   |                   |                   |
           |(12) Send          |                   |                   |
           |DATA=RTP           |                   |                   |
           |DA=192.0.2.17:12734|                   |                   |
           |S=10.0.1.1:4334    |                   |                   |
           |D=192.0.2.3:8776   |                   |                   |
           |------------------>|                   |                   |
           |                   |                   |                   |
           |                   |(13) Send          |                   |
           |                   |DATA=RTP           |                   |
           |                   |DA=192.0.2.17:12734|                   |
           |                   |S=192.0.2.1:63346  |                   |
           |                   |D=192.0.2.3:8776   |                   |
           |                   |------------------>|                   |
           |                   |                   |                   |
           |                   |                   |(14) RTP           |
           |                   |                   |S=192.0.2.3:32766  |
           |                   |                   |D=192.0.2.17:12734 |
           |                   |                   |------------------>|
           |                   |                   |                   |



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           |                   |                   |Permission         |
           |                   |                   |Created            |
           |                   |                   |192.0.2.17         |
           |                   |                   |                   |
           |                   |                   |(15) RTP           |
           |                   |                   |S=192.0.2.17:12734 |
           |                   |                   |D=192.0.2.3:32766  |
           |                   |                   |<------------------|
           |                   |                   |                   |
           |                   |(16) DataInd       |                   |
           |                   |DATA=RTP           |                   |
           |                   |RA=192.0.2.17:12734|                   |
           |                   |S=192.0.2.3:8776   |                   |
           |                   |D=192.0.2.1:63346  |                   |
           |                   |<------------------|                   |
           |(17) DataInd       |                   |                   |
           |DATA=RTP           |                   |                   |
           |RA=192.0.2.17:12734|                   |                   |
           |S=192.0.2.3:8776   |                   |                   |
           |D=10.0.1.1:4334    |                   |                   |
           |<------------------|                   |                   |
           |                   |                   |                   |
           |(18) SetAct        |                   |                   |
           |DA=192.0.2.17:12734|                   |                   |
           |S=10.0.1.1:4334    |                   |                   |
           |D=192.0.2.3:8776   |                   |                   |
           |------------------>|                   |                   |
           |                   |                   |                   |
           |                   |(19) SetAct        |                   |
           |                   |DA=192.0.2.17:12734|                   |
           |                   |S=192.0.2.1:63346  |                   |
           |                   |D=192.0.2.3:8776   |                   |
           |                   |------------------>|                   |
           |                   |                   |                   |
           |                   |(20) Response      |                   |
           |                   |S=192.0.2.3:8776   |                   |
           |                   |D=192.0.2.1:63346  |                   |
           |                   |<------------------|                   |
           |                   |                   |                   |
           |(21) Response      |                   |                   |
           |S=192.0.2.3:8776   |                   |                   |
           |D=10.0.1.1:4334    |                   |                   |
           |<------------------|                   |                   |
           |                   |                   |                   |
           |                   |                   |                   |
           |                   |                   |           after 3s|
           |                   |                   |                   |
           |                   |                   |                   |



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           |                   |                   |(22) RTP           |
           |                   |                   |S=192.0.2.17:12734 |
           |                   |                   |D=192.0.2.3:32766  |
           |                   |                   |<------------------|
           |                   |                   |                   |
           |                   |(23) RTP           |                   |
           |                   |S=192.0.2.3:8776   |                   |
           |                   |D=192.0.2.1:63346  |                   |
           |                   |<------------------|                   |
           |                   |                   |                   |
           |(24) RTP           |                   |                   |
           |S=192.0.2.3:8776   |                   |                   |
           |D=10.0.1.1:4334    |                   |                   |
           |<------------------|                   |                   |
           |                   |                   |                   |
           |                   |                   |                   |

                                 Figure 14

   The call flow is shown in Figure 14.  The client allocates a port
   from the local operating system on its private interface, obtaining
   4334.  It then attempts to secure a port for RTP traffic.  RTCP
   processing is not shown.  The client sends an Allocate request (1)
   with a source address (denoted by S) of 10.0.1.1:4334 and a
   destination (denoted by D) of 192.0.2.3:8776.  This passes through
   the NAT (2), which creates a mapping from the 192.0.2.1:63346 to the
   source IP address and port of the request, 10.0.1.1:4334.  This
   request is received at the STUN server, which challenges it (3),
   requesting credentials.  This response is passed to the client (4).
   The client retries the request, this time with credentials (5).  This
   arrives at the server (6).  The request is now authenticated.  The
   server provides a UDP allocation, 192.0.2.3:32766, and places it into
   the RELAY-ADDRESS (denoted by RA) in the response (7).  It also
   reflects the source IP address and port of the request into the
   MAPPED-ADDRESS (denoted by MA) in the response.  This passes through
   the NAT to the client (8).  The client now proceeds to perform a
   basic SIP call setup.  In message 9, it includes the relay address
   into the SDP of its INVITE.  The called party responds with a 200 OK,
   and includes its IP address - 192.0.2.17:12734.  The exchange
   completes with an ACK (11).

   Next, user A sends an RTP packet.  Since the active destination has
   not been set, the client decides to use the Send indication.  It does
   so, including the RTP packet as the contents of the DATA attribute.
   The REMOTE-ADDRESS attribute (denoted by DA) is set to 192.0.2.17:
   12734, learned from the 200 OK.  This is sent through the NAT
   (message 12) and arrives at the STUN server (message 13).  The server
   extracts the data contents, and sends the packet towards REMOTE-



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   ADDRESS (message 14).  Note how the source address and port in this
   packet is 192.0.2.3:32766, the allocated transport address given to
   the client.  The act of sending the packet with Send causes the STUN
   server to install a permission for 192.0.2.17.

   Indeed, the called party now sends an RTP packet toward the client
   (message 15).  This arrives at the STUN server.  Since a permission
   has been set for the IP address in the source of this packet, it is
   accepted.  As no active destination is set, the STUN server
   encapsulates the contents of the packet in a Data Indication (message
   16), and sends it towards the client.  The REMOTE-ADDRESS attribute
   (denoted by RA) indicates the source of the packet - 192.0.2.17:
   12734.  This is forwarded through the NAT to the client (message 17).

   The client decides to optimize the path for packets to and from
   192.0.2.17:12734.  So, it issues a Set Active Destination request
   (message 18) with a REMOTE-ADDRESS of 192.0.2.17:12734.  This passes
   through the NAT and arrives at the STUN server (message 19).  This
   generates a successful response (message 20) which is passed to the
   client (message 21).  At this point, the server and client are in the
   transitioning state.  A little over 3 seconds later (by default), the
   state machines transition back to "Set".  Until this point, packets
   from the called party would have been relayed back to the client in
   Data Indications.  Now, the next RTP packet shows up at the STUN
   server (message 22).  Since the source IP address and port match the
   active destination, the RTP packet is relayed towards the client
   without encapsulation (message 23 and 24).


16.  Acknowledgements

   The authors would like to thank Marc Petit-Huguenin for his comments
   and suggestions.


17.  References

17.1.  Normative References

   [1]   Rosenberg, J., "Simple Traversal Underneath Network Address
         Translators (NAT) (STUN)", draft-ietf-behave-rfc3489bis-05
         (work in progress), October 2006.

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






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17.2.  Informative References

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

   [4]   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.

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

   [6]   Kent, S., "IP Authentication Header", RFC 4302, December 2005.

   [7]   Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
         December 2005.

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

   [9]   Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-13 (work in
         progress), January 2007.

   [10]  Audet, F. and C. Jennings, "NAT Behavioral Requirements for
         Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress),
         October 2006.


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








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   Rohan Mahy
   Plantronics

   Email: rohan@ekabal.com


   Christian Huitema
   Microsoft
   One Microsoft Way
   Redmond, WA  98052-6399
   US

   Email: huitema@microsoft.com






































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