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Versions: 00 01 02 03 04 05 06 07 08 draft-ietf-behave-turn

MIDCOM                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: August 22, 2005                                         R. Mahy
                                                                Airspace
                                                              C. Huitema
                                                               Microsoft
                                                       February 21, 2005


                    Traversal Using Relay NAT (TURN)
                     draft-rosenberg-midcom-turn-07

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Traversal Using Relay NAT (TURN) is a protocol that allows for an
   element behind a NAT or firewall to receive incoming data over TCP or
   UDP connections.  It is most useful for elements behind symmetric
   NATs or firewalls that wish to be on the receiving end of a



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   connection to a single peer.  TURN does not allow for users to run
   servers on well known ports if they are behind a nat; it supports the
   connection of a user behind a nat to only a single peer.  In that
   regard, its role is to provide the same security functions provided
   by symmetric NATs and firewalls, but to "turn" them into
   port-restricted NATs.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  5
   5.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   6.  Message Overview . . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . .  7
     7.1   Shared Secret Request  . . . . . . . . . . . . . . . . . .  8
     7.2   Allocate Request . . . . . . . . . . . . . . . . . . . . . 10
       7.2.1   Overview . . . . . . . . . . . . . . . . . . . . . . . 10
       7.2.2   Initial Requests . . . . . . . . . . . . . . . . . . . 10
       7.2.3   Subsequent Requests  . . . . . . . . . . . . . . . . . 13
     7.3   Send Request . . . . . . . . . . . . . . . . . . . . . . . 14
     7.4   Receiving Packets and Connections  . . . . . . . . . . . . 15
     7.5   Lifetime Expiration  . . . . . . . . . . . . . . . . . . . 17
   8.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . . . 17
     8.1   Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 17
     8.2   Obtaining a One Time Password  . . . . . . . . . . . . . . 18
     8.3   Allocating a Binding . . . . . . . . . . . . . . . . . . . 19
     8.4   Processing Allocate Responses  . . . . . . . . . . . . . . 20
     8.5   Refreshing a Binding . . . . . . . . . . . . . . . . . . . 21
     8.6   Sending Data . . . . . . . . . . . . . . . . . . . . . . . 21
     8.7   Tearing Down a Binding . . . . . . . . . . . . . . . . . . 22
     8.8   Receiving and Sending Data . . . . . . . . . . . . . . . . 22
   9.  Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 23
     9.1   Message Types  . . . . . . . . . . . . . . . . . . . . . . 23
     9.2   Message Attributes . . . . . . . . . . . . . . . . . . . . 23
       9.2.1   LIFETIME . . . . . . . . . . . . . . . . . . . . . . . 23
       9.2.2   ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . 24
       9.2.3   MAGIC-COOKIE . . . . . . . . . . . . . . . . . . . . . 24
       9.2.4   BANDWIDTH  . . . . . . . . . . . . . . . . . . . . . . 24
       9.2.5   DESTINATION-ADDRESS  . . . . . . . . . . . . . . . . . 24
       9.2.6   SOURCE-ADDRESS . . . . . . . . . . . . . . . . . . . . 24
       9.2.7   DATA . . . . . . . . . . . . . . . . . . . . . . . . . 25
       9.2.8   NONCE  . . . . . . . . . . . . . . . . . . . . . . . . 25
       9.2.9   Response Codes . . . . . . . . . . . . . . . . . . . . 25
   10.   Security Considerations  . . . . . . . . . . . . . . . . . . 25
   11.   IAB Considerations . . . . . . . . . . . . . . . . . . . . . 27
     11.1  Problem Definition . . . . . . . . . . . . . . . . . . . . 27



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     11.2  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 27
     11.3  Brittleness Introduced by TURN . . . . . . . . . . . . . . 28
     11.4  Requirements for a Long Term Solution  . . . . . . . . . . 29
     11.5  Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 29
   12.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 29
   13.   References . . . . . . . . . . . . . . . . . . . . . . . . . 29
   13.1  Normative References . . . . . . . . . . . . . . . . . . . . 29
   13.2  Informative References . . . . . . . . . . . . . . . . . . . 30
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 30
       Intellectual Property and Copyright Statements . . . . . . . . 32









































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1.  Introduction

   Network Address Translators (NATs), while providing many benefits,
   also come with many drawbacks.  The most troublesome of those
   drawbacks is the fact that they break many existing IP applications,
   and make it difficult to deploy new ones.  Guidelines [9] have been
   developed that describe how to build "NAT friendly" protocols, but
   many protocols simply cannot be constructed according to those
   guidelines.  Examples of such protocols include multimedia
   applications and file sharing.

   Simple Traversal of UDP Through NAT (STUN) [1] provides one means for
   an application to traverse a NAT.  STUN allows a client to obtain a
   transport address (and IP address and port) which may be useful for
   receiving packets from a peer.  However, addresses obtained by STUN
   may not be usable by all peers.  Those addresses work depending on
   the topological conditions of the network.  Therefore, STUN by itself
   cannot provide a complete solution for NAT traversal.

   A complete solution requires a means by which a client can obtain a
   transport address from which it can receive media from any peer which
   can send packets to the public Internet.  This can only be
   accomplished by relaying data though a server that resides on the
   public Internet.  This specification describes Traversal Using Relay
   NAT (TURN), a protocol that allows a client to obtain IP addresses
   and ports from such a relay.

   Although TURN will almost always provide connectivity to a client, it
   comes at high cost to the provider of the TURN server.  It is
   therefore desirable to use TURN as a last resort only, preferring
   other mechanisms (such as STUN or direct connectivity) when possible.
   To accomplish that, the Interactive Connectivity Establishment (ICE)
   [13] methodology can be used to discover the optimal means of
   connectivity.

2.  Terminology

   In this document, the key words MUST, MUST NOT, REQUIRED, SHALL,
   SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to
   be interpreted as described in RFC 2119 [2] and indicate requirement
   levels for compliant TURN implementations.

3.  Definitions

      TURN Client: A TURN client (also just referred to as a client) is
      an entity that generates TURN requests.  A TURN client can be an
      end system, such as a Session Initiation Protocol (SIP) [6] User
      Agent, or can be a network element, such as a Back-to-Back User



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      Agent (B2BUA) SIP server.  The TURN protocol will provide the TURN
      client with IP addresses that route to it from the public
      Internet.

      TURN Server: A TURN Server (also just referred to as a server) is
      an entity that receives TURN requests, and sends TURN responses.
      The server is capable of acting as a data relay, receiving data on
      the address it provides to clients, and forwarding them to the
      clients.

      Transport Address: An IP address and port.


4.  Applicability Statement

   TURN is 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.  Examples of such protocols include SIP,
   which makes use of the Session Description Protocol (SDP) [7].  SDP
   carries and IP address on which the client will receive media packets
   from its peer.  Another example of a protocol meeting this criteria
   is the Real Time Streaming Protocol (RTSP) [8].

   When a client is behind a NAT, transport addresses obtained from the
   local operating system will not be publically routable, and
   therefore, not useful in these protocols.  TURN allows a client to
   obtain a transport address, from a server on the public Internet,
   which can be used in protocols meeting the above criteria.  However,
   the transport addresses obtained from TURN servers are not generally
   useful for receiving data from anywhere.  They are only useful for
   communicating with a single peer.  This is accomplished by having the
   TURN server emulate the behavior of a port-restricted NAT.  In
   particular, the TURN server will only relay packets from an external
   IP address and port towards the client if the client had previously
   sent a packet through the TURN server towards that IP address and
   port.  As a result of this, when a TURN server is placed in front of
   a symmetric NAT, the resulting combined system has identical security
   properties to a system that just had a port-restricted NAT.  Since
   clients behind such devices cannot run public servers, they cannot
   run them behind TURN servers either.

5.  Overview of Operation

   The typical TURN configuration is shown in Figure 1.  A TURN client
   is connected to private network 1.  This network connects to private
   network 2 through NAT 1.  Private network 2 connects to the public
   Internet through NAT 2.  On the public Internet is a TURN server.



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                              /-----\
                            // TURN  \\
                           |   Server  |
                            \\       //
                              \-----/


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

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

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


                                Figure 1

   TURN is a simple client-server protocol.  It is identical in syntax
   and general operation to STUN, in order to facilitate a joint
   implementation of both.  TURN defines a request message, called
   Allocate, which asks for a public IP address and port.  TURN can run
   over UDP and TCP, as it allows for a client to request address/port
   pairs for receiving both UDP and TCP.

   A TURN client first discovers the address of a TURN server.  This can
   be preconfigured, or it can be discovered using SRV records [3] This
   will allow for different TURN servers for UDP and TCP.  Once a TURN
   server is discovered, the client sends a TURN Allocate request to the
   TURN server.  TURN provides a mechanism for mutual authentication and
   integrity checks for both requests and responses, based on a shared
   secret.  Assuming the request is authenticated and has not been
   tampered with, the TURN server remembers the source transport address
   that the request came from (call this SA), and returns a public
   transport address, PA, in the TURN response.  The TURN server is
   responsible for guaranteeing that packets sent to PA route to the
   TURN server.  However, the TURN server will not relay any packets
   from PA to SA until the client sends a packet through the TURN server
   towards a correspondent.  To do that, a client sends a TURN SEND
   command, which includes a data packet and a destination IP address
   and port.  The TURN server, upon receipt of this command, will
   forward the packet to that IP address and port, add a "permission"



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   for that IP address and port, so that inbound packets from that
   address and port are permitted, and set the default destination
   address to that address and port.  From that point forward, non-TURN
   UDP packets sent from the client to the TURN server are relayed to
   this default IP address and port.  Packets received from this IP
   address and port are relayed to the client.  If a packet arrives from
   an IP address and port for which there is a permission, but which is
   not the current default destination IP address and port, the TURN
   server forwards this packet to the client wrapped in a DATA command,
   which informs the client of the source IP address and port.

   For TCP, the TURN server does not need to examine the data received;
   it merely forwards all data between the socket pairs it has
   associated together.  In the case of UDP, the TURN server looks for a
   magic cookie in the first 128 bytes of each UDP packet.  If present,
   it indicates that the packet is a TURN control packet, used for
   keepalives and teardown of the binding.  In the case of TCP, if
   either side closes a connection, the TURN server closes the other
   connection.  For both UDP and TCP, the TURN server can also time out
   a connection in the event data is not received after some configured
   time out period.  This period is sent to the client in the TURN
   response to the Allocate request.

6.  Message Overview

   TURN messages are identical to STUN messages in their syntax.  TURN
   defines several new messages - the Allocate Request, the Allocate
   Response, the Allocate Error Response, the Send Request, the Send
   Response, the Send Error Response and the Data Indication.  TURN also
   uses the Shared Secret Request, Shared Secret Response, and Shared
   Secret Error Response defined by STUN.  TURN makes use of some of the
   STUN attributes (MAPPED-ADDRESS, USERNAME, MESSAGE-INTEGRITY,
   ERROR-CODE, and UNKNOWN-ATTRIBUTES) and also defines several of its
   own.  Specifically, TURN adds the LIFETIME attribute, which allows
   the TURN server to tell the client when the binding will be released.
   It defines the MAGIC-COOKIE attribute, which allows the TURN client
   to find TURN messages in a stream of UDP packets.  It defines the
   BANDWIDTH attribute, which allows a client to inform the server of
   the expected bandwidth usage on the connection.  Finally, it defines
   the ALTERNATE-SERVER attribute, which allows the server to redirect
   the TURN client to connect to an alternate server.

7.  Server Behavior

   The server behavior depends on whether the request is a Shared Secret
   Request or an Allocate Request.





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7.1  Shared Secret Request

   Unlike a STUN server, a TURN server provides resources to clients
   that connect to it.  Therefore, only authorized clients can gain
   access to a TURN server.  This requires that TURN requests be
   authenticated.  TURN assumes the existence of a long-lived shared
   secret between the client and the TURN server in order to achieve
   this authentication.  The client uses this long-lived shared secret
   to authenticate itself in a Shared Secret Request, sent over TLS.
   The Shared Secret Response provides the client with a one-time
   username and password.  This one-time credential is then used by the
   server to authenticate an Allocate Request.  The usage of a separate
   long lived and one-time credentials prevents dictionary attacks,
   whereby an observer of a message and its HMAC could guess the
   password by an offline dictionary search.

   When a TURN server receives a Shared Secret Request, it first
   executes the processing described in the first three paragraphs of
   Section 8.2 of STUN.  This processing will ensure that the Shared
   Secret Request is received over TLS.

   Assuming it was, the server checks the Shared Secret Request for a
   MESSAGE-INTEGRITY attribute.  If not present, the server generates a
   Shared Secret Error Response with an ERROR-CODE attribute with
   response code 401.  That response MUST include a NONCE attribute,
   containing a nonce that the server wishes the client to reflect back
   in a subsequent Shared Secret Request (and therefore include the
   message integrity computation).  The response MUST include a REALM
   attribute, containing a realm from which the username and password
   are scoped [4].

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

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

   If the NONCE attribute was present, the server checks for the
   existence of the USERNAME attribute.  If it was not present, the
   server MUST generate a Shared Secret Error Response.  The Shared
   Secret Error Response MUST include an ERROR-CODE attribute with



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   response code 432.  It MUST include a NONCE attribute and a REALM
   attribute.

   If the USERNAME is present, the server computes the HMAC over the
   request as described in Section 11.2.8 of STUN.  The key is computed
   as MD5(unq(USERNAME-value) ":" unq(REALM-value) ":" passwd), where
   the password is the password associated with the username and realm
   provided in the request.  If the server does not have a record for
   that username within that realm, the server generates a Shared Secret
   Error Response.  That response MUST include an ERROR-CODE attribute
   with response code 436.  That response MUST include a NONCE attribute
   and a REALM attribute.

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

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

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

   In all cases, the Shared Secret Error Response is sent over the TLS
   connection on which the Shared Secret Request was received.

   The server proceeds to authorize the client.  The means for
   authorization are outside the scope of this specification.  It is
   anticipated that TURN servers will be run by providers that also
   provide an application service, such as SIP or RTSP.  In that case, a
   user would be authorized to use TURN if they are authorized to use
   the application service.

   The server then generates a Shared Secret Response as in Section 8.2
   of STUN.  This response will contain a USERNAME and PASSWORD, which
   are used by the client as a short-term shared secret in subsequent
   Allocate requests.  Note that STUN specifies that the server has to
   invalidate this username and password after 30 minutes.  This is not
   the case in TURN.  In TURN, the server MUST store the allocated
   username and password for a duration of at least 30 minutes.  Once an
   Allocate request has been authenticated using that username and
   password, if the result was an Allocate Error Response, the username
   and password are discarded.  If the result was an Allocate Response,



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   resulting in the creation of a new binding, the username and password
   become associated with that binding.  They can only be used to
   authenticate Allocate requests sent from the same source transport
   address in order to refresh or de-allocate that binding.  Once the
   binding is deleted, the username and password are discarded.

   This policy avoids replay attacks, whereby a recorded Allocate
   request is replayed in order to obtain a binding without proper
   authentication.  It also ensures that existing bindings can be
   refreshed without needed to continuously obtain one-time passwords
   from the TURN server.

7.2  Allocate Request

7.2.1  Overview

   Allocate requests are used to obtain an IP address and port that the
   client can use to receive UDP and TCP packets from any host on the
   network, even when the client is behind a symmetric NAT.  To do this,
   a TURN server allocates a local transport address, and passes it to
   the client in an Allocate Response.  The server also maintains, for
   each local transport address, a list of permissions.  These
   permissions are IP address and port combinations that the client has
   directed a request to, using the SEND primitive.  The server
   maintains an IP address and port called the default destination.
   This is the default destination for non-TURN packets received from
   the client.

   The server maintains a set of bindings.  These bindings are
   associations between the five-tuple of received Allocate requests
   (source IP address and port, destination IP address and port, and
   protocol), called the allocate five-tuple, and another five tuple,
   called the remote five-tuple.

   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 received with a source transport address which
   is not associated with any existing bindings.  A subsequent request
   is one received that is associated with an existing binding.

7.2.2  Initial Requests

   A TURN server MUST be prepared to receive Binding Requests over TCP
   and UDP.  The port on which to listen is based on the DNS SRV entries
   provided by the server.  Typically, this will be XXXX, the default
   TURN port.

   The server MUST check the Allocate Request for a MESSAGE-INTEGRITY



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   attribute.  If not present, the server generates a Allocate Error
   Response with an ERROR-CODE attribute with response code 401.

   If the MESSAGE-INTEGRITY attribute was present, the server checks for
   the existence of the USERNAME attribute.  If it was not present, the
   server MUST generate a Allocate Error Response.  The Allocate Error
   Response MUST include an ERROR-CODE attribute with response code 432.

   If the USERNAME is present, the server computes the HMAC over the
   request as described in Section 11.2.8 of STUN.  The key is equal to
   the password associated with the username in the request, where that
   username is a short term username allocated by the TURN server.  The
   username MUST be one which has been allocated by the server in a
   Shared Secret Response, but has not yet been used to authenticate an
   Allocate request.  If that username is not known by the server, or
   has already been used, the server generates an Allocate Error
   Response.  That response MUST include an ERROR-CODE attribute with
   response code 430.

   If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
   attribute in the request, the server MUST generate a Allocate Error
   Response with an ERROR-CODE attribute with response code 431.

   Assuming the message integrity check passed, processing continues.
   The server MUST check for any attributes in the request with values
   less than or equal to 0x7fff which it does not understand.  If it
   encounters any, the server MUST generate an Allocate Error Response,
   and it MUST include an ERROR-CODE attribute with a 420 response code.

   That response MUST contain an UNKNOWN-ATTRIBUTES attribute listing
   the attributes with values less than or equal to 0x7fff which were
   not understood.

   If the Allocate request arrived over TCP, the Allocate Error Response
   is sent on the connection from which the request arrived.  If the
   Allocate request arrived over UDP, the Allocate Error Response is
   sent to the transport address from which the request was received
   (i.e., the source IP address and port), and sent from the transport
   address on which the request was received (i.e., the destination IP
   address and port).

   Assuming the Allocate request was authenticated and was well-formed,
   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 port for the client.  The server MUST
   NOT allocate ports from the well-known port range (0-1023) and MUST



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   NOT allocate ports from the user registered port range (1024 through
   49151).

   If a port meeting the bandwidth constraints cannot be allocated, the
   server MUST generate a Allocate Error Response that includes an
   ERROR-CODE attribute with a response code of 300.  That response MAY
   include an ALTERNATE-SERVER attribute pointing to an alternate server
   which can be used by the client.

   Once the port is allocated, the server creates a binding for it.
   This binding is a mapping between two five-tuples - the allocate
   five-tuple and the remote five-tuple.  The allocate five-tuple is set
   to the five-tuple of the Allocate Request (that is, the protocol of
   the allocate five-tuple is set to the protocol of the Allocate
   Request (TCP or UDP), the source IP address and port of the allocate
   five-tuple are set to the source IP address and port in the Allocate
   Request, and the destination IP address and port of the allocate
   five-tuple are set to the destination IP address and port in the
   Allocate Request).  The protocol in the remote five-tuple is set to
   the protocol from the Allocate Request.  The source IP address of the
   remote five-tuple is set to the interface from which the port was
   allocated.  The source port of the remote five-tuple is set to the
   allocated port.  If the binding was allocated for TCP, the connection
   on which the Allocate request was received is associated with the
   allocate five-tuple in the binding.

   The server MUST remember the one-time username and password used to
   obtain the binding.

   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 binding, 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 cae, the
   resulting duration is added to the current time, and a timer is set
   to fire at or after that time.  Section 7.5 discusses behavior when
   the timer fires.

   Once the port has been obtained from the operating system and the
   activity timer started for the port binding, the server generates an
   Allocate Response.  The Allocate Response MUST contain the same
   transaction ID contained in the Allocate Request.  The length in the
   message header MUST contain the total length of the message in bytes,
   excluding the header.  The Allocate Response MUST have a message type
   of "Allocate Response".

   The server MUST add a MAPPED-ADDRESS attribute to the Allocate



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   Response.  The IP address component of this attribute MUST be set to
   the interface from which the base port was allocated.  The port
   component of this attribute MUST be set to the base port.

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

   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
   needed to prevent against denial-of-service attacks (See Section 10.

   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.

   The TURN server then sends the response.  If the Allocate request was
   received over TCP, the response is sent over that TCP connection.  If
   the Allocate request was received over UDP, the response is sent to
   the transport address from which the request was received (i.e., the
   source IP address and port), and sent from the transport address on
   which the request was received (i.e., the destination IP address and
   port).

   If the port was for TCP, the server MUST be prepared to receive a TCP
   connection request on that port.

7.2.3  Subsequent Requests

   Once a binding has been created for UDP and permissions installed,
   the client can send subsequent Allocate requests to the TURN server.
   To determine which packets are for the TURN server, and which need to
   be relayed, the server looks at the packet.  If the packet is shorter
   than 28 bytes, it is not a TURN request.  If it is longer than 28
   bytes, the server checks bytes 25-28.  If these bytes are equal to
   the MAGIC-COOKIE, the request is a TURN request.  Otherwise, it is a
   data packet, and is to be relayed.

   The server first authenticates the request.  This is done as in
   Section 7.2.2.  The request MUST be authenticated using the same
   one-time username and password used to allocate that binding
   previously.  That is, the five-tuple from the Allocate request is
   compared to the allocate five-tuples in existing bindings.  The
   matching binding is selected.  The one-time username and password
   associated with that binding MUST match the ones used in the request.




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   The server looks for the LIFETIME attribute in the Allocate Request.
   If not found, it determines the default refresh duration, in seconds,
   for this binding.  If the LIFETIME attribute was present in the
   request, and the value is larger than the maximum duration the server
   is willing to extend the lifetime of the binding, the server MAY
   lower it to that maximum.  However, the server MUST NOT increase the
   duration requested in the LIFETIME attribute.  The resulting duration
   is added to the current time, and the activity timer for this binding
   is reset to fire at or after that time.  Section 7.5 discusses
   behavior when the timer fires.

   Once the timer is set, the server MUST generate an Allocate Response.
   The Allocate Response MUST contain the same transaction ID contained
   in the Allocate Request.  The length in the message header MUST
   contain the total length of the message in bytes, excluding the
   header.  The Allocate Response MUST have a message type of "Allocate
   Response".  The response MUST contain a MAGIC-COOKIE as the first
   attribute.  It MUST contain a MAPPED-ADDRESS which contains the
   source IP address and port from the remote five-tuple of the binding.
   It MUST contain a LIFETIME attribute which contains the time from now
   until the point at which the binding will be deleted.  The final
   attribute MUST be a MESSAGE-INTEGRITY attribute, which MUST use the
   same one-time username and password used to authenticate the request.

   The TURN server then sends the response.  The response is sent to the
   transport address from which the request was received (i.e., the
   source IP address and port), and sent from the transport address on
   which the request was received (i.e., the destination IP address and
   port).

7.3  Send Request

   In order for the TURN server to relay packets to and from the client,
   it must be "primed" with one of more Send requests.  These requests
   are used with UDP, and carry a data payload.  In addition, they
   contain a destination IP address and port.  A Send Request is like
   any other TURN request.  A server can disambiguate a Send Request
   from a data packet by looking for the MAGIC-COOKIE attribute, as
   described in Section 7.2.3.

   Once the server has identified a request as a Send request, the
   server verifies that it has arrived with a source five-tuple
   corresponding to an existing allocation.  If there is no matching
   allocation, the server MUST generate a 437 (No Binding) Send Error
   Response.

   Next, the server authenticates the request.  This is done as in
   Section 7.2.2.  The request MUST be authenticated using the same



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   one-time username and password used to allocate that binding
   previously.  That is, the five-tuple from the Send request is
   compared to the allocate five-tuples in existing bindings.  The
   matching binding is selected.  The one-time username and password
   associated with that binding MUST match the ones used in the request.

   Once the request has been authenticated, the server validates it.
   The request should contain a DESTINATION-ADDRESS attribute and a DATA
   attribute.  If it doesn't, the server MUST reject the request with a
   400 (Bad Request) Send Error Response.

   Assuming the Send Request has been validated, the server then takes
   the contents of the DATA attribute, and creates a UDP packet whose
   payload equals that content.  The server sets the source IP address
   equal to the source IP from the remote five-tuple, and the source
   port equal to the source port from the remote five-tuple.  The
   destination address and port are set to the contents of the
   DESTINATION-ADDRESS.  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 clients responsibility to generate
   another Send Request if needed.

   The server then sets the default destination address to the IP
   address and port in the DESTINATION-ADDRESS.  It also adds the IP
   address and port from DESTINATION-ADDRESS to the list of permissions
   associated with this binding.

   Once the UDP packet is sent, the server generates a Send Response.
   The Send Response MUST have a message type of "Send Response".  The
   response MUST contain a MAGIC-COOKIE as the first attribute.  If the
   server needs to generate a Send Error Response, that message MUST
   contain a message type of "Send Error Response", and MUST contain a
   MAGIC-COOKIE as the first attribute.  It MUST contain an ERROR-CODE
   with the appropriate response code.  For UDP, both the Send Response
   and Send Error Response are sent back to the source IP and port where
   the request came from, and sent from the same address and port where
   the request was sent to.

7.4  Receiving Packets and Connections

   If a TURN server receives a TCP connection request on a port it has
   allocated, the server retrieves the binding whose remote five-tuple
   has a source address and source port that match the IP address and
   port to which the connection was made, and whose transport is TCP.
   If there is not already a TCP connection made to the remote
   five-tuple, the connection request is accepted.

   If a TURN server receives a UDP packet on a port it has allocated,



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   the server retrieves the binding whose remote five-tuple has a source
   address and source port that match the IP address and port to which
   the packet was sent, and whose transport is UDP.  If the source IP
   address and port of the UDP packet are not listed amongst the set of
   permissions for the binding, the UDP packet is discarded.  Otherwise,
   if the source IP address and port are listed, but are not equal to
   the current default IP address and port, the server transmits the
   packet to the client using a Data Indication message.  This is a TURN
   message that is not retransmitted by the server, and which does not
   generate a response.  As a result, like data packets which are
   forwarded, there is no reliability guarantee provided by the TURN
   server for this indication.  The Data Indication message MUST contain
   a DATA attribute whose contents are equal to the payload of the UDP
   packet.  The message MUST contain a SOURCE-ADDRESS attribute whose
   content is equal to the source IP address and port of the UDP packet
   received by the TURN server.  This packet is sent to the client using
   the allocate five-tuple.  That is, its destination address is equal
   to the source address from the allocate five-tuple, and its source
   address is equal to the destination address from the allocate
   five-tuple.

   If the packet received on the allocated port has a source IP address
   and port amongst the permissions for that binding, and that source IP
   and port is equal to the default IP address and port, the UDP packet
   is forwarded to the client and not encapsulated in a TURN packet.  To
   forward, the packet is sent with a source IP address and port equal
   to the destination IP address and port in the allocate five-tuple,
   and with a destination address and port equal to the source IP
   address and port in the allocate five-tuple.

   If a TURN server receives data on a TCP connection that was opened to
   a port it had allocated, the server MUST forward this data onto the
   connection associated with allocate-tuple in the binding.

   If a TURN server receives data on a TCP connection that is associated
   with an allocate five-tuple, the binding for that tuple is retrieved.
   If the destination IP address and port of that tuple have not been
   filled in yet, the data is discarded.  If the destination address and
   port have been filled in, the connection associated with the remote
   five-tuple is obtained, and the data is forwarded on that connection.

   Note that, because data is forwarded blindly across TCP bindings, TLS
   will successfully operate over a TURN allocated TCP port.

   Similarly, if a TURN server receives a UDP packet on one of its
   public TURN ports, it checks to see if the source IP address and port
   match those of the allocate five-tuples in an existing binding.  If
   there is a match, the the UDP packet is not a TURN request (see



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   Section 7.2.3 for details on how this determination is made), the
   default IP address and port are checked.  If they are not set, the
   UDP packet is discarded.  If they are set, the packet is forwarded.
   It is forwarded using the source IP address and port from the remote
   five-tuple, and a destination IP address and port equal to the
   default IP address and port.

   If a TCP connection associated with an allocate five-tuple is closed,
   the connection associated with the corresponding remote five-tuple is
   also closed.  At that point, the binding is destroyed.  Similarly, if
   the TCP connection associated with a remote five-tuple is closed, the
   connection associated with the corresponding allocate five-tuple is
   closed, and the binding is destroyed.

7.5  Lifetime Expiration

   When the activity timer for a binding fires, the server checks to see
   if there has been any activity on the binding since its creation, or
   since the last firing of the timer, whichever is more recent.
   Activity is defined as connection establishment, or packet
   transmission in either direction.  If there has been activity, the
   timer is set to fire once again in M seconds, where M is the value of
   the LIFETIME attribute returned in the most recent Allocate Response
   for this binding.

   If there has been no activity, the server MUST destroy the binding,
   along with its associated one-time password.  If the binding was over
   TCP, the server MUST close any connections it is holding to the
   client and to the remote client.

8.  Client Behavior

   Client behavior is broken into several separate steps.  First, the
   client obtains a one-time username and password.  Secondly, it
   generates initial Allocate Requests, and processes the responses.  It
   manages those addresses (refreshing and tearing them down), issues
   Send Requests, and processes TURN indications and data received on
   those addresses.

8.1  Discovery

   Generally, the client will be configured with a domain name of the
   provider of the TURN servers.  This domain name is resolved to an IP
   address and port of using the SRV procedures [3].  When sending a
   Shared Secret request, the service name is "turn" and the protocol is
   "tcp".  RFC 2782 spells out the details of how a set of SRV records
   are sorted and then tried.  However, it only states that the client
   should "try to connect to the (protocol, address, service)" without



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   giving any details on what happens in the event of failure.  Those
   details are described here for TURN.

   For TURN requests, failure occurs if there is a transport failure of
   some sort (generally, due to fatal ICMP errors in UDP or connection
   failures in TCP).  Failure also occurs if the the request does not
   solicit a response after 9.5 seconds.  If a failure occurs, the
   client SHOULD create a new request, which is identical to the
   previous, but has a different transaction ID and MESSAGE-INTEGRITY
   attribute.  That request is sent to the next element in the list as
   specified by RFC~2782.

8.2  Obtaining a One Time Password

   In order to allocate addresses, a client must obtain a one-time
   username and password from the TURN server.  A unique username and
   password are required for each distinct address allocated from the
   server.

   To obtain a one-time username and password, the client generates and
   sends a Shared Secret Request.  This is done as described in Section
   9.2 of STUN.  This request will have no attributes, and therefore,
   based on the processing in Section 7.1, the server will reject it
   with a Shared Secret Error Response with a 401 response code.  That
   response will contain a NONCE and a REALM.  The client SHOULD
   generate a new Shared Secret Request (with a new transaction ID),
   which contains the NONCE and REALM attributes copied from the 401
   response.  The request MUST include the USERNAME attribute, which
   contains a username supplied by the user for the specified realm.
   The request MUST include a MESSAGE-INTEGRITY attribute as the last
   attribute.  The key for the HMAC is computed as described in Section
   7.1.

   If the response (either to the initial request or to the second
   attempt with the credentials) is a Shared Secret Error Response, the
   processing depends on the the value of the response code in the
   ERROR-CODE attribute.  If the response code was a 430, the client
   SHOULD generate a new Shared Secret Request, using the username and
   password provided by the user, and the REALM and NONCE provided in
   the 430 response.  For a 431 or 436 response code, the client SHOULD
   alert the user.  For a 432, 434 and 435 response codes, if the client
   had omitted the USERNAME, REALM or NONCE attributes, respectively,
   from the previous request, it SHOULD retry, this time including the
   USERNAME, NONCE, REALM, and MESSAGE-INTEGRITY attributes.  For a 500
   response code, the client MAY wait several seconds and then retry the
   request.  For a 600 response code, the client MUST NOT retry the
   request, and SHOULD display the reason phrase to the user.  Unknown
   attributes between 400 and 499 are treated like a 400, unknown



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   attributes between 500 and 599 are treated like a 500, and unknown
   attributes between 600 and 699 are treated like a 600.  Any response
   between 100 and 399 MUST result in the cessation of request
   retransmissions, but otherwise is discarded.

   If a client receives a Shared Secret Response with an attribute whose
   type is greater than 0x7fff, the attribute MUST be ignored.  If the
   client receives a Shared Secret Response with an attribute whose type
   is less than or equal to 0x7fff, the response is ignored.

   If the response is a Shared Secret Response, it will contain the
   USERNAME and PASSWORD attributes.  The client can use these to
   authenticate an Allocate Request, as described below.

   A client MAY send multiple Shared Secret Requests over the same TLS
   connection, and MAY do so without waiting for responses to previous
   requests.  The client SHOULD close its connection when it has
   completed allocating usernames and passwords.

8.3  Allocating a Binding

   When a client wishes to obtain a transport address, it sends an
   Allocate Request to the TURN server.  Requests for TCP transport
   addresses MUST be sent over a TCP connection, and requests for UDP
   transport addresses MUST be sent over UDP.

   First, the client obtains a one-time username and password, using the
   mechanisms described in Section 8.2.  The client then formulates an
   Allocate Request.  The request MUST contain a transaction ID, unique
   for each request, and uniformly and randomly distributed between 0
   and 2**128 - 1.  The message type of the request MUST be "Allocate
   Request".  The length is set as described in Section 11.1 of STUN.

   The Allocate request MUST contain the MAGIC-COOKIE attribute as the
   first attribute.

   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 binding by
   including it in the LIFETIME attribute in the request.  If the no
   data is sent or received on the binding before expiration of the
   lifetime, the binding will be deleted by the client.

   The client MUST include a USERNAME attribute, containing a username
   obtained from a previous Shared Secret Response.  The request MUST
   include a MESSAGE-INTEGRITY attribute as the last attribute.  The key



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   is equal to the password obtained from the PASSWORD attribute of the
   Shared Secret Response.  The Allocate Request MUST be sent to the
   same IP address and port as the Shared Secret Request.  This is
   because one time passwords are expected to be host-specific.  Rules
   for retransmissions for Allocate Requests sent over UDP are identical
   to those for STUN Binding Requests.  Allocate Requests sent over TCP
   are not retransmitted.  Transaction timeouts are identical to those
   for STUN Binding Requests, independent of the transport protocol.

8.4  Processing Allocate Responses

   If the response is an Allocate Error Response, the client checks the
   response code from the ERROR-CODE attribute of the response.  For a
   400 response code, the client SHOULD display the reason phrase to the
   user.  For a 420 response code, the client SHOULD retry the request,
   this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
   attribute of the response.  For a 430 response code, the client
   SHOULD obtain a new one-time username and password, and retry the
   Allocate Request with a new transaction.  For 401 and 432 response
   codes, if the client had omitted the USERNAME or MESSAGE-INTEGRITY
   attribute as indicated by the error, it SHOULD try again with those
   attributes.  A new one-time username and password is needed in that
   case.  For a 431 response code, the client SHOULD alert the user, and
   MAY try the request again after obtaining a new username and
   password.  For a 300 response code, the client SHOULD attempt a new
   TURN transaction to the server indicated in the ALTERNATE-SERVER
   attribute.  For a 500 response code, the client MAY wait several
   seconds and then retry the request with a new username and password.
   For a 600 response code, the client MUST NOT retry the request, and
   SHOULD display the reason phrase to the user.  Unknown attributes
   between 400 and 499 are treated like a 400, unknown attributes
   between 500 and 599 are treated like a 500, and unknown attributes
   between 600 and 699 are treated like a 600.  Unknown attributes
   between 300 and 399 are treated like 300.  Any response between 100
   and 299 MUST result in the cessation of any request retransmissions,
   but otherwise is discarded.

   If a client receives a response with an attribute whose type is
   greater than 0x7fff, the attribute MUST be ignored.  If the client
   receives a response with an attribute whose type is less than or
   equal to 0x7fff, any request retransmissions MUST cease, but the
   entire response is otherwise ignored.

   If the response is an Allocate Response, the client MUST check the
   response for a MESSAGE-INTEGRITY attribute.  If not present, the
   client MUST discard the response.  If present, the client computes
   the HMAC over the response.  The key MUST be same as used to compute
   the MESSAGE-INTEGRITY attribute in the request.  If the computed HMAC



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   differs from the one in the response, the client MUST discard the
   response, and SHOULD alert the user about a possible attack.  If the
   computed HMAC matches the one from the response, processing
   continues.

   The MAPPED-ADDRESS in the Binding Response can be used by the client
   for receiving packets.  The server will expire the binding after
   LIFETIME seconds have passed with no activity.  The server will allow
   the user to send and receive no more than the amount of data
   indicated in the BANDWIDTH attribute.

8.5  Refreshing a Binding

   If there has been no activity on a UDP binding for a period of time
   equalling 3/4 of the lifetime of the binding (as conveyed in the
   LIFETIME attribute of the Allocate Response), the client SHOULD
   refresh the binding with another Allocate Request if it wishes to
   keep it.  Note that only UDP bindings can be refreshed.  For TCP,
   application-specific keepalives are needed.

   To perform a refresh, the client generates an Allocate Request as
   described in Section 8.3.  However, the one-time username and
   password used MUST be the same as those used in the successful
   Allocate Request for that binding.  The client will need to look for
   the TURN response amongst the data packets using the MAGIC-COOKIE, as
   described in Section 7.2.3.  Processing of that response is as
   defined in Section 8.4.  If the response was an Allocate Response,
   and the MAPPED-ADDRESS contains the same transport address as
   previously obtained, the binding has been refreshed.  The LIFETIME
   attribute indicates the amount of additional time the binding will
   live without activity.  If, however, the response was an Allocate
   Error Response with an ERROR-CODE indicating a 430 response, it means
   that the binding has expired at the server.  The client MAY use the
   procedures in Section 8.3 to obtain a new binding (this will require
   a new one-time username and password.  Other response codes do not
   imply that the binding has been expired, just that the refresh has
   failed.

8.6  Sending Data

   Before receiving any UDP data, a client has to send first.  To do
   that, it uses the Send Request.  This request MUST contain a
   transaction ID, unique for each request, and uniformly and randomly
   distributed between 0 and 2**128 - 1.  The message type of the
   request MUST be "Send Request".  The length is set as described in
   Section 11.1 of STUN.

   The Send request MUST contain the MAGIC-COOKIE attribute as the first



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   attribute.  The client MUST include a USERNAME attribute, containing
   the same username used in the Allocate request for this binding.  The
   request MUST include a MESSAGE-INTEGRITY attribute as the last
   attribute.  The key is equal to the password used for the Allocate
   request for this binding.  The Send Request MUST be sent to the same
   IP address and port as the Allocate Request, and MUST be sent from
   the same source IP and port used to send the Allocate request for the
   binding.  Rules for retransmissions for Send Requests sent over UDP
   are identical to those for STUN Binding Requests.  There is currently
   no support for Send Requests over TCP.  Transaction timeouts are
   identical to those for STUN Binding Requests, independent of the
   transport protocol.

   The Send Request MUST contain a DESTINATION-ADDRESS attribute, which
   contains the IP address and port that the data is being sent to.

   If the server successfully sends the data, the client will receive a
   Send Response.  Note that, as with responses to Allocate refreshes,
   the client will need to pick the Send Response (or Send Error
   Response) out of the packet stream by searching for the MAGIC-COOKIE
   in each received UDP packet.  If the response is a Send Error
   Response, it is processed as described in the first two paragraphs of
   Section 8.4.  If the response code is 438, the client is forbidden
   from using the Send Request, since lockdown has occurred.  The client
   can relay data to the peer by sending the data without a TURN message
   wrapper.  [[OPEN ISSUE: is there a need for the client to be told
   what the locked-down address is?]]

8.7  Tearing Down a Binding

   If a client no longer needs a binding, it SHOULD tear it down.  For
   TCP, this is done by closing the connection.  For UDP, this is done
   by performing a refresh, as described in Section 8.5, but with a
   LIFETIME attribute indicating a time of 0.

8.8  Receiving and Sending Data

   Once a binding has been allocated by an Allocate Response, the client
   MUST be prepared to receive data from the socket on which the
   Allocate Request was sent.  For UDP, the client MUST be prepared to
   disambiguate TURN messages from data for the lifetime of the binding.
   This disambiguation is done using the MAGIC-COOKIE, as described in
   Section 7.2.3.

   Once a Send request has been issued, the client MAY send data to its
   peer by sending data on that same socket.  Any UDP packets recieved
   by the server are forwarded to the default destination address until
   that address is changed by a subsequent Send command.



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   The client may receive a Data Indication message from the TURN
   server.  The client does not generate any kind of response to this
   message.  Its receipt implies that the server received a packet from
   a source which is not equal to the current default address.

9.  Protocol Details

   This section presents the detailed encoding of the message types,
   attributes, and response codes which are new to TURN.  The general
   message structure of TURN is identical to STUN [1].

9.1  Message Types

   TURN defines three new Message Types:


   0x0003  :  Allocate Request
   0x0103  :  Allocate Response
   0x0113  :  Allocate Error Response
   0x0004  :  Send Request
   0x0104  :  Send Response
   0x0114  :  Send Error Response
   0x0115  :  Data Indication


9.2  Message Attributes

   TURN defines the following message attributes:


   0x000d: LIFETIME
   0x000e: ALTERNATE-SERVER
   0x000f: MAGIC-COOKIE
   0x0010: BANDWIDTH
   0x0011: DESTINATION-ADDRESS
   0x0012: SOURCE-ADDRESS
   0x0013: DATA
   0x0014: NONCE


9.2.1  LIFETIME

   The lifetime attribute represents the duration for which the server
   will maintain a binding 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                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


9.2.2  ALTERNATE-SERVER

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

9.2.3  MAGIC-COOKIE

   The MAGIC-COOKIE is used by TURN clients and servers to disambiguate
   TURN traffic from data traffic.  Its value ix 0x72c64bc6.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1|1|1|0|0|1|0|1|1|0|0|0|1|1|0|0|1|0|0|1|0|1|1|1|1|0|0|0|1|1|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


9.2.4  BANDWIDTH

   The bandwidth attribute represents the peak bandwidth, measured in
   kbits per second, that the client expects to use on the binding.  The
   value represents the sum in the receive and send directions.
   [[Editors note: Need to define leaky bucket parameters for this.]]


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


9.2.5  DESTINATION-ADDRESS

   The DESTINATION-ADDRESS is present in Send Requests.  It specifies
   the address and port where the data is to be sent.  It is encoded in
   the same way as MAPPED-ADDRESS.

9.2.6  SOURCE-ADDRESS

   The SOURCE-ADDRESS is present in Data Indications.  It specifies the
   address and port from which a packet was received.  It is encoded in
   the same way as MAPPED-ADDRESS.





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9.2.7  DATA

   The DATA attribute is present in Send Requests 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).

9.2.8  NONCE

   The NONCE attribute is present in Shared Secret Requests and Shared
   Secret Error responses.  It contains a sequence of qdtext or
   quoted-pair, which are defined in [6].

9.2.9  Response Codes

   TURN defines the following new response codes:

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

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

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

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

      437 (No Binding): A Send Request was received by the server, but
      there is no binding in place for the source 5-tuple.

      439 (Illegal Port): A Send Request was received by the server, but
      lock-down has already occurred, and sending is disallowed.


10.  Security Considerations

   TURN servers allocate bandwidth and port resources to clients.
   Therefore, a TURN server requires authentication and authorization of
   TURN requests.  This authentication is provided by a client digest
   over TLS, which results in the generation of a one-time password that
   is used in a single subsequent Allocate Request.  This mechanism
   protects against eavesdropping attacks and man-in-the-middle attacks.
   The usage of one-time passwords ensures that the Allocate Requests,
   which 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.




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   Because TURN servers 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, but
   each requires a new one-time username and password.  It is
   RECOMMENDED that servers implement a cap on the number of one-time
   passwords that are allocated to any specific user at a time (around 5
   or 10 should be sufficient).  This will prevent floods of Allocate
   requests from a single user, in an attempt to use up the resources of
   the system.  A single malicious user could generate a single Allocate
   Request, obtain a binding, and then flood the server with data over
   this binding, in an attempt to deny others service.  However, this
   attack requires the attacker themselves to receive the data being
   sent at the server.  To ameliorate these kinds of attacks, servers
   SHOULD implement a bandwidth cap on each binding (conveyed to the
   client in the BANDWIDTH attribute of the Allocate Response), and
   discard packets beyond the threshold.

   A client will use the transport address learned from the
   MAPPED-ADDRESS attribute of the Binding 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 TLS and HMAC-based authentication and integrity
   checks provided in TURN.  They can guarantee the authenticity and
   integrity of the mapped addressses.  Note that TURN is not
   susceptible to the attacks described in Section 12.2.3, 12.2.4,
   12.2.5 or 12.2.6 of STUN.  These attacks are based on the fact that a
   STUN server mirrors the source IP address, which cannot be
   authenticated.  TURN does not use the source address of the Binding
   Request, and therefore, those attacks do not apply.

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

   TURN 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 TURN server, so that the
   user's IP addresses are never revealed.

   TCP transport addresses allocated by TURN will properly work with TLS
   and SSL.  However, any addresses allocated by TURN will not operate
   properly with IPSec Authentication Header (AH) [10] in transport
   mode.  IPSec ESP [11] and any tunnel-mode ESP or AH should still
   operate.






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11.  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 [12].
   TURN 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.

11.1  Problem Definition

   From RFC 3424 [12], 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 TURN 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.  TURN, by
   design, does not allow a client to run a server (such as a web or
   SMTP server) using a TURN address.  TURN is useful even when NAT is
   not present, to provide anonymity services.

11.2  Exit Strategy

   From [12], 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 TURN will be useful indefinitely, to provide
   anonymity services.  When used to facilitate NAT traversal, TURN does
   not iself provide an exit strategy.  That is provided by the
   Interactive Connectivity Establishment (ICE) [13] mechanism.  ICE
   allows two cooperating clients to interactively determine the best
   addresses to use when communicating.  ICE uses TURN-allocated
   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



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   will increasingly use other addresses (host local addresses).
   Therefore, clients will allocate TURN addresses, but not use them,
   and therefore, de-allocate them.  Servers will see a decrease in
   usage.  Once a provider sees that its TURN servers are not being used
   at all (that is, no media flows through them), they can simply remove
   them.  ICE will operate without TURN-allocated addresses.

11.3  Brittleness Introduced by TURN

   From [12], 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.

   TURN introduces brittleness in a few ways.  First, it adds another
   server element to any system, which adds another point of failure.
   TURN requires clients to demultiplex TURN packets and data based on
   hunting for a MAGIC-COOKIE in the TURN 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.  TURN relies on any
   NAT bindings existing for the duration of the bindings held by the
   TURN server.  Neither the client nor the TURN 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 address learned from TURN for some period, the address
   might become useless spontaneously.

   TURN 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 TURN addresses.  However, when used, this
   problem will exist.

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




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11.4  Requirements for a Long Term Solution

   From [12]}, 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 TURN continues to validate our belief in the
   requirements outlined in Section 14.4 of STUN.

11.5  Issues with Existing NAPT Boxes

   From [12], 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 will interfere with
   proper operation of any UNSAF mechanism, including TURN.  However, if
   a NAT tries to modify a MAPPED-ADDRESS in a TURN Allocate Response,
   this will be detected by the client as an attack.

12.  Examples

   TODO.

13.  References

13.1  Normative References

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

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

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

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



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13.2  Informative References

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

   [6]   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.

   [7]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [8]   Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.

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

   [10]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
         November 1998.

   [11]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
         (ESP)", RFC 2406, November 1998.

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

   [13]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Multimedia Session Establishment Protocols",
         draft-ietf-mmusic-ice-03 (work in progress), October 2004.


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
   Airspace

   EMail: rohan@ekabal.com


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

   EMail: huitema@microsoft.com






































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