





Internet Engineering Task Force (IETF)                           R. Mahy
Request for Comments: 5766                                  Unaffiliated
Category: Standards Track                                    P. Matthews
ISSN: 2070-1721                                           Alcatel-Lucent
                                                            J. Rosenberg
                                                             jdrosen.net
                                                              April 2010


               Traversal Using Relays around NAT (TURN):
     Relay Extensions to Session Traversal Utilities for NAT (STUN)

Abstract

   If a host is located behind a NAT, then in certain situations it can
   be impossible for that host to communicate directly with other hosts
   (peers).  In these situations, it is necessary for the host to use
   the services of an intermediate node that acts as a communication
   relay.  This specification defines a protocol, called TURN (Traversal
   Using Relays around NAT), that allows the host to control the
   operation of the relay and to exchange packets with its peers using
   the relay.  TURN differs from some other relay control protocols in
   that it allows a client to communicate with multiple peers using a
   single relay address.

   The TURN protocol was designed to be used as part of the ICE
   (Interactive Connectivity Establishment) approach to NAT traversal,
   though it also can be used without ICE.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5766.









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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Transports . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Allocations  . . . . . . . . . . . . . . . . . . . . . . .  9
     2.3.  Permissions  . . . . . . . . . . . . . . . . . . . . . . . 11
     2.4.  Send Mechanism . . . . . . . . . . . . . . . . . . . . . . 12
     2.5.  Channels . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.6.  Unprivileged TURN Servers  . . . . . . . . . . . . . . . . 15
     2.7.  Avoiding IP Fragmentation  . . . . . . . . . . . . . . . . 16
     2.8.  RTP Support  . . . . . . . . . . . . . . . . . . . . . . . 17
     2.9.  Anycast Discovery of Servers . . . . . . . . . . . . . . . 17
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.  General Behavior . . . . . . . . . . . . . . . . . . . . . . . 19
   5.  Allocations  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   6.  Creating an Allocation . . . . . . . . . . . . . . . . . . . . 23
     6.1.  Sending an Allocate Request  . . . . . . . . . . . . . . . 23
     6.2.  Receiving an Allocate Request  . . . . . . . . . . . . . . 24
     6.3.  Receiving an Allocate Success Response . . . . . . . . . . 28
     6.4.  Receiving an Allocate Error Response . . . . . . . . . . . 29
   7.  Refreshing an Allocation . . . . . . . . . . . . . . . . . . . 31
     7.1.  Sending a Refresh Request  . . . . . . . . . . . . . . . . 31
     7.2.  Receiving a Refresh Request  . . . . . . . . . . . . . . . 31
     7.3.  Receiving a Refresh Response . . . . . . . . . . . . . . . 32
   8.  Permissions  . . . . . . . . . . . . . . . . . . . . . . . . . 32
   9.  CreatePermission . . . . . . . . . . . . . . . . . . . . . . . 34
     9.1.  Forming a CreatePermission Request . . . . . . . . . . . . 34
     9.2.  Receiving a CreatePermission Request . . . . . . . . . . . 34
     9.3.  Receiving a CreatePermission Response  . . . . . . . . . . 35
   10. Send and Data Methods  . . . . . . . . . . . . . . . . . . . . 35
     10.1. Forming a Send Indication  . . . . . . . . . . . . . . . . 35
     10.2. Receiving a Send Indication  . . . . . . . . . . . . . . . 35



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     10.3. Receiving a UDP Datagram . . . . . . . . . . . . . . . . . 36
     10.4. Receiving a Data Indication  . . . . . . . . . . . . . . . 37
   11. Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     11.1. Sending a ChannelBind Request  . . . . . . . . . . . . . . 39
     11.2. Receiving a ChannelBind Request  . . . . . . . . . . . . . 39
     11.3. Receiving a ChannelBind Response . . . . . . . . . . . . . 40
     11.4. The ChannelData Message  . . . . . . . . . . . . . . . . . 41
     11.5. Sending a ChannelData Message  . . . . . . . . . . . . . . 41
     11.6. Receiving a ChannelData Message  . . . . . . . . . . . . . 42
     11.7. Relaying Data from the Peer  . . . . . . . . . . . . . . . 43
   12. IP Header Fields . . . . . . . . . . . . . . . . . . . . . . . 43
   13. New STUN Methods . . . . . . . . . . . . . . . . . . . . . . . 45
   14. New STUN Attributes  . . . . . . . . . . . . . . . . . . . . . 45
     14.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 45
     14.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 46
     14.3. XOR-PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . 46
     14.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
     14.5. XOR-RELAYED-ADDRESS  . . . . . . . . . . . . . . . . . . . 46
     14.6. EVEN-PORT  . . . . . . . . . . . . . . . . . . . . . . . . 46
     14.7. REQUESTED-TRANSPORT  . . . . . . . . . . . . . . . . . . . 47
     14.8. DONT-FRAGMENT  . . . . . . . . . . . . . . . . . . . . . . 47
     14.9. RESERVATION-TOKEN  . . . . . . . . . . . . . . . . . . . . 48
   15. New STUN Error Response Codes  . . . . . . . . . . . . . . . . 48
   16. Detailed Example . . . . . . . . . . . . . . . . . . . . . . . 48
   17. Security Considerations  . . . . . . . . . . . . . . . . . . . 55
     17.1. Outsider Attacks . . . . . . . . . . . . . . . . . . . . . 55
       17.1.1.  Obtaining Unauthorized Allocations  . . . . . . . . . 55
       17.1.2.  Offline Dictionary Attacks  . . . . . . . . . . . . . 56
       17.1.3.  Faked Refreshes and Permissions . . . . . . . . . . . 56
       17.1.4.  Fake Data . . . . . . . . . . . . . . . . . . . . . . 56
       17.1.5.  Impersonating a Server  . . . . . . . . . . . . . . . 57
       17.1.6.  Eavesdropping Traffic . . . . . . . . . . . . . . . . 58
       17.1.7.  TURN Loop Attack  . . . . . . . . . . . . . . . . . . 58
     17.2. Firewall Considerations  . . . . . . . . . . . . . . . . . 59
       17.2.1.  Faked Permissions . . . . . . . . . . . . . . . . . . 59
       17.2.2.  Blacklisted IP Addresses  . . . . . . . . . . . . . . 60
       17.2.3.  Running Servers on Well-Known Ports . . . . . . . . . 60
     17.3. Insider Attacks  . . . . . . . . . . . . . . . . . . . . . 60
       17.3.1.  DoS against TURN Server . . . . . . . . . . . . . . . 60
       17.3.2.  Anonymous Relaying of Malicious Traffic . . . . . . . 61
       17.3.3.  Manipulating Other Allocations  . . . . . . . . . . . 61
     17.4. Other Considerations . . . . . . . . . . . . . . . . . . . 61
   18. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 61
   19. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 62
   20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 63
   21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 64
     21.1. Normative References . . . . . . . . . . . . . . . . . . . 64
     21.2. Informative References . . . . . . . . . . . . . . . . . . 64



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1.  Introduction

   A host behind a NAT may wish to exchange packets with other hosts,
   some of which may also be behind NATs.  To do this, the hosts
   involved can use "hole punching" techniques (see [RFC5128]) in an
   attempt discover a direct communication path; that is, a
   communication path that goes from one host to another through
   intervening NATs and routers, but does not traverse any relays.

   As described in [RFC5128] and [RFC4787], hole punching techniques
   will fail if both hosts are behind NATs that are not well behaved.
   For example, if both hosts are behind NATs that have a mapping
   behavior of "address-dependent mapping" or "address- and port-
   dependent mapping", then hole punching techniques generally fail.

   When a direct communication path cannot be found, it is necessary to
   use the services of an intermediate host that acts as a relay for the
   packets.  This relay typically sits in the public Internet and relays
   packets between two hosts that both sit behind NATs.

   This specification defines a protocol, called TURN, that allows a
   host behind a NAT (called the TURN client) to request that another
   host (called the TURN server) act as a relay.  The client can arrange
   for the server to relay packets to and from certain other hosts
   (called peers) and can control aspects of how the relaying is done.
   The client does this by obtaining an IP address and port on the
   server, called the relayed transport address.  When a peer sends a
   packet to the relayed transport address, the server relays the packet
   to the client.  When the client sends a data packet to the server,
   the server relays it to the appropriate peer using the relayed
   transport address as the source.

   A client using TURN must have some way to communicate the relayed
   transport address to its peers, and to learn each peer's IP address
   and port (more precisely, each peer's server-reflexive transport
   address, see Section 2).  How this is done is out of the scope of the
   TURN protocol.  One way this might be done is for the client and
   peers to exchange email messages.  Another way is for the client and
   its peers to use a special-purpose "introduction" or "rendezvous"
   protocol (see [RFC5128] for more details).

   If TURN is used with ICE [RFC5245], then the relayed transport
   address and the IP addresses and ports of the peers are included in
   the ICE candidate information that the rendezvous protocol must
   carry.  For example, if TURN and ICE are used as part of a multimedia
   solution using SIP [RFC3261], then SIP serves the role of the
   rendezvous protocol, carrying the ICE candidate information inside
   the body of SIP messages.  If TURN and ICE are used with some other



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RFC 5766                          TURN                        April 2010


   rendezvous protocol, then [MMUSIC-ICE-NONSIP] provides guidance on
   the services the rendezvous protocol must perform.

   Though the use of a TURN server to enable communication between two
   hosts behind NATs is very likely to work, it comes at a high cost to
   the provider of the TURN server, since the server typically needs a
   high-bandwidth connection to the Internet.  As a consequence, it is
   best to use a TURN server only when a direct communication path
   cannot be found.  When the client and a peer use ICE to determine the
   communication path, ICE will use hole punching techniques to search
   for a direct path first and only use a TURN server when a direct path
   cannot be found.

   TURN was originally invented to support multimedia sessions signaled
   using SIP.  Since SIP supports forking, TURN supports multiple peers
   per relayed transport address; a feature not supported by other
   approaches (e.g., SOCKS [RFC1928]).  However, care has been taken to
   make sure that TURN is suitable for other types of applications.

   TURN was designed as one piece in the larger ICE approach to NAT
   traversal.  Implementors of TURN are urged to investigate ICE and
   seriously consider using it for their application.  However, it is
   possible to use TURN without ICE.

   TURN is an extension to the STUN (Session Traversal Utilities for
   NAT) protocol [RFC5389].  Most, though not all, TURN messages are
   STUN-formatted messages.  A reader of this document should be
   familiar with STUN.

2.  Overview of Operation

   This section gives an overview of the operation of TURN.  It is non-
   normative.

   In a typical configuration, a TURN client is connected to a private
   network [RFC1918] and through one or more NATs to the public
   Internet.  On the public Internet is a TURN server.  Elsewhere in the
   Internet are one or more peers with which the TURN client wishes to
   communicate.  These peers may or may not be behind one or more NATs.
   The client uses the server as a relay to send packets to these peers
   and to receive packets from these peers.










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RFC 5766                          TURN                        April 2010


                                        Peer A
                                        Server-Reflexive    +---------+
                                        Transport Address   |         |
                                        192.0.2.150:32102   |         |
                                            |              /|         |
                          TURN              |            / ^|  Peer A |
    Client's              Server            |           /  ||         |
    Host Transport        Transport         |         //   ||         |
    Address               Address           |       //     |+---------+
   10.1.1.2:49721       192.0.2.15:3478     |+-+  //     Peer A
            |               |               ||N| /       Host Transport
            |   +-+         |               ||A|/        Address
            |   | |         |               v|T|     192.168.100.2:49582
            |   | |         |               /+-+
 +---------+|   | |         |+---------+   /              +---------+
 |         ||   |N|         ||         | //               |         |
 | TURN    |v   | |         v| TURN    |/                 |         |
 | Client  |----|A|----------| Server  |------------------|  Peer B |
 |         |    | |^         |         |^                ^|         |
 |         |    |T||         |         ||                ||         |
 +---------+    | ||         +---------+|                |+---------+
                | ||                    |                |
                | ||                    |                |
                +-+|                    |                |
                   |                    |                |
                   |                    |                |
             Client's                   |            Peer B
             Server-Reflexive    Relayed             Transport
             Transport Address   Transport Address   Address
             192.0.2.1:7000      192.0.2.15:50000     192.0.2.210:49191

                                 Figure 1

   Figure 1 shows a typical deployment.  In this figure, the TURN client
   and the TURN server are separated by a NAT, with the client on the
   private side and the server on the public side of the NAT.  This NAT
   is assumed to be a "bad" NAT; for example, it might have a mapping
   property of "address-and-port-dependent mapping" (see [RFC4787]).

   The client talks to the server from a (IP address, port) combination
   called the client's HOST TRANSPORT ADDRESS.  (The combination of an
   IP address and port is called a TRANSPORT ADDRESS.)

   The client sends TURN messages from its host transport address to a
   transport address on the TURN server that is known as the TURN SERVER
   TRANSPORT ADDRESS.  The client learns the TURN server transport
   address through some unspecified means (e.g., configuration), and
   this address is typically used by many clients simultaneously.



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RFC 5766                          TURN                        April 2010


   Since the client is behind a NAT, the server sees packets from the
   client as coming from a transport address on the NAT itself.  This
   address is known as the client's SERVER-REFLEXIVE transport address;
   packets sent by the server to the client's server-reflexive transport
   address will be forwarded by the NAT to the client's host transport
   address.

   The client uses TURN commands to create and manipulate an ALLOCATION
   on the server.  An allocation is a data structure on the server.
   This data structure contains, amongst other things, the RELAYED
   TRANSPORT ADDRESS for the allocation.  The relayed transport address
   is the transport address on the server that peers can use to have the
   server relay data to the client.  An allocation is uniquely
   identified by its relayed transport address.

   Once an allocation is created, the client can send application data
   to the server along with an indication of to which peer the data is
   to be sent, and the server will relay this data to the appropriate
   peer.  The client sends the application data to the server inside a
   TURN message; at the server, the data is extracted from the TURN
   message and sent to the peer in a UDP datagram.  In the reverse
   direction, a peer can send application data in a UDP datagram to the
   relayed transport address for the allocation; the server will then
   encapsulate this data inside a TURN message and send it to the client
   along with an indication of which peer sent the data.  Since the TURN
   message always contains an indication of which peer the client is
   communicating with, the client can use a single allocation to
   communicate with multiple peers.

   When the peer is behind a NAT, then the client must identify the peer
   using its server-reflexive transport address rather than its host
   transport address.  For example, to send application data to Peer A
   in the example above, the client must specify 192.0.2.150:32102 (Peer
   A's server-reflexive transport address) rather than 192.168.100.2:
   49582 (Peer A's host transport address).

   Each allocation on the server belongs to a single client and has
   exactly one relayed transport address that is used only by that
   allocation.  Thus, when a packet arrives at a relayed transport
   address on the server, the server knows for which client the data is
   intended.

   The client may have multiple allocations on a server at the same
   time.







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RFC 5766                          TURN                        April 2010


2.1.  Transports

   TURN, as defined in this specification, always uses UDP between the
   server and the peer.  However, this specification allows the use of
   any one of UDP, TCP, or Transport Layer Security (TLS) over TCP to
   carry the TURN messages between the client and the server.

           +----------------------------+---------------------+
           | TURN client to TURN server | TURN server to peer |
           +----------------------------+---------------------+
           |             UDP            |         UDP         |
           |             TCP            |         UDP         |
           |        TLS over TCP        |         UDP         |
           +----------------------------+---------------------+

   If TCP or TLS-over-TCP is used between the client and the server,
   then the server will convert between these transports and UDP
   transport when relaying data to/from the peer.

   Since this version of TURN only supports UDP between the server and
   the peer, it is expected that most clients will prefer to use UDP
   between the client and the server as well.  That being the case, some
   readers may wonder: Why also support TCP and TLS-over-TCP?

   TURN supports TCP transport between the client and the server because
   some firewalls are configured to block UDP entirely.  These firewalls
   block UDP but not TCP, in part because TCP has properties that make
   the intention of the nodes being protected by the firewall more
   obvious to the firewall.  For example, TCP has a three-way handshake
   that makes in clearer that the protected node really wishes to have
   that particular connection established, while for UDP the best the
   firewall can do is guess which flows are desired by using filtering
   rules.  Also, TCP has explicit connection teardown; while for UDP,
   the firewall has to use timers to guess when the flow is finished.

   TURN supports TLS-over-TCP transport between the client and the
   server because TLS provides additional security properties not
   provided by TURN's default digest authentication; properties that
   some clients may wish to take advantage of.  In particular, TLS
   provides a way for the client to ascertain that it is talking to the
   correct server, and provides for confidentiality of TURN control
   messages.  TURN does not require TLS because the overhead of using
   TLS is higher than that of digest authentication; for example, using
   TLS likely means that most application data will be doubly encrypted
   (once by TLS and once to ensure it is still encrypted in the UDP
   datagram).





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RFC 5766                          TURN                        April 2010


   There is a planned extension to TURN to add support for TCP between
   the server and the peers [TURN-TCP].  For this reason, allocations
   that use UDP between the server and the peers are known as UDP
   allocations, while allocations that use TCP between the server and
   the peers are known as TCP allocations.  This specification describes
   only UDP allocations.

   TURN, as defined in this specification, only supports IPv4.  All IP
   addresses in this specification must be IPv4 addresses.  There is a
   planned extension to TURN to add support for IPv6 and for relaying
   between IPv4 and IPv6 [TURN-IPv6].

   In some applications for TURN, the client may send and receive
   packets other than TURN packets on the host transport address it uses
   to communicate with the server.  This can happen, for example, when
   using TURN with ICE.  In these cases, the client can distinguish TURN
   packets from other packets by examining the source address of the
   arriving packet: those arriving from the TURN server will be TURN
   packets.

2.2.  Allocations

   To create an allocation on the server, the client uses an Allocate
   transaction.  The client sends an Allocate request to the server, and
   the server replies with an Allocate success response containing the
   allocated relayed transport address.  The client can include
   attributes in the Allocate request that describe the type of
   allocation it desires (e.g., the lifetime of the allocation).  Since
   relaying data has security implications, the server requires that the
   client authenticate itself, typically using STUN's long-term
   credential mechanism, to show that it is authorized to use the
   server.

   Once a relayed transport address is allocated, a client must keep the
   allocation alive.  To do this, the client periodically sends a
   Refresh request to the server.  TURN deliberately uses a different
   method (Refresh rather than Allocate) for refreshes to ensure that
   the client is informed if the allocation vanishes for some reason.

   The frequency of the Refresh transaction is determined by the
   lifetime of the allocation.  The default lifetime of an allocation is
   10 minutes -- this value was chosen to be long enough so that
   refreshing is not typically a burden on the client, while expiring
   allocations where the client has unexpectedly quit in a timely
   manner.  However, the client can request a longer lifetime in the
   Allocate request and may modify its request in a Refresh request, and
   the server always indicates the actual lifetime in the response.  The
   client must issue a new Refresh transaction within "lifetime" seconds



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RFC 5766                          TURN                        April 2010


   of the previous Allocate or Refresh transaction.  Once a client no
   longer wishes to use an allocation, it should delete the allocation
   using a Refresh request with a requested lifetime of 0.

   Both the server and client keep track of a value known as the
   5-TUPLE.  At the client, the 5-tuple consists of the client's host
   transport address, the server transport address, and the transport
   protocol used by the client to communicate with the server.  At the
   server, the 5-tuple value is the same except that the client's host
   transport address is replaced by the client's server-reflexive
   address, since that is the client's address as seen by the server.

   Both the client and the server remember the 5-tuple used in the
   Allocate request.  Subsequent messages between the client and the
   server use the same 5-tuple.  In this way, the client and server know
   which allocation is being referred to.  If the client wishes to
   allocate a second relayed transport address, it must create a second
   allocation using a different 5-tuple (e.g., by using a different
   client host address or port).

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

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |-- Allocate request --------------->|             |             |
    |                                    |             |             |
    |<--------------- Allocate failure --|             |             |
    |                 (401 Unauthorized) |             |             |
    |                                    |             |             |
    |-- Allocate request --------------->|             |             |
    |                                    |             |             |
    |<---------- Allocate success resp --|             |             |
    |            (192.0.2.15:50000)      |             |             |
    //                                   //            //            //
    |                                    |             |             |
    |-- Refresh request ---------------->|             |             |
    |                                    |             |             |
    |<----------- Refresh success resp --|             |             |
    |                                    |             |             |

                                 Figure 2






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   In Figure 2, the client sends an Allocate request to the server
   without credentials.  Since the server requires that all requests be
   authenticated using STUN's long-term credential mechanism, the server
   rejects the request with a 401 (Unauthorized) error code.  The client
   then tries again, this time including credentials (not shown).  This
   time, the server accepts the Allocate request and returns an Allocate
   success response containing (amongst other things) the relayed
   transport address assigned to the allocation.  Sometime later, the
   client decides to refresh the allocation and thus sends a Refresh
   request to the server.  The refresh is accepted and the server
   replies with a Refresh success response.

2.3.  Permissions

   To ease concerns amongst enterprise IT administrators that TURN could
   be used to bypass corporate firewall security, TURN includes the
   notion of permissions.  TURN permissions mimic the address-restricted
   filtering mechanism of NATs that comply with [RFC4787].

   An allocation can have zero or more permissions.  Each permission
   consists of an IP address and a lifetime.  When the server receives a
   UDP datagram on the allocation's relayed transport address, it first
   checks the list of permissions.  If the source IP address of the
   datagram matches a permission, the application data is relayed to the
   client, otherwise the UDP datagram is silently discarded.

   A permission expires after 5 minutes if it is not refreshed, and
   there is no way to explicitly delete a permission.  This behavior was
   selected to match the behavior of a NAT that complies with [RFC4787].

   The client can install or refresh a permission using either a
   CreatePermission request or a ChannelBind request.  Using the
   CreatePermission request, multiple permissions can be installed or
   refreshed with a single request -- this is important for applications
   that use ICE.  For security reasons, permissions can only be
   installed or refreshed by transactions that can be authenticated;
   thus, Send indications and ChannelData messages (which are used to
   send data to peers) do not install or refresh any permissions.

   Note that permissions are within the context of an allocation, so
   adding or expiring a permission in one allocation does not affect
   other allocations.









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RFC 5766                          TURN                        April 2010


2.4.  Send Mechanism

   There are two mechanisms for the client and peers to exchange
   application data using the TURN server.  The first mechanism uses the
   Send and Data methods, the second way uses channels.  Common to both
   ways is the ability of the client to communicate with multiple peers
   using a single allocated relayed transport address; thus, both ways
   include a means for the client to indicate to the server which peer
   should receive the data, and for the server to indicate to the client
   which peer sent the data.

   The Send mechanism uses Send and Data indications.  Send indications
   are used to send application data from the client to the server,
   while Data indications are used to send application data from the
   server to the client.

   When using the Send mechanism, the client sends a Send indication to
   the TURN server containing (a) an XOR-PEER-ADDRESS attribute
   specifying the (server-reflexive) transport address of the peer and
   (b) a DATA attribute holding the application data.  When the TURN
   server receives the Send indication, it extracts the application data
   from the DATA attribute and sends it in a UDP datagram to the peer,
   using the allocated relay address as the source address.  Note that
   there is no need to specify the relayed transport address, since it
   is implied by the 5-tuple used for the Send indication.

   In the reverse direction, UDP datagrams arriving at the relayed
   transport address on the TURN server are converted into Data
   indications and sent to the client, with the server-reflexive
   transport address of the peer included in an XOR-PEER-ADDRESS
   attribute and the data itself in a DATA attribute.  Since the relayed
   transport address uniquely identified the allocation, the server
   knows which client should receive the data.

   Send and Data indications cannot be authenticated, since the long-
   term credential mechanism of STUN does not support authenticating
   indications.  This is not as big an issue as it might first appear,
   since the client-to-server leg is only half of the total path to the
   peer.  Applications that want proper security should encrypt the data
   sent between the client and a peer.

   Because Send indications are not authenticated, it is possible for an
   attacker to send bogus Send indications to the server, which will
   then relay these to a peer.  To partly mitigate this attack, TURN
   requires that the client install a permission towards a peer before
   sending data to it using a Send indication.





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  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |                                    |             |             |
    |-- CreatePermission req (Peer A) -->|             |             |
    |<-- CreatePermission success resp --|             |             |
    |                                    |             |             |
    |--- Send ind (Peer A)-------------->|             |             |
    |                                    |=== data ===>|             |
    |                                    |             |             |
    |                                    |<== data ====|             |
    |<-------------- Data ind (Peer A) --|             |             |
    |                                    |             |             |
    |                                    |             |             |
    |--- Send ind (Peer B)-------------->|             |             |
    |                                    | dropped     |             |
    |                                    |             |             |
    |                                    |<== data ==================|
    |                            dropped |             |             |
    |                                    |             |             |

                                 Figure 3

   In Figure 3, the client has already created an allocation and now
   wishes to send data to its peers.  The client first creates a
   permission by sending the server a CreatePermission request
   specifying Peer A's (server-reflexive) IP address in the XOR-PEER-
   ADDRESS attribute; if this was not done, the server would not relay
   data between the client and the server.  The client then sends data
   to Peer A using a Send indication; at the server, the application
   data is extracted and forwarded in a UDP datagram to Peer A, using
   the relayed transport address as the source transport address.  When
   a UDP datagram from Peer A is received at the relayed transport
   address, the contents are placed into a Data indication and forwarded
   to the client.  Later, the client attempts to exchange data with Peer
   B; however, no permission has been installed for Peer B, so the Send
   indication from the client and the UDP datagram from the peer are
   both dropped by the server.

2.5.  Channels

   For some applications (e.g., Voice over IP), the 36 bytes of overhead
   that a Send indication or Data indication adds to the application
   data can substantially increase the bandwidth required between the
   client and the server.  To remedy this, TURN offers a second way for
   the client and server to associate data with a specific peer.

   This second way uses an alternate packet format known as the
   ChannelData message.  The ChannelData message does not use the STUN



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   header used by other TURN messages, but instead has a 4-byte header
   that includes a number known as a channel number.  Each channel
   number in use is bound to a specific peer and thus serves as a
   shorthand for the peer's host transport address.

   To bind a channel to a peer, the client sends a ChannelBind request
   to the server, and includes an unbound channel number and the
   transport address of the peer.  Once the channel is bound, the client
   can use a ChannelData message to send the server data destined for
   the peer.  Similarly, the server can relay data from that peer
   towards the client using a ChannelData message.

   Channel bindings last for 10 minutes unless refreshed -- this
   lifetime was chosen to be longer than the permission lifetime.
   Channel bindings are refreshed by sending another ChannelBind request
   rebinding the channel to the peer.  Like permissions (but unlike
   allocations), there is no way to explicitly delete a channel binding;
   the client must simply wait for it to time out.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |                                    |             |             |
    |-- ChannelBind req ---------------->|             |             |
    | (Peer A to 0x4001)                 |             |             |
    |                                    |             |             |
    |<---------- ChannelBind succ resp --|             |             |
    |                                    |             |             |
    |-- [0x4001] data ------------------>|             |             |
    |                                    |=== data ===>|             |
    |                                    |             |             |
    |                                    |<== data ====|             |
    |<------------------ [0x4001] data --|             |             |
    |                                    |             |             |
    |--- Send ind (Peer A)-------------->|             |             |
    |                                    |=== data ===>|             |
    |                                    |             |             |
    |                                    |<== data ====|             |
    |<------------------ [0x4001] data --|             |             |
    |                                    |             |             |

                                 Figure 4

   Figure 4 shows the channel mechanism in use.  The client has already
   created an allocation and now wishes to bind a channel to Peer A.  To
   do this, the client sends a ChannelBind request to the server,
   specifying the transport address of Peer A and a channel number
   (0x4001).  After that, the client can send application data
   encapsulated inside ChannelData messages to Peer A: this is shown as



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   "[0x4001] data" where 0x4001 is the channel number.  When the
   ChannelData message arrives at the server, the server transfers the
   data to a UDP datagram and sends it to Peer A (which is the peer
   bound to channel number 0x4001).

   In the reverse direction, when Peer A sends a UDP datagram to the
   relayed transport address, this UDP datagram arrives at the server on
   the relayed transport address assigned to the allocation.  Since the
   UDP datagram was received from Peer A, which has a channel number
   assigned to it, the server encapsulates the data into a ChannelData
   message when sending the data to the client.

   Once a channel has been bound, the client is free to intermix
   ChannelData messages and Send indications.  In the figure, the client
   later decides to use a Send indication rather than a ChannelData
   message to send additional data to Peer A.  The client might decide
   to do this, for example, so it can use the DONT-FRAGMENT attribute
   (see the next section).  However, once a channel is bound, the server
   will always use a ChannelData message, as shown in the call flow.

   Note that ChannelData messages can only be used for peers to which
   the client has bound a channel.  In the example above, Peer A has
   been bound to a channel, but Peer B has not, so application data to
   and from Peer B would use the Send mechanism.

2.6.  Unprivileged TURN Servers

   This version of TURN is designed so that the server can be
   implemented as an application that runs in user space under commonly
   available operating systems without requiring special privileges.
   This design decision was made to make it easy to deploy a TURN
   server: for example, to allow a TURN server to be integrated into a
   peer-to-peer application so that one peer can offer NAT traversal
   services to another peer.

   This design decision has the following implications for data relayed
   by a TURN server:

   o  The value of the Diffserv field may not be preserved across the
      server;

   o  The Time to Live (TTL) field may be reset, rather than
      decremented, across the server;

   o  The Explicit Congestion Notification (ECN) field may be reset by
      the server;

   o  ICMP messages are not relayed by the server;



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   o  There is no end-to-end fragmentation, since the packet is re-
      assembled at the server.

   Future work may specify alternate TURN semantics that address these
   limitations.

2.7.  Avoiding IP Fragmentation

   For reasons described in [Frag-Harmful], applications, especially
   those sending large volumes of data, should try hard to avoid having
   their packets fragmented.  Applications using TCP can more or less
   ignore this issue because fragmentation avoidance is now a standard
   part of TCP, but applications using UDP (and thus any application
   using this version of TURN) must handle fragmentation avoidance
   themselves.

   The application running on the client and the peer can take one of
   two approaches to avoid IP fragmentation.

   The first approach is to avoid sending large amounts of application
   data in the TURN messages/UDP datagrams exchanged between the client
   and the peer.  This is the approach taken by most VoIP
   (Voice-over-IP) applications.  In this approach, the application
   exploits the fact that the IP specification [RFC0791] specifies that
   IP packets up to 576 bytes should never need to be fragmented.

   The exact amount of application data that can be included while
   avoiding fragmentation depends on the details of the TURN session
   between the client and the server: whether UDP, TCP, or TLS transport
   is used, whether ChannelData messages or Send/Data indications are
   used, and whether any additional attributes (such as the DONT-
   FRAGMENT attribute) are included.  Another factor, which is hard to
   determine, is whether the MTU is reduced somewhere along the path for
   other reasons, such as the use of IP-in-IP tunneling.

   As a guideline, sending a maximum of 500 bytes of application data in
   a single TURN message (by the client on the client-to-server leg) or
   a UDP datagram (by the peer on the peer-to-server leg) will generally
   avoid IP fragmentation.  To further reduce the chance of
   fragmentation, it is recommended that the client use ChannelData
   messages when transferring significant volumes of data, since the
   overhead of the ChannelData message is less than Send and Data
   indications.

   The second approach the client and peer can take to avoid
   fragmentation is to use a path MTU discovery algorithm to determine
   the maximum amount of application data that can be sent without
   fragmentation.



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   Unfortunately, because servers implementing this version of TURN do
   not relay ICMP messages, the classic path MTU discovery algorithm
   defined in [RFC1191] is not able to discover the MTU of the
   transmission path between the client and the peer.  (Even if they did
   relay ICMP messages, the algorithm would not always work since ICMP
   messages are often filtered out by combined NAT/firewall devices).

   So the client and server need to use a path MTU discovery algorithm
   that does not require ICMP messages.  The Packetized Path MTU
   Discovery algorithm defined in [RFC4821] is one such algorithm.

   The details of how to use the algorithm of [RFC4821] with TURN are
   still under investigation.  However, as a step towards this goal,
   this version of TURN supports a DONT-FRAGMENT attribute.  When the
   client includes this attribute in a Send indication, this tells the
   server to set the DF bit in the resulting UDP datagram that it sends
   to the peer.  Since some servers may be unable to set the DF bit, the
   client should also include this attribute in the Allocate request --
   any server that does not support the DONT-FRAGMENT attribute will
   indicate this by rejecting the Allocate request.

2.8.  RTP Support

   One of the envisioned uses of TURN is as a relay for clients and
   peers wishing to exchange real-time data (e.g., voice or video) using
   RTP.  To facilitate the use of TURN for this purpose, TURN includes
   some special support for older versions of RTP.

   Old versions of RTP [RFC3550] required that the RTP stream be on an
   even port number and the associated RTP Control Protocol (RTCP)
   stream, if present, be on the next highest port.  To allow clients to
   work with peers that still require this, TURN allows the client to
   request that the server allocate a relayed transport address with an
   even port number, and to optionally request the server reserve the
   next-highest port number for a subsequent allocation.

2.9.  Anycast Discovery of Servers

   This version of TURN has been designed to permit the future
   specification of a method of doing anycast discovery of a TURN server
   over UDP.

   Specifically, a TURN server can reject an Allocate request with the
   suggestion that the client try an alternate server.  To avoid certain
   types of attacks, the client must use the same credentials with the
   alternate server as it would have with the initial server.





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3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Readers are expected to be familiar with [RFC5389] and the terms
   defined there.

   The following terms are used in this document:

   TURN:  The protocol spoken between a TURN client and a TURN server.
      It is an extension to the STUN protocol [RFC5389].  The protocol
      allows a client to allocate and use a relayed transport address.

   TURN client:  A STUN client that implements this specification.

   TURN server:  A STUN server that implements this specification.  It
      relays data between a TURN client and its peer(s).

   Peer:  A host with which the TURN client wishes to communicate.  The
      TURN server relays traffic between the TURN client and its
      peer(s).  The peer does not interact with the TURN server using
      the protocol defined in this document; rather, the peer receives
      data sent by the TURN server and the peer sends data towards the
      TURN server.

   Transport Address:  The combination of an IP address and a port.

   Host Transport Address:  A transport address on a client or a peer.

   Server-Reflexive Transport Address:  A transport address on the
      "public side" of a NAT.  This address is allocated by the NAT to
      correspond to a specific host transport address.

   Relayed Transport Address:  A transport address on the TURN server
      that is used for relaying packets between the client and a peer.
      A peer sends to this address on the TURN server, and the packet is
      then relayed to the client.

   TURN Server Transport Address:  A transport address on the TURN
      server that is used for sending TURN messages to the server.  This
      is the transport address that the client uses to communicate with
      the server.

   Peer Transport Address:  The transport address of the peer as seen by
      the server.  When the peer is behind a NAT, this is the peer's
      server-reflexive transport address.



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   Allocation:  The relayed transport address granted to a client
      through an Allocate request, along with related state, such as
      permissions and expiration timers.

   5-tuple:  The combination (client IP address and port, server IP
      address and port, and transport protocol (currently one of UDP,
      TCP, or TLS)) used to communicate between the client and the
      server.  The 5-tuple uniquely identifies this communication
      stream.  The 5-tuple also uniquely identifies the Allocation on
      the server.

   Channel:  A channel number and associated peer transport address.
      Once a channel number is bound to a peer's transport address, the
      client and server can use the more bandwidth-efficient ChannelData
      message to exchange data.

   Permission:  The IP address and transport protocol (but not the port)
      of a peer that is permitted to send traffic to the TURN server and
      have that traffic relayed to the TURN client.  The TURN server
      will only forward traffic to its client from peers that match an
      existing permission.

   Realm:  A string used to describe the server or a context within the
      server.  The realm tells the client which username and password
      combination to use to authenticate requests.

   Nonce:  A string chosen at random by the server and included in the
      message-digest.  To prevent reply attacks, the server should
      change the nonce regularly.

4.  General Behavior

   This section contains general TURN processing rules that apply to all
   TURN messages.

   TURN is an extension to STUN.  All TURN messages, with the exception
   of the ChannelData message, are STUN-formatted messages.  All the
   base processing rules described in [RFC5389] apply to STUN-formatted
   messages.  This means that all the message-forming and message-
   processing descriptions in this document are implicitly prefixed with
   the rules of [RFC5389].

   [RFC5389] specifies an authentication mechanism called the long-term
   credential mechanism.  TURN servers and clients MUST implement this
   mechanism.  The server MUST demand that all requests from the client
   be authenticated using this mechanism, or that a equally strong or
   stronger mechanism for client authentication is used.




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   Note that the long-term credential mechanism applies only to requests
   and cannot be used to authenticate indications; thus, indications in
   TURN are never authenticated.  If the server requires requests to be
   authenticated, then the server's administrator MUST choose a realm
   value that will uniquely identify the username and password
   combination that the client must use, even if the client uses
   multiple servers under different administrations.  The server's
   administrator MAY choose to allocate a unique username to each
   client, or MAY choose to allocate the same username to more than one
   client (for example, to all clients from the same department or
   company).  For each allocation, the server SHOULD generate a new
   random nonce when the allocation is first attempted following the
   randomness recommendations in [RFC4086] and SHOULD expire the nonce
   at least once every hour during the lifetime of the allocation.

   All requests after the initial Allocate must use the same username as
   that used to create the allocation, to prevent attackers from
   hijacking the client's allocation.  Specifically, if the server
   requires the use of the long-term credential mechanism, and if a non-
   Allocate request passes authentication under this mechanism, and if
   the 5-tuple identifies an existing allocation, but the request does
   not use the same username as used to create the allocation, then the
   request MUST be rejected with a 441 (Wrong Credentials) error.

   When a TURN message arrives at the server from the client, the server
   uses the 5-tuple in the message to identify the associated
   allocation.  For all TURN messages (including ChannelData) EXCEPT an
   Allocate request, if the 5-tuple does not identify an existing
   allocation, then the message MUST either be rejected with a 437
   Allocation Mismatch error (if it is a request) or silently ignored
   (if it is an indication or a ChannelData message).  A client
   receiving a 437 error response to a request other than Allocate MUST
   assume the allocation no longer exists.

   [RFC5389] defines a number of attributes, including the SOFTWARE and
   FINGERPRINT attributes.  The client SHOULD include the SOFTWARE
   attribute in all Allocate and Refresh requests and MAY include it in
   any other requests or indications.  The server SHOULD include the
   SOFTWARE attribute in all Allocate and Refresh responses (either
   success or failure) and MAY include it in other responses or
   indications.  The client and the server MAY include the FINGERPRINT
   attribute in any STUN-formatted messages defined in this document.

   TURN does not use the backwards-compatibility mechanism described in
   [RFC5389].






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   TURN, as defined in this specification, only supports IPv4.  The
   client's IP address, the server's IP address, and all IP addresses
   appearing in a relayed transport address MUST be IPv4 addresses.

   By default, TURN runs on the same ports as STUN: 3478 for TURN over
   UDP and TCP, and 5349 for TURN over TLS.  However, TURN has its own
   set of Service Record (SRV) names: "turn" for UDP and TCP, and
   "turns" for TLS.  Either the SRV procedures or the ALTERNATE-SERVER
   procedures, both described in Section 6, can be used to run TURN on a
   different port.

   To ensure interoperability, a TURN server MUST support the use of UDP
   transport between the client and the server, and SHOULD support the
   use of TCP and TLS transport.

   When UDP transport is used between the client and the server, the
   client will retransmit a request if it does not receive a response
   within a certain timeout period.  Because of this, the server may
   receive two (or more) requests with the same 5-tuple and same
   transaction id.  STUN requires that the server recognize this case
   and treat the request as idempotent (see [RFC5389]).  Some
   implementations may choose to meet this requirement by remembering
   all received requests and the corresponding responses for 40 seconds.
   Other implementations may choose to reprocess the request and arrange
   that such reprocessing returns essentially the same response.  To aid
   implementors who choose the latter approach (the so-called "stateless
   stack approach"), this specification includes some implementation
   notes on how this might be done.  Implementations are free to choose
   either approach or choose some other approach that gives the same
   results.

   When TCP transport is used between the client and the server, it is
   possible that a bit error will cause a length field in a TURN packet
   to become corrupted, causing the receiver to lose synchronization
   with the incoming stream of TURN messages.  A client or server that
   detects a long sequence of invalid TURN messages over TCP transport
   SHOULD close the corresponding TCP connection to help the other end
   detect this situation more rapidly.

   To mitigate either intentional or unintentional denial-of-service
   attacks against the server by clients with valid usernames and
   passwords, it is RECOMMENDED that the server impose limits on both
   the number of allocations active at one time for a given username and
   on the amount of bandwidth those allocations can use.  The server
   should reject new allocations that would exceed the limit on the
   allowed number of allocations active at one time with a 486
   (Allocation Quota Exceeded) (see Section 6.2), and should discard
   application data traffic that exceeds the bandwidth quota.



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5.  Allocations

   All TURN operations revolve around allocations, and all TURN messages
   are associated with an allocation.  An allocation conceptually
   consists of the following state data:

   o  the relayed transport address;

   o  the 5-tuple: (client's IP address, client's port, server IP
      address, server port, transport protocol);

   o  the authentication information;

   o  the time-to-expiry;

   o  a list of permissions;

   o  a list of channel to peer bindings.

   The relayed transport address is the transport address allocated by
   the server for communicating with peers, while the 5-tuple describes
   the communication path between the client and the server.  On the
   client, the 5-tuple uses the client's host transport address; on the
   server, the 5-tuple uses the client's server-reflexive transport
   address.

   Both the relayed transport address and the 5-tuple MUST be unique
   across all allocations, so either one can be used to uniquely
   identify the allocation.

   The authentication information (e.g., username, password, realm, and
   nonce) is used to both verify subsequent requests and to compute the
   message integrity of responses.  The username, realm, and nonce
   values are initially those used in the authenticated Allocate request
   that creates the allocation, though the server can change the nonce
   value during the lifetime of the allocation using a 438 (Stale Nonce)
   reply.  Note that, rather than storing the password explicitly, for
   security reasons, it may be desirable for the server to store the key
   value, which is an MD5 hash over the username, realm, and password
   (see [RFC5389]).

   The time-to-expiry is the time in seconds left until the allocation
   expires.  Each Allocate or Refresh transaction sets this timer, which
   then ticks down towards 0.  By default, each Allocate or Refresh
   transaction resets this timer to the default lifetime value of 600
   seconds (10 minutes), but the client can request a different value in
   the Allocate and Refresh request.  Allocations can only be refreshed
   using the Refresh request; sending data to a peer does not refresh an



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   allocation.  When an allocation expires, the state data associated
   with the allocation can be freed.

   The list of permissions is described in Section 8 and the list of
   channels is described in Section 11.

6.  Creating an Allocation

   An allocation on the server is created using an Allocate transaction.

6.1.  Sending an Allocate Request

   The client forms an Allocate request as follows.

   The client first picks a host transport address.  It is RECOMMENDED
   that the client pick a currently unused transport address, typically
   by allowing the underlying OS to pick a currently unused port for a
   new socket.

   The client then picks a transport protocol to use between the client
   and the server.  The transport protocol MUST be one of UDP, TCP, or
   TLS-over-TCP.  Since this specification only allows UDP between the
   server and the peers, it is RECOMMENDED that the client pick UDP
   unless it has a reason to use a different transport.  One reason to
   pick a different transport would be that the client believes, either
   through configuration or by experiment, that it is unable to contact
   any TURN server using UDP.  See Section 2.1 for more discussion.

   The client also picks a server transport address, which SHOULD be
   done as follows.  The client receives (perhaps through configuration)
   a domain name for a TURN server.  The client then uses the DNS
   procedures described in [RFC5389], but using an SRV service name of
   "turn" (or "turns" for TURN over TLS) instead of "stun" (or "stuns").
   For example, to find servers in the example.com domain, the client
   performs a lookup for '_turn._udp.example.com',
   '_turn._tcp.example.com', and '_turns._tcp.example.com' if the client
   wants to communicate with the server using UDP, TCP, or TLS-over-TCP,
   respectively.

   The client MUST include a REQUESTED-TRANSPORT attribute in the
   request.  This attribute specifies the transport protocol between the
   server and the peers (note that this is NOT the transport protocol
   that appears in the 5-tuple).  In this specification, the REQUESTED-
   TRANSPORT type is always UDP.  This attribute is included to allow
   future extensions to specify other protocols.

   If the client wishes the server to initialize the time-to-expiry
   field of the allocation to some value other than the default



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   lifetime, then it MAY include a LIFETIME attribute specifying its
   desired value.  This is just a request, and the server may elect to
   use a different value.  Note that the server will ignore requests to
   initialize the field to less than the default value.

   If the client wishes to later use the DONT-FRAGMENT attribute in one
   or more Send indications on this allocation, then the client SHOULD
   include the DONT-FRAGMENT attribute in the Allocate request.  This
   allows the client to test whether this attribute is supported by the
   server.

   If the client requires the port number of the relayed transport
   address be even, the client includes the EVEN-PORT attribute.  If
   this attribute is not included, then the port can be even or odd.  By
   setting the R bit in the EVEN-PORT attribute to 1, the client can
   request that the server reserve the next highest port number (on the
   same IP address) for a subsequent allocation.  If the R bit is 0, no
   such request is made.

   The client MAY also include a RESERVATION-TOKEN attribute in the
   request to ask the server to use a previously reserved port for the
   allocation.  If the RESERVATION-TOKEN attribute is included, then the
   client MUST omit the EVEN-PORT attribute.

   Once constructed, the client sends the Allocate request on the
   5-tuple.

6.2.  Receiving an Allocate Request

   When the server receives an Allocate request, it performs the
   following checks:

   1.  The server MUST require that the request be authenticated.  This
       authentication MUST be done using the long-term credential
       mechanism of [RFC5389] unless the client and server agree to use
       another mechanism through some procedure outside the scope of
       this document.

   2.  The server checks if the 5-tuple is currently in use by an
       existing allocation.  If yes, the server rejects the request with
       a 437 (Allocation Mismatch) error.

   3.  The server checks if the request contains a REQUESTED-TRANSPORT
       attribute.  If the REQUESTED-TRANSPORT attribute is not included
       or is malformed, the server rejects the request with a 400 (Bad
       Request) error.  Otherwise, if the attribute is included but
       specifies a protocol other that UDP, the server rejects the
       request with a 442 (Unsupported Transport Protocol) error.



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   4.  The request may contain a DONT-FRAGMENT attribute.  If it does,
       but the server does not support sending UDP datagrams with the DF
       bit set to 1 (see Section 12), then the server treats the DONT-
       FRAGMENT attribute in the Allocate request as an unknown
       comprehension-required attribute.

   5.  The server checks if the request contains a RESERVATION-TOKEN
       attribute.  If yes, and the request also contains an EVEN-PORT
       attribute, then the server rejects the request with a 400 (Bad
       Request) error.  Otherwise, it checks to see if the token is
       valid (i.e., the token is in range and has not expired and the
       corresponding relayed transport address is still available).  If
       the token is not valid for some reason, the server rejects the
       request with a 508 (Insufficient Capacity) error.

   6.  The server checks if the request contains an EVEN-PORT attribute.
       If yes, then the server checks that it can satisfy the request
       (i.e., can allocate a relayed transport address as described
       below).  If the server cannot satisfy the request, then the
       server rejects the request with a 508 (Insufficient Capacity)
       error.

   7.  At any point, the server MAY choose to reject the request with a
       486 (Allocation Quota Reached) error if it feels the client is
       trying to exceed some locally defined allocation quota.  The
       server is free to define this allocation quota any way it wishes,
       but SHOULD define it based on the username used to authenticate
       the request, and not on the client's transport address.

   8.  Also at any point, the server MAY choose to reject the request
       with a 300 (Try Alternate) error if it wishes to redirect the
       client to a different server.  The use of this error code and
       attribute follow the specification in [RFC5389].

   If all the checks pass, the server creates the allocation.  The
   5-tuple is set to the 5-tuple from the Allocate request, while the
   list of permissions and the list of channels are initially empty.

   The server chooses a relayed transport address for the allocation as
   follows:

   o  If the request contains a RESERVATION-TOKEN, the server uses the
      previously reserved transport address corresponding to the
      included token (if it is still available).  Note that the
      reservation is a server-wide reservation and is not specific to a
      particular allocation, since the Allocate request containing the
      RESERVATION-TOKEN uses a different 5-tuple than the Allocate
      request that made the reservation.  The 5-tuple for the Allocate



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      request containing the RESERVATION-TOKEN attribute can be any
      allowed 5-tuple; it can use a different client IP address and
      port, a different transport protocol, and even different server IP
      address and port (provided, of course, that the server IP address
      and port are ones on which the server is listening for TURN
      requests).

   o  If the request contains an EVEN-PORT attribute with the R bit set
      to 0, then the server allocates a relayed transport address with
      an even port number.

   o  If the request contains an EVEN-PORT attribute with the R bit set
      to 1, then the server looks for a pair of port numbers N and N+1
      on the same IP address, where N is even.  Port N is used in the
      current allocation, while the relayed transport address with port
      N+1 is assigned a token and reserved for a future allocation.  The
      server MUST hold this reservation for at least 30 seconds, and MAY
      choose to hold longer (e.g., until the allocation with port N
      expires).  The server then includes the token in a RESERVATION-
      TOKEN attribute in the success response.

   o  Otherwise, the server allocates any available relayed transport
      address.

   In all cases, the server SHOULD only allocate ports from the range
   49152 - 65535 (the Dynamic and/or Private Port range [Port-Numbers]),
   unless the TURN server application knows, through some means not
   specified here, that other applications running on the same host as
   the TURN server application will not be impacted by allocating ports
   outside this range.  This condition can often be satisfied by running
   the TURN server application on a dedicated machine and/or by
   arranging that any other applications on the machine allocate ports
   before the TURN server application starts.  In any case, the TURN
   server SHOULD NOT allocate ports in the range 0 - 1023 (the Well-
   Known Port range) to discourage clients from using TURN to run
   standard services.

      NOTE: The IETF is currently investigating the topic of randomized
      port assignments to avoid certain types of attacks (see
      [TSVWG-PORT]).  It is strongly recommended that a TURN implementor
      keep abreast of this topic and, if appropriate, implement a
      randomized port assignment algorithm.  This is especially
      applicable to servers that choose to pre-allocate a number of
      ports from the underlying OS and then later assign them to
      allocations; for example, a server may choose this technique to
      implement the EVEN-PORT attribute.





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   The server determines the initial value of the time-to-expiry field
   as follows.  If the request contains a LIFETIME attribute, then the
   server computes the minimum of the client's proposed lifetime and the
   server's maximum allowed lifetime.  If this computed value is greater
   than the default lifetime, then the server uses the computed lifetime
   as the initial value of the time-to-expiry field.  Otherwise, the
   server uses the default lifetime.  It is RECOMMENDED that the server
   use a maximum allowed lifetime value of no more than 3600 seconds (1
   hour).  Servers that implement allocation quotas or charge users for
   allocations in some way may wish to use a smaller maximum allowed
   lifetime (perhaps as small as the default lifetime) to more quickly
   remove orphaned allocations (that is, allocations where the
   corresponding client has crashed or terminated or the client
   connection has been lost for some reason).  Also, note that the time-
   to-expiry is recomputed with each successful Refresh request, and
   thus the value computed here applies only until the first refresh.

   Once the allocation is created, the server replies with a success
   response.  The success response contains:

   o  An XOR-RELAYED-ADDRESS attribute containing the relayed transport
      address.

   o  A LIFETIME attribute containing the current value of the time-to-
      expiry timer.

   o  A RESERVATION-TOKEN attribute (if a second relayed transport
      address was reserved).

   o  An XOR-MAPPED-ADDRESS attribute containing the client's IP address
      and port (from the 5-tuple).

      NOTE: The XOR-MAPPED-ADDRESS attribute is included in the response
      as a convenience to the client.  TURN itself does not make use of
      this value, but clients running ICE can often need this value and
      can thus avoid having to do an extra Binding transaction with some
      STUN server to learn it.

   The response (either success or error) is sent back to the client on
   the 5-tuple.

      NOTE: When the Allocate request is sent over UDP, section 7.3.1 of
      [RFC5389] requires that the server handle the possible
      retransmissions of the request so that retransmissions do not
      cause multiple allocations to be created.  Implementations may
      achieve this using the so-called "stateless stack approach" as
      follows.  To detect retransmissions when the original request was
      successful in creating an allocation, the server can store the



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      transaction id that created the request with the allocation data
      and compare it with incoming Allocate requests on the same
      5-tuple.  Once such a request is detected, the server can stop
      parsing the request and immediately generate a success response.
      When building this response, the value of the LIFETIME attribute
      can be taken from the time-to-expiry field in the allocate state
      data, even though this value may differ slightly from the LIFETIME
      value originally returned.  In addition, the server may need to
      store an indication of any reservation token returned in the
      original response, so that this may be returned in any
      retransmitted responses.

      For the case where the original request was unsuccessful in
      creating an allocation, the server may choose to do nothing
      special.  Note, however, that there is a rare case where the
      server rejects the original request but accepts the retransmitted
      request (because conditions have changed in the brief intervening
      time period).  If the client receives the first failure response,
      it will ignore the second (success) response and believe that an
      allocation was not created.  An allocation created in this matter
      will eventually timeout, since the client will not refresh it.
      Furthermore, if the client later retries with the same 5-tuple but
      different transaction id, it will receive a 437 (Allocation
      Mismatch), which will cause it to retry with a different 5-tuple.
      The server may use a smaller maximum lifetime value to minimize
      the lifetime of allocations "orphaned" in this manner.

6.3.  Receiving an Allocate Success Response

   If the client receives an Allocate success response, then it MUST
   check that the mapped address and the relayed transport address are
   in an address family that the client understands and is prepared to
   handle.  This specification only covers the case where these two
   addresses are IPv4 addresses.  If these two addresses are not in an
   address family which the client is prepared to handle, then the
   client MUST delete the allocation (Section 7) and MUST NOT attempt to
   create another allocation on that server until it believes the
   mismatch has been fixed.

      The IETF is currently considering mechanisms for transitioning
      between IPv4 and IPv6 that could result in a client originating an
      Allocate request over IPv6, but the request would arrive at the
      server over IPv4, or vice versa.

   Otherwise, the client creates its own copy of the allocation data
   structure to track what is happening on the server.  In particular,
   the client needs to remember the actual lifetime received back from
   the server, rather than the value sent to the server in the request.



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   The client must also remember the 5-tuple used for the request and
   the username and password it used to authenticate the request to
   ensure that it reuses them for subsequent messages.  The client also
   needs to track the channels and permissions it establishes on the
   server.

   The client will probably wish to send the relayed transport address
   to peers (using some method not specified here) so the peers can
   communicate with it.  The client may also wish to use the server-
   reflexive address it receives in the XOR-MAPPED-ADDRESS attribute in
   its ICE processing.

6.4.  Receiving an Allocate Error Response

   If the client receives an Allocate error response, then the
   processing depends on the actual error code returned:

   o  (Request timed out): There is either a problem with the server, or
      a problem reaching the server with the chosen transport.  The
      client considers the current transaction as having failed but MAY
      choose to retry the Allocate request using a different transport
      (e.g., TCP instead of UDP).

   o  300 (Try Alternate): The server would like the client to use the
      server specified in the ALTERNATE-SERVER attribute instead.  The
      client considers the current transaction as having failed, but
      SHOULD try the Allocate request with the alternate server before
      trying any other servers (e.g., other servers discovered using the
      SRV procedures).  When trying the Allocate request with the
      alternate server, the client follows the ALTERNATE-SERVER
      procedures specified in [RFC5389].

   o  400 (Bad Request): The server believes the client's request is
      malformed for some reason.  The client considers the current
      transaction as having failed.  The client MAY notify the user or
      operator and SHOULD NOT retry the request with this server until
      it believes the problem has been fixed.

   o  401 (Unauthorized): If the client has followed the procedures of
      the long-term credential mechanism and still gets this error, then
      the server is not accepting the client's credentials.  In this
      case, the client considers the current transaction as having
      failed and SHOULD notify the user or operator.  The client SHOULD
      NOT send any further requests to this server until it believes the
      problem has been fixed.






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   o  403 (Forbidden): The request is valid, but the server is refusing
      to perform it, likely due to administrative restrictions.  The
      client considers the current transaction as having failed.  The
      client MAY notify the user or operator and SHOULD NOT retry the
      same request with this server until it believes the problem has
      been fixed.

   o  420 (Unknown Attribute): If the client included a DONT-FRAGMENT
      attribute in the request and the server rejected the request with
      a 420 error code and listed the DONT-FRAGMENT attribute in the
      UNKNOWN-ATTRIBUTES attribute in the error response, then the
      client now knows that the server does not support the DONT-
      FRAGMENT attribute.  The client considers the current transaction
      as having failed but MAY choose to retry the Allocate request
      without the DONT-FRAGMENT attribute.

   o  437 (Allocation Mismatch): This indicates that the client has
      picked a 5-tuple that the server sees as already in use.  One way
      this could happen is if an intervening NAT assigned a mapped
      transport address that was used by another client that recently
      crashed.  The client considers the current transaction as having
      failed.  The client SHOULD pick another client transport address
      and retry the Allocate request (using a different transaction id).
      The client SHOULD try three different client transport addresses
      before giving up on this server.  Once the client gives up on the
      server, it SHOULD NOT try to create another allocation on the
      server for 2 minutes.

   o  438 (Stale Nonce): See the procedures for the long-term credential
      mechanism [RFC5389].

   o  441 (Wrong Credentials): The client should not receive this error
      in response to a Allocate request.  The client MAY notify the user
      or operator and SHOULD NOT retry the same request with this server
      until it believes the problem has been fixed.

   o  442 (Unsupported Transport Address): The client should not receive
      this error in response to a request for a UDP allocation.  The
      client MAY notify the user or operator and SHOULD NOT reattempt
      the request with this server until it believes the problem has
      been fixed.

   o  486 (Allocation Quota Reached): The server is currently unable to
      create any more allocations with this username.  The client
      considers the current transaction as having failed.  The client
      SHOULD wait at least 1 minute before trying to create any more
      allocations on the server.




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   o  508 (Insufficient Capacity): The server has no more relayed
      transport addresses available, or has none with the requested
      properties, or the one that was reserved is no longer available.
      The client considers the current operation as having failed.  If
      the client is using either the EVEN-PORT or the RESERVATION-TOKEN
      attribute, then the client MAY choose to remove or modify this
      attribute and try again immediately.  Otherwise, the client SHOULD
      wait at least 1 minute before trying to create any more
      allocations on this server.

   An unknown error response MUST be handled as described in [RFC5389].

7.  Refreshing an Allocation

   A Refresh transaction can be used to either (a) refresh an existing
   allocation and update its time-to-expiry or (b) delete an existing
   allocation.

   If a client wishes to continue using an allocation, then the client
   MUST refresh it before it expires.  It is suggested that the client
   refresh the allocation roughly 1 minute before it expires.  If a
   client no longer wishes to use an allocation, then it SHOULD
   explicitly delete the allocation.  A client MAY refresh an allocation
   at any time for other reasons.

7.1.  Sending a Refresh Request

   If the client wishes to immediately delete an existing allocation, it
   includes a LIFETIME attribute with a value of 0.  All other forms of
   the request refresh the allocation.

   The Refresh transaction updates the time-to-expiry timer of an
   allocation.  If the client wishes the server to set the time-to-
   expiry timer to something other than the default lifetime, it
   includes a LIFETIME attribute with the requested value.  The server
   then computes a new time-to-expiry value in the same way as it does
   for an Allocate transaction, with the exception that a requested
   lifetime of 0 causes the server to immediately delete the allocation.

7.2.  Receiving a Refresh Request

   When the server receives a Refresh request, it processes as per
   Section 4 plus the specific rules mentioned here.

   The server computes a value called the "desired lifetime" as follows:
   if the request contains a LIFETIME attribute and the attribute value
   is 0, then the "desired lifetime" is 0.  Otherwise, if the request
   contains a LIFETIME attribute, then the server computes the minimum



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   of the client's requested lifetime and the server's maximum allowed
   lifetime.  If this computed value is greater than the default
   lifetime, then the "desired lifetime" is the computed value.
   Otherwise, the "desired lifetime" is the default lifetime.

   Subsequent processing depends on the "desired lifetime" value:

   o  If the "desired lifetime" is 0, then the request succeeds and the
      allocation is deleted.

   o  If the "desired lifetime" is non-zero, then the request succeeds
      and the allocation's time-to-expiry is set to the "desired
      lifetime".

   If the request succeeds, then the server sends a success response
   containing:

   o  A LIFETIME attribute containing the current value of the time-to-
      expiry timer.

      NOTE: A server need not do anything special to implement
      idempotency of Refresh requests over UDP using the "stateless
      stack approach".  Retransmitted Refresh requests with a non-zero
      "desired lifetime" will simply refresh the allocation.  A
      retransmitted Refresh request with a zero "desired lifetime" will
      cause a 437 (Allocation Mismatch) response if the allocation has
      already been deleted, but the client will treat this as equivalent
      to a success response (see below).

7.3.  Receiving a Refresh Response

   If the client receives a success response to its Refresh request with
   a non-zero lifetime, it updates its copy of the allocation data
   structure with the time-to-expiry value contained in the response.

   If the client receives a 437 (Allocation Mismatch) error response to
   a request to delete the allocation, then the allocation no longer
   exists and it should consider its request as having effectively
   succeeded.

8.  Permissions

   For each allocation, the server keeps a list of zero or more
   permissions.  Each permission consists of an IP address and an
   associated time-to-expiry.  While a permission exists, all peers
   using the IP address in the permission are allowed to send data to





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   the client.  The time-to-expiry is the number of seconds until the
   permission expires.  Within the context of an allocation, a
   permission is uniquely identified by its associated IP address.

   By sending either CreatePermission requests or ChannelBind requests,
   the client can cause the server to install or refresh a permission
   for a given IP address.  This causes one of two things to happen:

   o  If no permission for that IP address exists, then a permission is
      created with the given IP address and a time-to-expiry equal to
      Permission Lifetime.

   o  If a permission for that IP address already exists, then the time-
      to-expiry for that permission is reset to Permission Lifetime.

   The Permission Lifetime MUST be 300 seconds (= 5 minutes).

   Each permission's time-to-expiry decreases down once per second until
   it reaches 0; at which point, the permission expires and is deleted.

   CreatePermission and ChannelBind requests may be freely intermixed on
   a permission.  A given permission may be initially installed and/or
   refreshed with a CreatePermission request, and then later refreshed
   with a ChannelBind request, or vice versa.

   When a UDP datagram arrives at the relayed transport address for the
   allocation, the server extracts the source IP address from the IP
   header.  The server then compares this address with the IP address
   associated with each permission in the list of permissions for the
   allocation.  If no match is found, relaying is not permitted, and the
   server silently discards the UDP datagram.  If an exact match is
   found, then the permission check is considered to have succeeded and
   the server continues to process the UDP datagram as specified
   elsewhere (Section 10.3).  Note that only addresses are compared and
   port numbers are not considered.

   The permissions for one allocation are totally unrelated to the
   permissions for a different allocation.  If an allocation expires,
   all its permissions expire with it.

      NOTE: Though TURN permissions expire after 5 minutes, many NATs
      deployed at the time of publication expire their UDP bindings
      considerably faster.  Thus, an application using TURN will
      probably wish to send some sort of keep-alive traffic at a much
      faster rate.  Applications using ICE should follow the keep-alive
      guidelines of ICE [RFC5245], and applications not using ICE are
      advised to do something similar.




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9.  CreatePermission

   TURN supports two ways for the client to install or refresh
   permissions on the server.  This section describes one way: the
   CreatePermission request.

   A CreatePermission request may be used in conjunction with either the
   Send mechanism in Section 10 or the Channel mechanism in Section 11.

9.1.  Forming a CreatePermission Request

   The client who wishes to install or refresh one or more permissions
   can send a CreatePermission request to the server.

   When forming a CreatePermission request, the client MUST include at
   least one XOR-PEER-ADDRESS attribute, and MAY include more than one
   such attribute.  The IP address portion of each XOR-PEER-ADDRESS
   attribute contains the IP address for which a permission should be
   installed or refreshed.  The port portion of each XOR-PEER-ADDRESS
   attribute will be ignored and can be any arbitrary value.  The
   various XOR-PEER-ADDRESS attributes can appear in any order.

9.2.  Receiving a CreatePermission Request

   When the server receives the CreatePermission request, it processes
   as per Section 4 plus the specific rules mentioned here.

   The message is checked for validity.  The CreatePermission request
   MUST contain at least one XOR-PEER-ADDRESS attribute and MAY contain
   multiple such attributes.  If no such attribute exists, or if any of
   these attributes are invalid, then a 400 (Bad Request) error is
   returned.  If the request is valid, but the server is unable to
   satisfy the request due to some capacity limit or similar, then a 508
   (Insufficient Capacity) error is returned.

   The server MAY impose restrictions on the IP address allowed in the
   XOR-PEER-ADDRESS attribute -- if a value is not allowed, the server
   rejects the request with a 403 (Forbidden) error.

   If the message is valid and the server is capable of carrying out the
   request, then the server installs or refreshes a permission for the
   IP address contained in each XOR-PEER-ADDRESS attribute as described
   in Section 8.  The port portion of each attribute is ignored and may
   be any arbitrary value.

   The server then responds with a CreatePermission success response.
   There are no mandatory attributes in the success response.




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      NOTE: A server need not do anything special to implement
      idempotency of CreatePermission requests over UDP using the
      "stateless stack approach".  Retransmitted CreatePermission
      requests will simply refresh the permissions.

9.3.  Receiving a CreatePermission Response

   If the client receives a valid CreatePermission success response,
   then the client updates its data structures to indicate that the
   permissions have been installed or refreshed.

10.  Send and Data Methods

   TURN supports two mechanisms for sending and receiving data from
   peers.  This section describes the use of the Send and Data
   mechanisms, while Section 11 describes the use of the Channel
   mechanism.

10.1.  Forming a Send Indication

   The client can use a Send indication to pass data to the server for
   relaying to a peer.  A client may use a Send indication even if a
   channel is bound to that peer.  However, the client MUST ensure that
   there is a permission installed for the IP address of the peer to
   which the Send indication is being sent; this prevents a third party
   from using a TURN server to send data to arbitrary destinations.

   When forming a Send indication, the client MUST include an XOR-PEER-
   ADDRESS attribute and a DATA attribute.  The XOR-PEER-ADDRESS
   attribute contains the transport address of the peer to which the
   data is to be sent, and the DATA attribute contains the actual
   application data to be sent to the peer.

   The client MAY include a DONT-FRAGMENT attribute in the Send
   indication if it wishes the server to set the DF bit on the UDP
   datagram sent to the peer.

10.2.  Receiving a Send Indication

   When the server receives a Send indication, it processes as per
   Section 4 plus the specific rules mentioned here.

   The message is first checked for validity.  The Send indication MUST
   contain both an XOR-PEER-ADDRESS attribute and a DATA attribute.  If
   one of these attributes is missing or invalid, then the message is
   discarded.  Note that the DATA attribute is allowed to contain zero
   bytes of data.




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   The Send indication may also contain the DONT-FRAGMENT attribute.  If
   the server is unable to set the DF bit on outgoing UDP datagrams when
   this attribute is present, then the server acts as if the DONT-
   FRAGMENT attribute is an unknown comprehension-required attribute
   (and thus the Send indication is discarded).

   The server also checks that there is a permission installed for the
   IP address contained in the XOR-PEER-ADDRESS attribute.  If no such
   permission exists, the message is discarded.  Note that a Send
   indication never causes the server to refresh the permission.

   The server MAY impose restrictions on the IP address and port values
   allowed in the XOR-PEER-ADDRESS attribute -- if a value is not
   allowed, the server silently discards the Send indication.

   If everything is OK, then the server forms a UDP datagram as follows:

   o  the source transport address is the relayed transport address of
      the allocation, where the allocation is determined by the 5-tuple
      on which the Send indication arrived;

   o  the destination transport address is taken from the XOR-PEER-
      ADDRESS attribute;

   o  the data following the UDP header is the contents of the value
      field of the DATA attribute.

   The handling of the DONT-FRAGMENT attribute (if present), is
   described in Section 12.

   The resulting UDP datagram is then sent to the peer.

10.3.  Receiving a UDP Datagram

   When the server receives a UDP datagram at a currently allocated
   relayed transport address, the server looks up the allocation
   associated with the relayed transport address.  The server then
   checks to see whether the set of permissions for the allocation allow
   the relaying of the UDP datagram as described in Section 8.

   If relaying is permitted, then the server checks if there is a
   channel bound to the peer that sent the UDP datagram (see
   Section 11).  If a channel is bound, then processing proceeds as
   described in Section 11.7.

   If relaying is permitted but no channel is bound to the peer, then
   the server forms and sends a Data indication.  The Data indication
   MUST contain both an XOR-PEER-ADDRESS and a DATA attribute.  The DATA



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   attribute is set to the value of the 'data octets' field from the
   datagram, and the XOR-PEER-ADDRESS attribute is set to the source
   transport address of the received UDP datagram.  The Data indication
   is then sent on the 5-tuple associated with the allocation.

10.4.  Receiving a Data Indication

   When the client receives a Data indication, it checks that the Data
   indication contains both an XOR-PEER-ADDRESS and a DATA attribute,
   and discards the indication if it does not.  The client SHOULD also
   check that the XOR-PEER-ADDRESS attribute value contains an IP
   address with which the client believes there is an active permission,
   and discard the Data indication otherwise.  Note that the DATA
   attribute is allowed to contain zero bytes of data.

      NOTE: The latter check protects the client against an attacker who
      somehow manages to trick the server into installing permissions
      not desired by the client.

   If the Data indication passes the above checks, the client delivers
   the data octets inside the DATA attribute to the application, along
   with an indication that they were received from the peer whose
   transport address is given by the XOR-PEER-ADDRESS attribute.

11.  Channels

   Channels provide a way for the client and server to send application
   data using ChannelData messages, which have less overhead than Send
   and Data indications.

   The ChannelData message (see Section 11.4) starts with a two-byte
   field that carries the channel number.  The values of this field are
   allocated as follows:

      0x0000 through 0x3FFF: These values can never be used for channel
      numbers.

      0x4000 through 0x7FFF: These values are the allowed channel
      numbers (16,383 possible values).

      0x8000 through 0xFFFF: These values are reserved for future use.

   Because of this division, ChannelData messages can be distinguished
   from STUN-formatted messages (e.g., Allocate request, Send
   indication, etc.) by examining the first two bits of the message:

      0b00: STUN-formatted message (since the first two bits of a STUN-
      formatted message are always zero).



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      0b01: ChannelData message (since the channel number is the first
      field in the ChannelData message and channel numbers fall in the
      range 0x4000 - 0x7FFF).

      0b10: Reserved

      0b11: Reserved

   The reserved values may be used in the future to extend the range of
   channel numbers.  Thus, an implementation MUST NOT assume that a TURN
   message always starts with a 0 bit.

   Channel bindings are always initiated by the client.  The client can
   bind a channel to a peer at any time during the lifetime of the
   allocation.  The client may bind a channel to a peer before
   exchanging data with it, or after exchanging data with it (using Send
   and Data indications) for some time, or may choose never to bind a
   channel to it.  The client can also bind channels to some peers while
   not binding channels to other peers.

   Channel bindings are specific to an allocation, so that the use of a
   channel number or peer transport address in a channel binding in one
   allocation has no impact on their use in a different allocation.  If
   an allocation expires, all its channel bindings expire with it.

   A channel binding consists of:

   o  a channel number;

   o  a transport address (of the peer); and

   o  A time-to-expiry timer.

   Within the context of an allocation, a channel binding is uniquely
   identified either by the channel number or by the peer's transport
   address.  Thus, the same channel cannot be bound to two different
   transport addresses, nor can the same transport address be bound to
   two different channels.

   A channel binding lasts for 10 minutes unless refreshed.  Refreshing
   the binding (by the server receiving a ChannelBind request rebinding
   the channel to the same peer) resets the time-to-expiry timer back to
   10 minutes.

   When the channel binding expires, the channel becomes unbound.  Once
   unbound, the channel number can be bound to a different transport
   address, and the transport address can be bound to a different
   channel number.  To prevent race conditions, the client MUST wait 5



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   minutes after the channel binding expires before attempting to bind
   the channel number to a different transport address or the transport
   address to a different channel number.

   When binding a channel to a peer, the client SHOULD be prepared to
   receive ChannelData messages on the channel from the server as soon
   as it has sent the ChannelBind request.  Over UDP, it is possible for
   the client to receive ChannelData messages from the server before it
   receives a ChannelBind success response.

   In the other direction, the client MAY elect to send ChannelData
   messages before receiving the ChannelBind success response.  Doing
   so, however, runs the risk of having the ChannelData messages dropped
   by the server if the ChannelBind request does not succeed for some
   reason (e.g., packet lost if the request is sent over UDP, or the
   server being unable to fulfill the request).  A client that wishes to
   be safe should either queue the data or use Send indications until
   the channel binding is confirmed.

11.1.  Sending a ChannelBind Request

   A channel binding is created or refreshed using a ChannelBind
   transaction.  A ChannelBind transaction also creates or refreshes a
   permission towards the peer (see Section 8).

   To initiate the ChannelBind transaction, the client forms a
   ChannelBind request.  The channel to be bound is specified in a
   CHANNEL-NUMBER attribute, and the peer's transport address is
   specified in an XOR-PEER-ADDRESS attribute.  Section 11.2 describes
   the restrictions on these attributes.

   Rebinding a channel to the same transport address that it is already
   bound to provides a way to refresh a channel binding and the
   corresponding permission without sending data to the peer.  Note
   however, that permissions need to be refreshed more frequently than
   channels.

11.2.  Receiving a ChannelBind Request

   When the server receives a ChannelBind request, it processes as per
   Section 4 plus the specific rules mentioned here.

   The server checks the following:

   o  The request contains both a CHANNEL-NUMBER and an XOR-PEER-ADDRESS
      attribute;





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   o  The channel number is in the range 0x4000 through 0x7FFE
      (inclusive);

   o  The channel number is not currently bound to a different transport
      address (same transport address is OK);

   o  The transport address is not currently bound to a different
      channel number.

   If any of these tests fail, the server replies with a 400 (Bad
   Request) error.

   The server MAY impose restrictions on the IP address and port values
   allowed in the XOR-PEER-ADDRESS attribute -- if a value is not
   allowed, the server rejects the request with a 403 (Forbidden) error.

   If the request is valid, but the server is unable to fulfill the
   request due to some capacity limit or similar, the server replies
   with a 508 (Insufficient Capacity) error.

   Otherwise, the server replies with a ChannelBind success response.
   There are no required attributes in a successful ChannelBind
   response.

   If the server can satisfy the request, then the server creates or
   refreshes the channel binding using the channel number in the
   CHANNEL-NUMBER attribute and the transport address in the XOR-PEER-
   ADDRESS attribute.  The server also installs or refreshes a
   permission for the IP address in the XOR-PEER-ADDRESS attribute as
   described in Section 8.

      NOTE: A server need not do anything special to implement
      idempotency of ChannelBind requests over UDP using the "stateless
      stack approach".  Retransmitted ChannelBind requests will simply
      refresh the channel binding and the corresponding permission.
      Furthermore, the client must wait 5 minutes before binding a
      previously bound channel number or peer address to a different
      channel, eliminating the possibility that the transaction would
      initially fail but succeed on a retransmission.

11.3.  Receiving a ChannelBind Response

   When the client receives a ChannelBind success response, it updates
   its data structures to record that the channel binding is now active.
   It also updates its data structures to record that the corresponding
   permission has been installed or refreshed.





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   If the client receives a ChannelBind failure response that indicates
   that the channel information is out-of-sync between the client and
   the server (e.g., an unexpected 400 "Bad Request" response), then it
   is RECOMMENDED that the client immediately delete the allocation and
   start afresh with a new allocation.

11.4.  The ChannelData Message

   The ChannelData message is used to carry application data between the
   client and the server.  It has the following format:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Channel Number        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                       Application Data                        /
   /                                                               /
   |                                                               |
   |                               +-------------------------------+
   |                               |
   +-------------------------------+

   The Channel Number field specifies the number of the channel on which
   the data is traveling, and thus the address of the peer that is
   sending or is to receive the data.

   The Length field specifies the length in bytes of the application
   data field (i.e., it does not include the size of the ChannelData
   header).  Note that 0 is a valid length.

   The Application Data field carries the data the client is trying to
   send to the peer, or that the peer is sending to the client.

11.5.  Sending a ChannelData Message

   Once a client has bound a channel to a peer, then when the client has
   data to send to that peer it may use either a ChannelData message or
   a Send indication; that is, the client is not obligated to use the
   channel when it exists and may freely intermix the two message types
   when sending data to the peer.  The server, on the other hand, MUST
   use the ChannelData message if a channel has been bound to the peer.

   The fields of the ChannelData message are filled in as described in
   Section 11.4.





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   Over TCP and TLS-over-TCP, the ChannelData message MUST be padded to
   a multiple of four bytes in order to ensure the alignment of
   subsequent messages.  The padding is not reflected in the length
   field of the ChannelData message, so the actual size of a ChannelData
   message (including padding) is (4 + Length) rounded up to the nearest
   multiple of 4.  Over UDP, the padding is not required but MAY be
   included.

   The ChannelData message is then sent on the 5-tuple associated with
   the allocation.

11.6.  Receiving a ChannelData Message

   The receiver of the ChannelData message uses the first two bits to
   distinguish it from STUN-formatted messages, as described above.  If
   the message uses a value in the reserved range (0x8000 through
   0xFFFF), then the message is silently discarded.

   If the ChannelData message is received in a UDP datagram, and if the
   UDP datagram is too short to contain the claimed length of the
   ChannelData message (i.e., the UDP header length field value is less
   than the ChannelData header length field value + 4 + 8), then the
   message is silently discarded.

   If the ChannelData message is received over TCP or over TLS-over-TCP,
   then the actual length of the ChannelData message is as described in
   Section 11.5.

   If the ChannelData message is received on a channel that is not bound
   to any peer, then the message is silently discarded.

   On the client, it is RECOMMENDED that the client discard the
   ChannelData message if the client believes there is no active
   permission towards the peer.  On the server, the receipt of a
   ChannelData message MUST NOT refresh either the channel binding or
   the permission towards the peer.

   On the server, if no errors are detected, the server relays the
   application data to the peer by forming a UDP datagram as follows:

   o  the source transport address is the relayed transport address of
      the allocation, where the allocation is determined by the 5-tuple
      on which the ChannelData message arrived;

   o  the destination transport address is the transport address to
      which the channel is bound;





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   o  the data following the UDP header is the contents of the data
      field of the ChannelData message.

   The resulting UDP datagram is then sent to the peer.  Note that if
   the Length field in the ChannelData message is 0, then there will be
   no data in the UDP datagram, but the UDP datagram is still formed and
   sent.

11.7.  Relaying Data from the Peer

   When the server receives a UDP datagram on the relayed transport
   address associated with an allocation, the server processes it as
   described in Section 10.3.  If that section indicates that a
   ChannelData message should be sent (because there is a channel bound
   to the peer that sent to the UDP datagram), then the server forms and
   sends a ChannelData message as described in Section 11.5.

12.  IP Header Fields

   This section describes how the server sets various fields in the IP
   header when relaying between the client and the peer or vice versa.
   The descriptions in this section apply: (a) when the server sends a
   UDP datagram to the peer, or (b) when the server sends a Data
   indication or ChannelData message to the client over UDP transport.
   The descriptions in this section do not apply to TURN messages sent
   over TCP or TLS transport from the server to the client.

   The descriptions below have two parts: a preferred behavior and an
   alternate behavior.  The server SHOULD implement the preferred
   behavior, but if that is not possible for a particular field, then it
   SHOULD implement the alternative behavior.

   Time to Live (TTL) field

      Preferred Behavior: If the incoming value is 0, then the drop the
      incoming packet.  Otherwise, set the outgoing Time to Live/Hop
      Count to one less than the incoming value.

      Alternate Behavior: Set the outgoing value to the default for
      outgoing packets.

   Differentiated Services Code Point (DSCP) field [RFC2474]

      Preferred Behavior: Set the outgoing value to the incoming value,
      unless the server includes a differentiated services classifier
      and marker [RFC2474].





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      Alternate Behavior: Set the outgoing value to a fixed value, which
      by default is Best Effort unless configured otherwise.

      In both cases, if the server is immediately adjacent to a
      differentiated services classifier and marker, then DSCP MAY be
      set to any arbitrary value in the direction towards the
      classifier.

   Explicit Congestion Notification (ECN) field [RFC3168]

      Preferred Behavior: Set the outgoing value to the incoming value,
      UNLESS the server is doing Active Queue Management, the incoming
      ECN field is ECT(1) (=0b01) or ECT(0) (=0b10), and the server
      wishes to indicate that congestion has been experienced, in which
      case set the outgoing value to CE (=0b11).

      Alternate Behavior: Set the outgoing value to Not-ECT (=0b00).

   IPv4 Fragmentation fields

      Preferred Behavior: When the server sends a packet to a peer in
      response to a Send indication containing the DONT-FRAGMENT
      attribute, then set the DF bit in the outgoing IP header to 1.  In
      all other cases when sending an outgoing packet containing
      application data (e.g., Data indication, ChannelData message, or
      DONT-FRAGMENT attribute not included in the Send indication), copy
      the DF bit from the DF bit of the incoming packet that contained
      the application data.

      Set the other fragmentation fields (Identification, More
      Fragments, Fragment Offset) as appropriate for a packet
      originating from the server.

      Alternate Behavior: As described in the Preferred Behavior, except
      always assume the incoming DF bit is 0.

      In both the Preferred and Alternate Behaviors, the resulting
      packet may be too large for the outgoing link.  If this is the
      case, then the normal fragmentation rules apply [RFC1122].

   IPv4 Options

      Preferred Behavior: The outgoing packet is sent without any IPv4
      options.

      Alternate Behavior: Same as preferred.





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13.  New STUN Methods

   This section lists the codepoints for the new STUN methods defined in
   this specification.  See elsewhere in this document for the semantics
   of these new methods.

   0x003  :  Allocate          (only request/response semantics defined)
   0x004  :  Refresh           (only request/response semantics defined)
   0x006  :  Send              (only indication semantics defined)
   0x007  :  Data              (only indication semantics defined)
   0x008  :  CreatePermission  (only request/response semantics defined
   0x009  :  ChannelBind       (only request/response semantics defined)

14.  New STUN Attributes

   This STUN extension defines the following new attributes:

     0x000C: CHANNEL-NUMBER
     0x000D: LIFETIME
     0x0010: Reserved (was BANDWIDTH)
     0x0012: XOR-PEER-ADDRESS
     0x0013: DATA
     0x0016: XOR-RELAYED-ADDRESS
     0x0018: EVEN-PORT
     0x0019: REQUESTED-TRANSPORT
     0x001A: DONT-FRAGMENT
     0x0021: Reserved (was TIMER-VAL)
     0x0022: RESERVATION-TOKEN

   Some of these attributes have lengths that are not multiples of 4.
   By the rules of STUN, any attribute whose length is not a multiple of
   4 bytes MUST be immediately followed by 1 to 3 padding bytes to
   ensure the next attribute (if any) would start on a 4-byte boundary
   (see [RFC5389]).

14.1.  CHANNEL-NUMBER

   The CHANNEL-NUMBER attribute contains the number of the channel.  The
   value portion of this attribute is 4 bytes long and consists of a 16-
   bit unsigned integer, followed by a two-octet RFFU (Reserved For
   Future Use) field, which MUST be set to 0 on transmission and MUST be
   ignored on reception.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Channel Number         |         RFFU = 0              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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14.2.  LIFETIME

   The LIFETIME attribute represents the duration for which the server
   will maintain an allocation in the absence of a refresh.  The value
   portion of this attribute is 4-bytes long and consists of a 32-bit
   unsigned integral value representing the number of seconds remaining
   until expiration.

14.3.  XOR-PEER-ADDRESS

   The XOR-PEER-ADDRESS specifies the address and port of the peer as
   seen from the TURN server.  (For example, the peer's server-reflexive
   transport address if the peer is behind a NAT.)  It is encoded in the
   same way as XOR-MAPPED-ADDRESS [RFC5389].

14.4.  DATA

   The DATA attribute is present in all Send and Data indications.  The
   value portion of this attribute is variable length and consists of
   the application data (that is, the data that would immediately follow
   the UDP header if the data was been sent directly between the client
   and the peer).  If the length of this attribute is not a multiple of
   4, then padding must be added after this attribute.

14.5.  XOR-RELAYED-ADDRESS

   The XOR-RELAYED-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 XOR-MAPPED-ADDRESS
   [RFC5389].

14.6.  EVEN-PORT

   This attribute allows the client to request that the port in the
   relayed transport address be even, and (optionally) that the server
   reserve the next-higher port number.  The value portion of this
   attribute is 1 byte long.  Its format is:














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      0
      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |R|    RFFU     |
     +-+-+-+-+-+-+-+-+

   The value contains a single 1-bit flag:

   R: If 1, the server is requested to reserve the next-higher port
      number (on the same IP address) for a subsequent allocation.  If
      0, no such reservation is requested.

   The other 7 bits of the attribute's value must be set to zero on
   transmission and ignored on reception.

   Since the length of this attribute is not a multiple of 4, padding
   must immediately follow this attribute.

14.7.  REQUESTED-TRANSPORT

   This attribute is used by the client to request a specific transport
   protocol for the allocated transport address.  The value of this
   attribute is 4 bytes with the following format:
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Protocol   |                    RFFU                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Protocol field specifies the desired protocol.  The codepoints
   used in this field are taken from those allowed in the Protocol field
   in the IPv4 header and the NextHeader field in the IPv6 header
   [Protocol-Numbers].  This specification only allows the use of
   codepoint 17 (User Datagram Protocol).

   The RFFU field MUST be set to zero on transmission and MUST be
   ignored on reception.  It is reserved for future uses.

14.8.  DONT-FRAGMENT

   This attribute is used by the client to request that the server set
   the DF (Don't Fragment) bit in the IP header when relaying the
   application data onward to the peer.  This attribute has no value
   part and thus the attribute length field is 0.







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14.9.  RESERVATION-TOKEN

   The RESERVATION-TOKEN attribute contains a token that uniquely
   identifies a relayed transport address being held in reserve by the
   server.  The server includes this attribute in a success response to
   tell the client about the token, and the client includes this
   attribute in a subsequent Allocate request to request the server use
   that relayed transport address for the allocation.

   The attribute value is 8 bytes and contains the token value.

15.  New STUN Error Response Codes

   This document defines the following new error response codes:

   403  (Forbidden): The request was valid but cannot be performed due
      to administrative or similar restrictions.

   437  (Allocation Mismatch): A request was received by the server that
      requires an allocation to be in place, but no allocation exists,
      or a request was received that requires no allocation, but an
      allocation exists.

   441  (Wrong Credentials): The credentials in the (non-Allocate)
      request do not match those used to create the allocation.

   442  (Unsupported Transport Protocol): The Allocate request asked the
      server to use a transport protocol between the server and the peer
      that the server does not support.  NOTE: This does NOT refer to
      the transport protocol used in the 5-tuple.

   486  (Allocation Quota Reached): No more allocations using this
      username can be created at the present time.

   508  (Insufficient Capacity): The server is unable to carry out the
      request due to some capacity limit being reached.  In an Allocate
      response, this could be due to the server having no more relayed
      transport addresses available at that time, having none with the
      requested properties, or the one that corresponds to the specified
      reservation token is not available.

16.  Detailed Example

   This section gives an example of the use of TURN, showing in detail
   the contents of the messages exchanged.  The example uses the network
   diagram shown in the Overview (Figure 1).





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   For each message, the attributes included in the message and their
   values are shown.  For convenience, values are shown in a human-
   readable format rather than showing the actual octets; for example,
   "XOR-RELAYED-ADDRESS=192.0.2.15:9000" shows that the XOR-RELAYED-
   ADDRESS attribute is included with an address of 192.0.2.15 and a
   port of 9000, here the address and port are shown before the xor-ing
   is done.  For attributes with string-like values (e.g.,
   SOFTWARE="Example client, version 1.03" and
   NONCE="adl7W7PeDU4hKE72jdaQvbAMcr6h39sm"), the value of the attribute
   is shown in quotes for readability, but these quotes do not appear in
   the actual value.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |                                    |             |             |
    |--- Allocate request -------------->|             |             |
    |    Transaction-Id=0xA56250D3F17ABE679422DE85     |             |
    |    SOFTWARE="Example client, version 1.03"       |             |
    |    LIFETIME=3600 (1 hour)          |             |             |
    |    REQUESTED-TRANSPORT=17 (UDP)    |             |             |
    |    DONT-FRAGMENT                   |             |             |
    |                                    |             |             |
    |<-- Allocate error response --------|             |             |
    |    Transaction-Id=0xA56250D3F17ABE679422DE85     |             |
    |    SOFTWARE="Example server, version 1.17"       |             |
    |    ERROR-CODE=401 (Unauthorized)   |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="adl7W7PeDU4hKE72jdaQvbAMcr6h39sm"      |             |
    |                                    |             |             |
    |--- Allocate request -------------->|             |             |
    |    Transaction-Id=0xC271E932AD7446A32C234492     |             |
    |    SOFTWARE="Example client 1.03"  |             |             |
    |    LIFETIME=3600 (1 hour)          |             |             |
    |    REQUESTED-TRANSPORT=17 (UDP)    |             |             |
    |    DONT-FRAGMENT                   |             |             |
    |    USERNAME="George"               |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="adl7W7PeDU4hKE72jdaQvbAMcr6h39sm"      |             |
    |    MESSAGE-INTEGRITY=...           |             |             |
    |                                    |             |             |
    |<-- Allocate success response ------|             |             |
    |    Transaction-Id=0xC271E932AD7446A32C234492     |             |
    |    SOFTWARE="Example server, version 1.17"       |             |
    |    LIFETIME=1200 (20 minutes)      |             |             |
    |    XOR-RELAYED-ADDRESS=192.0.2.15:50000          |             |
    |    XOR-MAPPED-ADDRESS=192.0.2.1:7000             |             |
    |    MESSAGE-INTEGRITY=...           |             |             |




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   The client begins by selecting a host transport address to use for
   the TURN session; in this example, the client has selected 10.1.1.2:
   49721 as shown in Figure 1.  The client then sends an Allocate
   request to the server at the server transport address.  The client
   randomly selects a 96-bit transaction id of
   0xA56250D3F17ABE679422DE85 for this transaction; this is encoded in
   the transaction id field in the fixed header.  The client includes a
   SOFTWARE attribute that gives information about the client's
   software; here the value is "Example client, version 1.03" to
   indicate that this is version 1.03 of something called the Example
   client.  The client includes the LIFETIME attribute because it wishes
   the allocation to have a longer lifetime than the default of 10
   minutes; the value of this attribute is 3600 seconds, which
   corresponds to 1 hour.  The client must always include a REQUESTED-
   TRANSPORT attribute in an Allocate request and the only value allowed
   by this specification is 17, which indicates UDP transport between
   the server and the peers.  The client also includes the DONT-FRAGMENT
   attribute because it wishes to use the DONT-FRAGMENT attribute later
   in Send indications; this attribute consists of only an attribute
   header, there is no value part.  We assume the client has not
   recently interacted with the server, thus the client does not include
   USERNAME, REALM, NONCE, or MESSAGE-INTEGRITY attribute.  Finally,
   note that the order of attributes in a message is arbitrary (except
   for the MESSAGE-INTEGRITY and FINGERPRINT attributes) and the client
   could have used a different order.

   Servers require any request to be authenticated.  Thus, when the
   server receives the initial Allocate request, it rejects the request
   because the request does not contain the authentication attributes.
   Following the procedures of the long-term credential mechanism of
   STUN [RFC5389], the server includes an ERROR-CODE attribute with a
   value of 401 (Unauthorized), a REALM attribute that specifies the
   authentication realm used by the server (in this case, the server's
   domain "example.com"), and a nonce value in a NONCE attribute.  The
   server also includes a SOFTWARE attribute that gives information
   about the server's software.

   The client, upon receipt of the 401 error, re-attempts the Allocate
   request, this time including the authentication attributes.  The
   client selects a new transaction id, and then populates the new
   Allocate request with the same attributes as before.  The client
   includes a USERNAME attribute and uses the realm value received from
   the server to help it determine which value to use; here the client
   is configured to use the username "George" for the realm
   "example.com".  The client also includes the REALM and NONCE
   attributes, which are just copied from the 401 error response.
   Finally, the client includes a MESSAGE-INTEGRITY attribute as the
   last attribute in the message, whose value is a Hashed Message



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   Authentication Code - Secure Hash Algorithm 1 (HMAC-SHA1) hash over
   the contents of the message (shown as just "..." above); this HMAC-
   SHA1 computation includes a password value.  Thus, an attacker cannot
   compute the message integrity value without somehow knowing the
   secret password.

   The server, upon receipt of the authenticated Allocate request,
   checks that everything is OK, then creates an allocation.  The server
   replies with an Allocate success response.  The server includes a
   LIFETIME attribute giving the lifetime of the allocation; here, the
   server has reduced the client's requested 1-hour lifetime to just 20
   minutes, because this particular server doesn't allow lifetimes
   longer than 20 minutes.  The server includes an XOR-RELAYED-ADDRESS
   attribute whose value is the relayed transport address of the
   allocation.  The server includes an XOR-MAPPED-ADDRESS attribute
   whose value is the server-reflexive address of the client; this value
   is not used otherwise in TURN but is returned as a convenience to the
   client.  The server includes a MESSAGE-INTEGRITY attribute to
   authenticate the response and to ensure its integrity; note that the
   response does not contain the USERNAME, REALM, and NONCE attributes.
   The server also includes a SOFTWARE attribute.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |--- CreatePermission request ------>|             |             |
    |    Transaction-Id=0xE5913A8F460956CA277D3319     |             |
    |    XOR-PEER-ADDRESS=192.0.2.150:0  |             |             |
    |    USERNAME="George"               |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="adl7W7PeDU4hKE72jdaQvbAMcr6h39sm"      |             |
    |    MESSAGE-INTEGRITY=...           |             |             |
    |                                    |             |             |
    |<-- CreatePermission success resp.--|             |             |
    |    Transaction-Id=0xE5913A8F460956CA277D3319     |             |
    |    MESSAGE-INTEGRITY=...           |             |             |

   The client then creates a permission towards Peer A in preparation
   for sending it some application data.  This is done through a
   CreatePermission request.  The XOR-PEER-ADDRESS attribute contains
   the IP address for which a permission is established (the IP address
   of peer A); note that the port number in the attribute is ignored
   when used in a CreatePermission request, and here it has been set to
   0; also, note how the client uses Peer A's server-reflexive IP
   address and not its (private) host address.  The client uses the same
   username, realm, and nonce values as in the previous request on the
   allocation.  Though it is allowed to do so, the client has chosen not
   to include a SOFTWARE attribute in this request.




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   The server receives the CreatePermission request, creates the
   corresponding permission, and then replies with a CreatePermission
   success response.  Like the client, the server chooses not to include
   the SOFTWARE attribute in its reply.  Again, note how success
   responses contain a MESSAGE-INTEGRITY attribute (assuming the server
   uses the long-term credential mechanism), but no USERNAME, REALM, and
   NONCE attributes.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |--- Send indication --------------->|             |             |
    |    Transaction-Id=0x1278E9ACA2711637EF7D3328     |             |
    |    XOR-PEER-ADDRESS=192.0.2.150:32102            |             |
    |    DONT-FRAGMENT                   |             |             |
    |    DATA=...                        |             |             |
    |                                    |-- UDP dgm ->|             |
    |                                    |  data=...   |             |
    |                                    |             |             |
    |                                    |<- UDP dgm --|             |
    |                                    |  data=...   |             |
    |<-- Data indication ----------------|             |             |
    |    Transaction-Id=0x8231AE8F9242DA9FF287FEFF     |             |
    |    XOR-PEER-ADDRESS=192.0.2.150:32102            |             |
    |    DATA=...                        |             |             |

   The client now sends application data to Peer A using a Send
   indication.  Peer A's server-reflexive transport address is specified
   in the XOR-PEER-ADDRESS attribute, and the application data (shown
   here as just "...") is specified in the DATA attribute.  The client
   is doing a form of path MTU discovery at the application layer and
   thus specifies (by including the DONT-FRAGMENT attribute) that the
   server should set the DF bit in the UDP datagram to send to the peer.
   Indications cannot be authenticated using the long-term credential
   mechanism of STUN, so no MESSAGE-INTEGRITY attribute is included in
   the message.  An application wishing to ensure that its data is not
   altered or forged must integrity-protect its data at the application
   level.

   Upon receipt of the Send indication, the server extracts the
   application data and sends it in a UDP datagram to Peer A, with the
   relayed transport address as the source transport address of the
   datagram, and with the DF bit set as requested.  Note that, had the
   client not previously established a permission for Peer A's server-
   reflexive IP address, then the server would have silently discarded
   the Send indication instead.






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   Peer A then replies with its own UDP datagram containing application
   data.  The datagram is sent to the relayed transport address on the
   server.  When this arrives, the server creates a Data indication
   containing the source of the UDP datagram in the XOR-PEER-ADDRESS
   attribute, and the data from the UDP datagram in the DATA attribute.
   The resulting Data indication is then sent to the client.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |--- ChannelBind request ----------->|             |             |
    |    Transaction-Id=0x6490D3BC175AFF3D84513212     |             |
    |    CHANNEL-NUMBER=0x4000           |             |             |
    |    XOR-PEER-ADDRESS=192.0.2.210:49191            |             |
    |    USERNAME="George"               |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="adl7W7PeDU4hKE72jdaQvbAMcr6h39sm"      |             |
    |    MESSAGE-INTEGRITY=...           |             |             |
    |                                    |             |             |
    |<-- ChannelBind success response ---|             |             |
    |    Transaction-Id=0x6490D3BC175AFF3D84513212     |             |
    |    MESSAGE-INTEGRITY=...           |             |             |

   The client now binds a channel to Peer B, specifying a free channel
   number (0x4000) in the CHANNEL-NUMBER attribute, and Peer B's
   transport address in the XOR-PEER-ADDRESS attribute.  As before, the
   client re-uses the username, realm, and nonce from its last request
   in the message.

   Upon receipt of the request, the server binds the channel number to
   the peer, installs a permission for Peer B's IP address, and then
   replies with ChannelBind success response.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |--- ChannelData ------------------->|             |             |
    |    Channel-number=0x4000           |--- UDP datagram --------->|
    |    Data=...                        |    Data=...               |
    |                                    |             |             |
    |                                    |<-- UDP datagram ----------|
    |                                    |    Data=... |             |
    |<-- ChannelData --------------------|             |             |
    |    Channel-number=0x4000           |             |             |
    |    Data=...                        |             |             |

   The client now sends a ChannelData message to the server with data
   destined for Peer B.  The ChannelData message is not a STUN message,
   and thus has no transaction id.  Instead, it has only three fields: a
   channel number, data, and data length; here the channel number field



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   is 0x4000 (the channel the client just bound to Peer B).  When the
   server receives the ChannelData message, it checks that the channel
   is currently bound (which it is) and then sends the data onward to
   Peer B in a UDP datagram, using the relayed transport address as the
   source transport address and 192.0.2.210:49191 (the value of the XOR-
   PEER-ADDRESS attribute in the ChannelBind request) as the destination
   transport address.

   Later, Peer B sends a UDP datagram back to the relayed transport
   address.  This causes the server to send a ChannelData message to the
   client containing the data from the UDP datagram.  The server knows
   to which client to send the ChannelData message because of the
   relayed transport address at which the UDP datagram arrived, and
   knows to use channel 0x4000 because this is the channel bound to
   192.0.2.210:49191.  Note that if there had not been any channel
   number bound to that address, the server would have used a Data
   indication instead.

  TURN                                 TURN           Peer          Peer
  client                               server          A             B
    |--- Refresh request --------------->|             |             |
    |    Transaction-Id=0x0864B3C27ADE9354B4312414     |             |
    |    SOFTWARE="Example client 1.03"  |             |             |
    |    USERNAME="George"               |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="adl7W7PeDU4hKE72jdaQvbAMcr6h39sm"      |             |
    |    MESSAGE-INTEGRITY=...           |             |             |
    |                                    |             |             |
    |<-- Refresh error response ---------|             |             |
    |    Transaction-Id=0x0864B3C27ADE9354B4312414     |             |
    |    SOFTWARE="Example server, version 1.17"       |             |
    |    ERROR-CODE=438 (Stale Nonce)    |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="npSw1Xw239bBwGYhjNWgz2yH47sxB2j"       |             |
    |                                    |             |             |
    |--- Refresh request --------------->|             |             |
    |    Transaction-Id=0x427BD3E625A85FC731DC4191     |             |
    |    SOFTWARE="Example client 1.03"  |             |             |
    |    USERNAME="George"               |             |             |
    |    REALM="example.com"             |             |             |
    |    NONCE="npSw1Xw239bBwGYhjNWgz2yH47sxB2j"       |             |
    |    MESSAGE-INTEGRITY=...           |             |             |
    |                                    |             |             |
    |<-- Refresh success response -------|             |             |
    |    Transaction-Id=0x427BD3E625A85FC731DC4191     |             |
    |    SOFTWARE="Example server, version 1.17"       |             |
    |    LIFETIME=600 (10 minutes)       |             |             |




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   Sometime before the 20 minute lifetime is up, the client refreshes
   the allocation.  This is done using a Refresh request.  As before,
   the client includes the latest username, realm, and nonce values in
   the request.  The client also includes the SOFTWARE attribute,
   following the recommended practice of always including this attribute
   in Allocate and Refresh messages.  When the server receives the
   Refresh request, it notices that the nonce value has expired, and so
   replies with 438 (Stale Nonce) error given a new nonce value.  The
   client then reattempts the request, this time with the new nonce
   value.  This second attempt is accepted, and the server replies with
   a success response.  Note that the client did not include a LIFETIME
   attribute in the request, so the server refreshes the allocation for
   the default lifetime of 10 minutes (as can be seen by the LIFETIME
   attribute in the success response).

17.  Security Considerations

   This section considers attacks that are possible in a TURN
   deployment, and discusses how they are mitigated by mechanisms in the
   protocol or recommended practices in the implementation.

   Most of the attacks on TURN are mitigated by the server requiring
   requests be authenticated.  Thus, this specification requires the use
   of authentication.  The mandatory-to-implement mechanism is the long-
   term credential mechanism of STUN.  Other authentication mechanisms
   of equal or stronger security properties may be used.  However, it is
   important to ensure that they can be invoked in an inter-operable
   way.

17.1.  Outsider Attacks

   Outsider attacks are ones where the attacker has no credentials in
   the system, and is attempting to disrupt the service seen by the
   client or the server.

17.1.1.  Obtaining Unauthorized Allocations

   An attacker might wish to obtain allocations on a TURN server for any
   number of nefarious purposes.  A TURN server provides a mechanism for
   sending and receiving packets while cloaking the actual IP address of
   the client.  This makes TURN servers an attractive target for
   attackers who wish to use it to mask their true identity.

   An attacker might also wish to simply utilize the services of a TURN
   server without paying for them.  Since TURN services require
   resources from the provider, it is anticipated that their usage will
   come with a cost.




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   These attacks are prevented using the long-term credential mechanism,
   which allows the TURN server to determine the identity of the
   requestor and whether the requestor is allowed to obtain the
   allocation.

17.1.2.  Offline Dictionary Attacks

   The long-term credential mechanism used by TURN is subject to offline
   dictionary attacks.  An attacker that is capable of eavesdropping on
   a message exchange between a client and server can determine the
   password by trying a number of candidate passwords and seeing if one
   of them is correct.  This attack works when the passwords are low
   entropy, such as a word from the dictionary.  This attack can be
   mitigated by using strong passwords with large entropy.  In
   situations where even stronger mitigation is required, TLS transport
   between the client and the server can be used.

17.1.3.  Faked Refreshes and Permissions

   An attacker might wish to attack an active allocation by sending it a
   Refresh request with an immediate expiration, in order to delete it
   and disrupt service to the client.  This is prevented by
   authentication of refreshes.  Similarly, an attacker wishing to send
   CreatePermission requests to create permissions to undesirable
   destinations is prevented from doing so through authentication.  The
   motivations for such an attack are described in Section 17.2.

17.1.4.  Fake Data

   An attacker might wish to send data to the client or the peer, as if
   they came from the peer or client, respectively.  To do that, the
   attacker can send the client a faked Data Indication or ChannelData
   message, or send the TURN server a faked Send Indication or
   ChannelData message.

   Since indications and ChannelData messages are not authenticated,
   this attack is not prevented by TURN.  However, this attack is
   generally present in IP-based communications and is not substantially
   worsened by TURN.  Consider a normal, non-TURN IP session between
   hosts A and B.  An attacker can send packets to B as if they came
   from A by sending packets towards A with a spoofed IP address of B.
   This attack requires the attacker to know the IP addresses of A and
   B.  With TURN, an attacker wishing to send packets towards a client
   using a Data indication needs to know its IP address (and port), the
   IP address and port of the TURN server, and the IP address and port
   of the peer (for inclusion in the XOR-PEER-ADDRESS attribute).  To
   send a fake ChannelData message to a client, an attacker needs to
   know the IP address and port of the client, the IP address and port



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   of the TURN server, and the channel number.  This particular
   combination is mildly more guessable than in the non-TURN case.

   These attacks are more properly mitigated by application-layer
   authentication techniques.  In the case of real-time traffic, usage
   of SRTP [RFC3711] prevents these attacks.

   In some situations, the TURN server may be situated in the network
   such that it is able to send to hosts to which the client cannot
   directly send.  This can happen, for example, if the server is
   located behind a firewall that allows packets from outside the
   firewall to be delivered to the server, but not to other hosts behind
   the firewall.  In these situations, an attacker could send the server
   a Send indication with an XOR-PEER-ADDRESS attribute containing the
   transport address of one of the other hosts behind the firewall.  If
   the server was to allow relaying of traffic to arbitrary peers, then
   this would provide a way for the attacker to attack arbitrary hosts
   behind the firewall.

   To mitigate this attack, TURN requires that the client establish a
   permission to a host before sending it data.  Thus, an attacker can
   only attack hosts with which the client is already communicating,
   unless the attacker is able to create authenticated requests.
   Furthermore, the server administrator may configure the server to
   restrict the range of IP addresses and ports to which it will relay
   data.  To provide even greater security, the server administrator can
   require that the client use TLS for all communication between the
   client and the server.

17.1.5.  Impersonating a Server

   When a client learns a relayed address from a TURN server, it uses
   that relayed address in application protocols to receive traffic.
   Therefore, an attacker wishing to intercept or redirect that traffic
   might try to impersonate a TURN server and provide the client with a
   faked relayed address.

   This attack is prevented through the long-term credential mechanism,
   which provides message integrity for responses in addition to
   verifying that they came from the server.  Furthermore, an attacker
   cannot replay old server responses as the transaction id in the STUN
   header prevents this.  Replay attacks are further thwarted through
   frequent changes to the nonce value.








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17.1.6.  Eavesdropping Traffic

   TURN concerns itself primarily with authentication and message
   integrity.  Confidentiality is only a secondary concern, as TURN
   control messages do not include information that is particularly
   sensitive.  The primary protocol content of the messages is the IP
   address of the peer.  If it is important to prevent an eavesdropper
   on a TURN connection from learning this, TURN can be run over TLS.

   Confidentiality for the application data relayed by TURN is best
   provided by the application protocol itself, since running TURN over
   TLS does not protect application data between the server and the
   peer.  If confidentiality of application data is important, then the
   application should encrypt or otherwise protect its data.  For
   example, for real-time media, confidentiality can be provided by
   using SRTP.

17.1.7.  TURN Loop Attack

   An attacker might attempt to cause data packets to loop indefinitely
   between two TURN servers.  The attack goes as follows.  First, the
   attacker sends an Allocate request to server A, using the source
   address of server B.  Server A will send its response to server B,
   and for the attack to succeed, the attacker must have the ability to
   either view or guess the contents of this response, so that the
   attacker can learn the allocated relayed transport address.  The
   attacker then sends an Allocate request to server B, using the source
   address of server A.  Again, the attacker must be able to view or
   guess the contents of the response, so it can send learn the
   allocated relayed transport address.  Using the same spoofed source
   address technique, the attacker then binds a channel number on server
   A to the relayed transport address on server B, and similarly binds
   the same channel number on server B to the relayed transport address
   on server A.  Finally, the attacker sends a ChannelData message to
   server A.

   The result is a data packet that loops from the relayed transport
   address on server A to the relayed transport address on server B,
   then from server B's transport address to server A's transport
   address, and then around the loop again.

   This attack is mitigated as follows.  By requiring all requests to be
   authenticated and/or by randomizing the port number allocated for the
   relayed transport address, the server forces the attacker to either
   intercept or view responses sent to a third party (in this case, the
   other server) so that the attacker can authenticate the requests and
   learn the relayed transport address.  Without one of these two
   measures, an attacker can guess the contents of the responses without



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   needing to see them, which makes the attack much easier to perform.
   Furthermore, by requiring authenticated requests, the server forces
   the attacker to have credentials acceptable to the server, which
   turns this from an outsider attack into an insider attack and allows
   the attack to be traced back to the client initiating it.

   The attack can be further mitigated by imposing a per-username limit
   on the bandwidth used to relay data by allocations owned by that
   username, to limit the impact of this attack on other allocations.
   More mitigation can be achieved by decrementing the TTL when relaying
   data packets (if the underlying OS allows this).

17.2.  Firewall Considerations

   A key security consideration of TURN is that TURN should not weaken
   the protections afforded by firewalls deployed between a client and a
   TURN server.  It is anticipated that TURN servers will often be
   present on the public Internet, and clients may often be inside
   enterprise networks with corporate firewalls.  If TURN servers
   provide a 'backdoor' for reaching into the enterprise, TURN will be
   blocked by these firewalls.

   TURN servers therefore emulate the behavior of NAT devices that
   implement address-dependent filtering [RFC4787], a property common in
   many firewalls as well.  When a NAT or firewall implements this
   behavior, packets from an outside IP address are only allowed to be
   sent to an internal IP address and port if the internal IP address
   and port had recently sent a packet to that outside IP address.  TURN
   servers introduce the concept of permissions, which provide exactly
   this same behavior on the TURN server.  An attacker cannot send a
   packet to a TURN server and expect it to be relayed towards the
   client, unless the client has tried to contact the attacker first.

   It is important to note that some firewalls have policies that are
   even more restrictive than address-dependent filtering.  Firewalls
   can also be configured with address- and port-dependent filtering, or
   can be configured to disallow inbound traffic entirely.  In these
   cases, if a client is allowed to connect the TURN server,
   communications to the client will be less restrictive than what the
   firewall would normally allow.

17.2.1.  Faked Permissions

   In firewalls and NAT devices, permissions are granted implicitly
   through the traversal of a packet from the inside of the network
   towards the outside peer.  Thus, a permission cannot, by definition,
   be created by any entity except one inside the firewall or NAT.  With
   TURN, this restriction no longer holds.  Since the TURN server sits



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   outside the firewall, at attacker outside the firewall can now send a
   message to the TURN server and try to create a permission for itself.

   This attack is prevented because all messages that create permissions
   (i.e., ChannelBind and CreatePermission) are authenticated.

17.2.2.  Blacklisted IP Addresses

   Many firewalls can be configured with blacklists that prevent a
   client behind the firewall from sending packets to, or receiving
   packets from, ranges of blacklisted IP addresses.  This is
   accomplished by inspecting the source and destination addresses of
   packets entering and exiting the firewall, respectively.

   This feature is also present in TURN, since TURN servers are allowed
   to arbitrarily restrict the range of addresses of peers that they
   will relay to.

17.2.3.  Running Servers on Well-Known Ports

   A malicious client behind a firewall might try to connect to a TURN
   server and obtain an allocation which it then uses to run a server.
   For example, a client might try to run a DNS server or FTP server.

   This is not possible in TURN.  A TURN server will never accept
   traffic from a peer for which the client has not installed a
   permission.  Thus, peers cannot just connect to the allocated port in
   order to obtain the service.

17.3.  Insider Attacks

   In insider attacks, a client has legitimate credentials but defies
   the trust relationship that goes with those credentials.  These
   attacks cannot be prevented by cryptographic means but need to be
   considered in the design of the protocol.

17.3.1.  DoS against TURN Server

   A client wishing to disrupt service to other clients might obtain an
   allocation and then flood it with traffic, in an attempt to swamp the
   server and prevent it from servicing other legitimate clients.  This
   is mitigated by the recommendation that the server limit the amount
   of bandwidth it will relay for a given username.  This won't prevent
   a client from sending a large amount of traffic, but it allows the
   server to immediately discard traffic in excess.

   Since each allocation uses a port number on the IP address of the
   TURN server, the number of allocations on a server is finite.  An



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   attacker might attempt to consume all of them by requesting a large
   number of allocations.  This is prevented by the recommendation that
   the server impose a limit of the number of allocations active at a
   time for a given username.

17.3.2.  Anonymous Relaying of Malicious Traffic

   TURN servers provide a degree of anonymization.  A client can send
   data to peers without revealing its own IP address.  TURN servers may
   therefore become attractive vehicles for attackers to launch attacks
   against targets without fear of detection.  Indeed, it is possible
   for a client to chain together multiple TURN servers, such that any
   number of relays can be used before a target receives a packet.

   Administrators who are worried about this attack can maintain logs
   that capture the actual source IP and port of the client, and perhaps
   even every permission that client installs.  This will allow for
   forensic tracing to determine the original source, should it be
   discovered that an attack is being relayed through a TURN server.

17.3.3.  Manipulating Other Allocations

   An attacker might attempt to disrupt service to other users of the
   TURN server by sending Refresh requests or CreatePermission requests
   that (through source address spoofing) appear to be coming from
   another user of the TURN server.  TURN prevents this by requiring
   that the credentials used in CreatePermission, Refresh, and
   ChannelBind messages match those used to create the initial
   allocation.  Thus, the fake requests from the attacker will be
   rejected.

17.4.  Other Considerations

   Any relay addresses learned through an Allocate request will not
   operate properly with IPsec Authentication Header (AH) [RFC4302] in
   transport or tunnel mode.  However, tunnel-mode IPsec Encapsulating
   Security Payload (ESP) [RFC4303] should still operate.

18.  IANA Considerations

   Since TURN is an extension to STUN [RFC5389], the methods,
   attributes, and error codes defined in this specification are new
   methods, attributes, and error codes for STUN.  IANA has added these
   new protocol elements to the IANA registry of STUN protocol elements.

   The codepoints for the new STUN methods defined in this specification
   are listed in Section 13.




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   The codepoints for the new STUN attributes defined in this
   specification are listed in Section 14.

   The codepoints for the new STUN error codes defined in this
   specification are listed in Section 15.

   IANA has allocated the SRV service name of "turn" for TURN over UDP
   or TCP, and the service name of "turns" for TURN over TLS.

   IANA has created a registry for TURN channel numbers, initially
   populated as follows:

      0x0000 through 0x3FFF: Reserved and not available for use, since
      they conflict with the STUN header.

      0x4000 through 0x7FFF: A TURN implementation is free to use
      channel numbers in this range.

      0x8000 through 0xFFFF: Unassigned.

   Any change to this registry must be made through an IETF Standards
   Action.

19.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing"
   (UNSAF), which is the general process by which a client attempts to
   determine its address in another realm on the other side of a NAT
   through a collaborative protocol-reflection mechanism [RFC3424].  The
   TURN 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.  These
   considerations and the responses for TURN are documented in this
   section.

   Consideration 1: 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.  Such
   generalizations lead to the prolonged dependence on and usage of the
   supposed short-term fix -- meaning that it is no longer accurate to
   call it "short-term".

   Response: TURN is a protocol for communication between a relay (=
   TURN server) and its client.  The protocol allows a client that is
   behind a NAT to obtain and use a public IP address on the relay.  As
   a convenience to the client, TURN also allows the client to determine
   its server-reflexive transport address.




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   Consideration 2: 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.

   Response: TURN will no longer be needed once there are no longer any
   NATs.  Unfortunately, as of the date of publication of this document,
   it no longer seems very likely that NATs will go away any time soon.
   However, the need for TURN will also decrease as the number of NATs
   with the mapping property of Endpoint-Independent Mapping [RFC4787]
   increases.

   Consideration 3: 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.

   Response: TURN is "brittle" in that it requires the NAT bindings
   between the client and the server to be maintained unchanged for the
   lifetime of the allocation.  This is typically done using keep-
   alives.  If this is not done, then the client will lose its
   allocation and can no longer exchange data with its peers.

   Consideration 4: Identify requirements for longer-term, sound
   technical solutions; contribute to the process of finding the right
   longer-term solution.

   Response: The need for TURN will be reduced once NATs implement the
   recommendations for NAT UDP behavior documented in [RFC4787].
   Applications are also strongly urged to use ICE [RFC5245] to
   communicate with peers; though ICE uses TURN, it does so only as a
   last resort, and uses it in a controlled manner.

   Consideration 5: Discussion of the impact of the noted practical
   issues with existing deployed NATs and experience reports.

   Response: Some NATs deployed today exhibit a mapping behavior other
   than Endpoint-Independent mapping.  These NATs are difficult to work
   with, as they make it difficult or impossible for protocols like ICE
   to use server-reflexive transport addresses on those NATs.  A client
   behind such a NAT is often forced to use a relay protocol like TURN
   because "UDP hole punching" techniques [RFC5128] do not work.

20.  Acknowledgements

   The authors would like to thank the various participants in the
   BEHAVE working group for their many comments on this document.  Marc
   Petit-Huguenin, Remi Denis-Courmont, Jason Fischl, Derek MacDonald,
   Scott Godin, Cullen Jennings, Lars Eggert, Magnus Westerlund, Benny



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   Prijono, and Eric Rescorla have been particularly helpful, with Eric
   suggesting the channel allocation mechanism, Cullen suggesting an
   earlier version of the EVEN-PORT mechanism, and Marc spending many
   hours implementing the preliminary versions to look for problems.
   Christian Huitema was an early contributor to this document and was a
   co-author on the first few versions.  Finally, the authors would like
   to thank Dan Wing for both his contributions to the text and his huge
   help in restarting progress on this document after work had stalled.

21.  References

21.1.  Normative References

   [RFC5389]            Rosenberg, J., Mahy, R., Matthews, P., and D.
                        Wing, "Session Traversal Utilities for NAT
                        (STUN)", RFC 5389, October 2008.

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

   [RFC2474]            Nichols, K., Blake, S., Baker, F., and D. Black,
                        "Definition of the Differentiated Services Field
                        (DS Field) in the IPv4 and IPv6 Headers",
                        RFC 2474, December 1998.

   [RFC3168]            Ramakrishnan, K., Floyd, S., and D. Black, "The
                        Addition of Explicit Congestion Notification
                        (ECN) to IP", RFC 3168, September 2001.

   [RFC1122]            Braden, R., "Requirements for Internet Hosts -
                        Communication Layers", STD 3, RFC 1122,
                        October 1989.

21.2.  Informative References

   [RFC1191]            Mogul, J. and S. Deering, "Path MTU discovery",
                        RFC 1191, November 1990.

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

   [RFC1918]            Rekhter, Y., Moskowitz, R., Karrenberg, D.,
                        Groot, G., and E. Lear, "Address Allocation for
                        Private Internets", BCP 5, RFC 1918,
                        February 1996.





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

   [RFC4787]            Audet, F. and C. Jennings, "Network Address
                        Translation (NAT) Behavioral Requirements for
                        Unicast UDP", BCP 127, RFC 4787, January 2007.

   [RFC5245]            Rosenberg, J., "Interactive Connectivity
                        Establishment (ICE): A Protocol for Network
                        Address Translator (NAT) Traversal for
                        Offer/Answer Protocols", RFC 5245, April 2010.

   [TURN-TCP]           Perreault, S. and J. Rosenberg, "Traversal Using
                        Relays around NAT (TURN) Extensions for TCP
                        Allocations", Work in Progress, March 2010.

   [TURN-IPv6]          Perreault, S., Camarillo, G., and O. Novo,
                        "Traversal Using Relays around NAT (TURN)
                        Extension for IPv6", Work in Progress, March
                        2010.

   [TSVWG-PORT]         Larsen, M. and F. Gont, "Port Randomization",
                        Work in Progress, April 2010.

   [RFC5128]            Srisuresh, P., Ford, B., and D. Kegel, "State of
                        Peer-to-Peer (P2P) Communication across Network
                        Address Translators (NATs)", RFC 5128,
                        March 2008.

   [RFC1928]            Leech, M., Ganis, M., Lee, Y., Kuris, R.,
                        Koblas, D., and L. Jones, "SOCKS Protocol
                        Version 5", RFC 1928, March 1996.

   [RFC3550]            Schulzrinne, H., Casner, S., Frederick, R., and
                        V. Jacobson, "RTP: A Transport Protocol for
                        Real-Time Applications", STD 64, RFC 3550,
                        July 2003.

   [RFC3711]            Baugher, M., McGrew, D., Naslund, M., Carrara,
                        E., and K. Norrman, "The Secure Real-time
                        Transport Protocol (SRTP)", RFC 3711,
                        March 2004.

   [RFC4302]            Kent, S., "IP Authentication Header", RFC 4302,
                        December 2005.




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   [RFC4303]            Kent, S., "IP Encapsulating Security Payload
                        (ESP)", RFC 4303, December 2005.

   [RFC4821]            Mathis, M. and J. Heffner, "Packetization Layer
                        Path MTU Discovery", RFC 4821, March 2007.

   [RFC3261]            Rosenberg, J., Schulzrinne, H., Camarillo, G.,
                        Johnston, A., Peterson, J., Sparks, R., Handley,
                        M., and E. Schooler, "SIP: Session Initiation
                        Protocol", RFC 3261, June 2002.

   [MMUSIC-ICE-NONSIP]  Rosenberg, J., "Guidelines for Usage of
                        Interactive Connectivity Establishment (ICE) by
                        non Session Initiation Protocol (SIP)
                        Protocols", Work in Progress, July 2008.

   [RFC4086]            Eastlake, D., Schiller, J., and S. Crocker,
                        "Randomness Requirements for Security", BCP 106,
                        RFC 4086, June 2005.

   [Frag-Harmful]       Kent and Mogul, "Fragmentation Considered
                        Harmful".  Proc. SIGCOMM '87, vol. 17, No. 5,
                        October 1987

   [Port-Numbers]       "IANA Port Numbers Registry",
                        <http://www.iana.org>.

   [Protocol-Numbers]   "IANA Protocol Numbers Registry", 2005,
                        <http://www.iana.org>.






















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Authors' Addresses

   Rohan Mahy
   Unaffiliated

   EMail: rohan@ekabal.com


   Philip Matthews
   Alcatel-Lucent
   600 March Road
   Ottawa, Ontario
   Canada

   EMail: philip_matthews@magma.ca


   Jonathan Rosenberg
   jdrosen.net
   Monmouth, NJ
   USA

   EMail: jdrosen@jdrosen.net
   URI:   http://www.jdrosen.net



























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