DCCP Working Group                                          G. Fairhurst
Internet-Draft                                                 G. Renker
Intended status: Standards Track                  University of Aberdeen
Expires: May 17, 2008                                  November 16, 2007

  An Update for DCCP Connection Establishment to Assist NAT & Firewall

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   This document proposes an update to the Datagram Congestion Control
   Protocol (DCCP), a connection-oriented and datagram-based transport
   protocol.  The update assists DCCP applications that need to traverse
   middleboxes (e.g.  Network Address Translators or firewalls), where
   peering endpoints need to nearly simultaneously initiate the
   connection to establish middlebox state required for communication.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope of this Document . . . . . . . . . . . . . . . . . .  3
     1.2.  Scope of the Problem to be Tackled . . . . . . . . . . . .  4
     1.3.  Discussion of Traversal Techniques for Traditional NAT
           Devices  . . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.3.1.  Near Simultaneous-Open of Connections  . . . . . . . .  5
       1.3.2.  Role Reversal  . . . . . . . . . . . . . . . . . . . .  6
   2.  Solution for DCCP NAT Traversal  . . . . . . . . . . . . . . .  7
     2.1.  Conventions and Terminology  . . . . . . . . . . . . . . .  7
     2.2.  DCCP-Listen Packet Format  . . . . . . . . . . . . . . . .  7
     2.3.  Protocol Method  . . . . . . . . . . . . . . . . . . . . .  8
       2.3.1.  Protocol Method for DCCP-Server Endpoints  . . . . . .  8
       2.3.2.  Protocol Method for DCCP-Client Endpoints  . . . . . .  9
       2.3.3.  Processing by Routers and Middleboxes  . . . . . . . .  9
     2.4.  Examples of Use  . . . . . . . . . . . . . . . . . . . . .  9
     2.5.  Backwards Compatibility with RFC 4340  . . . . . . . . . . 10
   3.  Discussion of Design Decisions . . . . . . . . . . . . . . . . 12
     3.1.  Generation of Listen Packets . . . . . . . . . . . . . . . 12
     3.2.  Repetition of Listen Packets . . . . . . . . . . . . . . . 12
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

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1.  Introduction

   UDP Network Address Translator (NAT) traversal is well understood and
   widely implemented.  NAT traversal for connection-oriented protocols
   (e.g.  TCP) uses similar principles, but in some cases requires more
   complex and expensive solutions, such as dedicated relay servers.

   DCCP [RFC4340] is both datagram-based and connection-oriented; and
   thus NAT traversal of DCCP faces the same problems as TCP NAT
   traversal, without being able to reap the benefits of datagram-based
   NAT traversal as in UDP.  In addition, DCCP has the disadvantage of
   not being able to perform a simultaneous-open, a TCP-inherent
   characteristic which greatly simplifies TCP NAT traversal.

   After discussing the problem space for DCCP NAT traversal, this
   document proposes a solution to natively support NAT traversal in
   DCCP.  Among the discussed solutions, it requires the least changes
   and is based on an indicator message.  A new type of DCCP packet is
   introduced, which is sent by the server and addressed to the expected
   client.  Since the use of this packet type is tied to an optional
   condition, it facilitates robust and native support for NAT
   traversal, while at the same time leaving the standard operational
   procedure of DCCP untouched.

1.1.  Scope of this Document

   The solution proposed by this document assists in connection
   establishment when one or both peers are located behind a middlebox.
   While the solution is specifically targeted at NAT traversal, due to
   the similarity of involved principles, it may also be of similar use
   to the traversal of other types of middlebox, such as firewalls.

   For the scope of this document we consider traditional (outbound)
   types of NAT as defined in [RFC2663] and further discussed in
   [RFC3022].  We consider NAT traversal to involve one or more NAT
   devices of this type in the path (i.e. hierarchies of nested NAT
   devices are possible).  It is assumed that all NATs in the path
   between endpoints are BEHAVE-compliant [NAT-APP].

   We consider NAT devices which provide a minimum of DCCP protocol
   support, in that layer-4 checksums can be updated to account for
   changes in the pseudo-header.  Since this document tackles an
   inherent problem of DCCP NAT traversal, we do not specify any further
   requirements for DCCP support in NAT devices.  These may be described
   by a separate document.

   The method described by this document is relevant to both client/
   server and peer-to-peer applications, such as VoIP, file sharing, or

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   online gaming.  This update assists connections that utilise prior
   out-of-band signaling between the client and server (e.g. a well-
   known rendezvous server, [RFC3261], [H.323]) to notify both endpoints
   of the connection parameters ([RFC3235], [NAT-APP]).

1.2.  Scope of the Problem to be Tackled

   We refer to DCCP hosts located behind one or more NAT devices as
   having "private" addresses, and to DCCP hosts located in the global
   address realm as having "public" addresses.

   We consider DCCP NAT traversal for the following scenarios:

   1.  Private client connects to public server.

   2.  Public server connects to private client.

   3.  Private client connects to private server.

   A defining characteristic of traditional NAT devices [RFC3022] is
   that private hosts can connect to external hosts, but not vice versa.
   Hence the case (1) is always possible, whereas cases (2) and (3)
   require NAT traversal techniques.  In this document we do not
   consider use of pre-configured static NAT address maps, which would
   also allow outside hosts to connect to the private network in cases
   (2) and (3).

   A DCCP implementation conforming to [RFC4340] can perform NAT
   traversal with the help of a public relay server.  The update
   described by this document allows simple NAT traversal, without
   indirection via relay servers.

1.3.  Discussion of Traversal Techniques for Traditional NAT Devices

   This section is a brief review of some existing techniques to
   establish connectivity across NAT devices, the basic idea being to
   make peer-to-peer sessions look like "outbound" sessions on each NAT

   Often a rendezvous server, located in the public address realm, is
   used to enable clients to discover their NAT topology and the
   addresses of peers.

   The term 'hole punching' was coined in [FSK05] and refers to creating
   soft state in a traditional NAT device by initiating an outbound
   connection.  A well-behaved NAT can subsequently exploit this to
   allow a reverse connection back to the host in the private address

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   The adaptation of the basic hole punching principle to TCP NAT
   traversal was introduced in section 4 of [FSK05] and relies on the
   simultaneous-open feature of TCP [RFC0793].  UDP and TCP hole
   punching use nearly the same protocol.  The main difference lies in
   the way the clients perform connectivity checks after obtaining the
   address pairs from the server; whereas in UDP a single socket is
   sufficient, TCP clients require several sockets for the same address/
   port tuple:

   o  a passive socket to listen for connectivity tests from peers and

   o  multiple active connections from the same address to test
      connectivity of other peers.

   The SYN sent out by client A to its peer B creates soft state in A's
   NAT.  At the same time, B tries to connect to A:

   o  if the SYN from B has left B's NAT before the arrival of A's SYN,
      both endpoints perform simultaneous-open (4-way handshake of SYN/

   o  otherwise A's SYN may not enter B's NAT, which leads to B
      performing a normal open (SYN_SENT => ESTABLISHED) and A
      performing a simultaneous-open (SYN_SENT => SYN_RCVD =>

   In the latter case it is necessary that the NAT does not interfere
   with a RST segment (REQ-4 in [GBF+07]).  The simultaneous-open
   solution is convenient due to its simplicity, and is thus a preferred
   mode of operation in the TCP extension for ICE (section 2 of

   We note that a simultaneous-open is not the only existing solution
   for TCP NAT traversal [GTF04], [GF05]; other approaches are reviewed
   in the next subsection.

   The considerations in this section and the following subsections
   motivate a discussion as to whether and how the DCCP state machine
   can be enhanced to natively facilitate easier middlebox traversal.

1.3.1.  Near Simultaneous-Open of Connections

   Among the various TCP NAT traversal approaches, simultaneous-open
   suggests itself due to its simplicity [GF05], [NAT-APP].  A
   characteristic of simultaneous-open is that the clear distinction
   between client and server is erased: both sides enter through active
   (SYN_SENT) as well as passive (SYN_RCVD) states.  This characteristic
   is in conflict with several ideas underlying DCCP, as a clear

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   separation between client and server has been one of the initial
   design decisions ([RFC4340], 4.6).  Furthermore, several mechanisms
   implicitly rely on clearly-defined client/server roles:

   o  Feature Negotiation: with few exceptions, almost all of DCCP's
      negotiable features use the "server-priority" reconciliation rule
      ([RFC4340], 6.3.1), whereby peers exchange their preference lists
      of feature values, and the server decides the outcome.

   o  Closing States: only servers may generate CloseReq packets (asking
      the peer to hold timewait state), while clients are only permitted
      to send Close or Reset packets to terminate a connection
      ([RFC4340], 8.3).

   o  Service Codes: servers may be associated with multiple service
      codes, while clients must be associated with exactly one
      ([RFC4340], 8.1.2).

   o  Init Cookies: may only be used by the server and on DCCP-Response
      packets ([RFC4340], 8.1.4).

   The latter two points are not obstacles per se, but hinder the
   transition from a passive to an active socket.  The assumption that
   "all DCCP hosts are clients", on the other hand, must be dismissed
   since it limits application programming.  As a consequence, retro-
   fitting simultaneous-open into DCCP does not seem a very sensible

1.3.2.  Role Reversal

   After the simultaneous-open, one of the simplest TCP NAT traversal
   schemes involves role traversal ([Epp05] and [GTF04]), where a peer
   first opens an active connection for the single purpose of punching a
   hole in the firewall, and then reverts to a listening socket to
   accept incoming connections arriving via the new path.

   This solution has several disadvantages for DCCP.  First, a DCCP
   client would be required to change its role from initially 'client'
   to 'server'.  This makes common server processing difficult: it is
   not clear how to assign multiple service codes (this would have to be
   done after the transition); a similar issue arises with Init Cookies.

   Further, the the client must not yet have started feature
   negotiation, since its choice of initial options may rely on its role
   (i.e. if an endpoint knows it is the server, it can make a priori
   assumptions about the preference lists of features it is negotiating
   with the client, thereby enforcing a particular policy).  We
   therefore do not recommend this approach for DCCP.

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2.  Solution for DCCP NAT Traversal

   This section revises packet processing for a DCCP-client and Server,
   to assist in connection establishment when one or both peers are
   located behind a middlebox.  The method does not employ role
   reversal; both endpoints start out with their designated roles, as
   specified in [RFC4340].

2.1.  Conventions and Terminology

   The document uses the terms and definitions provided in [RFC4340].
   Familiarity with this specification is assumed.  In particular, the
   following convention from ([RFC4340], 3.2) is used:

      "Each DCCP connection runs between two hosts, which we often name
      DCCP A and DCCP B. Each connection is actively initiated by one of
      the hosts, which we call the client; the other, initially passive
      host is called the server."

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.2.  DCCP-Listen Packet Format

   The document updates DCCP by adding a new packet type: DCCP-Listen,
   whose format is shown below

   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
   |          Source Port          |           Dest Port           |
   |  Data Offset  | CCVal | CsCov |           Checksum            |
   | Res | Type  |X|             Sequence Number                   |

                         DCCP-Listen Packet Format

   The Reserved (Res) field is specified by [RFC4340], and its
   successors.  This document does not modify that definition.

   The Type field has the value XX-IANA-assigned-XX, which indicates
   that this is a DCCP-Listen packet.

   For a DCCP-Listen type of packet the following protocol fields MUST
   be set to zero:

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      Data Offset (the connection has not been established),

      CCVal (the connection has not been established),

      CsCov (there is no payload),

      Sequence Number (no initial sequence number has been negotiated).

   DCCP-Listen packets MUST set X to 1 (short sequence numbers are not
   permitted), and endpoints MUST ignore any such packets with X set to

   A DCCP-Listen Packet MUST NOT include any DCCP options (since this
   packet does not modify the receiver protocol state) and MUST NOT
   include application data.

2.3.  Protocol Method

   We use the term "session" as defined in ([RFC2663], 2.3): DCCP
   sessions are uniquely identified by the tuple of <source IP-address,
   source port, target IP-address, target port>.

   DCCP, in addition, introduces service codes ([RFC4340], 8.1.2) which
   can be used to identify different services that may be connected
   using the same port.

   We call the five-tuple <source IP-address, source port, service code,
   target IP-address, target port> a fully specified DCCP connection,
   and refer to endpoints that have been assigned all five parameters as
   a "fully specified endpoint".  DCCP-Listen packets are only sent for
   the specific case of fully specified DCCP-server endpoints.

2.3.1.  Protocol Method for DCCP-Server Endpoints

   This document updates [RFC4340] for the case of fully specified DCCP-
   server endpoints.

   Prior to connection setup, it is common for DCCP-server endpoints to
   not be fully specified: before the connection is established, a
   server usually sets the target address / target port to wildcard
   numbers (i.e. leaves these unspecified); the endpoint only becomes
   fully specified after performing the handshake with an incoming, new
   connection.  For such cases, this document does not update the server
   behaviour described in [RFC4340], i.e. no DCCP-Listen packet is sent.

   A fully specified DCCP-server endpoint permits only one specific
   client (matching the target IP-address / target port) to set up the
   connection.  When such a server is in the LISTEN state, it SHOULD use

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   the method specified in this document, and send a single DCCP-Listen
   packet to the remote endpoint.

   The server SHOULD treat ICMP error messages received in response to a
   DCCP-Listen packet as "soft errors", that do not abort a connection.

2.3.2.  Protocol Method for DCCP-Client Endpoints

   A DCCP-client that implements [RFC4340] and receives a DCCP-Listen
   would issue a DCCP-Reset (as it would for all unknown types of

   This document updates [RFC4340], so that a DCCP-client in any state
   that receives a DCCP-Listen packet, MUST silently discard the packet.

   The packet indicates only a willingness to accept a connection; if
   the client has already established a connection, this has no meaning.
   If the client is awaiting the response to a DCCP-Request, the client
   does not need to take specific action, and should continue to use the
   protocol method defined in [RFC4340].

   Receipt of a DCCP-Listen packet by a server in the LISTEN state must
   necessarily lead to a Reset (Reset Code 7, "Connection Refused" is
   suggested).  This is the expected behaviour for [RFC4340].

2.3.3.  Processing by Routers and Middleboxes

   Routers and middleboxes both need to forward DCCP packets.  This
   document does not specify the rules for forwarding these packets, but
   notes that DCCP-Listen packets do not require special treatment and
   should be forwarded end-to-end across the internet path.  Middleboxes
   may utilise the connection information (address, port, Service Code)
   to establish local forwarding state.

2.4.  Examples of Use

   In the examples below, DCCP A is the client and DCCP B is the server.
   NAT/Firewall device NA is placed before DCCP A, and NAT/Firewall
   device NB is placed before DCCP B.

   Both NA and NB use a policy that permits DCCP packets to traverse the
   device for outgoing links, but only permit incoming DCCP packets when
   a previous packet has been sent for the same connection.

   DCCP A and DCCP B decide to communicate using some out-of-band
   mechanism, whereupon the client and server are started.  DCCP A
   initiates a connection by sending a DCCP-Request.  DCCP B actively
   indicates its state by sending a Listen message.  This fulfils the

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   requirement of punching a hole in NB so that DCCP A can retransmit
   the DCCP-Request and connect through it.

          DCCP A                                       DCCP B
          ------               NA      NB              ------
          +------------------+  +-+    +-+  +-----------------+
          |(1) Initiation    |  | |    | |  |                 |
          |DCCP-Request -->  +--+-+---X| |  |                 |
          |                  |<-+-+----+-+--+<-- DCCP-Listen  |
          |                  |  | |    | |  |                 |
          |DCCP-Request -->  +--+-+----+-+->|                 |
          |                  |<-+-+----+-+--+<-- DCCP-Response|
          |DCCP-Ack -->      +--+-+----+-+->|                 |
          |                  |  | |    | |  |                 |
          |(2) Data transfer |  | |    | |  |                 |
          |DCCP-Data -->     +--+-+----+-+->|                 |
          +------------------+  +-+    +-+  +-----------------+

       Sequence of events when a client is started before the server

   The diagram below reverses this sequencing:

          DCCP A                                       DCCP B
          ------               NA      NB              ------
          +------------------+  +-+    +-+  +-----------------+
          |(1) Initiation    |  | |    | |  |                 |
          |                  |  | |X---+-+--+<-- DCCP-Listen  |
          |DCCP-Request -->  +--+-+----+-+->|                 |
          |                  | <+-+----+-+--+<-- DCCP-Response|
          |DCCP-Ack -->      +--+-+----+-+> |                 |
          |                  |  | |    | |  |                 |
          |(2) Data transfer |  | |    | |  |                 |
          |DCCP-Data -->     +--+-+----+-+> |                 |
          +------------------+  +-+    +-+  +-----------------+

       Sequence of events when a server is started before the client

2.5.  Backwards Compatibility with RFC 4340

   This proposal does not modify communication between a server
   conforming to [RFC4340] and a client that implements the update
   described in this document.

   This proposal also does not modify communication for any server that
   implements a passive-open without fully binding the addresses, ports
   and service codes to be used.

   A change in the protocol exchange does occur when a server implements

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   this update and binds to a client that implements [RFC4340].

   If DCCP A and B were not behind NAT devices, the receipt of a DCCP-
   Listen packet at A by a server that implements [RFC4340] would lead
   to a DCCP-Reset (presumably Code 3, "No Connection", since Code 7,
   "Connection Refused" applies to DCCP-Requests only).  This would
   abort the connection.

   Since the outgoing DCCP-Listen packet is expected to open a hole in
   the firewall, the same conclusion is expected when using NATs; as in
   the examples presented in the previous section.

   The authors do not however expect these issues to introduce practical
   deployment problems.

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3.  Discussion of Design Decisions

   This section identifies a number of design decisions that were made
   in defining the current approach.

3.1.  Generation of Listen Packets

   Since DCCP-Listen packets solve a particular problem (NAT and/or
   firewall traversal), the generation of DCCP-Listen packets on passive
   sockets has been tied to a condition (i.e. the server is aware of the
   expected client connection request), so as to not interfere with the
   general case of "normal" DCCP connections.

   In the TCP world, the analogue is a transition from LISTEN to
   SYN_SENT by virtue of sending data: "A fully specified passive call
   can be made active by the subsequent execution of a SEND" ([RFC0793],

   Unlike TCP, this proposal does not perform a role-change from passive
   to active.

   Like TCP, we require that DCCP-Listen packets are only sent by a
   DCCP-server when the endpoint is fully specified (Section 2.3).

   If this condition were considered too weak, it could be coupled with
   a socket option to trigger generation of DCCP-Listen packets on fully
   specified passive sockets.

3.2.  Repetition of Listen Packets

   Section 4.3 of [RFC4340] defines the LISTEN state as:

      "Represents server sockets in the passive listening state.  LISTEN
      and CLOSED are not associated with any particular DCCP

   This document revises this definition with regard to the protocol
   method described in Section 2.3.  In the current revision of this
   document, this is represented as an additional transition (to
   the LISTEN state), but the authors also acknowledge the possibility
   of introducing a new state.

   This state would allow a DCCP-Listen packet to be re-issued
   periodically, to refresh firewall state.  This approach would add
   robustness to the proposed mechanism, but has the disadvantage that
   the DCCP-Listen packets could then contribute unwanted load in a mis-
   configured network and procedures may be needed to limit the rate and
   number of packets sent.

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4.  IANA Considerations

   This document requires IANA action by allocation of a new Packet Type
   from the IANA Packet Types Registry.  The Registry entry is to
   reference this document.  This allocation requires IESG review and
   approval and standards-track IETF RFC publication.

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5.  Security Considerations

   The method specified in this document exposes the state of a DCCP
   server that has been explicitly configured to accept a connection
   from a client.  This requires prior out-of-band signaling between the
   client and server (e.g. via SIP) to establish this state.  The method
   generates a packet addressed to the expected client.

   This increases the vulnerability of the DCCP host by revealing which
   ports are in a passive LISTEN state to the expected client (this
   information is not encrypted, and therefore could be seen on the path
   to the client through the network).  We do not believe this change
   significantly increases the complexity or vulnerability of a DCCP
   implementation that conforms to [RFC4340].

   Reception of a DCCP-Listen packet would trigger a DCCP-Reset in
   [RFC4340], but is ignored by the method described in this protocol.
   This does not introduce new security concerns.

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6.  References

6.1.  Normative References

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

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

6.2.  Informative References

   [Epp05]    Eppinger, J-L., "TCP Connections for P2P Apps: A Software
              Approach to Solving the NAT Problem", Carnegie Mellon
              University/ISRI Technical Report CMU-ISRI-05-104,
              January 2005.

   [FSK05]    Ford, B., Srisuresh, P., and D. Kegel, "Peer-to-Peer
              Communication Across Network Address Translators",
              Proceedings of USENIX-05, pages 179-192, 2005.

   [GBF+07]   Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", Work In
              Progress, draft-ietf-behave-tcp-07, April 2007.

   [GF05]     Guha, S. and P. Francis, "Characterization and Measurement
              of TCP Traversal through NATs and Firewalls", Proceedings
              of Internet Measurement Conference (IMC-05), pages 199-
              211, 2005.

   [GTF04]    Guha, S., Takeda, Y., and P. Francis, "NUTSS: A SIP based
              approach to UDP and TCP connectivity", Proceedings of
              SIGCOMM-04 Workshops, Portland, OR, pages 43-48, 2004.

   [H.323]    ITU-T, "Packet-based Multimedia Communications Systems",
              Recommendation H.323, July 2003.

   [NAT-APP]  Ford, B., Srisuresh, P., and D. Kegel, "Application Design
              Guidelines for Traversal through Network Address
              Translators", Work In Progress, draft-ford-behave-app-05,
              March 2007.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

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   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235, January 2002.

   [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.

   [Ros07]    Rosenberg, J., "TCP Candidates with Interactive
              Connectivity Establishment (ICE)", Work In
              Progress, draft-ietf-mmusic-ice-tcp-04, July 2007.

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Appendix A.  Change Log

   This is the first public draft.

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Authors' Addresses

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE

   Email: gorry@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

   Gerrit Renker
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE

   Email: gerrit@erg.abdn.ac.uk
   URI:   http://www.erg.abdn.ac.uk

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Full Copyright Statement

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