[Search] [txt|pdf|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03                                                   
Internet Draft                                                   B. Ford
Document: draft-ford-midcom-p2p-02.txt                            M.I.T.
Expires: September 25, 2004                                 P. Srisuresh
                                                          Caymas Systems
                                                                D. Kegel
                                                              March 2004

Peer-to-Peer(P2P) communication across Network Address Translators(NAT)

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.  Internet-Drafts are working documents of
   the Internet Engineering Task Force (IETF), its areas, and its
   working groups.  Note that other groups may also distribute working
   documents as Internet-Drafts.

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

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   Distribution of this document is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This memo documents the methods used by the current peer-to-peer
   (P2P) applications to communicate in the presence of network
   address translators (NAT). In addition, the memo suggests
   guidelines to application designers and NAT implementers on the
   measures they could take to enable immediate, wide deployment of
   P2P applications with or without requiring the use of special
   proxy, relay or midcom protocols.

Ford, Srisuresh & Kegel                                         [Page 1]

Internet-Draft    P2P communication across NAT devices        March 2004

Table of Contents

   1.  Introduction .................................................
   2.  Terminology ..................................................
   3.  Techniques for P2P communication across NAT devices ..........
       3.1.  Relaying ...............................................
       3.2.  Connection reversal ....................................
       3.3.  UDP Hole Punching ......................................
             3.3.1.  Peers behind different NATs ....................
             3.3.2.  Peers behind the same NAT ......................
             3.3.3.  Peers separated by multiple NATs ...............
             3.3.4.  Consistent port bindings .......................
       3.4.  UDP Port number prediction .............................
       3.5.  Simultaneous TCP open ..................................
   4.  Application design guidelines ................................
       4.1. What works with P2P NAT devices .........................
       4.2. Applications behind the same NAT ........................
       4.3. Peer discovery ..........................................
       4.4. TCP applications using sockets API ......................
       4.5. Use of midcom protocol ..................................
   5.  NAT design guidelines ........................................
       5.1. Deprecate the use of symmetric NATs .....................
       5.2. Add incremental Cone-NAT support to symmetric NAT devices
       5.3. Support Address and port bindings .......................
             5.3.1. Preserving Port Numbers .........................
             5.3.2. Support TCP port bindings .......................
       5.4. Large timeout for P2P applications ......................
       5.5. Support loopback translation ............................
       5.6. Support midcom protocol .................................
   6.  Security considerations ......................................

1. Introduction

   Present-day Internet has seen ubiquitous deployment of network
   address translators (NAT), driven primarily by the ongoing depletion
   of the IPv4 address space. There are a variety of NAT devices in
   use. Readers are urged to refer [NAT-TERM] to learn of NAT varieties
   and their definition. Of the various NAT devices, traditional NAT
   [TRAD-NAT] is the most common type of NAT device. The asymmetric
   addressing and connectivity regimes established by the NAT devices
   has created unique problems for peer-to-peer (P2P) applications and
   protocols, such as teleconferencing and multiplayer on-line gaming.
   These issues are likely to persist even into the IPv6 world, where
   NAT is often used as an IPv4 compatibility mechanism [NAT-PT], and
   firewalls will still be commonplace even after NAT is no longer

Ford, Srisuresh & Kegel                                         [Page 2]

Internet-Draft    P2P communication across NAT devices        March 2004

   Currently deployed NAT devices are designed primarily around the
   client/server paradigm, in which relatively anonymous client machines
   inside a private network initiate connections to public servers with
   stable IP addresses and DNS names. NAT devices enroute provide
   dynamic address assignment. The anonymity and inaccessibility of
   the internal hosts behind a NAT device is not a problem for client
   software such as web browsers, which only need to initiate outgoing
   connections. This inaccessibility is sometimes seen as a privacy

   In the peer-to-peer paradigm, however, Internet hosts that would
   normally be considered "clients" need to establish communication
   sessions directly with each other. The initiator and the responder
   might lie behind different NAT devices with neither endpoint
   having a permanent IP address or other form of public network
   presence. A common on-line gaming architecture, for example,
   is for the participating application hosts to contact a well-known
   server for initialization and administration purposes. Subsequent
   to this, the hosts establish direct connections with each other
   for fast and efficient propagation of updates during game play.
   Similarly, a file sharing application might contact a well-known
   server for resource discovery or searching, but establish direct
   connections with peer hosts for data transfer. NAT devices create
   problems for peer-to-peer connections because hosts behind a
   NAT device normally have no permanently visible public ports on the
   Internet to which incoming TCP or UDP connections from other peers
   can be directed.  RFC 3235 [NAT-APPL] briefly addresses this issue,
   but does not offer any general solutions.

   In this document we address the P2P/NAT problem in two ways.
   First, we summarize known methods by which P2P applications can
   work around the presence of NAT devices. Second, we provide a set
   of application design guidelines based on these practices to make
   P2P applications operate more robustly over currently-deployed
   NAT devices. Further, we provide design guidelines for future
   NAT device implementers to allow them to support P2P applications
   more effectively. Our focus is to enable immediate and wide
   deployment of P2P applications requiring to traverse NAT devices.

2. Terminology

   Readers are urged to refer [NAT-TERM] for information on NAT
   taxonomy and terminology. Traditional NAT is the most common type
   of NAT device deployed. Readers may refer [NAT-TRAD] for detailed
   information on traditional NAT. Traditional NAT has two main
   varieties - Basic NAT and Network Address/Port Translator (NAPT).

   NAPT is by far the most commonly deployed NAT device. NAPT allows

Ford, Srisuresh & Kegel                                         [Page 3]

Internet-Draft    P2P communication across NAT devices        March 2004

   multiple internal hosts to share a single public IP address
   simultaneously. When an internal host opens an outgoing TCP or UDP
   session through a NAPT, the NAPT assigns the session a public IP
   address and port number so that subsequent response packets from
   the external endpoint can be received by the NAPT, translated, and
   forwarded to the internal host. The effect is that the NAPT
   establishes a NAT session to translate the (private IP address,
   private port number) tuple to (public IP address, public port
   number) tuple and vice versa for the duration of the session. An
   issue of relevance to P2P applications is how the NAT behaves when
   an internal host initiates multiple simultaneous sessions from a
   single (private IP, private port) pair to multiple distinct
   endpoints on the external network.

   Additional terms that further classify NAPT implementation are
   defined in more recent work [STUN] and are summarized below.

   Cone NAT
      The fundamental property of Cone NAT is that it reuses port
      port binding between a (private IP, private port) tuple and a
      (public IP, public port) tuple for multiple sessions an
      application may initiate from the same private IP address and
      port number.

      For example, suppose Client A in the diagram below initiates two
      simultaneous outgoing sessions through a cone NAT, from the same
      internal endpoint ( to two different
      external servers, S1 and S2.  The cone NAT assigns just one public
      endpoint tuple,, to both of these sessions,
      ensuring that the "identity" of the client's port is maintained
      across address translation. Since Basic-NAT devices do not modify
      port numbers as packets traverse the device, Basic-NAT device
      can be viewed as a degenerate form of Cone NAT.

Ford, Srisuresh & Kegel                                         [Page 4]

Internet-Draft    P2P communication across NAT devices        March 2004

           Server S1                                     Server S2                    
               |                                             |
               |                                             |
          ^  Session 1 (A-S1)  ^      |      ^  Session 2 (A-S2)  ^
          |  |      |      |  |
          | |      |      | |
                                | |
                                |              |
                                | Any type of  |
                                |   Cone NAT   |
          ^  Session 1 (A-S1)  ^      |      ^  Session 2 (A-S2)  ^
          |  |      |      |  |
          |    |      |      |    |
                                   Client A

      Figure 1: Cone NAT - Reuse of port binding for multiple sessions

   Symmetric NAT
      A symmetric NAT, in contrast, does not use port bindings.
      Instead, it assigns a new public port to each new session.
      For example, suppose Client A initiates two outgoing sessions
      from the same port as above, one with S1 and one with S2. A
      symmetric NAT might allocate the public endpoint to session 1, and then allocate a different
      public endpoint, when the application
      initiates session 2.  The NAT is able to differentiate
      between the two sessions for translation purposes because the
      external endpoints involved in the sessions (those of S1
      and S2) differ, even as the endpoint identity of the client
      application is lost across the address translation boundary.

Ford, Srisuresh & Kegel                                         [Page 5]

Internet-Draft    P2P communication across NAT devices        March 2004

           Server S1                                     Server S2                    
               |                                             |
               |                                             |
          ^  Session 1 (A-S1)  ^      |      ^  Session 2 (A-S2)  ^
          |  |      |      |  |
          | |      |      | |
                               |  |
                               |               |
                               | Symmetric     |
                               | NAT           |
          ^  Session 1 (A-S1)  ^      |      ^  Session 2 (A-S2)  ^
          |  |      |      |  |
          |    |      |      |    |
                                   Client A

       Figure 2: Symmetric NAT - Port binding not in use for sessions

   Cone NAT is further classified according to how liberally the NAT
   accepts incoming traffic directed to an already-established (public
   IP, public port) pair.  This classification generally applies only
   to UDP traffic, since NATs and firewalls reject incoming TCP
   connection attempts unconditionally unless specifically configured
   to do otherwise. The following Cone NAT variations are defined in
   [STUN], but restated here for additional explanation.

   Full Cone NAT
      Subsequent to establishing port binding at the start of an
      outgoing session, a full cone NAT will accept incoming traffic
      to the corresponding public port from ANY external endpoint on
      the public network. Full cone NAT is also sometimes referred
      as "promiscuous" NAT.

   Address Restricted Cone NAT
      Subsequent to establishing port binding at the start of an
      outgoing session, Address Restricted Cone NAT will accept
      incoming traffic to the corresponding public port from only
      those external endpoints whose IP address match the address
      of a node to which the internal host has previously sent one

Ford, Srisuresh & Kegel                                         [Page 6]

Internet-Draft    P2P communication across NAT devices        March 2004

      or more outgoing packets.

   Port Restricted Cone NAT
      Subsequent to establishing port binding at the start of an
      outgoing session, Port Restricted Cone NAT will accept
      incoming traffic to the corresponding public port from only
      those external endpoints to which the internal host has
      previously sent one or more outgoing packets. Port Restricted
      Cone NAT is the true-to-spirit implementation of NAPT, as
      defined. Port Restricted Cone NAT provides internal nodes the
      same level of protection against unsolicited incoming traffic
      as does a symmetric NAT, while maintaining a private port's
      identity across multiple sessions.

   Finally, we define the following new terms for classifying
   P2P-relevant behavior across middleboxes. Readers are urged to
   refer [MIDCOM-FW] for information on middlebox terms and
   communication framework.

      P2P-application as used in this document is an application in
      which each P2P participant registers with a public
      registration server, and subsequently uses either its
      private endpoint, or public endpoint, or both, to establish
      peering sessions.

      A P2P-Middlebox is middlebox that permits the traversal of
      P2P applications.

      A P2P-firewall is a P2P-Middlebox that provides firewall
      functionality but performs no address translation.

      A P2P-NAT is a P2P-Middlebox that provides NAT functionality.

   Loopback translation / Hairpin translation
      When a host in the private domain of a NAT device attempts to
      connect with another host behind the same NAT device using
      the public address of the host, the NAT device performs the
      equivalent of a "Twice-nat" translation on the packet as
      follows. The originating host's private endpoint is translated
      into its assigned public endpoint, and the target host's public
      endpoint is translated into its private endpoint, before
      the packet is forwarded to the target host. We refer the above
      translation performed by a NAT device as "Loopback translation".
      This is also referred sometimes as "Hairpin translation".

Ford, Srisuresh & Kegel                                         [Page 7]

Internet-Draft    P2P communication across NAT devices        March 2004

3. Techniques for P2P Communication across NAT devices

   This section reviews in detail the currently known techniques for
   implementing peer-to-peer communication over existing NAT devices,
   from the perspective of the application or protocol designer. The
   readers will note that the applications assume an Address/Port
   Restricted Cone NAT in majority of the cases below.

3.1. Relaying

   The most reliable, but least efficient, method of implementing peer-
   to-peer communication in the presence of a NAT device is to make the
   peer-to-peer communication look to the network like client/server
   communication through relaying.  For example, suppose two client
   hosts, A and B, have each initiated TCP or UDP connections with a
   well-known server S having a permanent IP address.  The clients
   reside on separate private networks, however, and their respective
   NAT devices prevent either client from directly initiating a
   connection to the other.

                                 Server S
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |     |   |     | |
        |                                                         |
      +--------------+                                 +--------------+
      | |                                 |  |
      |              |                                 |              |
      | Address/Port |                                 | Address/port |
      | Restricted   |                                 | Restricted   |
      | Cone-NAT A   |                                 | Cone-NAT B   |
      +--------------+                                 +--------------+
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |      |   |      | |
        |                                                         |
        |                                                         |
     Client A                                                 Client B                              

   Figure 3: Use Client-Server session for Indirect-P2P communication

Ford, Srisuresh & Kegel                                         [Page 8]

Internet-Draft    P2P communication across NAT devices        March 2004

   Instead of attempting a direct connection, the two clients can simply
   use the server S to relay messages between them.  For example, to
   send a message to client B, client A simply sends the message to
   server S along its already-established client/server connection, and
   server S then sends the message on to client B using its existing
   client/server connection with B.

   This method has the advantage that it will always work as long as
   both clients have connectivity to the server.  Its obvious
   disadvantages are that it consumes the server's processing power and
   network bandwidth, and communication latency between the peering
   clients is likely to be increased even if the server is well-
   connected. The TURN protocol [TURN] defines a method of implementing
   relaying in a relatively secure fashion.

3.2. Connection reversal

   The following connection reversal technique for a direct P2P
   communication works only when one of the clients (i.e., peers) is
   behind a NAT device. For example, suppose client A is behind a NAT
   but client B has a globally routable IP address, as in figure 4.

Ford, Srisuresh & Kegel                                         [Page 9]

Internet-Draft    P2P communication across NAT devices        March 2004

                                 Server S
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |     |   |      | |
        |                                                         |
        | ^ P2P Session (A-B)      ^   |  P2P Session (B-A)     | |
        | |      |   |    | |
        | |     |   v     v |
        |                                                         |
      +--------------+                                            |
      | |                                            |
      |              |                                            |
      | Address/Port |                                            |
      | Restricted   |                                            |
      | Cone-NAT A   |                                            |
      +--------------+                                            |
        |                                                         |
        | ^ Relay-Req Session(A-S) ^                              |
        | |      |                              |
        | |      |                              |
        |                                                         |
        | ^ P2P Session (A-B)      ^                              |
        | |      |                              |
        | |      |                              |
        |                                                         |
     Private Client A                                 Public Client B                          

   Figure 4: Force private client to initiate session for Direct-P2P

   Client A has private IP address, and the application is
   using TCP port 1234.  This client has established a connection with
   server S at public IP address and port 1235.  NAT A has
   assigned TCP port 62000, at its own public IP address,
   to serve as the temporary public endpoint address for A's session
   with S: therefore, server S believes that client A is at IP address using port 62000.  Client B, however, has its own
   permanent IP address,, and the peer-to-peer application
   on B is accepting TCP connections at port 1234.

   Now suppose client B would like to initiate a peer-to-peer
   communication session with client A.  B might first attempt to
   contact client A either at the address client A believes itself to

Ford, Srisuresh & Kegel                                        [Page 10]

Internet-Draft    P2P communication across NAT devices        March 2004

   have, namely, or at the address of A as observed by
   server S, namely  In either case, however, the
   connection will fail.  In the first case, traffic directed to IP
   address will simply be dropped by the network because is not a publicly routable IP address.  In the second case,
   the TCP SYN request from B will arrive at NAT A directed to port
   62000, but NAT A will reject the connection request because only
   outgoing connections are allowed.

   After attempting and failing to establish a direct connection to A,
   client B can use server S to relay a request to client A to initiate
   a "reversed" connection to client B.  Client A, upon receiving this
   relayed request through S, opens a TCP connection to client B at B's
   public IP address and port number.  NAT A allows the connection to
   proceed because it is originating inside the firewall, and client B
   can receive the connection because it is not behind a NAT device.

   A variety of current peer-to-peer systems implement this technique.
   Its main limitation, of course, is that it only works as long as only
   one of the communicating peers is behind a NAT: in the increasingly
   common case where both peers are behind NATs, the method fails.
   Because connection reversal is not a general solution to the problem,
   it is NOT recommended as a primary strategy.  Applications may choose
   to attempt connection reversal, but should be able to fall back
   automatically on another mechanism such as relaying if neither a
   "forward" nor a "reverse" connection can be established.

3.3. UDP hole punching

   The third technique, and the one of primary interest in this
   document, is widely known as "UDP Hole Punching."  UDP hole punching
   relies on the properties of common firewalls and cone NATs to allow
   appropriately designed peer-to-peer applications to "punch holes"
   through the NAT device and establish direct connectivity with each
   other, even when both communicating hosts may lie behind NAT devices.
   This technique was mentioned briefly in section 5.1 of RFC 3027 [NAT-
   PROT], and has been informally described elsewhere on the Internet
   [KEGEL] and used in some recent protocols [TEREDO, ICE].  As the name
   implies, unfortunately, this technique works reliably only with UDP.

   We will consider two specific scenarios, and how applications can be
   designed to handle both of them gracefully.  In the first situation,
   representing the common case, two clients desiring direct peer-to-
   peer communication reside behind two different NATs.  In the second,
   the two clients actually reside behind the same NAT, but do not
   necessarily know that they do.

3.3.1. Peers behind different NATs

Ford, Srisuresh & Kegel                                        [Page 11]

Internet-Draft    P2P communication across NAT devices        March 2004

   Suppose clients A and B both have private IP addresses and lie behind
   different network address translators.  The peer-to-peer application
   running on clients A and B and on server S each use UDP port 1234.  A
   and B have each initiated UDP communication sessions with server S,
   causing NAT A to assign its own public UDP port 62000 for A's session
   with S, and causing NAT B to assign its port 31000 to B's session
   with S, respectively.

                                 Server S
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |     |   |     | |
        |                                                         |
        | ^ P2P Session (A-B)      ^   ^  P2P Session (B-A)     ^ |
        | |     |   |    | |
        | |     |   |     | |
        |                                                         |
      +--------------+                                 +--------------+
      | |                                 |  |
      |              |                                 |              |
      | Address/Port |                                 | Address/port |
      | Restricted   |                                 | Restricted   |
      | Cone-NAT A   |                                 | Cone-NAT B   |
      +--------------+                                 +--------------+
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |      |   |      | |
        |                                                         |
        | ^ P2P Session (A-B)      ^   ^  P2P Session (B-A)     ^ |
        | |     |   |    | |
        | |      |   |     | |
        |                                                         |
     Client A                                                 Client B                              

   Figure 5: Coordinate simultaneous outgoing sessions for Direct-P2P

   Now suppose that client A wants to establish a UDP communication
   session directly with client B.  If A simply starts sending UDP
   messages to B's public address,, then NAT B will
   typically discard these incoming messages (unless it is a full cone
   NAT), because the source address and port number does not match those

Ford, Srisuresh & Kegel                                        [Page 12]

Internet-Draft    P2P communication across NAT devices        March 2004

   of S, with which the original outgoing session was established.
   Similarly, if B simply starts sending UDP messages to A's public
   address, then NAT A will typically discard these messages.

   Suppose A starts sending UDP messages to B's public address, however,
   and simultaneously relays a request through server S to B, asking B
   to start sending UDP messages to A's public address.  A's outgoing
   messages directed to B's public address ( cause NAT
   A to open up a new communication session between A's private address
   and B's public address.  At the same time, B's messages to A's public
   address ( cause NAT B to open up a new
   communication session between B's private address and A's public
   address.  Once the new UDP sessions have been opened up in each
   direction, client A and B can communicate with each other directly
   without further burden on the "introduction" server S.

   The UDP hole punching technique has several useful properties.  Once
   a direct peer-to-peer UDP connection has been established between two
   clients behind NAT devices, either party on that connection can in
   turn take over the role of "introducer" and help the other party
   establish peer-to-peer connections with additional peers, minimizing
   the load on the initial introduction server S.  The application does
   not need to attempt to detect the kind of NAT device it is behind,
   if any [STUN], since the procedure above will establish peer-to-peer
   communication channels equally well if either or both clients do not
   happen to be behind a NAT device.  The hole punching technique
   even works automatically with multiple NATs, where one or both
   clients are removed from the public Internet via two or more levels
   of address translation.

3.3.2. Peers behind the same NAT

   Now consider the scenario in which the two clients (probably
   unknowingly) happen to reside behind the same NAT, and are therefore
   located in the same private IP address space.  Client A has
   established a UDP session with server S, to which the common NAT has
   assigned public port number 62000.  Client B has similarly
   established a session with S, to which the NAT has assigned public
   port number 62001.

Ford, Srisuresh & Kegel                                        [Page 13]

Internet-Draft    P2P communication across NAT devices        March 2004

                                Server S
        ^ Relay-Req Session(A-S) ^  | ^ Relay-Req Session(B-S) ^
        |      |  | |      |
        |     |  | |    |
                             | |
                             |              |
                             | Address/Port |
                             | Restricted   |
                             | Cone-NAT     |
      |                                                          |
      |                                                          |
      | ^ Relay-Req Session(A-S) ^    ^ Relay-Req Session(B-S) ^ |
      | |      |    |      | |
      | |      |    |      | |
      |                                                          |
      | ^ P2P Session-try1(A-B)  ^    ^  P2P Session-try1 (B-A)^ |
      | |      |    |     | |
      | |      |    |     | |
      |                                                          |
      | ^ P2P Session-try2 (A-B) ^    ^  P2P Session-try2 (B-A)^ |
      | |     |    |    | |
      | |      |    |     | |
      |                                                          |
   Client A                                                   Client B                               

   Figure 6: Register private identity & NAT identity with Relay server.

   Suppose that A and B use the UDP hole punching technique as outlined
   above to establish a communication channel using server S as an
   introducer.  Then A and B will learn each other's public IP addresses
   and port numbers as observed by server S, and start sending each
   other messages at those public addresses.  The two clients will be
   able to communicate with each other this way as long as the NAT
   allows hosts on the internal network to open translated UDP sessions
   with other internal hosts and not just with external hosts. We refer
   to this situation as "loopback translation," because packets arriving
   at the NAT from the private network are translated and then "looped
   back" to the private network rather than being passed through to the
   public network.  For example, when A sends a UDP packet to B's public
   address, the packet initially has a source IP address and port number

Ford, Srisuresh & Kegel                                        [Page 14]

Internet-Draft    P2P communication across NAT devices        March 2004

   of and a destination of  The NAT
   receives this packet, translates it to have a source of (A's public address) and a destination of, and then forwards it on to B.  Even if loopback
   translation is supported by the NAT, this translation and forwarding
   step is obviously unnecessary in this situation, and is likely to add
   latency to the dialog between A and B as well as burdening the NAT.

   The solution to this problem is straightforward, however.  When A and
   B initially exchange address information through server S, they
   should include their own IP addresses and port numbers as "observed"
   by themselves, as well as their addresses as observed by S.  The
   clients then simultaneously start sending packets to each other at
   each of the alternative addresses they know about, and use the first
   address that leads to successful communication.  If the two clients
   are behind the same NAT, then the packets directed to their private
   addresses are likely to arrive first, resulting in a direct
   communication channel not involving the NAT.  If the two clients are
   behind different NATs, then the packets directed to their private
   addresses will fail to reach each other at all, but the clients will
   hopefully establish connectivity using their respective public
   addresses.  It is important that these packets be authenticated in
   some way, however, since in the case of different NATs it is entirely
   possible for A's messages directed at B's private address to reach
   some other, unrelated node on A's private network, or vice versa.

3.3.3. Peers separated by multiple NATs

   In some topologies involving multiple NAT devices, it is not
   possible for two clients to establish an "optimal" P2P route between
   them without specific knowledge of the topology.  Consider for
   example the following situation.

Ford, Srisuresh & Kegel                                        [Page 15]

Internet-Draft    P2P communication across NAT devices        March 2004

                                Server S
        ^ Relay-Req Session(A-S) ^ | ^ Relay-Req Session(B-S) ^
        |      | | |      |
        |     | | |     |
                            | |
                            |              |
                            | Address/Port |
                            | Restricted   |
                            | Cone-NAT X   |
                            | (Supporting  |
                            | Loopback     |
                            | Translation) |
      |                                                         |
      | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
      | |      |   |      | |
      | |     |   |     | |
      |                                                         |
      | ^ P2P Session (A-B)      ^   ^  P2P Session (B-A)     ^ |
      | |    |   |   | |
      | |    |   |   | |
      |                                                         |
   +--------------+                                  +--------------+
   |  |                                  |  |
   |              |                                  |              |
   | Address/Port |                                  | Address/Port |
   | Restricted   |                                  | Restricted   |
   | Cone-NAT A   |                                  | Cone-NAT B   |
   +--------------+                                  +--------------+
       |                                                        |
       | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S)^ |
       | |      |   |     | |
       | |      |   |     | |
       |                                                        |
       | ^ P2P Session (A-B)      ^   ^  P2P Session (B-A)    ^ |
       | |    |   |   | |
       | |     |   |    | |
       |                                                        |
   Client A                                                  Client B                              

   Figure 7: Use of Loopback translation to facilitate Direct-P2P

Ford, Srisuresh & Kegel                                        [Page 16]

Internet-Draft    P2P communication across NAT devices        March 2004

   Suppose NAT X is a large industrial Cone NAT deployed by an internet
   service provider (ISP) to multiplex many customers onto a few public
   IP addresses, and NATs A and B are small consumer NAT gateways
   deployed independently by two of the ISP's customers to multiplex
   their private home networks onto their respective ISP-provided IP
   addresses.  Only server S and NAT X have globally routable IP
   addresses; the "public" IP addresses used by NAT A and NAT B are
   actually private to the ISP's addressing realm, while client A's and
   B's addresses in turn are private to the addressing realms of NAT A
   and B, respectively.  Each client initiates an outgoing connection to
   server S as before, causing NATs A and B each to create a single
   public/private translation, and causing NAT X to establish a
   public/private translation for each session.

   Now suppose clients A and B attempt to establish a direct peer-to-
   peer UDP connection.  The optimal method would be for client A to
   send messages to client B's public address at NAT B, in the ISP's addressing realm, and for client B to
   send messages to A's public address at NAT B, namely  Unfortunately, A and B have no way to learn these
   addresses, because server S only sees the "global" public addresses
   of the clients, and  Even if A
   and B had some way to learn these addresses, there is still no
   guarantee that they would be usable because the address assignments
   in the ISP's private addressing realm might conflict with unrelated
   address assignments in the clients' private realms.  The clients
   therefore have no choice but to use their global public addresses as
   seen by S for their P2P communication, and rely on NAT X to provide
   loopback translation.

3.3.4. Consistent port bindings

   The hole punching technique has one caveat in that it works only if
   the traversing NAT is cone NAT. That is because Cone NAT reuses
   port bindings. When a symmetric NAT is enroute, it is impossible
   for a P2P application to reuse an already-established translation
   endpoint for communication with different external destinations.
   Since Cone NATs are the most widespread, the UDP hole punching
   technique is fairly broadly applicable; nevertheless a substantial
   fraction of deployed NATs are symmetric NATs and do not support
   the hole punching technique.

3.4. UDP port number prediction

   A variant of the UDP hole punching technique discussed above exists
   that allows peer-to-peer UDP sessions to be created in the presence
   of some symmetric NATs.  This method is sometimes called the "N+1"

Ford, Srisuresh & Kegel                                        [Page 17]

Internet-Draft    P2P communication across NAT devices        March 2004

   technique [BIDIR] and is explored in detail by Takeda [SYM-STUN].
   The method works by analyzing the behavior of the NAT and attempting
   to predict the public port numbers it will assign to future sessions.
   Consider again the situation in which two clients, A and B, each
   behind a separate NAT, have each established UDP connections with a
   permanently addressable server S:

                                 Server S
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |     |   |     | |
        |                                                         |
        |                                                         |
      +--------------+                                 +-------------+
      | |                                 | |
      |              |                                 |             |
      |  Symmetric   |                                 |  Symmetric  |
      |    NAT A     |                                 |    NAT B    |
      +--------------+                                 +-------------+
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |      |   |      | |
        |                                                         |
     Client A                                                 Client B                             

   Figure 8: Use Peer's Symmetric-NAT Identity to predict P2P port

   NAT A has assigned its own UDP port 62000 to the communication
   session between A and S, and NAT B has assigned its port 31000 to
   the session between B and S. By communicating through server S, A
   and B learn each other's public IP addresses and port numbers as
   observed by S. Client A now starts sending UDP messages to port
   31001 at address (note the port number increment), and
   client B simultaneously starts sending messages to port 62001 at
   address If NATs A and B assign port numbers to new
   sessions sequentially, and if not much time has passed since the
   A-S and B-S sessions were initiated, then a working bi-directional
   communication channel between A and B should result. A's messages
   to B cause NAT A to open up a new session, to which NAT A will
   (hopefully) assign public port number 62001, because 62001 is next
   in sequence after the port number 62000 it previously assigned to
   the session between A and S. Similarly, B's messages to A will

Ford, Srisuresh & Kegel                                        [Page 18]

Internet-Draft    P2P communication across NAT devices        March 2004

   cause NAT B to open a new session, to which it will (hopefully)
   assign port number 31001. If both clients have correctly guessed
   the port numbers each NAT assigns to the new sessions, then a
   bi-directional UDP communication channel will have been
   established as shown below.

Ford, Srisuresh & Kegel                                        [Page 19]

Internet-Draft    P2P communication across NAT devices        March 2004

                                 Server S
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |     |   |     | |
        |                                                         |
        | ^ P2P Session (A-B)      ^   ^  P2P Session (B-A)     ^ |
        | |     |   |    | |
        | |     |   |     | |
        |                                                         |
      +--------------+                                 +-------------+
      | |                                 | |
      |              |                                 |             |
      |  Symmetric   |                                 |  Symmetric  |
      |    NAT A     |                                 |   NAT B     |
      +--------------+                                 +-------------+
        |                                                         |
        | ^ Relay-Req Session(A-S) ^   ^ Relay-Req Session(B-S) ^ |
        | |      |   |      | |
        | |      |   |      | |
        |                                                         |
        | ^ P2P Session (A-B)      ^   ^  P2P Session (B-A)     ^ |
        | |     |   |    | |
        | |      |   |     | |
        |                                                         |
     Client A                                                 Client B                              

   Figure 9: Use Port Prediction on Symmetric NATs to setup Direct-p2p

   Clearly, there are many things that can cause this trick to fail.
   If the predicted port number at either NAT already happens to be in
   use by an unrelated session, then the NAT will skip over that port
   number and the connection attempt will fail.  If either NAT sometimes
   or always chooses port numbers non-sequentially, then the trick will
   fail.  If a different client behind NAT A (or B respectively) opens
   up a new outgoing UDP connection to any external destination after A
   (B) establishes its connection with S but before sending its first
   message to B (A), then the unrelated client will inadvertently
   "steal" the desired port number.  This trick is therefore much less
   likely to work when either NAT involved is under load.

   Since in practice a P2P application implementing this trick would

Ford, Srisuresh & Kegel                                        [Page 20]

Internet-Draft    P2P communication across NAT devices        March 2004

   still need to work if the NATs are cone NATs, or if one is a cone NAT
   and the other is a symmetric NAT, the application would need to
   detect beforehand what kind of NAT is involved on either end [STUN]
   and modify its behavior accordingly, increasing the complexity of the
   algorithm and the general brittleness of the network.  Finally, port
   number prediction has no chance of working if either client is behind
   two or more levels of NAT and the NAT(s) closest to the client are
   symmetric.  For all of these reasons, it is NOT recommended that new
   applications implement this trick; it is mentioned here for
   historical and informational purposes.

3.5. Simultaneous TCP open

   There is a method that can be used in some cases to establish direct
   peer-to-peer TCP connections between a pair of nodes that are both
   behind existing NAT devices.  Most TCP sessions start with one
   endpoint sending a SYN packet, to which the other party responds with
   a SYN-ACK packet.  It is possible and legal, however, for two
   endpoints to start a TCP session by simultaneously sending each other
   SYN packets, to which each party subsequently responds with a
   separate ACK.  This procedure is known as a "simultaneous open."

   If a NAT device receives a TCP SYN packet from outside the private
   network attempting to initiate an incoming TCP connection, the
   NAT device will normally reject the connection attempt by either
   dropping the SYN packet or sending back a TCP RST (connection reset)
   packet.  If, however, the SYN packet arrives with source and
   destination addresses and port numbers that correspond to a TCP
   session that the NAT device believes is already active, then the
   NAT device will allow the packet to pass through.  In particular, if
   the NAT device has just recently seen and transmitted an outgoing SYN
   packet with the same addresses and port numbers, then it will
   consider the session active and allow the incoming SYN through.  If
   clients A and B can each correctly predict the public port number
   that its respective NAT device will assign the next outgoing TCP
   connection, and if each client initiates an outgoing TCP connection
   with the other client timed so that each client's outgoing SYN passes
   through its local NAT device before either SYN reaches the opposite
   NAT device, then a working peer-to-peer TCP connection will result.

   Unfortunately, this trick may be even more fragile and timing-
   sensitive than the UDP port number prediction trick described above.
   First, unless both NAT devices implement Cone NAT behavior on their
   TCP traffic, all the same things can go wrong with each side's
   attempt to predict the public port numbers that the respective NATs
   will assign to the new sessions.  In addition, if either client's
   SYN arrives at the opposite NAT device too quickly, then the remote
   NAT device may reject the SYN with a RST packet, causing the local

Ford, Srisuresh & Kegel                                        [Page 21]

Internet-Draft    P2P communication across NAT devices        March 2004

   NAT device in turn to close the new session and make future SYN
   retransmission attempts using the same port numbers futile. Finally,
   even though support for simultaneous open is technically a mandatory
   part of the TCP specification [TCP], it is not implemented correctly
   in some common operating systems. For this reason, this trick is
   mentioned here only for historical reasons. It is NOT recommended
   for use by applications. Applications that require efficient, direct
   peer-to-peer communication over existing NATs should use UDP.

4. Application design guidelines

4.1. What works with P2P NAT devices

  Since UDP hole punching is the most efficient existing method of
  establishing direct peer-to-peer communication between two nodes
  that are both behind NATs, and it works with a wide variety of
  existing NATs, it is recommended that applications use this
  technique if efficient peer-to-peer communication is required,
  but be prepared to fall back on simple relaying when direct
  communication cannot be established.

4.2. Peers behind the same NAT

  In practice there may be a fairly large number of users who
  have not two IP addresses, but three or more. In these cases,
  it is hard or impossible to tell which addresses to send to
  the registration server. The applications should send all its
  addresses, in such a case.

4.3. Peer discovery

  Applications sending packets to several addresses to discover
  which one is best to use for a given peer may become a
  significant source of 'space junk' littering the net, as the
  peer may have chosen to use routable addresses improperly as
  an internal LAN (e.g., which is assigned to the DOD).
  Thus applications should exercise caution when sending the
  speculative hello packets.

4.4. TCP applications using sockets API

  The socket API, used widely by application developers, is designed
  with client-server applications in mind. In its native form, only
  a single socket can bind to a TCP or UDP port. An application is
  not allowed to have multiple sockets binding to the same port
  (TCP or UDP) to initiate simultaneous sessions with multiple
  external nodes (or) use one socket to listen on the port and the

Ford, Srisuresh & Kegel                                        [Page 22]

Internet-Draft    P2P communication across NAT devices        March 2004

  other sockets to initiate outgoing sessions.

  The above single-socket-to-port bind restriction is not a problem
  however with UDP, because UDP is a datagram based protocol. UDP P2P
  application designers could use a single socket to send as well as
  receive datagrams from multiple peers using recvfrom() and sendto()

  This is not the case with TCP. With TCP, each incoming and outgoing
  connection is to be associated with a separate socket. In many of
  the Operating Systems (OS), sockets API addresses this problem with
  SO_REUSEADDR option on the socket or a OS specific SetReuseAddress
  call. Readers using the UNIX OS may refer [STEVENS] for additional
  details on socket options. Readers should also refer OS specific
  documentation for the API details. In summary, it is possible for a
  P2P application to use multiple sockets to reuse a TCP port. Say,
  open two TCP stream sockets bound to the same port, do a listen()
  on one and a connect() from the other.

4.5. Use of midcom protocol

  If the applications know the NAT devices they would be traversing
  and these NAT devices implement the midcom protocol ([MIDCOM]),
  applications could use the midcom protocol to ease their way through
  the NAT devices.

  For example, P2P applications require that NAT devices preserve
  endpoint port bindings. If midcom is supported on the NAT devices,
  P2P applications can exercise control over port binding (or address
  binding) parameters such as lifetime, maxidletime, and
  directionality so the applications can both connect to external
  peers as well as receive connections from external peers; and do
  not need to send periodic keep-alives to keep the port binding
  alive. When the application no longer needs the binding, the
  application could simply dismantle the binding, also using the
  midcom protocol.

5. NAT Design Guidelines

   This section discusses considerations in the design of network
   address translators, as they affect peer-to-peer applications.

5.1. Deprecate the use of symmetric NATs

   Symmetric NATs gained popularity with client-server applications
   such as web browsers, which only need to initiate outgoing
   connections. However, in the recent times, P2P applications such

Ford, Srisuresh & Kegel                                        [Page 23]

Internet-Draft    P2P communication across NAT devices        March 2004

   as Instant messaging and audio conferencing have been in wide
   use. Symmetric NATs do not support the concept of retaining
   endpoint identity and are not suitable for P2P applications.
   Deprecating symmetric NATs is recommended in order to support
   P2P applications.

   A P2P-NAT must implement Cone NAT behavior, allowing
   applications to establish robust P2P connectivity using the
   UDP hole punching technique. Ideally, a P2P-NAT should also
   allow applications to make P2P connections via both TCP and

5.2. Add incremental Cone NAT support to Symmetric NAT devices

   One way for a symmetric NAT device to extend support to P2P
   applications would be to divide its assignable port
   namespace, reserving a portion of its ports for one-to-one
   sessions and a different set of ports for one-to-many

   Further, a NAT device may be explicitly configured with
   applications and hosts that need the P2P feature, so the
   NAT device can auto magically assign a P2P port from the
   right port block.

5.3. Support Address and port bindings

   The primary and most important recommendation of this document for
   NAT designers is that they maintain address and/or port
   bindings in their NAT implementations. When a node on the
   private network initiates connection to a new external
   destination, using the same source IP address and TCP/UDP port as
   an existing translated TCP/UDP session, the NAT should ensure
   that the new TCP/UDP session reuses the address/port binding
   of the existing session.

5.3.1. Preserving port numbers

   Some NATs, when establishing a new UDP session, attempt to assign the
   same public port number as the corresponding private port number, if
   that port number happens to be available.  For example, if client A
   at address initiates an outgoing UDP session with a datagram
   from port number 1234, and the NAT's public port number 1234 happens
   to be available, then the NAT uses port number 1234 at the NAT's
   public IP address as the translated endpoint address for the session.
   This behavior might be beneficial to some legacy UDP applications
   that expect to communicate only using specific UDP port numbers, but
   it is not recommended that applications depend on this behavior since

Ford, Srisuresh & Kegel                                        [Page 24]

Internet-Draft    P2P communication across NAT devices        March 2004

   it is only possible for a NAT to preserve the port number if at most
   one node on the internal network is using that port number.

   In addition, a NAT should NOT try to preserve the port number in a
   new session if doing so would conflict with an existing port
   binding. For example, suppose client A at internal port 1234 has
   established a session with external server S, and NAT A has created
   a port binding to public port 62000, because public port number
   1234 on the NAT was not available at the time. Now, suppose port
   number 1234 on the NAT subsequently becomes available, and while the
   session between A and S is still active, client A initiates a new
   session from the same internal port (1234) to a different external
   node B. In this case, because a port binding has already been
   established between client A's port 1234 and the NAT's public port
   62000, this binding should be preserved and the new session should
   reuse the port binding (to port 62000).  The NAT should not assign
   public port 1234 to this new session just because port 1234 has
   become available. Such a behavior would not be likely to benefit the
   application in any way since the application has already been
   operating with a translated port number, and it would break any
   attempts the application might make to establish peer-to-peer
   connections using the UDP hole punching technique.

5.3.2. Support TCP port bindings

   Cone NAT implementers should maintain port bindings for TCP
   sessions just as with UDP sessions. TCP port bindings on a
   Cone NAT will increase the NAT's ability to support P2P TCP
   application deployment.

5.4. Large timeout for P2P applications

   We recommend the NAT device implementers to use a minimum timeout
   of, say, 5 minutes (300 seconds) for P2P applications, i.e.,
   configure the NAT device with this idle-timeout for the port
   bindings for the ports set aside for P2P use. NAT device
   implementers are often tempted to use a shorter one, as they are
   accustomed to doing currently. But, short timeouts are
   problematic. Consider a P2P application that involved 16 peers.
   They will flood the network with keepalive packets every 10
   seconds to avoid NAT timeouts.  This is so because one might
   send them 5 times as often as the NAT device's timeout just in
   case the keepalives are dropped in the network.

5.5. Support loopback translation

   We strongly recommend that NAT implementers support
   loopback translation, allowing hosts behind a NAT device to

Ford, Srisuresh & Kegel                                        [Page 25]

Internet-Draft    P2P communication across NAT devices        March 2004

   communicate with other hosts behind the same NAT device through
   their public, possibly translated endpoints. Support for
   loopback translation is particularly important in the case
   of large-capacity NATs that are likely to be deployed as the
   first level of a multi-level NAT scenario. As described in
   section 3.3.3, hosts behind the same first-level NAT but
   different second-level NATs have no way to communicate with
   each other by UDP hole punching, even if all the NAT devices
   preserve endpoint identities, unless the first-level NAT
   also supports loopback translation.

5.6. Support midcom protocol

   We recommend that NAT implementers support midcom protocol,
   the details of which are currently in specification stage.
   Readers may refer the midcom working group [MIDCOM] to monitor
   the status of protocol specification. Support for midcom
   protocol in NAT devices will provide substantial additional
   flexibility for the P2P applications to control NAT
   resources effectively. Readers may refer section 4.5 on how
   P2P applications can benefit from NAT devices supporting
   midcom protocol.

6. Security Considerations

   Following the recommendations in this document should not
   inherently create new security issues, for either the
   applications or the NAT devices. Nevertheless, new security
   risks may be created if the techniques described here are
   not adhered to with sufficient care. This section describes
   security risks the applications could inadvertently create
   in attempting to support P2P communication across NAT devices,
   and implications for the security policies of P2P-friendly
   NAT devices.

6.1. IP address aliasing

   P2P applications must use appropriate authentication mechanisms
   to protect their P2P connections from accidental confusion with
   other P2P connections as well as from malicious connection
   hijacking or denial-of-service attacks. NAT-friendly P2P
   applications effectively must interact with multiple distinct
   IP address domains, but are not generally aware of the exact
   topology or administrative policies defining these address
   domains.  While attempting to establish P2P connections via
   UDP hole punching, applications send packets that may frequently
   arrive at an entirely different host than the intended one.

Ford, Srisuresh & Kegel                                        [Page 26]

Internet-Draft    P2P communication across NAT devices        March 2004

   For example, many consumer-level NAT devices provide DHCP
   services that are configured by default to hand out site-local
   IP addresses in a particular address range. Say, a particular
   consumer NAT device, by default, hands out IP addresses starting
   with Most private home networks using that NAT
   device will have a host with that IP address, and many of these
   networks will probably have a host at address as
   well. If host A at address on one private network
   attempts to establish a connection by UDP hole punching with
   host B at on a different private network, then as
   part of this process host A will send discovery packets to
   address on its local network, and host B will send
   discovery packets to address on its network. Clearly,
   these discovery packets will not reach the intended machine since
   the two hosts are on different private networks, but they are very
   likely to reach SOME machine on these respective networks at the
   standard UDP port numbers used by this application, potentially
   causing confusion. especially if the application is also running
   on those other machines and does not properly authenticate its

   This risk due to aliasing is therefore present even without a
   malicious attacker. If one endpoint, say host A, is actually
   malicious, then without proper authentication the attacker could
   cause host B to connect and interact in unintended ways with
   another host on its private network having the same IP address
   as the attacker's (purported) private address. Since the two
   endpoint hosts A and B presumably discovered each other through
   a public server S, and neither S nor B has any means to verify
   A's reported private address, all P2P applications must assume
   that any IP address they find to be suspect until they successfully
   establish authenticated two-way communication.

6.2. Denial-of-service attacks

   P2P applications and the public servers that support them must
   protect themselves against denial-of-service attacks, and ensure
   that they cannot be used by an attacker to mount denial-of-service
   attacks against other targets. To protect themselves, P2P
   applications and servers must avoid taking any action requiring
   significant local processing or storage resources until
   authenticated two-way communication is established. To avoid being
   used as a tool for denial-of-service attacks, P2P applications and
   servers must minimize the amount and rate of traffic they send to
   any newly-discovered IP address until after authenticated two-way
   communication is established with the intended target.

Ford, Srisuresh & Kegel                                        [Page 27]

Internet-Draft    P2P communication across NAT devices        March 2004

   For example, P2P applications that register with a public rendezvous
   server can claim to have any private IP address, or perhaps multiple
   IP addresses. A well-connected host or group of hosts that can
   collectively attract a substantial volume of P2P connection attempts
   (e.g., by offering to serve popular content) could mount a
   denial-of-service attack on a target host C simply by including C's
   IP address in their own list of IP addresses they register with the
   rendezvous server. There is no way the rendezvous server can verify
   the IP addresses, since they could well be legitimate private
   network addresses useful to other hosts for establishing
   network-local communication. The P2P application protocol must
   therefore be designed to size- and rate-limit traffic to unverified
   IP addresses in order to avoid the potential damage such a
   concentration effect could cause.

6.3. Man-in-the-middle attacks

   Any network device on the path between a P2P client and a
   rendezvous server can mount a variety of man-in-the-middle
   attacks by pretending to be a NAT.  For example, suppose
   host A attempts to register with rendezvous server S, but a
   network-snooping attacker is able to observe this registration
   request. The attacker could then flood server S with requests
   that are identical to the client's original request except with
   a modified source IP address, such as the IP address of the
   attacker itself.  If the attacker can convince the server to
   register the client using the attacker's IP address, then the
   attacker can make itself an active component on the path of all
   future traffic from the server AND other P2P hosts to the
   original client, even if the attacker was originally only able
   to snoop the path from the client to the server.

   The client cannot protect itself from this attack by
   authenticating its source IP address to the rendezvous server,
   because in order to be NAT-friendly the application must allow
   intervening NATs to change the source address silently.  This
   appears to be an inherent security weakness of the NAT paradigm.
   The only defense against such an attack is for the client to
   authenticate and potentially encrypt the actual content of its
   communication using appropriate higher-level identities, so that
   the interposed attacker is not able to take advantage of its
   position.  Even if all application-level communication is
   authenticated and encrypted, however, this attack could still be
   used as a traffic analysis tool for observing who the client is
   communicating with.

6.4. Impact on NAT device security

Ford, Srisuresh & Kegel                                        [Page 28]

Internet-Draft    P2P communication across NAT devices        March 2004

   Designing NAT devices to preserve endpoint identities does not
   weaken the security provided by the NAT device. For example, a
   Port-Restricted Cone NAT is inherently no more "promiscuous"
   than a Symmetric NAT in its policies for allowing either
   incoming or outgoing traffic to pass through the NAT device.
   As long as outgoing UDP sessions are enabled and the NAT device
   maintains consistent binding between internal and external
   UDP ports, the NAT device will filter out any incoming UDP packets
   that do not match the active sessions initiated from within the
   enclave. Filtering incoming traffic aggressively while maintaining
   consistent port bindings thus allows a NAT device to be
   "peer-to-peer friendly" without compromising the principle of
   rejecting unsolicited incoming traffic.

   Maintaining consistent port binding could arguably increase the
   predictability of traffic emerging from the NAT device, by revealing
   the relationships between different UDP sessions and hence about
   the behavior of applications running within the enclave. This
   predictability could conceivably be useful to an attacker in
   exploiting other network or application level vulnerabilities.
   If the security requirements of a particular deployment scenario
   are so critical that such subtle information channels are of
   concern, however, then the NAT device almost certainly should not be
   configured to allow unrestricted outgoing UDP traffic in the
   first place. Such a NAT device should only allow communication
   originating from specific applications at specific ports, or
   via tightly-controlled application-level gateways.  In this
   situation there is no hope of generic, transparent peer-to-peer
   connectivity across the NAT device (or transparent client/server
   connectivity for that matter); the NAT device must either
   implement appropriate application-specific behavior or disallow
   communication entirely.

7. Acknowledgments

   The authors wish to thank Henrik, Dave, and Christian Huitema
   for their valuable feedback.

8. References

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

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

Ford, Srisuresh & Kegel                                        [Page 29]

Internet-Draft    P2P communication across NAT devices        March 2004

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

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

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

[NAT-PT]   G. Tsirtsis and P. Srisuresh, "Network Address
           Translation - Protocol Translation (NAT-PT)", RFC 2766,
           February 2000.

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

[STEVENS]  W. Richard Stevens, "UNIX Network Programming, Volume 1,
           Second Edition: Networking APIs: Sockets and XTI",
           Prentice Hall, 1998.

[BIDIR]    Peer-to-Peer Working Group, NAT/Firewall Working Committee,
           "Bidirectional Peer-to-Peer Communication with Interposing
           Firewalls and NATs", August 2001.

[KEGEL]    Dan Kegel, "NAT and Peer-to-Peer Networking", July 1999.

[TCP]      "Transmission Control Protocol", RFC 793, September 1981.

[MIDCOM]   Middlebox Communication (midcom) working group,

[TURN]     J. Rosenberg, J. Weinberger, R. Mahy, and C. Huitema,
           "Traversal Using Relay NAT (TURN)",
           draft-rosenberg-midcom-turn-01 (Work In Progress),
           March 2003.

9. Author's Address

   Bryan Ford
   Laboratory for Computer Science
   Massachusetts Institute of Technology

Ford, Srisuresh & Kegel                                        [Page 30]

Internet-Draft    P2P communication across NAT devices        March 2004

   77 Massachusetts Ave.
   Cambridge, MA 02139
   Phone: (617) 253-5261
   E-mail: baford@mit.edu
   Web: http://www.brynosaurus.com/

   Pyda Srisuresh
   Caymas Systems, Inc.
   11799-A North McDowell Blvd.
   Petaluma, CA 94954
   Phone: (707) 283-5063
   E-mail: srisuresh@yahoo.com

   Dan Kegel
   901 S. Sycamore Ave.
   Los Angeles, CA 90036
   Phone: 323 931-6717
   Email: dank@kegel.com
   Web: http://www.kegel.com/

Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an

Ford, Srisuresh & Kegel                                        [Page 31]

Internet-Draft    P2P communication across NAT devices        March 2004


Ford, Srisuresh & Kegel                                        [Page 32]