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BEHAVE                                                           B. Ford
Internet-Draft                                                    M.I.T.
Expires: April 23,  2006                                    P. Srisuresh
                                                              Consultant
                                                                D. Kegel
                                                               kegel.com
                                                            October 2005


              Application Design Guidelines for Traversal
                  through Network Address Translators
                  <draft-ford-behave-app-01.txt>


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Abstract

   This document defines best current practices by which application
   designers can create applications that communicate reliably and
   efficiently in the presence of Network Address Translators (NAT),
   particularly when the application has a need for "peer-to-peer"-
   style (P2P) communication patterns in addition to traditional
   client/server-style communication. There is no single NAT traversal
   algorithm that will make a P2P application reliably work over all
   deployed NATs. The guidelines described in this document allow a P2P



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   application to work reliably over a majority of existing NATs, as
   well as all future NATs that conform to the behave requirements
   specified in companion documents. The NAT traversal techniques
   described here do not require the use of special proxy or relay
   protocols, do not require specific knowledge about the network
   topology or the number and type of NATs in the path, and do not
   require any modifications to IP or transport-layer protocols
   on the end hosts.

Table of Contents

   1.  Introduction and Scope........................................
       1.1. BEHAVE-compliant versus Legacy NATs .....................
   2.  General NAT Traversal Concepts ...............................
       2.1. NAT Functions Influencing Traversal Logic ...............
       2.2. Communication Between Peers Behind Distinct NATs ........
       2.3. Short-Circuiting Sessions on Private Networks ...........
       2.4. Authenticating Peer-to-Peer Connections .................
       2.5. NAT Behavior Detection ..................................
   3.  NAT Traversal for UDP ........................................
       3.1. UDP Idle Timeouts .......................................
   4.  NAT Traversal for TCP ........................................
       4.1.  Ensuring Robustness ....................................
   5.  Summary of Requirements ......................................
   6.  Security Considerations ......................................
       6.1. Denial-of-service attacks ...............................
       6.2. Man-in-the-middle attacks ...............................
   7.  Normative references .........................................
   8.  Informative references .......................................


1. Introduction and Scope

   The present-day Internet has seen the ubiquitous deployment of
   network address translators (NAT), driven by a variety of practical
   challenges such as security and the ongoing depletion of the IPv4
   address space.  The asymmetric addressing and connectivity regimes
   established by NATs, however, cause problems for many applications,
   particularly those such as teleconferencing [SIP, H.323] and
   multiplayer on-line gaming systems.  Such application protocols
   require "peer-to-peer" communication directly between arbitrary
   hosts, and not just traditional "client/server" communication between
   an arbitrary "client" host and a "well-known" server with a global IP
   address and DNS name.  RFC 3235 [NAT-APPL] already proposes some NAT-
   friendly design guidelines for applications, but merely recommends
   against using peer-to-peer communication without providing a
   workable solution to this problem.




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   Given the increasing demand for applications that require P2P
   communication, in conjunction with the ubiquity of NATs, applications
   are increasingly implementing and deploying various workarounds to
   this problem.  Most of these workarounds take the form of a NAT
   traversal or "hole punching" algorithm, by which two "peers" lying
   behind one or more NATs cooperate with a well-known "rendezvous
   server" to set up a direct peer-to-peer communication path between
   them.  The rendezvous server is crucial to the initial path setup but
   does not take part in the subsequent peer-to-peer data stream.

   There are many different NAT traversal algorithms already in use and
   currently being explored. However, due to the lack of standardization
   for NAT behavior up to this point, none of these algorithms can be
   guaranteed to work reliably over all currently deployed NATs.
   Further, without standardization of NAT traversal algorithms there
   is a strong danger that the proliferation of traversal algorithms
   may further compound the reliability and predictability problems
   that NAT created in the first place.

   This document focuses exclusively on NAT traversal techniques that do
   not require the application to communicate explicitly with the NATs
   in the path.  Protocols that allow applications to obtain external
   communication endpoints through explicit interaction with NATs in the
   path are outside the scope of this document.  Several such protocols
   exist and are documented elsewhere [SOCKS, RSIP, MIDCOM, UPNP], but
   so far none of these protocols have become widely accepted.

   This document defines a set of best current practices for
   implementing NAT traversal in applications.  The specific
   recommendations are described at length in the sections below and
   later summarized concisely in Section 5.

1.1. BEHAVE-compliant versus Legacy NATs

   The NAT traversal practices set forth here are relatively simple to
   implement and are guaranteed to work reliably across all
   BEHAVE-compliant NATs, BEHAVE-compliant NATs are those NAT devices
   that conform to the behavioral requirements set out in the
   companion documents [BEH-TCP, BEH-UDP, BEH-ICMP]. These techniques
   are also known to work in practice with a wide variety of existing
   NATs in the most common interconnection scenarios, even when those
   NATs are only partially BEHAVE-compliant.

   To be considered a "BEHAVE-compliant application", an application
   MUST be designed to operate reliably when all NATs
   in its communication paths are BEHAVE-compliant.  It is also
   RECOMMENDED that new applications assume that all NATs in the path
   are BEHAVE-compliant, since non-BEHAVE-compliant NATs are expected



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   to be deprecated quickly. Adding complexity to applications for
   the purpose of handling legacy NATs risks introducing additional
   unpredictability into the network.

   This document does not specifically prohibit applications from
   implementing more elaborate NAT traversal algorithms that may
   function over a wider variety of non-BEHAVE-compliant, "legacy"
   NATs. Some known techniques for operating over such poorly-behaved
   NATs are outlined briefly in the companion document [BEH-STATE],
   and are described more thoroughly in [NUTSS, P2PNAT, NATBLAST,
   NATTRAV]. Applications implementing fancier protocols such as
   these, however, MUST ensure that their traversal algorithms
   operate just as efficiently as the ones specified here over
   BEHAVE-compliant NATs, and do not create new security
   vulnerabilities or unnecessarily burden network components in
   the path.

   REQ-1  Applications MUST be designed to operate reliably over BEHAVE-
          compliant NATs.  New applications are RECOMMENDED to assume
          that all NATs in the path are BEHAVE-compliant.

2. General NAT Traversal Concepts

   This section describes requirements and techniques for NAT traversal
   that are independent of transport protocol; subsequent sections will
   specifically address NAT traversal for the UDP and TCP transport
   protocols.  For more detailed background information on current
   practices in use by existing applications, please refer to the
   companion document [BEH-STATE].

2.1. NAT Functions Influencing Traversal Logic

   Commonly-deployed NATs integrate two distinct logical functions,
   each of which interferes with peer-to-peer communication in a
   different way and thus requires NAT traversal support in
   applications.

   Address Translation:

      A NAT modifies the IP-level and often the transport-level header
      information in packets flowing across the boundary, in order to
      enable many "private" hosts behind the NAT to share the use of a
      smaller number of public IP addresses (often just one).  Hosts
      behind the NAT therefore usually have no unique, permanently
      usable address on the public Internet, but instead can only
      communicate through temporary public endpoints that the NAT
      assigns them dynamically as a result of communication attempts
      initiated by hosts on the private network.



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      When two hosts reside on two different private networks behind
      distinct NATs, neither of them has a permanent address that the
      other can reach at any time, so in order to establish peer-to-peer
      connections the hosts must rely on the temporary public endpoints
      their NATs assign them as a result of prior outgoing
      client/server-style communication sessions.  Discovering,
      exchanging, and using these temporary public endpoints generally
      requires that the two hosts first collaborate through a well-known
      server on the public Internet that both hosts can reach, as
      described below.

   Filtering of Unsolicited Traffic:

      The filtering function in a Traditional NAT device restricts
      communication between a private internal network and the public
      Internet by dropping incoming sessions that are deemed
      "unsolicited". All packets arriving from the public Internet
      are dropped unless they are part of an existing communication
      session that was previously initiated by a host on the private
      network.

      When two hosts reside on two different private networks behind
      distinct NATs, an attempt by either host to initiate a peer-to-
      peer connection to the other will usually fail, even if the
      connection attempt is directed to the correct temporary public
      endpoint assigned by the opposite host's NAT, because the opposite
      host's NATs will interpret this attempt as unsolicited incoming
      traffic and reject it.  NAT traversal in this case requires the
      two hosts to cooperate, typically by communicating initially
      through a well-known server on the public Internet that they can
      both reach, so as to make their peer-to-peer connection appear to
      each host's NAT as if it was initiated from within that host's own
      private network.

   The cooperation of two hosts to create a peer-to-peer connection
   across NATs does not constitute a violation of the filtering policy
   imposed by the NAT. Network firewall functionality in general is
   outside the scope of this document, and this document does not
   condone any attempts by application developers to subvert security
   policies that may be imposed by NATs or firewalls.

2.2. Communication Between Peers Behind Distinct NATs

   Although the details of NAT traversal vary from one transport
   protocol to another depending on how NATs recognize and handle
   sessions for that transport, the basic approach to NAT traversal is
   transport-independent.  We merely assume for now that each transport



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   uses session endpoints consisting of an (IP address, port number)
   pair to identify and differentiate communication sessions, and that
   each communication session is uniquely identified by its two
   endpoints.  We focus here specifically on the one NAT traversal
   algorithm recommended here for new applications.

   Suppose client hosts A and B both have private IP addresses and lie
   behind different NATs, as shown below.

                                 Server S
                              18.181.0.31:1234
                                     |
        +----------------------------+----------------------------+
        |                                                         |
        | ^ Registry Session(A-S) ^     ^ Registry Session(B-S) ^ |
        | |  18.181.0.31:1234     |     |  18.181.0.31:1234     | |
        | | 155.99.25.11:62000    |     |  138.76.29.7:31000    | |
        |                                                         |
        | ^ P2P Session (A-B)     ^     ^  P2P Session (B-A)    ^ |
        | |  138.76.29.7:31000    |     |  155.99.25.11:62000   | |
        | | 155.99.25.11:62000    |     |  138.76.29.7:31000    | |
        |                                                         |
    +------------------+                          +------------------+
    | 155.99.25.11     |                          |     138.76.29.7  |
    |                  |                          |                  |
    | BEHAVE-compliant |                          | BEHAVE-compliant |
    | NAT A            |                          | NAT B            |
    +------------------+                          +------------------+
        |                                                         |
        | ^ Registry Session(A-S) ^     ^ Registry Session(B-S) ^ |
        | |  18.181.0.31:1234     |     |  18.181.0.31:1234     | |
        | |     10.0.0.1:1234     |     |     10.1.1.3:1234     | |
        |                                                         |
        | ^ P2P Session (A-B)     ^     ^  P2P Session (B-A)    ^ |
        | |  138.76.29.7:31000    |     |  155.99.25.11:62000   | |
        | |     10.0.0.1:1234     |     |      10.1.1.3:1234    | |
        |                                                         |
     Client A                                                 Client B
     10.0.0.1:1234                                        10.1.1.3:1234

   Figure 1: Simultaneous outgoing sessions to accomplish direct-P2P


   A peer-to-peer application running on clients A and B, and also on a
   well-known rendezvous server S, each use port number 1234 at their
   own IP address to form their primary local communication endpoint.  A
   and B have each initiated communication sessions from their local
   endpoint server S's endpoint.  As a result of A's outgoing connection



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   attempt to S, NAT A has dynamically assigned port 62000 at its own
   public IP address, 155.99.25.11, to A's session with S, so that S
   sees this session as having been initiated from the endpoint
   155.99.25.11:62000, rather than from A's original private endpoint of
   10.0.0.1:1234.  Similarly, B's outgoing connection to S causes NAT B
   to assign port 31000 at its own IP address to B's session with S,
   thus forming B's public endpoint of 138.76.29.7:31000.

   Now suppose that host A wants to establish a communication session
   directly with host B. If A just naively initiates a new
   communication session to the endpoint B believes itself to be using,
   namely 10.1.1.3:1234, then A's connection attempt will either reach
   the wrong host - a different host on A's own private network that
   happens to have the private IP address 10.1.1.3 - or it will reach no
   host at all, because B's private IP address 10.1.1.3 is not routable
   over the Internet.

   Even if A learns B's temporary public endpoint, 138.76.29.7:31000,
   from server S, and attempts to initiate a communication session to
   that destination endpoint, NAT B may reject this attempt because
   the IP packet's source and destination endpoints do not match
   those of an existing session previously initiated from within the
   private network. The destination endpoint of A's connection attempt
   to B matches the source endpoint of B's existing session with S,
   but the source endpoint of A's connection attempt is of course
   different. Similarly, if B makes a unilateral connection attempt
   to A's public endpoint, NAT A may similarly reject B's attempt.
   As it turns out, this difficulty could arise even if NAT-A and
   NAT-B are replaced by firewalls with the standard filtering
   policy of rejecting unsolicited incoming communication attempts.

   In order to operate reliably across NATs and firewalls that reject
   unsolicited incoming communication, the client hosts A and B
   collaborate with an external server S to learn each other's public
   AND private endpoints, and then each of the two client hosts
   initiate "approximately simultaneous" connection attempts from their
   existing primary local endpoints (the same local endpoints they used
   previously for the connection to S), and directed at all of the known
   endpoints (public and private) for the other host.  In the scenario
   illustrated above, A's connection attempt to B's public endpoint is
   interpreted by NAT A as a legitimate, outgoing session whose private
   source endpoint (10.0.0.1:1234) is the same as that of A's existing
   session with S, but whose public destination endpoint
   (138.76.29.7:31000) is different.  If NAT A is BEHAVE-compliant, it
   will translate A's private source endpoint for this new session in
   the same way that it did for A's existing session with S, so that the
   new session appears on the public Internet to be a session between
   A's public endpoint, 155.99.25.11:62000, and B's public endpoint,



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   138.76.29.7:31000.

   In similar fashion, B's "approximately simultaneous" connection
   attempt from its private endpoint, 10.1.1.3:1234, to A's public
   endpoint, 155.99.25.11:62000, results in NAT B opening a new
   translation session that reuses the existing public endpoint for B,
   138.76.29.7:31000, which NAT B previously assigned to B's session
   with S.  NAT B is now set up to allow communication between A's
   public endpoint and B's private endpoint, and on the public Internet
   this session has the endpoints 155.99.25.11:62000 and
   138.76.29.7:31000, the same as the endpoints of the session that A
   initiated above toward B's public endpoint.  Both NATs are thus set
   up to permit communication between these two public endpoints,
   translating and forwarding the traffic comprising this session to the
   respective client hosts on the private networks as appropriate.

   This traversal technique relies on certain aspects of NAT behavior
   described fully in the companion documents [BEH-GEN, BEH-TCP, BEH-
   UDP].  The technique also relies on the transport protocol allowing a
   connection to be initiated actively by two endpoints, rather than
   asymmetrically in traditional client/server fashion.  Fortunately
   both of the two ubiquitous transports, TCP and UDP, allow symmetric
   connection initiation in this way.

   REQ-2  Application hosts wishing to establish peer-to-peer
          communication MUST support NAT traversal by implementing the
          "approximately simultaneous connection attempts" technique
          described above with the help of a mutually accessible
          registry or "rendezvous server" in the external network.

2.3. Short-Circuiting Sessions on Private Networks

   Although the network topology illustrated in the figure above is
   typical of the situation seen by P2P applications, it is by no means
   the only possible scenario.  Only one of the client hosts may be
   behind, or one or more of the clients may be located behind two or
   more levels of NATs, any number of which may be shared between the
   two clients.  The general NAT traversal algorithm described above
   will work reliably in all of the common topological scenarios
   provided that the NATs involved are BEHAVE-compliant. One other
   particularly common scenario is worth special consideration however.
   In the situation illustrated below the two clients (probably
   unknowingly) happen to reside behind the same NAT, and are therefore
   located in the same private IP address space.







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                                Server S
                            18.181.0.31:1234
                                   |
        ^ Registry Session(A-S) ^  | ^ Registry Session(B-S) ^
        |  18.181.0.31:1234     |  | |  18.181.0.31:1234     |
        | 155.99.25.11:62000    |  | |  155.99.25.11:62001   |
                                   |
                           +------------------+
                           |   155.99.25.11   |
                           |                  |
                           | Behave-Compliant |
                           |       NAT        |
                           +------------------+
                                    |
      +-----------------------------+----------------------------+
      |                                                          |
      |                                                          |
      | ^ Registry Session(A-S) ^      ^ Registry Session(B-S) ^ |
      | |  18.181.0.31:1234     |      |  18.181.0.31:1234     | |
      | |     10.0.0.1:1234     |      |     10.1.1.3:1234     | |
      |                                                          |
      | ^ P2P Session-try1(A-B) ^      ^ P2P Session-try1 (B-A)^ |
      | |     10.1.1.3:1234     |      |     10.0.0.1:1234     | |
      | |     10.0.0.1:1234     |      |     10.1.1.3:1234     | |
      |                                                          |
      | ^ P2P Session-try2(A-B) ^      ^  P2P Session-try2(B-A)^ |
      | | 155.99.25.11:62001    |      |  155.99.25.11:62000   | |
      | |     10.0.0.1:1234     |      |     10.1.1.3:1234     | |
      |                                                          |
   Client A                                                   Client B
   10.0.0.1:1234                                         10.1.1.3:1234

   Figure 2: Register private identity & NAT identity with Relay server


   In this scenario, client A has established a session with well-known
   server S as before, 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.  Suppose that
   A and B use the NAT traversal technique outlined above to establish a
   communication channel using server S as an introducer.  If A and B
   only attempt simultaneous connections to each other's public
   endpoints, 155.99.25.11:62001 and 155.99.25.11:62000 respectively,
   then their connection attempts will succeed only if the NAT supports
   hairpin translation, as described in [BEH-STATE, BEH-TOP].  Although
   hairpin translation is required for a NAT to be considered fully
   BEHAVE-compliant, this feature is not yet widely supported by



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   commonly deployed NATs at the time of this writing.  Additionally,
   the resulting connection between A and B will be sub-optimal in this
   case because all traffic will unnecessarily pass through and be
   translated by the NAT, whereas the two endpoint hosts are perfectly
   capable of communicating directly on their common IP network without
   the NAT intervention.

   To address this problem, it is recommended that P2P applications make
   simultaneous connection attempts both to the private endpoints and
   the public endpoints of the desired peer, and use whichever
   connection succeeds first in establishing bi-directional
   communication between the correct peers. If the two client hosts
   happen to be located in the same private network, their connection
   attempt using each others' private endpoints is likely to succeed
   first because it follows a shorter network path not involving the
   NAT. If the NAT does not support hairpin translation, the connection
   attempts using the hosts' private endpoints will be the only one to
   succeed.

   REQ-3  Applications implementing NAT traversal MUST exchange their
          local IP addresses and transport-level port numbers as known
          to themselves, in addition to the global IP address and
          port number tuples they register with the rendezvous server.
          Further, peering applications MUST make "approximately
          simultaneous" connection attempts to each other using both
          private and external communication endpoints. In addition,
          applications SHOULD exchange and make connection attempts
          to multiple endpoint addresses for each peer, in case the
          hosts have multiple IP addresses or are registered with
          multiple rendezvous servers at different locations in the
          network topology.

2.4. Authenticating Peer-to-Peer Connections

   It is extremely important not only for security but also for general
   robustness that applications implementing a NAT traversal protocol
   authenticate any peer-to-peer connections they establish, using some
   higher-level application-specific notion of host or user identity.
   To operate reliably and securely, applications MUST consider any IP
   addresses and port numbers they use for communication with other
   hosts to be merely "locators" for hosts, serving as hints indicating
   how the desired host might be reached, and not as a reliable
   "identifier" for the target host or user.

   Applications MUST NOT in particular merely assume that the first
   communication attempt that establishes transport-level connectivity
   and elicits a response from a particular target endpoint (IP address
   and port number) necessarily represents a connection to the desired



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   host.  Consider the following topological scenario, for example,
   which is in fact extremely common in today's Internet.

                                Server S
                              18.181.0.31:1234
                                     |
        +----------------------------+----------------------------+
        |                                                         |
        |                                                         |
    +------------------+                          +------------------+
    | 155.99.25.11     |                          |     138.76.29.7  |
    |                  |                          |                  |
    | BEHAVE-compliant |                          | BEHAVE-compliant |
    | NAT A            |                          | NAT B            |
    +------------------+                          +------------------+
                |                                          |
    -------+----+-----------+                              |
           |                |                              |
       Client X         Client A                       Client B
       10.1.1.10:1234   10.0.0.11:1234                 10.1.1.10:1234

   Figure 3: Clients behind different NATs can bear same local endpoint

   In this scenario, suppose that NAT A and NAT B are both "off-the-
   shelf" consumer NAT routers from the same vendor, which the vendor
   has configured by default to act as DHCP servers that hand out
   private IP addresses starting at 10.1.1.10.  (Most users of such
   devices know little or nothing about IP addresses, and therefore are
   very unlikely to reconfigure their NATs any more than is necessary to
   get them to connect to the Internet.)  As before, Client A wishes to
   establish a peer-to-peer connection with Client B with the help of
   Server S.  Client A happened to receive private IP address 10.1.1.11
   on NAT A's private network, after Client X had already been assigned
   private IP address 10.1.1.10. Client B happens to be the only host
   on NAT B's private network, and thus received the first available
   private IP address, 10.1.1.10. Client X happens to be running the
   same P2P application as is running on clients A and B, and thus has
   port 1234 allocated and ready to initiate and accept peer-to-peer
   connections.

   Suppose Client A follows the NAT traversal approach described above
   to establish a peer-to-peer session with Client B.  As per the
   suggested protocol, A and B each make approximately simultaneous
   connection attempts both to each other's public and private
   endpoints.  B's connection attempt to A's private endpoint,
   10.1.1.11:1234, will of course fail because there is no host
   10.1.1.11 on NAT B's private network and that IP address is not
   globally routable.  A's connection attempt to B's public endpoint and



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   B's connection attempt to A's public endpoint will eventually succeed
   in establishing the desired peer-to-peer connection if the two NATs
   are BEHAVE-compliant.  However, A's connection attempt to B's private
   endpoint, 10.1.1.10:1234, will succeed at the transport layer but
   connect to the wrong host: namely client X, the host on NAT A's
   private network that happens to have the same private IP address as B
   does on NAT B's network.  Furthermore, this bogus connection to
   client X is likely to succeed much more quickly than the actually
   desired connection to client B, because X is on the same private
   network as A.  If the application running on client A does not
   properly authenticate its peer-to-peer connections using some higher-
   level notion of identity that is independent of IP address, then
   client A is likely to assume that its transport-level connection to X
   is the desired peer-to-peer connection, cancel its attempt to connect
   to B's public endpoint, and subsequently become very confused when
   the peer it connected to fails to behave like client B.

   Given the prevalence of NAT routers that are pre-configured by their
   vendors to hand out private IP addresses via DHCP in more-or-less
   deterministic fashion from a standard private IP address block,
   different hosts on different private networks are very likely to have
   the same private IP addresses, making the above scenario extremely
   likely for P2P applications to encounter.  P2P applications therefore
   MUST authenticate their transport-layer connections before assuming
   they have successfully connected to the desired host.  Strong
   cryptographic authentication using standard algorithms is of course
   preferred.

   REQ-4  Applications making peer-to-peer connections MUST authenticate
          each other using some higher-level application-specific notion
          of identity, before concluding that a successfully-established
          transport-level connection has actually reached the intended
          host.

2.5. NAT Behavior Detection

   In many existing NAT traversal protocols for both TCP and UDP, each
   client attempts to determine experimentally certain properties of any
   NATs it is located behind before attempting to establish peer-to-peer
   connections with other clients.  For example, even when a NAT does
   not re-use the same public endpoint for all sessions involving a
   given private endpoint as required for BEHAVE compliance, it is
   sometimes possible to predict which port the NAT will assign to a new
   session.

   Extensive testing of various existing NATs, however, has revealed
   that there is no truly robust way a client can predict how a legacy
   NAT will behave in the future based on such experimental tests.  Some



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   legacy NATs behave differently depending on the local port number the
   application is using on the client, and can even switch behaviors
   dynamically depending on unpredictable timing and network conditions.
   Therefore, while applications MAY use such experimental tests in an
   attempt to make peer-to-peer connections work across a wider variety
   of legacy NATs, they MUST ensure that such methods do not interfere
   with the performance or reliability of the application over BEHAVE-
   compliant NATs.

   REQ-5  Applications SHOULD NOT attempt to predict the future behavior
          of NATs in the path through empirical tests. If they do,
          applications MUST ensure that any such tests do not delay or
          otherwise impede the efficiency of NAT traversal over BEHAVE-
          compliant NATs.

3. NAT Traversal for UDP

   NAT traversal for UDP, also commonly known as UDP "hole punching",
   was mentioned briefly in section 5.1 of RFC 3027 [NAT-PROT], and
   first publicly documented informally on the Internet [KEGEL].
   Because of UDP's simplicity and its connectionless nature, NAT
   traversal for UDP is somewhat simpler, more widely understood, and
   hence more universally supported by NATs and applications than is NAT
   traversal for TCP, though the principles are the same for both
   transports.  NAT traversal for UDP has been used in several recent
   experimental Internet protocols [TEREDO, ICE] along with various
   proprietary or non-standardized protocols.  The NAT traversal
   approach recommended in this document is also described informally in
   [P2PNAT], and other variations of hole punching are explored more
   thoroughly in other recent research papers [NUTSS, NATBLASTER,
   NATTRAV].

   To set up a peer-to-peer UDP session between two clients A and B, we
   assume that the clients have each bound to a particular primary local
   UDP port, and that the clients have each initiated a UDP session from
   this primary local port to a well-known rendezvous server S, as
   described earlier.  Each client then learns the other's public and
   private UDP endpoints from the server S, and simply begins sending
   UDP datagrams, from their respective primary local ports (the same
   ports they used to contact S), to all of the other client's known
   endpoints.  If one or both of the clients is behind a BEHAVE-
   compliant NAT, the outgoing datagrams from each client will "open a
   hole" through a firewall or establish a translation session
   through the NAT, causing the NAT to forward subsequent incoming
   datagrams from the opposite client as desired.

3.1. UDP Idle Timeouts




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   Because of its inherently connectionless nature, NATs have no fully
   reliable way to determine when a UDP communication session crossing
   the NAT has terminated, other than simply by assuming the session is
   over if it observes a sufficiently long idle period.  Applications
   whose UDP communication sessions may experience long idle periods
   must therefore account for this idle timeout.

   As specified in [BEH-UDP], any BEHAVE-compliant NAT is required to
   have an idle timeout of at least two minutes, but idle timeouts as
   small as 30 seconds have been observed in existing NATs.
   Additionally, BEHAVE-compliant NATs are only required to reset the
   idle timer on the observance of outgoing traffic leaving the private
   network; the NAT may ignore incoming traffic for this purpose, in
   order to prevent external hosts from being able to hold UDP sessions
   open unilaterally and thus consume NAT resources indefinitely.
   BEHAVE-compliant NATs are required to support Address Restricted
   Cone NAT Behavior, which essentially resets the idle timer for ALL
   sessions for a particular binding (i.e., involving a particular
   private port) whenever outbound traffic is seen from that private
   port on ANY such session. However, there are many existing NATs
   which exhibit Port Restricted Cone NAT behavior. Cone NAT devices
   maintain UDP idle timers independently on each session.

   A NAT's UDP idle timeouts affects P2P applications implementing NAT
   traversal in two main ways:

   Rendezvous Server Registration Sessions:

      Client hosts implementing UDP hole punching typically register
      with one or more well-known rendezvous servers, S in the above
      scenarios, and expect to be notified by S when a second client
      wishes to open a peer-to-peer connection to the first. However,
      if a NAT's UDP idle timer times out while the first client is
      waiting for incoming connections, then the client will not
      receive the notification from S of the second client's desire
      to connect. The client therefore MUST send periodic outbound
      "keep-alive" packets to the rendezvous server(s) in order to
      ensure that the registration session remains open while the
      application is active.

      If a UDP application merely desires to be compatible with BEHAVE-
      compliant NATs, then its outbound keep-alive packets need not
      elicit a response from the server unless the application is
      concerned about detecting if the server disappears.  Also, if a
      UDP application maintains active registration sessions with more
      than one well-known rendezvous server simultaneously, then it is
      sufficient for the application to send outbound keep-alive packets
      on only one of these registration sessions as long as all of them



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      were initiated from the same local UDP port.

      If the application desires to function reliably over a broader
      range of existing NATs, however, then it may wish to send periodic
      outbound keep-alives to each of the rendezvous servers it is
      registered with, and ensure that these keep-alives elicit
      responses from those servers.  This behavior increases
      compatibility at the expense of consuming additional network
      bandwidth.

   REQ-6  Applications wishing to accept connections from other peers
          after registering via UDP with one or more rendezvous servers
          MUST send periodic outgoing UDP "keep-alive" packets to at
          least one rendezvous server, at least once within the
          BEHAVE-compliant NAT UDP timeout (2 minutes).

   Peer-to-Peer Sessions:

      Once two client hosts have used a rendezvous server to set up a
      peer-to-peer UDP communication session between them, this peer-to-
      peer session is similarly vulnerable to being closed by any of the
      NATs along the path if it goes idle for too long.  If the
      application is only concerned about compatibility with BEHAVE-
      compliant NATs and already sends regular outbound keep-alives to a
      registration server as described above, then those keep-alives are
      sufficient to maintain the peer-to-peer sessions the application
      has opened, as long as those peer-to-peer sessions involve the
      same local UDP endpoint.

      If an application desires to be more broadly compatible with
      existing NATs such as Port Restricted Cone NATs, and has only a
      few peer-to-peer sessions active at once, then the application
      may use keep-alives for each of the active peering sessions
      to keep the sessions open. If an application has
      many idle peer-to-peer sessions at once, however, then it is
      recommended that the application NOT use keep-alives on peer-to-
      peer sessions, but instead simply be prepared to re-establish
      peer-to-peer sessions as needed after an idle period, by simply
      re-running the NAT traversal protocol via the original rendezvous
      server.

   REQ-7  Applications whose peer-to-peer UDP sessions may go idle for
          more than the BEHAVE-compliant NAT UDP timeout (2 minutes) at
          a time, SHOULD re-run the original NAT traversal protocol in
          an attempt to re-establish the peer-to-peer session, if they
          detect that connectivity has been lost.

4. NAT Traversal for TCP



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   NAT traversal for TCP, or "TCP hole punching," is not yet as well-
   understood or widely supported as is UDP hole punching.
   Nevertheless, the general technique described in section 2 above
   works for TCP as well as UDP, as long as any NATs in the path are
   well-behaved.  The recommended NAT traversal algorithm for TCP,
   described here, makes use of the symmetric TCP connection initiation
   feature of TCP as specified in RFC 793 [TCP] and RFC 1122 [REQ-COMM].
   This algorithm is guaranteed to work reliably as long as all NATs in
   the path are BEHAVE-compliant [BEH-TCP], and as long as the end-hosts
   correctly implement the TCP protocol.

   Other more complex TCP hole punching algorithms have been developed
   and explored elsewhere [NUTSS, NATBLAST, NATTRAV].  These algorithms
   use various tricks to work around the nonstandard behaviors of many
   existing NATs, and/or to work around bugs in the TCP implementations
   of certain existing operating systems.  Applications MAY implement
   more complex algorithms such as these in order to achieve broader
   compatibility with existing NATs and hosts, but applications MUST
   ensure that any such alternative algorithm still works reliably and
   efficiently over BEHAVE-compliant NATs without substantially
   burdening the network and any NATs on the path.

   To prepare for TCP NAT traversal, a P2P client application first
   binds to an arbitrary local port, which becomes the application's
   primary local port, and then uses that port both to listen for
   incoming peer-to-peer connections, and to initiate outgoing
   connections to rendezvous servers and other peers.  Because standard
   sockets APIs usually associate TCP sockets with individual TCP
   sessions rather than with a local port as with UDP, the application
   must typically open multiple TCP sockets - one listen socket and one
   or more connect-sockets - and explicitly bind them to the same local
   port, using a special socket option usually named SO_REUSEADDR or
   SO_REUSEPORT.

   Once the TCP application has bound to its primary local port, started
   listening on it, and opened connections to one or more rendezvous
   servers, it can initiate or accept peer-to-peer connections as
   follows.  If two clients, A and B, wish to establish a peer-to-peer
   connection with the help of a common rendezvous server S, they first
   exchange their public and private TCP endpoints through S as
   described earlier in section 2.  Each client then simultaneously
   attempts to initiate outgoing TCP connections from its primary local
   port to each of the opposite client's known TCP endpoints (public and
   private).  As long as any NATs in the path are well-behaved, each
   client's outgoing TCP connection attempt will open firewall and/or
   translation sessions through any NATs it is located behind,
   eventually resulting in a working bi-directional TCP connection



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   through all intervening NATs on the path, in the same way as for UDP.

   Because of timing dependencies and differences in TCP
   implementations, applications may observe slightly different (but
   functionally equivalent) results when a P2P connection is
   successfully established using this method.  If client B is not
   actually located behind a firewall or NAT, for example, and client
   A's first attempt to connect directly to B reaches B before its peer-
   to-peer connection request relayed through S reaches B, then B will
   accept A's connection via its outstanding listen socket, in
   traditional client/server fashion.  Even if A's connection request
   (SYN packet) to B crosses B's corresponding request to A, resulting
   in a TCP simultaneous open at the protocol level, some end-host
   operating systems may still "deliver" the resulting connection to the
   application via the application's outstanding listen socket for its
   primary local port, rather than via the socket by which the
   application explicitly initiated a connection to the opposite client.
   The application must be prepared to handle all such possible cases
   gracefully.

   REQ-8  Application hosts implementing peer-to-peer communication via
          TCP SHOULD simultaneously listen for incoming peer-to-peer
          connections and open connections to rendezvous servers and
          other peers from the same primary port.

   REQ-9  Applications SHOULD establish peer-to-peer TCP connections by
          making "approximately simultaneous" connection attempts from
          each peer to all of the known endpoint addresses (both
          original and translated) for the opposite peer's primary port.
          Applications MAY alternatively establish peer-to-peer TCP
          connections via other, asymmetric methods if one or both
          endpoint hosts does not correctly support simultaneous TCP
          open.

4.1. Ensuring Robustness

   Some existing NATs actively reject an apparently-unsolicited incoming
   TCP connection by sending back TCP RST or ICMP error packets to the
   connection initiator, rather than simply by dropping the incoming
   SYN.  This behavior can cause one of the clients to observe bogus
   timing-dependent connection failures.  While this NAT behavior is
   deprecated and not allowed for BEHAVE-compliant NATs, P2P
   applications can fairly easily make themselves robust against this
   behavior.  If a client's attempt to initiate a peer-to-peer
   connection fails with a "Connection Refused" or "Network Unreachable"
   or similar network-related error before some application-defined
   peer-to-peer connection timeout has expired, the application SHOULD
   simply retry the same outgoing connection attempt.  The application



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   MUST ensure that at least one second has passed between each such
   retry, however, in order to avoid accidentally flooding the network
   with SYNs if the cause of the error is close to the client and is
   thus reported very quickly after each attempt.

   REQ-10 Applications SHOULD re-try peer-to-peer TCP connection
          attempts that fail due to network conditions other than
          timeout, but MUST NOT re-try connecting to a given peer
          more than once per second.


5. Summary of Requirements

An application that supports all of the mandatory requirements of this
specification (the "MUST" requirements), is "compliant with this
specification" or "BEHAVE-compliant".  An application that supports all
of the mandatory and optional recommendations of this specification
(including the "SHOULD" or "RECOMMENDED" ones) is "fully compliant with
all the mandatory and recommended requirements of this specification."

   REQ-1  Applications MUST be designed to operate reliably over BEHAVE-
          compliant NATs.  New applications are RECOMMENDED to assume
          that all NATs in the path are BEHAVE-compliant.

   REQ-2  Application hosts wishing to establish peer-to-peer
          communication with the help of a mutually accessible registry
          or "rendezvous server", MUST support traversal of NATs that
          may be in the paths of one or both hosts, by implementing the
          "approximately simultaneous connection attempts" technique
          described above.

   REQ-3  Applications implementing NAT traversal MUST exchange both
          their IP addresses and transport-level port numbers as known
          to themselves, and their IP addresses and port numbers as
          known to the common rendezvous server, and make "approximately
          simultaneous" connection attempts to each other using both
          sets of communication endpoints.  Applications SHOULD
          additionally be able to exchange and make connection attempts
          to more than two endpoints for each peer, in case the hosts
          have multiple IP addresses or are registered with multiple
          rendezvous servers at different locations in the network
          topology.

   REQ-4  Applications making peer-to-peer connections MUST authenticate
          each other using some higher-level application-specific notion
          of identity, before concluding that a successfully-established
          transport-level connection has actually reached the intended
          host.



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   REQ-5  Applications SHOULD NOT attempt to predict the future behavior
          of NATs in the path through empirical tests.  If they do,
          applications MUST ensure that any such tests do not delay or
          otherwise impede the efficiency of NAT traversal over BEHAVE-
          compliant NATs.

   REQ-6  Applications wishing to accept connections from other peers
          after registering via UDP with one or more rendezvous servers
          MUST send periodic outgoing UDP "keep-alive" packets to at
          least one rendezvous server, at least once within the
          BEHAVE-compliant NAT UDP timeout (2 minutes).

   REQ-7  Applications whose peer-to-peer UDP sessions may go idle for
          more than the BEHAVE-compliant NAT UDP timeout (2 minutes) at
          a time, SHOULD re-run the original NAT traversal protocol in
          an attempt to re-establish the peer-to-peer session, if they
          detect that connectivity has been lost.

   REQ-8  Application hosts implementing peer-to-peer communication via
          TCP SHOULD simultaneously listen for incoming peer-to-peer
          connections and open connections to rendezvous servers and
          other peers from the same primary port.

   REQ-9 Applications SHOULD establish peer-to-peer TCP connections by
          making "approximately simultaneous" connection attempts from
          each peer to all of the known endpoint addresses (both
          original and translated) for the opposite peer's primary port.
          Applications MAY alternatively establish peer-to-peer TCP
          connections via other, asymmetric methods if one or both
          endpoint hosts does not correctly support simultaneous TCP
          open.

   REQ-10 Applications SHOULD re-try peer-to-peer TCP connection
          attempts that fail due to network conditions other than
          timeout, but MUST NOT re-try connecting to a given peer more
          than once per second.

6. Security Considerations

   This document does not inherently create new security issues.
   This section describes security risks the applications could
   inadvertently create in attempting to support P2P communication
   across NAT devices.

6.1. Denial-of-service attacks

   P2P applications and the public registry servers that support them



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   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 registry 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.

   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.

6.2. 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



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   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.


7. Normative References

[BEH-ICMP]  P. Srisuresh, B. Ford, and S. Sivakumar, "NAT Behavioral
            Requirements for ICMP",
            draft-srisuresh-behave-nat-icmp-00.txt (Work In
            Progress), October 2005.

[BEH-TCP]   P.Srisuresh, S. Sivakumar, K. Biswas, and B. Ford, "NAT
            Behavioral Requirements for TCP",
            draft-sivakumar-behave-nat-tcp-req-02.txt (Work In
            Progress), October 2005.

[BEH-UDP]   F. Audet and C. Jennings, "NAT Behavioral Requirements for
            Unicast UDP", draft-ietf-behave-nat-udp-04.txt (Work In
            Progress), September 2005.

[KEGEL]     Dan Kegel, "NAT and Peer-to-Peer Networking", July 1999.
            http://www.alumni.caltech.edu/~dank/peer-nat.html

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


8. Informative References

[BEH-STATE] P. Srisuresh, B. Ford, and D. Kegel, "State of Peer-to-Peer
            (P2P) communication across Network Address Translators
            (NATs)", draft-srisuresh-behave-p2p-state-01.txt (Work In
            Progress), October 2005.

[BEH-TOP]   B. Ford and P. Srisuresh, "Topological Complications from
            Network Address Translation", draft-ford-behave-top-00.txt
            (Work In Progress), February 2005.

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




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[ICE]       J. Rosenberg, "Interactive Connectivity Establishment (ICE):
            A Methodology for Network Address Translator (NAT) Traversal
            for Offer/Answer Protocols", draft-ietf-mmusic-ice-06.txt
            (work in Progress), October 2005.

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

[NATBLAST]  Andrew Biggadike and Daniel Ferullo and Geoffrey Wilson and
            Adrian Perrig, "NATBLASTER: Establishing TCP Connections
            Between Hosts Behind NATs", ACM SIGCOMM Asia Workshop, April
            2005.

[NUTSS]     Saikat Guha and Yutaka Takeday and Paul Francis, "NUTSS: A
            SIP-based Approach to UDP and TCP Network Connectivity",
            SIGCOMM 2004 Workshops, August 2004.

[NATTRAV]   Jeffrey L. Eppinger, "TCP Connections for P2P Apps: A
            Software Approach to Solving the NAT Problem", Carnegie
            Mellon Tech Report CMU-ISRI-05-104, January 2005.

[P2PNAT]    Bryan Ford, Pyda Srisuresh, and Dan Kegel, "Peer-to-Peer
            Communication Across Network Address Translators", USENIX
            Annual Technical Conference, April 2005.

[REQ-COMM]  R. Braden, Editor, "Requirements for Internet Hosts -
            Communication Layers", RFC 1122, October 1989.

[RSIP]      M. Borella, J. Lo, D. Grabelsky, and G. Montenegro, "Realm
            Specific IP: Framework", RFC 3102, October 2001.

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

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

[TEREDO]    C. Huitema, "Teredo: Tunneling IPv6 over UDP through NATs",
            draft-ietf-ngtrans-shipworm-08.txt (Work In Progress),
            September 2002.



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[UPNP]      UPnP Forum, "Internet Gateway Device (IGD) Standardized
            Device Control Protocol V 1.0", November 2001.
            http://www.upnp.org/standardizeddcps/igd.asp


Author's Address

   Bryan Ford
   Computer Science and Artificial Intelligence Laboratory
   Massachusetts Institute of Technology
   77 Massachusetts Ave.
   Cambridge, MA 02139
   U.S.A.
   Phone: (617) 253-5261
   E-mail: baford@mit.edu
   Web: http://www.brynosaurus.com/

   Pyda Srisuresh
   Consultant
   20072 Pacifica Dr.
   Cupertino, CA 95014
   U.S.A.
   Phone: (408)836-4773
   E-mail: srisuresh@yahoo.com

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

Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED



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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

















































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