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Versions: 00 draft-ietf-behave-nat-udp

BEHAVE                                                          F. Audet
Internet-Draft                                           Nortel Networks
Expires: April 15, 2005                                      C. Jennings
                                                           Cisco Systems
                                                        October 15, 2004


              NAT Behavioral Requirements for Unicast UDP
                        draft-ietf-behave-nat-00

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 15, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document defines basic terminology for describing different
   types of behavior for NATs when handling unicast UDP.  It also
   defines a set of requirements for NATs that would allow many
   applications, such as multimedia communications or online gaming, to
   work consistently.  Developing NATs that meet this set of
   requirements will greatly increase the likelihood that applications
   will function properly.




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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.   Network Address and Port Translation Behavior  . . . . . . .   6
     3.1  Address and Port Binding . . . . . . . . . . . . . . . . .   6
     3.2  Port Assignment  . . . . . . . . . . . . . . . . . . . . .   8
     3.3  Bind Refresh Direction . . . . . . . . . . . . . . . . . .   9
     3.4  Bind Refresh Scope . . . . . . . . . . . . . . . . . . . .  10
   4.   Filtering Behavior . . . . . . . . . . . . . . . . . . . . .  10
     4.1  Filtering of Unsolicited Packets . . . . . . . . . . . . .  10
     4.2  NAT Filter Refresh . . . . . . . . . . . . . . . . . . . .  11
   5.   Hairpinning Behavior . . . . . . . . . . . . . . . . . . . .  11
   6.   Application Level Gateways . . . . . . . . . . . . . . . . .  12
   7.   Deterministic Properties . . . . . . . . . . . . . . . . . .  12
   8.   ICMP Behavior  . . . . . . . . . . . . . . . . . . . . . . .  13
   9.   Fragmentation of Packets . . . . . . . . . . . . . . . . . .  14
     9.1  Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . .  14
     9.2  Smaller Network MTU  . . . . . . . . . . . . . . . . . . .  14
   10.  Receiving Fragmented Packets . . . . . . . . . . . . . . . .  14
   11.  Requirements . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1   Requirement Discussion . . . . . . . . . . . . . . . . .  17
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  19
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  20
   14.  IAB Considerations . . . . . . . . . . . . . . . . . . . . .  20
   15.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  21
   16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  21
   16.1   Normative References . . . . . . . . . . . . . . . . . . .  21
   16.2   Informational References . . . . . . . . . . . . . . . . .  21
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  23
        Intellectual Property and Copyright Statements . . . . . . .  24



















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1.  Introduction

   Network Address Translators (NAT) are well known to cause very
   significant problems with applications that carry IP addresses in the
   payload RFC 3027 [5].  Applications that suffer from this problem
   include Voice Over IP and Multimedia Over IP (e.g., SIP [6] and H.323
   [11]), as well as online gaming.

   Many techniques are used to attempt to make realtime multimedia
   applications, online games, and other applications work across NATs.
   Application Level Gateways [3] are one such mechanism.  STUN [7]
   describes a UNilateral Self-Address Translation (UNSAF) mechanism [2]
   .  UDP Relays have also been used to enable applications across NATs,
   but these are generally seen as a solution of last resort.  ICE [9]
   describes a methodology for using many of these techniques and
   avoiding a UDP Relay unless the type of NAT is such that it forces
   the use of such a UDP Relay.  This specification defines requirements
   for improving NATs.  Meeting these requirements ensures that
   applications will not be forced to use UDP media relay.

   Several recommendations regarding NATs for Peer-to-Peer media were
   made in [10] and this specification derives some of its requirements
   from that draft.

   As pointed out in UNSAF [2] , "From observations of deployed
   networks, it is clear that different NAT boxes' implementation vary
   widely in terms of how they handle different traffic and addressing
   cases." This wide degree of variability is one part of what
   contributes to the overall brittleness introduced by NATs and makes
   it extremely difficult to predict how any given protocol will behave
   on a network traversing NATs.  Discussions with many of the major NAT
   vendors have made it clear that they would prefer to deploy NATs that
   were deterministic and caused the least harm to applications while
   still meeting the requirements that caused their customers to deploy
   NATs in the first place.  The problem the NAT vendors face is they
   are not sure how best to do that or how to document how their NATs
   behave.

   The goals of this document are to define a set of common terminology
   for describing the behavior of NATs and to produce a set of
   requirements on a specific set of behaviors for NATs.  The
   requirements represent what many vendors are already doing, and it is
   not expected that it should be any more difficult to build a NAT that
   meets these requirements or that these requirements should affect
   performance.

   The authors strongly believe that if there were a common set of
   requirements that were simple and useful for voice, video, and games,



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   the bulk of the NAT vendors would choose to meet those requirements.
   This document will simplify the analysis of protocols for deciding
   whether or not they work in this environment and will allow providers
   of services that have NAT traversal issues to make statements about
   where their applications will work and where they will not, as well
   as to specify requirements for NATs.

1.1  Scope

   This specification only covers Traditional NATs [4].  Bi-directional,
   Twice NAT, and Multihomed NAT [3] are outside the scope of this
   document.

   Approaches using directly signaled control off the middle boxes such
   as Midcom, UPnP, or in-path signaling are out of scope.

   UDP Relays are out of the scope of this document.

   Application aspects are out of scope as the focus is strictly on the
   NAT itself.

   This document only covers the UDP unicast aspects of NAT traversal
   and does not cover TCP, ICMP, IPSEC, or other protocols.  Since the
   document is on UDP unicast only, packet inspection below the UDP
   layer (including RTP) is also out-of-scope.

   This document does not cover Firewalls.  However, it does cover the
   inherent filtering aspects of NATs.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

   It is assumed that the reader is familiar with the terminology
   described in RFC 2663 [3] and RFC 3022 [4].  This specification
   attempts to preserve the terminology used in those RFCs.

   This document uses the term "session" as defined in RFC 2663 [3]:
   "TCP/UDP sessions are uniquely identified by the tuple of (source IP
   address, source TCP/UDP ports, target IP address, target TCP/UDP
   Port)."

   This document uses the term "address binding" as defined in RFC 2663
   RFC 3022: "Address binding is the phase in which a local IP address
   is associated with an external address, or vice versa, for purpose of
   translation."



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   The term NAT is used to refer to both traditional address translation
   and address port translation.  The authors understand that there was
   a time when these were considered different, but terminology has
   changed over time, and the term NAT has subsumed port translation as
   part of it.

   RFC 3489 [7] defines a terminology for different NAT variations.  In
   particular, it uses the terms "Full Cone", "Restricted Cone", "Port
   Restricted Cone" and "Symmetric" to refer to different variations of
   NATs.  Unfortunately, this terminology has been the source of much
   confusion.  This terminology does not distinguish between the NAT
   behavior and the filtering behavior of the device.  It was found that
   many devices' behaviors do not exactly fit into the described
   variations.  For example, a device could be symmetric from a
   filtering point of view and Cone from a NAT point of view.  Other
   aspects of NATs are not covered by this terminology: for example,
   many NATs will switch over from basic NAT (preserving ports) to NAPT
   (mapping ports) in order to preserve ports when possible.

   This specification will therefore not use the Cone/Symmetric
   terminology.  Furthermore, many other important behaviors are not
   fully described by the Cone/Symmetric terminology.  This
   specification refers to specific individual NAT behaviors instead of
   using the Cone/Symmetric terminology.

   Note: RFC 3489 defines a "Symmetric NAT" in effectively two parts:

   1.  All requests from the same internal IP address and port to a
       specific destination IP address and port are mapped to the same
       external IP address and port.  If a host sends a packet with the
       same source address and port to different destination addresses
       or ports, a different mapping is used for each.
   2.  Furthermore, only the external host that receives a packet can
       send a UDP packet back to the internal host.

   Condition 1 is the NAT behavior and condition 2 is the filtering
   behavior.  However, they are not necessarily dependent: we have
   observed NATs that will conform to condition (1) but not to (2).
   Using RFC 3489, this type of NAT would be detected as a "Cone NAT"
   since it uses condition (2).  Using a different algorithm such as the
   one described in NATCHECK [12] which uses condition (1), the same NAT
   would be detected as a "Symmetric NAT".  If the endpoint receiving
   the media has a permissive policy on accepting media, condition (2)
   is more appropriate, but if it has a restrictive policy, condition
   (1) is more appropriate.






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3.  Network Address and Port Translation Behavior

   This section describes the various NAT behaviors applicable to
   dynamic NAT; static NAT is outside the scope of this document.

3.1  Address and Port Binding

   When an internal endpoint opens an outgoing UDP session through a
   NAT, the NAT assigns the session an external IP address and port
   number so that subsequent response packets from the external endpoint
   can be received by the NAT, translated and forwarded to the internal
   endpoint.  This is a binding between an internal IP address and port
   (IP:port) and external IP:port tuple.  It establishes the translation
   that will be performed by the NAT for the duration of the session.
   For many applications, it is important to distinguish the behavior of
   the NAT when there are multiple simultaneous sessions established to
   different external endpoints.

   The key behavior to describe is the criteria for re-use of a binding
   for new sessions to external endpoints, after establishing a first
   binding between an internal X:x address and port and an external
   Y1:y1 address tuple.  Let's assume that the internal IP address and
   port X:x is mapped to X1':x1' for this first session.  The endpoint
   then sends from X:x to an external address Y2:y2 and gets a mapping
   of X2':x2' on the NAT.  The relationship between X1':x1' and X2':x2'
   for various combinations of the relationship between Y1:y1 and Y2:y2
   is critical for describing the NAT behavior.  This arrangement is
   illustrated in the following diagram:



                                            E
         +------+                 +------+  x
         |  Y1  |                 |  Y2  |  t
         +--+---+                 +---+--+  e
            | Y1:y1            Y2:y2  |     r
            +----------+   +----------+     n
                       |   |                a
               X1':x1' |   | X2':x2'        l
                    +--+---+-+
         ...........|   NAT  |...............
                    +--+---+-+              I
                       |   |                n
                   X:x |   | X:x            t
                      ++---++               e
                      |  X  |               r
                      +-----+               n
                                            a



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                                            l


   The following address and port binding behavior are defined:

   External NAT binding is endpoint independent: The NAT reuses the port
      binding for subsequent sessions initiated from the same internal
      IP address and port (X:x) to any external IP address and port.
      Specifically, X1':x1' equals X2':x2' for all values of Y2:y2.
      (From an RFC 3489 NAT perspective, this is a "Cone NAT" where the
      sub-type is really based on the filtering behavior.)

   External NAT binding is endpoint address dependent: The NAT reuses
      the port binding for subsequent sessions initiated from the same
      internal IP address and port (X:x) only for sessions to the same
      external IP address, regardless of the external port.
      Specifically, X1':x1' equals X2':x2' if, and only if, Y2 equals
      Y1.  (From an RFC 3489 NAT perspective, but not necessarily a
      filtering perspective, this is a "Symmetric NAT".)

   External NAT binding is endpoint address and port dependent: The NAT
      reuses the port binding for subsequent sessions initiated from the
      same internal IP address and port (X:x) only for sessions to the
      same external and port.  Specifically, X1':x1' equals X2':x2' if,
      and only if, Y2:y2 equals Y1:y1.  (From an RFC 3489 NAT
      perspective, but not necessarily a filtering perspective, this is
      a "Symmetric NAT".)

   The three possibilities are abbreviated as NB=I, NB=AD, and NB=APD,
   respectively.  NB stands for Nat Binding, I for independent, AD for
   Address Dependent, and APD for Address Port Dependent.

   It is important to note that these three possible choices make no
   difference to the security properties of the NAT.  The security
   properties are fully determined by which packets the NAT allows in
   and which it does not.  This is determined by the filtering behavior
   in the filtering portions of the NAT.

   Some NATs are capable of assigning IP addresses from a pool of IP
   addresses on the external side of the NAT, as opposed to just a
   single IP address.  This is especially common with larger NATs.  Some
   NATs use the external IP address binding in an arbitrary fashion
   (i.e.  randomly): one internal IP address could have multiple
   external IP address bindings active at the same time for different
   sessions.  These NATs have an "IP address pooling" behavior of
   "arbitrary".  Other NATs use the same external IP address binding for
   all sessions associated with the same internal IP address in any
   circumstance, even if ports are running out, in which case they would



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   fail to establish a session.  These NATs have an "IP address pooling"
   behavior of "Paired, strict." Yet other NATs use the same external IP
   address binding for all sessions associated with the same IP internal
   IP address only when possible.  These NATs have an "IP address
   pooling" behavior of "Paired, best effort." NATs that use an "IP
   address pooling" behavior of "arbitrary" can cause issues for
   applications that use multiple ports from the same endpoint but have
   no way to negotiate IP addresses individually (e.g., RTP and RTCP
   ports).

3.2  Port Assignment

   Some NATs attempt to preserve the port number used internally when
   assigning a binding to an external IP address and port (e.g., X:x to
   X':x).  A basic NAT, for example, will preserve the same port and
   will assign a different IP address from a pool of external IP
   addresses in case of port collision (e.g.  X1:x to X1':x and X2:x to
   X2':x).  This is only possible as long as the NAT has enough external
   IP addresses.  If the port x is already in use on all available
   external IP addresses, then the NAT needs to switch from Basic NAT to
   a Network Address and Port Translator (NAPT) mode (i.e., X1:x to X':x
   and X2:x to X':x').  This is referred to as "port preservation".  It
   does not guarantee that the external port x' will always be the same
   as the internal port x but only that the NAT will preserve the port
   if possible.

   A NAT that does not attempt to make the external port numbers match
   the internal port numbers in any case (i.e., X1:x to X':x1', X2:x to
   X':x2') is referred to as "no port preservation".

   Tools such as network sniffers identify traffic based on the
   destination port, not the source port, so port preservation does not
   help these tools.

   Some particularly nasty NATs use port overloading, i.e.  they always
   use port preservation even in the case of collision (i.e., X1:x to
   X':x, and X2:x to X':x).  These NATs rely on the source of the
   response from the external endpoint (Y:y, Z:z) to forward a packet to
   the proper internal endpoint (X1 or X2).  Port overloading fails if
   the two internal endpoints are establishing sessions to the same
   external destination.  This is referred to as "port overloaded".

   Most applications fail in some cases with "Port overloaded".  It is
   clear that "port overloaded" behavior will result in many problems.

   Some NATs preserve the parity of the UDP port, i.e., an even port
   will be mapped to an even port, and an odd port will be mapped to an
   odd port.  This behavior respects the RFC 3550 [8] rule that RTP use



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   even ports, and RTCP use odd ports.  There is some very unfortunate
   wording in RFC 3550 section 11, which can cause some problematic
   behavior in RTP clients.  The wording could be interpreted as saying
   that if a client receives an odd port for sending RTP, it SHOULD
   change it to the next lower even number.  This would obviously result
   in the loss of RTP/UDP.  In the event that a NAT supporting port
   parity preservation is running out of available ports for a specific
   parity, it may either fail to assign a port, or it could decide not
   respect port parity.  We refer to these properties as "port parity
   preservation" of "No", "Yes, strict" and "Yes, best effort".

   When NATs do allocate a new source port, there is the issue of which
   IANA-defined range of port to choose.  The ranges are "well-known"
   from 0 to 1023, "registered" from 1024 to 49151, and "dynamic/
   private" from 49152 through 65535.  For most protocols, these are
   destination ports and not source ports, so mapping a source port to a
   source port that is already registered is unlikely to have any bad
   effects.  There has been some suggestion that some IDS systems with
   deep packet inspection devices may end up looking at the ports.  This
   is clearly true for destination ports but less understood for source
   ports.  Some NATs may choose to use only the ports in the dynamic
   range; the only down side of this practice is that it limits the
   number of ports available.  Other NAT devices may use everything but
   the well-known range and may prefer to use the dynamics range first
   or possibly avoid the actual registered ports in the registered
   range.

3.3  Bind Refresh Direction

   NAT UDP binding timeout implementations vary but include the timer's
   value and the way the binding timer is refreshed to keep the binding
   alive.

   The binding timer is defined as the time a binding will stay active
   without packets traversing the NAT.  There is great variation in the
   values used by different NATs.

   Some NATs keep the binding active (i.e., refresh the timer value)
   when a packet goes from the internal side of the NAT to the external
   side of the NAT.  This is referred to as having an Outbound NAT
   refresh behavior of "True".

   Some NATs keep the binding active when a packet goes from the
   external side of the NAT to the internal side of the NAT.  This is
   referred to as having an Inbound NAT refresh direction behavior of
   "True".

   Some NATs keep the binding active on both, in which case both



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   properties are "True".

3.4  Bind Refresh Scope

   If the binding is refreshed for all sessions on that bind by any
   outbound traffic, the NAT is said to have a NAT refresh method
   behavior of "Per binding".  If the binding is refreshed only on a
   specific session on that particular bind by any outbound traffic, the
   NAT is said to have a "Per session" NAT refresh method behavior.

4.  Filtering Behavior

   This section describes various filtering behaviors observed in NATs.

4.1  Filtering of Unsolicited Packets

   When an internal endpoint opens an outgoing UDP session through a
   NAT, the NAT assigns a filtering rule for the binding between an
   internal IP:port (X:x) and external IP:port (Y:y) tuple.

   The key behavior to describe is what criteria are used by the NAT to
   filter packets originating from specific external endpoints.

   External filtering is open: The NAT does not filter any packets.

   External filtering is endpoint independent: The NAT filters out only
      packets not destined to the internal address and port X:x,
      regardless of the external IP address and port source (Z:z).  The
      NAT forwards any packets destined to X:x.  In other words, sending
      packets from the internal side of the NAT to any external IP
      address is sufficient to allow any packets back to the internal
      endpoint.  (From an RFC 3489 filtering perspective, this is a
      "Full Cone NAT".)

   External filtering is endpoint address dependent: The NAT filters out
      packets not destined to the internal address X:x.  Additionally,
      the NAT will filter out packets from Y:y destined for the internal
      endpoint X:x if X:x has not sent packets to Y previously
      (independently of the port used by Y).  In other words, for
      receiving packets from a specific external endpoint, it is
      necessary for the internal endpoint to send packets first to that
      specific external endpoint's IP address.  (From an RFC 3489
      filtering perspective, this is a "Restricted Cone NAT".)

   External filtering is endpoint address and port dependent: This is
      similar to the previous behavior, except that the external port is
      also relevant.  The NAT filters out packets not destined for the
      internal address X:x.  Additionally, the NAT will filter out



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      packets from Y:y destined for the internal endpoint X:x if X:x has
      not sent packets to Y:y previously.  In other words, for receiving
      packets from a specific external endpoint, it is necessary for the
      internal endpoint to send packets first to that external
      endpoint's IP address and port.  (From an RFC 3489 filtering
      perspective, this is both a "Port Restricted Cone NAT" and a
      "Symmetric NAT" as they have the same filtering behavior.)

   These are abbreviated as EF=O, EF=I, EF=AD, EF=APD respectively.  In
   the case of a NAT, "open" cannot forward a packet unless there is a
   NAT binding, so for all practical purposes, a NAT will never be
   "open" but will be one of the others.

4.2  NAT Filter Refresh

   The time for which a NAT filter is valid can be refreshed based on
   packets that are inbound, outbound, or going either direction.  In
   the case of EF=AD or EF=APD NATs, the scope of the refresh could
   include the filters for just the particular port and destination or
   for all the ports and destinations sharing the same external address
   and port on the NAT.

5.  Hairpinning Behavior

   If two hosts (called X1 and X2) are behind the same NAT and
   exchanging traffic, the NAT may allocate an address on the outside of
   the NAT for X2, called X2':x2'.  If X1 sends traffic to X2':x2', it
   goes to the NAT, which must relay the traffic from X1 to X2.  This is
   referred to as hairpinning and is illustrated below.



                                            NAT
           +----+ from X1:x1 to X2':x2'   +-----+ X1':x1'
           | X1 |>>>>>>>>>>>>>>>>>>>>>>>>>>>>>--+---
           +----+                         |  v  |
                                          |  v  |
                                          |  v  |
                                          |  v  |
           +----+ from X2':x2' to X1:x1   |  v  | X2':x2'
           | X2 |<<<<<<<<<<<<<<<<<<<<<<<<<<<<<--+---
           +----+                         +-----+

   Hairpinning allows two endpoints on the internal side of the NAT to
   communicate even if they only use each other's external IP addresses
   and ports.

   More formally, a NAT that supports hairpinning forwards packets



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   originating from an internal address, X1:x1, destined for an external
   address X2':x2' that has an active binding to an internal address
   X2:x2, back to that internal address X2:x2.  (Note that typically,
   X1'=X2'.)

   Furthermore, the NAT may present the hairpinned packet with either an
   internal or an external source IP address and port.  The hairpinning
   NAT behavior can therefore be either "External source IP address and
   port" or "Internal source IP address and port".  "Internal source IP
   address and port" may cause problems by confusing an implementation
   that is expecting an external IP address and port.

6.  Application Level Gateways

   Certain NATs have implemented Application Level Gateways (ALGs) for
   various protocols, including protocols for negotiating peer-to-peer
   UDP sessions.

   Certain NATs have these ALGs turned on permanently, others have them
   turned on by default but let them be be turned off, and others have
   them turned off by default but let them be turned on.

   NAT ALGs may interfere with UNSAF methods and must therefore be used
   with extreme caution.

7.  Deterministic Properties

   The diagnosis is further complicated by the fact that under some
   conditions the same NAT will exhibit different behaviors.  This has
   been seen on NATs that preserve ports or have specific algorithms for
   selecting a port other than a free one.  If the external port that
   the NAT wishes to use is already in use by another session, the NAT
   must select a different port.  This results in different code paths
   for this conflict case, which results in different behavior.

   For example, if three hosts X1, X2, and X3 all send from the same
   port x, through a port preserving NAT with only one external IP
   address, called X1', the first one to send (i.e., X1) will get an
   external port of x but the next two will get x2' and x3' (where these
   are not equal to x).  There are NATs where the External NAT Binding
   characteristics and the External Filter characteristics change
   between the X1:x and the X2:x binding.  To make matters worse, there
   are NATs where the behavior may be the same on the X1:x and X2:x
   binds but different on the third X3:x binding.

   Some NATs that try to reuse external ports flow from two internal IP
   addresses to two different external IP addresses.  For example, X1:x
   is going to Y1:y1 and X2:x is going to Y2:y2, where Y1:y1 does not



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   equal Y2:y2.  Some NATs will map X1:x to X1':x and will also map X2:x
   to X1':x.  This works in the case where the NAT Binding is address
   port dependent.  However some NATs change their behavior when this
   type of port reuse is happening.  The NAT may look like it has NAT
   Bindings that are independent when this type of reuse is not
   happening but may change to Address Port Dependent when this reuse
   happens.

   Any NAT that changes the NAT Binding or the External Filtering at any
   point in time or under any particular conditions is referred to as a
   "non-deterministic" NAT.  NATs that don't are called "deterministic".

   Non-deterministic NATs generally change behavior when a conflict of
   some sort happens, i.e.  when the port that would normally be used is
   already in use by another bind.  The NAT binding and External
   Filtering in the absence of conflict is referred to as the Primary
   behavior.  The behavior after the first conflict is referred to as
   Secondary and after the second conflict is referred to as Tertiary.
   No NATs have been observed that change on further conflicts but
   additional testing may be required.

8.  ICMP Behavior

   There are cases in which a host inside the NAT sends a packet to the
   NAT that gets relayed towards a host on the external side of the NAT
   that results in an ICMP Destination Unreachable message being
   returned to the NAT.  Most NATs will send an appropriate ICMP
   Destination Unreachable message to the internal host that sent the
   original packet.  NATs that do not filter out this ICMP Destination
   Unreachable message when it is in reply to a IP packet sent are
   referred to as "Support Destination Unreachable" (abbreviated SU).

   Incoming Destination Unreachable messages can be ignored after some
   period of time after the packet which elicited the Destination
   Unreachable message.  This IMCP timeout needs to be greater than the
   RTT for any destination the NAT may attempt to send IP packets to.
   Keep in mind satellite links when setting this timeout.

   Applications use the destination unreachable message to decide that
   they can stop trying to retransmit to a particular IP address and can
   fail over to a secondary address.  If a destination unreachable
   message is not received, the fail over will take too long for many
   applications.  Another key use of this message is for MTU discovery
   (described in RFC 1191 [14]).  MTU discovery is important for
   allowing applications to avoid the fragmentation problems discussed
   in the next section.  The arrival of a destination unreachable packet
   cannot destroy the NAT binding, as the occasional arrival of these
   messages is normal for black-hole discovery RFC 2923 [17].



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   There is no significant security advantage to blocking these ICMP
   Destination Unreachable packets.

9.  Fragmentation of Packets

   When sending a packet, there are two situations that may cause IP
   fragmentation for packets from the inside to the outside.  It is
   worth noting that many IP stacks do not use Path MTU Discovery with
   UDP packets.

9.1  Smaller Adjacent MTU

   The first situation is when the MTU of the adjacent link is too
   small.  This can occur if the NAT is doing PPPoE, or if the NAT has
   been configured with a small MTU to reduce serialization delay when
   sending large packets and small, higher-priority packets.

   The packet could have its Don't Fragment bit set to 1 (DF=1) or 0
   (DF=0).

   If the packet has DF=1, the NAT should send back an ICMP message
   "fragmentation needed and DF set" message to the host RFC 792 [18].

   If the packet has DF=0, the NAT should fragment the packet and send
   the fragments, in order.  This is the same function a router performs
   in a similar situation RFC 1812 [19].

   NATs that operate as described in this section are described as
   "Supports Fragmentation" (abbreviated SF).

9.2  Smaller Network MTU

   The second situation is when the MTU in the middle of the network is
   too small.  If DF=0, the router adjacent to the small-MTU segment
   will fragment the packet and forward the fragments RFC 1812 [19].

   If DF=1, the router adjacent to the small-MTU segment will send the
   ICMP message "fragmentation needed and DF set" back towards the NAT.
   The NAT needs to forward this ICMP message to the inside host.

   The classification of NATs that perform this behavior is covered in
   the ICMP section of this document.

10.  Receiving Fragmented Packets

   For a variety of reasons, a NAT may receive a fragmented UDP packet.
   The IP packet containing the UDP header could arrive first or last
   depending on network conditions, packet ordering, and the



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   implementation of the IP stack that generated the fragments.

   A NAT that is capable only of receiving UDP fragments in order (that
   is, with the UDP header in the first packet) and forwarding each of
   the fragments to the internal host is described as Received Fragments
   Ordered (abbreviated RF=O).

   A NAT that is capable of receiving UDP fragments in or out of order
   and forwarding the individual packets (or a reassembled packet) to
   the internal host is referred to as Receive Fragments Out of Order
   (abbreviated RF=OO).  See the Security Considerations section of this
   document for a discussion of this behavior.

   A NAT that is neither of these is referred to as Receive Fragments
   None (abbreviated RF=N).

11.  Requirements

   The requirements in this section are aimed at minimizing the damage
   caused by NATs to applications such as realtime communications and
   online gaming.

   It should be understood, however, that applications normally do not
   know in advance if the NAT conforms to the recommendations defined in
   this section.  Peer-to-peer media applications still need to use
   normal procedures such as ICE [9].

      REQ-1: A NAT MUST have an "External NAT Binding is endpoint
      independent" behavior (NB=I).

      REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling"
      behavior of "Paired, best effort".  Note that this requirement is
      not applicable to NATs that do not support IP address pooling.

         REQ-2a: A NAT MAY have an "IP address pooling" behavior of
         "Yes, strict."
         REQ-2b: A NAT MUST NOT have an "IP address pooling" behavior of
         "No".

      REQ-3: It is RECOMMENDED that a NAT have a "No port preservation"
      behavior.

         REQ-3a: A NAT MAY use a "Port preservation" behavior.
         REQ-3b: A NAT MUST NOT have a "Port overloaded" behavior.
         REQ-3c: If the NAT changes the source port, it MUST NOT
         allocate the new port from within the range of 0-1023.





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      REQ-4: It is RECOMMENDED that a NAT have a "Port parity
      preservation" behavior of "Yes, best effort".

         REQ-4a: A NAT MAY have a "Port parity preservation" behavior of
         "Yes, strict."
         REQ-4b: A NAT MUST NOT have a "Port parity preservation"
         behavior of "No".

      REQ-5: A dynamic NAT UDP binding timer MUST NOT expire in less
      than 2 minutes.

         REQ-5a: The value of the NAT UDP binding timer MAY be
         configurable.
         REQ-5b: A default value of 5 minutes for the NAT UDP binding
         timer is RECOMMENDED.

      REQ-6: The NAT UDP timeout binding MUST have a NAT Outbound
      refresh behavior of "True".

         REQ-6a: The NAT UDP timeout binding MAY have a NAT Inbound
         refresh behavior of "True".
         REQ-6b: The NAT UDP timeout binding MUST have a NAT refresh
         method behavior of "Per binding" (i.e.  refresh all sessions
         active on a particular bind).

      REQ-7: It is RECOMMENDED that a NAT have an "External filtering is
      endpoint address dependent" behavior.  (EF=AD)

         REQ-7a: A NAT MAY have an "External filtering is endpoint
         independent" behavior.  (EF=I)
         REQ-7b: A NAT MAY have an "External filtering is endpoint
         address and port dependent" behavior.  (EF=APD)

      REQ-8: The NAT UDP filter timeout behavior MUST be the same as the
      NAT UDP binding timeout.

      REQ-9: A NAT MUST support "Hairpinning" behavior.

         REQ-9a: A NAT Hairpinning behavior MUST be "External source IP
         address and port".

      REQ-10: A NAT MUST have the capability to turn off individually
      all ALGs it supports, except for DNS and IPsec.

         REQ-10a: Any NAT ALG for SIP MUST be turned off by default.

      REQ-11: A NAT MUST have deterministic behavior.




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      REQ-12: A NAT MUST support ICMP Destination Unreachable (SU).

         REQ-12a: The ICMP timeout SHOULD be greater than 2 seconds.
      REQ-13: A NAT MUST support fragmentation of packets larger than
      link MTU (SF).

      REQ-14: A NAT MUST support receiving in order fragments, so it
      MUST be RF=O or RF=OO.

         REQ-14a: A NAT MAY support receiving fragmented packets that
         are out of order and be of type RF=OO.


11.1  Requirement Discussion

   This section describes why each of these requirements was chosen and
   the consequences of violating any of them:

      REQ-1: In order for UNSAF methods to work, REQ-1 needs to be met.
      Failure to meet REQ-1 will force the use of a Media Relay which is
      very often impractical.

      REQ-2: This will allow applications that use multiple ports
      originating from the same internal IP address to also have the
      same external IP address, but without running out of ports
      unnecessarily.

      REQ-3: NATs that implement port preservation have to deal with
      conflicts on ports, and the multiple code paths this introduces
      often result in nondeterministic behavior.

         REQ-3a: Port preservation can work, but the NAT implementors
         need to be very careful that it does not become a
         nondeterministic NAT.
         REQ-3b: REQ-2b must be met in order to enable two applications
         on the internal side of the NAT both to use the same port to
         try to communicate with the same destination.
         REQ-3c: This requirement is because some applications may not
         be able to use ports in the well-known range because of
         priviledge restrictions.

      REQ-4: This will respect the RTP/RTCP parity convention when
      possible, but without running out of ports unnecessarily.

      REQ-5: This requirement is to ensure that the timeout is long
      enough to avoid too frequent timer refresh packets.




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         REQ-5a: Configuration is desirable for adapting to specific
         networks and troubleshooting.
         REQ-5b: This default is to avoid too frequent timer refresh
         packets.

      REQ-6: Outbound refresh is necessary for allowing the client to
      keep the binding alive.

         REQ-6a: Inbound refresh may be useful for applications where
         there is no outgoing UDP traffic.
         REQ-6b: Using the refresh on a per binding basis avoids the
         need for separate keep alives for all the available sessions.

      REQ-7: Filtering based on the IP address is felt to have the
      maximum balance between security and usefulness.  See below.

         REQ-7a: Filtering independently of the external IP address and
         port is not as secure: an unauthorized packet could get at a
         specific port while the port was kept open if it was lucky
         enough to find the port open.
         REQ-7b: In theory, filtering based on both IP address and port
         is more secure than filtering based only on the IP address
         (because the external endpoint could in reality be two
         endpoints behind another NAT, where one of the two endpoints is
         an attacker).  However, such a restrictive policy could
         interfere with certain applications that use more than one
         port.

      REQ-8: This is to avoid overly complex applications.

      REQ-9: This requirement is to allow communications between two
      endpoints behind the same NAT when they are trying each other's
      external IP addresses.

         REQ-9a: Using the external IP address is necessary for
         applications with a restrictive policy of not accepting packets
         from IP addresses that differ from what is expected.

      REQ-10: NAT ALGs may interfere with UNSAF methods.

         REQ-10a: A SIP NAT ALG will interfere with UNSAF methods.

      REQ-11: Non-deterministic NATs are very difficult to troubleshoot
      because they require more intensive testing.  This
      non-deterministic behavior is the root cause of much of the
      uncertainty that NATs introduce about whether or not applications
      will work.



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      REQ-12: This is easy to do, is used for many things including MTU
      discovery and rapid detection of error conditions, and has no
      negative consequences.

      REQ-13 and REQ-13: Fragmented packets become more common with
      large video packets and should continue to work.  Applications can
      use MTU discovery to work around this.


12.  Security Considerations

   NATs are often deployed to achieve security goals.  Most of the
   recommendations and requirements in this document do not affect the
   security properties of these devices, but a few of them do have
   security implications and are discussed in this section.

   This work recommends that the timers for binding be refreshed only on
   outgoing packets and does not make recommendations about whether or
   not inbound packets should update the timers.  If inbound packets
   update the timers, an external attacker can keep the binding alive
   forever and attack future devices that may end up with the same
   internal address.  A device that was also the DHCP server for the
   private address space could mitigate this by cleaning any bindings
   when a DHCP lease expired.  For unicast UDP traffic (the scope of
   this document), it may not seem relevant to support inbound timer
   refresh; however, for multicast UDP, the question is harder.  It is
   expected that future documents discussing NAT behavior with multicast
   traffic will refine the requirements around handling of the inbound
   refresh timer.  Some devices today do update the timers on inbound
   packets.

   This work recommends that the NAT filters be specific to the external
   IP only and not the external IP and port.  It can be argued that this
   is less secure than using the IP and port.  Devices that wish to
   filter on IP and port do still comply with these requirements.

   Non-deterministic NATs are risky from a security point of view.  They
   are very difficult to test because they are, well, non-deterministic.
   Testing by a person configuring one may result in the person thinking
   it is behaving as desired, yet under different conditions, which an
   attacker can create, the NAT may behave differently.  These
   requirements recommend that devices be deterministic.

   The work requires that NATs have an "external NAT binding is endpoint
   independent" behavior.  This does not reduce the security of devices.
   Which packets are allowed to flow across the device is determined by
   the external filtering behavior, which is independent of the binding



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   behavior.

   When a fragmented packet is received from the external side and the
   packets are out of order so that the initial fragment does not arrive
   first, many systems simply discard the out of order packets.
   Moreover, since some networks deliver small packets ahead of large
   ones, there can be many out of order fragments.  NATs that are
   capable of delivering these out of order packets are possible but
   they need to store the out of order fragments, which can open up a
   DoS opportunity.  Fragmentation has been a tool used in many attacks,
   some involving passing fragmented packets through NATs and others
   involving DoS attacks based on the state needed to reassemble the
   fragments.  NAT implementers should be aware of RFC 3128 [16] and RFC
   1858 [15].

13.  IANA Considerations

   There are no IANA considerations.

14.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a client attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism [2].

   This specification does not in itself constitute an UNSAF
   application.  It consists of a series of requirements for NATs aimed
   at minimizing the negative impact that those devices have on
   peer-to-peer media applications, especially when those applications
   are using UNSAF methods.

   Section 3 of UNSAF lists several practical issues with solutions to
   NAT problems.  This document makes recommendations to reduce the
   uncertainty and problems introduced by these practical issues with
   NATs.  In addition, UNSAF lists five architectural considerations.
   Although this is not an UNSAF proposal, it is interesting to consider
   the impact of this work on these architectural considerations.

   Arch-1: The scope of this is limited to UDP packets in NATs like the
      ones widely deployed today.  The "fix" helps constrain the
      variability of NATs for true UNSAF solutions such as STUN.
   Arch-2: This will exit at the same rate that NATs exit.  It does not
      imply any protocol machinery that would continue to live after
      NATs were gone or make it more difficult to remove them.






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   Arch-3: This does not reduce the overall brittleness of NATs but will
      hopefully reduce some of the more outrageous NAT behaviors and
      make it easer to discuss and predict NAT behavior in given
      situations.
   Arch-4: This work and the results [13] of various NATs represent the
      most comprehensive work at IETF on what the real issues are with
      NATs for applications like VoIP.  This work and STUN have pointed
      out more than anything else the brittleness NATs introduce and the
      difficulty of addressing these issues.
   Arch-5: This work and the test results [13] provide a reference model
      for what any UNSAF proposal might encounter in deployed NATs.

15.  Acknowledgments

   Dan Wing contributed substantial text on IP fragmentation.

   The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and
   Dan Kegel for the NATP2P [10] draft, from which a lot of the material
   in this specification is derived.  Thanks to Rohan Mahy, Jonathan
   Rosenberg, Mary Barnes, Dan Wing, Melinda Shore, Lyndsay Campbell,
   Geoff Huston, Jiri Kuthan, Harald Welte and Spencer Dawkins for their
   important contributions.

16.  References

16.1  Normative References

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

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

16.2  Informational References

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

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

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

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



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

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

   [9]   Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network Address Translator (NAT) Traversal for
         the Session Initiation Protocol (SIP)",
         draft-ietf-mmusic-ice-02 (work in progress), July 2004.

   [10]  Ford, B., Srisuresh, P. and D. Kegel, "Peer-to-Peer(P2P)
         communication across Network Address Translators(NATs)",
         draft-ford-midcom-p2p-03 (work in progress), June 2004.

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

   [12]  Ford, B. and D. Andersen, "Nat Check Web Site:
         http://midcom-p2p.sourceforge.net", June 2004.

   [13]  Jennings, C., "NAT Classification Results using STUN",
         draft-jennings-midcom-stun-results-01 (work in progress), July
         2004.

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

   [15]  Ziemba, G., Reed, D. and P. Traina, "Security Considerations
         for IP Fragment Filtering", RFC 1858, October 1995.

   [16]  Miller, I., "Protection Against a Variant of the Tiny Fragment
         Attack (RFC 1858)", RFC 3128, June 2001.

   [17]  Reynolds, J. and J. Postel, "Assigned numbers", RFC 923,
         October 1984.

   [18]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
         792, September 1981.

   [19]  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
         June 1995.







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Authors' Addresses

   Francois Audet
   Nortel Networks
   4655 Great America Parkway
   Santa Clara, CA  95054
   USA

   Phone: +1-408-495-3756
   EMail: audet@nortelnetworks.com


   Cullen Jennings
   Cisco Systems
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134
   USA

   Phone: +1-408-902-3341
   EMail: fluffy@cisco.com






























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