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Versions: (draft-ietf-behave-nat) 00 01 02 03 04 05 06 07 08 RFC 4787

BEHAVE                                                     F. Audet, Ed.
Internet-Draft                                           Nortel Networks
Expires: October 13, 2005                                    C. Jennings
                                                           Cisco Systems
                                                          April 11, 2005


              NAT Behavioral Requirements for Unicast UDP
                      draft-ietf-behave-nat-udp-01

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

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   This Internet-Draft will expire on October 13, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document defines basic terminology for describing different
   types of NAT behavior when handling Unicast UDP, and defines a set of
   requirements that would allow many applications, such as multimedia
   communications or on-line gaming, to work consistently.  Developing
   NATs that meet this set of requirements will greatly increase the



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   likelihood that these applications will function properly.

Table of Contents

   1.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Network Address and Port Translation Behavior  . . . . . . . .  5
     4.1   Address and Port Mapping . . . . . . . . . . . . . . . . .  5
     4.2   Port Assignment  . . . . . . . . . . . . . . . . . . . . .  7
       4.2.1   Port Assignment Behavior . . . . . . . . . . . . . . .  7
       4.2.2   Port Parity  . . . . . . . . . . . . . . . . . . . . .  9
       4.2.3   Port Contiguity  . . . . . . . . . . . . . . . . . . .  9
     4.3   Mapping Refresh Direction  . . . . . . . . . . . . . . . . 10
     4.4   Mapping Refresh Scope  . . . . . . . . . . . . . . . . . . 10
   5.  Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 11
     5.1   Filtering of Unsolicited Packets . . . . . . . . . . . . . 11
     5.2   NAT Filter Refresh . . . . . . . . . . . . . . . . . . . . 11
   6.  Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 12
   7.  Application Level Gateways . . . . . . . . . . . . . . . . . . 12
   8.  Deterministic Properties . . . . . . . . . . . . . . . . . . . 13
   9.  ICMP Behavior  . . . . . . . . . . . . . . . . . . . . . . . . 14
   10.   Fragmentation of Packets . . . . . . . . . . . . . . . . . . 14
     10.1  Smaller Adjacent MTU . . . . . . . . . . . . . . . . . . . 14
     10.2  Smaller Network MTU  . . . . . . . . . . . . . . . . . . . 15
   11.   Receiving Fragmented Packets . . . . . . . . . . . . . . . . 15
   12.   Requirements . . . . . . . . . . . . . . . . . . . . . . . . 15
     12.1  Requirement Discussion . . . . . . . . . . . . . . . . . . 17
   13.   Security Considerations  . . . . . . . . . . . . . . . . . . 19
   14.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 20
   15.   IAB Considerations . . . . . . . . . . . . . . . . . . . . . 20
   16.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 21
   17.   References . . . . . . . . . . . . . . . . . . . . . . . . . 21
     17.1  Normative References . . . . . . . . . . . . . . . . . . . 21
     17.2  Informational References . . . . . . . . . . . . . . . . . 21
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 23
       Intellectual Property and Copyright Statements . . . . . . . . 24














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1.  Applicability Statement

   The purpose of this specification is to define a set of requirements
   for NATs that would allow many applications, such as multimedia
   communications or on-line gaming, to work consistently.  Developing
   NATs that meet this set of requirements will greatly increase the
   likelihood that these applications will function properly.

   The requirements of this specification apply generally to all NAT
   variations, including the ones described in RFC 2663 [3] (Traditional
   NAT, Basic NAT, NAPT, Bi-directional NAT, Twice NAT, and Multihomed
   NATs).  However, it is not within the scope of this specification to
   address all issues specific to all possible NAT variations.

   This document is meant to cover NATs of any size, from small
   residential NATs to large Enterprise NATs.  However, it should be
   understood that Enterprise NATs normally provide much more than just
   NAT capabilities: for example, they typically provide Firewall
   capabilities.  Firewalls is specifically out-of-scope of this
   specification: however, this specification does cover the inherent
   filtering aspects of NAT.

   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, IPSEC, or other protocols.  Since the
   document is for UDP only, packet inspection below the UDP layer
   (including RTP) is also out-of-scope.

2.  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
   [20]), 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



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   NATs, but these are generally seen as a solution of last resort.  ICE
   [16] 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.

   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.

   This document forms a common set of requirements that are simple and
   useful for voice, video, and games, which can be implemented by NAT
   vendors.  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 their own NAT requirements.

3.  Terminology

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

   Readers are urged to refer to RFC 2263 [3] for information on NAT
   taxonomy and terminology.  Traditional NAT is the most common type of
   NAT device deployed.  Readers may refer to RFC 3022 [4] for detailed
   information on traditional NAT.  Traditional NAT has two main
   varieties - Basic NAT and Network Address/Port Translator (NAPT).




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   NAPT is by far the most commonly deployed NAT device.  NAPT allows
   multiple internal hosts to share a single public IP address
   simultaneously.  When an internal host opens an outgoing TCP or UDP
   session through a NAPT, the NAPT assigns the session a public IP
   address and port number so that subsequent response packets from the
   external endpoint can be received by the NAPT, translated, and
   forwarded to the internal host.  The effect is that the NAPT
   establishes a NAT session to translate the (private IP address,
   private port number) tuple to (public IP address, public port number)
   tuple and vice versa for the duration of the session.  An issue of
   relevance to peer-to-peer applications is how the NAT behaves when an
   internal host initiates multiple simultaneous sessions from a single
   (private IP, private port) endpoint to multiple distinct endpoints on
   the external network.  In this specification, the term "NAT" refers
   to both "Basic NAT" and "Network Address/Port Translator (NAPT)".

   This document uses the term "session" as defined in RFC 2663: "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 and port mapping" as the
   translation between an external address and port and an internal
   address and port.  Note that this is not the same as an "address
   binding" as defined in RFC 2663.

   Earlier documents used the terms "Full Cone", "Restricted Cone",
   "Port Restricted Cone" and "Symmetric" to refer to different
   variations of NATs applicable to UDP only.  Unfortunately, this
   terminology has been the source of much confusion as it proved
   inadequate at describing real-life NAT behavior.  This specification
   therefore refers to specific individual NAT behaviors instead of
   using the Cone/Symmetric terminology.

4.  Network Address and Port Translation Behavior

   This section describes the various NAT behaviors applicable to NAT.

4.1  Address and Port Mapping

   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 mapping 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



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   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 mapping
   for new sessions to external endpoints, after establishing a first
   mapping 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
                                      l

   The following address and port mapping behavior are defined:

      External NAT mapping is endpoint independent:
         The NAT reuses the port mapping 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.

      External NAT mapping is endpoint address dependent:
         The NAT reuses the port mapping 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.




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      External NAT mapping is endpoint address and port dependent:
         The NAT reuses the port mapping 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.

   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 mapping in an arbitrary fashion
   (i.e. randomly): one internal IP address could have multiple external
   IP address mappings active at the same time for different sessions.
   These NATs have an "IP address pooling" behavior of "Arbitrary".
   Some large Enterprise NATs use an IP address pooling behavior of
   "Arbitrary" as a means of hiding the IP address assigned to specific
   endpoints by making their assignment less predictable.  Other NATs
   use the same external IP address mapping for all sessions associated
   with the same internal IP address.  These NATs have an "IP address
   pooling" behavior of "Paired."  NATs that use an "IP address pooling"
   behavior of "arbitrary" can cause issues for applications that use
   multiple ports from the same endpoint but do not negotiate IP
   addresses individually (e.g., some applications using RTP and RTCP).

4.2  Port Assignment

4.2.1  Port Assignment Behavior

   This section uses the following diagram for reference.
















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

   Some NATs attempt to preserve the port number used internally when
   assigning a mapping to an external IP address and port (e.g.,
   x=x1=x2=x1'=x2', or more succinctly, a mapping of 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., X'=X1'=X2' and x=x1=x2 but x1'!=x2', or
   a mapping of X1:x to X':x1' and X2:x to X':x2').  This port
   assignment behavior 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".

   Some NATs use "Port overloading", i.e. they always use port
   preservation even in the case of collision (i.e., X'=X1'=X2' and
   x=x1=x2=x1'=x2', or a mapping of X1:x to X':x, and X2:x to X':x).
   These NATs rely on the source of the response from the external
   endpoint (Y1:y1, Y2:y2) 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.

   Most applications fail in some cases with "Port Overloading".  It is



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   clear that "Port Overloading" behavior will result in many problems.
   For example it will fail if two internal endpoints try to reach the
   same external destination, e.g., a server used by both endpoints such
   as a SIP proxy, or a web server, etc.)

   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.  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.  Other NATs preserve the port range if it is in the well-known
   range.  It should be noted that port 0 is reserved and must not be
   used.

4.2.2  Port Parity

   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
   even ports, and RTCP use odd ports.  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 rule that RTP use even ports and RTCP use odd ports when
   the application takes a single port number as a parameter and derives
   the RTP and RTCP port pair from that number.  RFC 3550 allows any
   port numbers to be used for RTP and RTCP if the two numbers are
   specified separately, for example using RFC 3605 [9].  However, some
   implementations do not include RFC 3605 and do not recognize when the
   peer has specified the RTCP port separately using RFC 3605.  If such
   an implementation receives an odd RTP port number from the peer
   (perhaps after having been translated by a NAT), and then follows the
   RFC 3550 rule to change the RTP port to the next lower even number,
   this would obviously result in the loss of RTP.  NAT-friendly
   application aspects are outside the scope of this document.  It is
   expected that this issue will fade away with time, as implementations
   improve.  Preserving the port parity allows for supporting
   communication with peers that do not support explicit specification
   of both RTP and RTCP port numbers.

4.2.3  Port Contiguity

   Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1.



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   These NATs do things like sequential assignment, port reservation and
   so forth.  Sequential port assignment assumes that the application
   will open a mapping for RTP first and then open a mapping for RTCP.
   It is not practical to enforce this requirement on all applications.
   Furthermore, there is a glare problem if many applications (or
   endpoints) are trying to open mapping simultaneously.  Port
   reservation is also problematic since it is wasteful, especially
   considering that a NAT can not reliably distinguish between RTP over
   UDP and other UDP packets where there is no contiguity rule.  For
   those reasons, it would be too complex to attempt to preserve the
   contiguity rule by suggesting specific NAT behavior, and it would
   certainly break the deterministic behavior rule.

   In order to support both RTP and RTCP, it will therefore be necessary
   that applications follows rules to negotiate both RTP and RTCP
   separately, and account for the very real possibility that the
   RTCP=RTP+1 rule will be broken.  As this is an application
   requirement, it is outside of the scope of this document.

4.3  Mapping Refresh Direction

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

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

   Some NATs keep the mapping 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 a NAT Outbound
   refresh behavior of "True".

   Some NATs keep the mapping 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 a NAT Inbound Refresh Behavior of "True".

   Some NATs keep the mapping active on both, in which case both
   properties are "True".

4.4  Mapping Refresh Scope

   If the mapping is refreshed for all sessions on that mapping by any
   outbound traffic, the NAT is said to have a NAT Mapping Refresh Scope
   of "Per mapping".  If the mapping is refreshed only on a specific
   session on that particular mapping by any outbound traffic, the NAT
   is said to have a "Per session" NAT mapping Refresh Scope.



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5.  Filtering Behavior

   This section describes various filtering behaviors observed in NATs.

5.1  Filtering of Unsolicited Packets

   When an internal endpoint opens an outgoing UDP session through a
   NAT, the NAT assigns a filtering rule for the mapping 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 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.

      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.

      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 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.

5.2  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 "External Filtering" of "Address dependent" or "Address
   and port dependent" NATs, the scope of the refresh could include the
   filters for just the particular port and destination or for all the



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   ports and destinations sharing the same external address and port on
   the NAT.

6.  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 X1':x1' to X2:x2   |  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
   originating from an internal address, X1:x1, destined for an external
   address X2':x2' that has an active mapping to an internal address
   X2:x2, back to that internal address X2:x2.  Note that typically X1'
   is the same as 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.

7.  Application Level Gateways

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

   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



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   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.

8.  Deterministic Properties

   The classification of NATs 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 mapping
   characteristics and the External Filter characteristics change
   between the X1:x and the X2:x mapping.  To make matters worse, there
   are NATs where the behavior may be the same on the X1:x and X2:x
   mappings but different on the third X3:x mapping.

   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
   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 mapping 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
   mappings 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 mapping or the External Filtering at any
   point in time 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 mapping.  The NAT mapping 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 it is



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   certainly possible that they exist.

9.  ICMP Behavior

   When a NAT sends a UDP packet towards a host on the other side of the
   NAT, an ICMP message may be sent in response to that packet.  That
   ICMP message may be sent by the destination host or by any router
   along the network path.  The NAT's default configuration SHOULD NOT
   filter ICMP messages based on their source IP address.  Such ICMP
   messages SHOULD be rewritten by the NAT (specifically the IP headers
   and the ICMP payload) and forwarded to the appropriate internal or
   external host.  The NAT needs to perform this function for as long as
   the UDP mapping is active.  Receipt of any sort of ICMP message MUST
   NOT destroy the NAT binding.  A NAT which performs the functions
   described in the paragraph above is referred to as "UDP Support
   Destination Unreachable".

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

   Additionally, blocking ICMP Destination Unreachable packets can
   interfere with application failover, UDP Path MTU Discovery (see
   RFC1191 [10] and RFC1435 [15]), and with traceroute.  Blocking any
   ICMP message is discouraged, and blocking ICMP Destination
   Unreachable is strongly discouraged.

10.  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.

10.1  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 as described in
   RFC 792 [13].

   If the packet has DF=0, the NAT should fragment the packet and send



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   the fragments, in order.  This is the same function a router performs
   in a similar situation RFC 1812 [14].

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

10.2  Smaller Network MTU

   The second situation is when the MTU on some link in the middle of
   the network that is not the adjacent link is too small.  If DF=0, the
   router adjacent to the small-MTU segment will fragment the packet and
   forward the fragments RFC 1812.

   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.

11.  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
   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".

   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".
   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".

12.  Requirements

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

   It should be understood, however, that applications normally do not



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   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 [16].

   A NAT that supports all of the mandatory requirements of this
   specification (i.e., the "MUST"), is "compliant with this
   specification."  A NAT that supports all of the requirements of this
   specification (i.e., included the "RECOMMENDED") is "fully compliant
   with all the mandatory and recommended requirements of this
   specification."

   REQ-1  A NAT MUST have an "External NAT mapping is endpoint
          independent" behavior.
   REQ-2  It is RECOMMENDED that a NAT have an "IP address pooling"
          behavior of "Paired".  Note that this requirement is not
          applicable to NATs that do not support IP address pooling.
   REQ-3  IA NAT MUST NOT have a "Port assignment" behavior of "Port
          overloading".
          a) If the host's source port was in the range 1-1023, it is
             RECOMMENDED the NAT's source port also be in the same
             range.  If the host's source port was in the range 1024-
             65535, it is RECOMMENDED that the NAT's source port also be
             in that range.
   REQ-4  It is RECOMMENDED that a NAT have a "Port parity preservation"
          behavior of "Yes".
   REQ-5  A NAT UDP mapping timer MUST NOT expire in less than 2
          minutes.
          a) The value of the NAT UDP mapping timer MAY be configurable.
          b) A default value of 5 minutes for the NAT UDP mapping timer
             is RECOMMENDED.
   REQ-6  The NAT mapping Refresh Direction MUST have a "NAT Outbound
          refresh behavior" of "True".
          a) The NAT mapping Refresh Direction MAY have a "NAT Inbound
             refresh behavior" of "True".
          b) The NAT mapping Refresh Direction MUST have a "NAT refresh
             method behavior" of "Per mapping" (i.e. refresh all
             sessions active on a particular mapping).
   REQ-7  It is RECOMMENDED that a NAT have an "External filtering is
          endpoint address dependent" behavior.
   REQ-8  A NAT MUST support "Hairpinning".
          a) A NAT Hairpinning behavior MUST be "External source IP
             address and port".
   REQ-9  If a NAT includes ALGs, it is RECOMMENDED that all of those
          ALGs (except for DNS [19] and FTP [18]) be disabled by
          default.






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          a) If a NAT includes ALGs, it is RECOMMENDED that the NAT
             allow the user to enable or disable each ALG separately.
   REQ-10 A NAT MUST have deterministic behavior, i.e., it MUST NOT
          change the NAT mapping or the External External Filtering
          Behavior at any point in time or under any particular
          conditions.
   REQ-11 It is RECOMMENDED that a NAT support ICMP Destination
          Unreachable.
          a) The ICMP timeout SHOULD be greater than 2 seconds.
   REQ-12 A NAT MUST support fragmentation of packets larger than link
          MTU.
   REQ-13 A NAT MUST support receiving in order fragments, so it MUST be
          "Received Fragment Ordered" or "Received Fragment Out of
          Order".
          a) A NAT MAY support receiving fragmented packets that are out
             of order and be of type "Received Fragment Out of Order".

12.1  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.  This is to avoid breaking peer-to-
          peer applications which are not capable of negotiating the IP
          address for RTP and the IP address for RTCP separately.  As
          such it is envisioned that this requirement will become less
          important as applications become NAT-friendlier with time.
          The main reason why this requirement is here is because in a
          peer-to-peer application, you are subject to the other peer's
          mistake.  In particular, in the context of SIP, if my
          application supports the extensions defined in RFC 3605 [9]
          for indicating RTP and RTCP addresses and ports separately,
          but the other peer does not, there may still be breakage in
          the form of lost of the RTP stream.  This requirements will
          avoid the loss of RTP in this context, although the loss of
          RTCP may be inevitable in this particular example.  It is also
          worth noting that RFC 3605 is unfortunately not a mandatory
          part of SIP (i.e., RFC 3261).  This requirement will therefore
          address a particularly nasty problem that will prevail for a
          significant amount of time.






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   REQ-3  This requirement 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.
          NATs that implement port preservation have to deal with
          conflicts on ports, and the multiple code paths this
          introduces often result in nondeterministic behavior.
          However, it should be understood that a NAT that when a port
          is randomly assigned, it may just randomly happen to be
          assigned the same port.  Applications must therefore be able
          to deal with both port preservation, and no port preservation.
          a) Certain applications expect the source UDP port to be in
             the well-known range.  See RFC 2623 for an example.
   REQ-4  This is to avoid breaking peer-to-peer applications which do
          not explicity and separately specify RTP and RTCP port numbers
          and which follow the RFC 3550 rule to decrement an odd RTP
          port to make it even.  The same considerations as per the IP
          address pooling requirement apply.
   REQ-5  This requirement is to ensure that the timeout is long enough
          to avoid too frequent timer refresh packets.
          a) Configuration is desirable for adapting to specific
             networks and troubleshooting.
          b) This default is to avoid too frequent timer refresh
             packets.
   REQ-6  Outbound refresh is necessary for allowing the client to keep
          the mapping alive.
          a) Inbound refresh may be useful for applications where there
             is no outgoing UDP traffic.
          b) Using the refresh on a per mapping basis avoids the need
             for separate keep alive packets for all the available
             sessions.
   REQ-7  Filtering based on the IP address is felt to have the maximum
          balance between security and usefulness.  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.  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 requirement is to allow communications between two
          endpoints behind the same NAT when they are trying each
          other's external IP addresses.






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          a) 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-9  NAT ALGs may interfere with UNSAF methods.
          a) This requirement allows the user to enable ALGs which are
             necessary to aid operation of some applications without
             enabling ALGs which interfere with operation of other
             applications.
   REQ-10 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.
   REQ-11 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-12 Fragmented packets become more common with large video packets
          and should continue to work.  Applications can use MTU
          discovery to work around this problem.
   REQ-13 See Security Considerations.

13.  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 mapping 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 mapping 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 mappings
   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.




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   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 mapping 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 mapping
   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 [12] and RFC
   1858 [11].

14.  IANA Considerations

   There are no IANA considerations.

15.  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



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   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.
   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 [17] 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 [17] provide a reference model
           for what any UNSAF proposal might encounter in deployed NATs.

16.  Acknowledgments

   The editor would like to acknowledge Bryan Ford, Pyda Srisuresh and
   Dan Kegel for the their multiple contributions on peer-to-peer
   communications accross a NAT, from which a lot of the material in
   this specification is derived.

   Dan Wing contributed substantial text on IP fragmentation and ICMP
   behavior.

   Thanks to Rohan Mahy, Jonathan Rosenberg, Mary Barnes, Melinda Shore,
   Lyndsay Campbell, Geoff Huston, Jiri Kuthan, Harald Welte, Steve
   Casner, Robert Sanders and Spencer Dawkins for their important
   contributions.

17.  References

17.1  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.





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17.2  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.

   [7]   Rosenberg, J., Huitema, C., and R. Mahy, "STUN - Simple
         Traversal of User Datagram Protocol (UDP) Through Network
         Address Translators (NATs)", draft-ietf-behave-rfc3489bis (work
         in progress), February 2003.

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

   [9]   Huitema, C., "Real Time Control Protocol (RTCP) attribute in
         Session Description Protocol (SDP)", RFC 3605, October 2003.

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

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

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

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

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

   [15]  Knowles, S., "IESG Advice from Experience with Path MTU
         Discovery", March 1993.

   [16]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network Address Translator (NAT) Traversal for
         the Session Initiation Protocol (SIP)",



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         draft-ietf-mmusic-ice-04 (work in progress), February 2005.

   [17]  Jennings, C., "NAT Classification Results using STUN",
         draft-jennings-behave-test-results-00 (work in progress),
         February 2005.

   [18]  Postel, J. and J. Reynolds, "FILE TRANSFER PROTOCOL (FTP)",
         RFC 959, October 1985.

   [19]  Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND
         SPECIFICATION", RFC 1035, November 1987.

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


Authors' Addresses

   Francois Audet (editor)
   Nortel Networks
   4655 Great America Parkway
   Santa Clara, CA  95054
   US

   Phone: +1 408 495 3756
   Email: audet@nortel.com


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

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














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