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rfc:rfc4787

Network Working Group F. Audet, Ed. Request for Comments: 4787 Nortel Networks BCP: 127 C. Jennings Category: Best Current Practice Cisco Systems

                                                          January 2007
     Network Address Translation (NAT) Behavioral Requirements
                          for Unicast UDP

Status of This Memo

 This document specifies an Internet Best Current Practices for the
 Internet Community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).

Abstract

 This document defines basic terminology for describing different
 types of Network Address Translation (NAT) behavior when handling
 Unicast UDP and also defines a set of requirements 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 these
 applications will function properly.

Audet & Jennings Best Current Practice [Page 1] RFC 4787 NAT UDP Unicast Requirements January 2007

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  . . . . . . . . . . . . . . . . . . . . .  9
     4.2.1.  Port Assignment Behavior . . . . . . . . . . . . . . .  9
     4.2.2.  Port Parity  . . . . . . . . . . . . . . . . . . . . . 11
     4.2.3.  Port Contiguity  . . . . . . . . . . . . . . . . . . . 11
   4.3.  Mapping Refresh  . . . . . . . . . . . . . . . . . . . . . 12
   4.4.  Conflicting Internal and External IP Address Spaces  . . . 13
 5.  Filtering Behavior . . . . . . . . . . . . . . . . . . . . . . 15
 6.  Hairpinning Behavior . . . . . . . . . . . . . . . . . . . . . 16
 7.  Application Level Gateways . . . . . . . . . . . . . . . . . . 17
 8.  Deterministic Properties . . . . . . . . . . . . . . . . . . . 18
 9.  ICMP Destination Unreachable Behavior  . . . . . . . . . . . . 19
 10. Fragmentation of Outgoing Packets  . . . . . . . . . . . . . . 20
 11. Receiving Fragmented Packets . . . . . . . . . . . . . . . . . 20
 12. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 21
 13. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
 14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 25
 15. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 26
 16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   16.1. Normative References . . . . . . . . . . . . . . . . . . . 26
   16.2. Informative References . . . . . . . . . . . . . . . . . . 26

Audet & Jennings Best Current Practice [Page 2] RFC 4787 NAT UDP Unicast Requirements January 2007

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 online 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 to Traditional NATs as
 described in [RFC2663].
 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
 functionalities.  A comprehensive description of firewall behaviors
 and associated requirements is specifically out-of-scope for this
 specification.  However, this specification does cover basic firewall
 aspects present in NATs (see Section 5).
 Approaches using directly signaled control of middle boxes are out of
 scope.
 UDP Relays (e.g., Traversal Using Relay NAT [TURN]) are out of scope.
 Application aspects are out of scope, as the focus here is strictly
 on the NAT itself.
 This document only covers aspects of NAT traversal related to Unicast
 UDP [RFC0768] over IP [RFC0791] and their dependencies on other
 protocols.

2. Introduction

 Network Address Translators (NATs) are well known to cause very
 significant problems with applications that carry IP addresses in the
 payload (see [RFC3027]).  Applications that suffer from this problem
 include Voice Over IP and Multimedia Over IP (e.g., SIP [RFC3261] and
 H.323 [ITU.H323]), 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 [RFC2663] are one such mechanism.  STUN
 [RFC3489bis] describes a UNilateral Self-Address Fixing (UNSAF)
 mechanism [RFC3424].  Teredo [RFC4380] describes an UNSAF mechanism
 consisting of tunnelling IPv6 [RFC2460] over UDP/IPv4.  UDP Relays
 have also been used to enable applications across NATs, but these are
 generally seen as a solution of last resort.  Interactive

Audet & Jennings Best Current Practice [Page 3] RFC 4787 NAT UDP Unicast Requirements January 2007

 Connectivity Establishment [ICE] 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
 relay.
 As pointed out in UNSAF [RFC3424], "From observations of deployed
 networks, it is clear that different NAT box implementations vary
 widely in terms of how they handle different traffic and addressing
 cases".  This wide degree of variability is one factor in the overall
 brittleness introduced by NATs and makes it extremely difficult to
 predict how any given protocol will behave on a network traversing
 NAT.  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 NAT vendors face is that they are not sure how
 best to do that or how to document their NATs' behavior.
 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.
 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 [RFC2119].
 Readers are urged to refer to [RFC2663] for information on NAT
 taxonomy and terminology.  Traditional NAT is the most common type of
 NAT device deployed.  Readers may refer to [RFC3022] for detailed
 information on traditional NAT.  Traditional NAT has two main
 varieties -- Basic NAT and Network Address/Port Translator (NAPT).
 NAPT is by far the most commonly deployed NAT device.  NAPT allows
 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

Audet & Jennings Best Current Practice [Page 4] RFC 4787 NAT UDP Unicast Requirements January 2007

 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 a (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.
 This document uses IANA terminology for port ranges, i.e., "Well
 Known Ports" is 0-1023, "Registered" is 1024-49151, and "Dynamic
 and/or Private" is 49152-65535, as defined in
 http://www.iana.org/assignments/port-numbers.
 STUN [RFC3489] 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 has proven 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 NATs.

4.1. Address and Port Mapping

 When an internal endpoint opens an outgoing 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

Audet & Jennings Best Current Practice [Page 5] RFC 4787 NAT UDP Unicast Requirements January 2007

 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 reuse 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 are 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
                       Address and Port Mapping
 The following address and port mapping behavior are defined:
    Endpoint-Independent Mapping:
       The NAT reuses the port mapping for subsequent packets sent
       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.

Audet & Jennings Best Current Practice [Page 6] RFC 4787 NAT UDP Unicast Requirements January 2007

    Address-Dependent Mapping:
       The NAT reuses the port mapping for subsequent packets sent
       from the same internal IP address and port (X:x) to the same
       external IP address, regardless of the external port.
       Specifically, X1':x1' equals X2':x2' if and only if, Y2 equals
       Y1.
    Address and Port-Dependent Mapping:
       The NAT reuses the port mapping for subsequent packets sent
       from the same internal IP address and port (X:x) to the same
       external IP address and port while the mapping is still active.
       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.
 REQ-1:  A NAT MUST have an "Endpoint-Independent Mapping" behavior.
 Justification:  In order for UNSAF methods to work, REQ-1 needs to be
    met.  Failure to meet REQ-1 will force the use of a UDP relay,
    which is very often impractical.
 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 that do not
 negotiate IP addresses individually (e.g., some applications using
 RTP and RTCP).

Audet & Jennings Best Current Practice [Page 7] RFC 4787 NAT UDP Unicast Requirements January 2007

 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.
 Justification:  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 that 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 that 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 [RFC3605] for indicating RTP and RTCP addresses and
    ports separately, but the other peer does not, there may still be
    breakage in the form of the stream losing RTCP packets.  This
    requirement 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 [RFC3261].  Therefore, this requirement will
    address a particularly nasty problem that will prevail for a
    significant period of time.

Audet & Jennings Best Current Practice [Page 8] RFC 4787 NAT UDP Unicast Requirements January 2007

4.2. Port Assignment

4.2.1. Port Assignment Behavior

 This section uses the following diagram for reference.
                                    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
                            Port Assignment
 Some NATs attempt to preserve the port number used internally when
 assigning a mapping to an external IP address and port (e.g., x1=x1',
 x2=x2').  This port assignment behavior is referred to as "port
 preservation".  In case of port collision, these NATs attempt a
 variety of techniques for coping.  For example, some NATs will
 overridden the previous mapping to preserve the same port.  Other
 NATs will assign a different IP address from a pool of external IP
 addresses; this is only possible as long as the NAT has enough
 external IP addresses; if the port is already in use on all available
 external IP addresses, then these NATs will pick a different port
 (i.e., they don't do port preservation anymore).
 Some NATs use "Port overloading", i.e., they always use port
 preservation even in the case of collision (i.e., X1'=X2' and
 x1=x2=x1'=x2').  Most applications will fail if the NAT uses "Port
 overloading".
 A NAT that does not attempt to make the external port numbers match
 the internal port numbers in any case is referred to as "no port
 preservation".

Audet & Jennings Best Current Practice [Page 9] RFC 4787 NAT UDP Unicast Requirements January 2007

 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 downside 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 dynamic 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.  [RFC0768] specifies that the source port is set to zero if no
 reply packets are expected.  In this case, it does not matter what
 the NAT maps it to, as the source port will not be used.  However,
 many common OS APIs do not allow a user to send from port zero,
 applications do not use port zero, and the behavior of various
 existing NATs with regards to a packet with a source of port zero is
 unknown.  This document does not specify any normative behavior for a
 NAT when handling a packet with a source port of zero which means
 that applications cannot count on any sort of deterministic behavior
 for these packets.
 REQ-3:  A NAT MUST NOT have a "Port assignment" behavior of "Port
    overloading".
    a) If the host's source port was in the range 0-1023, it is
       RECOMMENDED the NAT's source port 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 be in that range.
 Justification:  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
    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 the discussion of Network File System
       port expectations in [RFC2623] for an example.

Audet & Jennings Best Current Practice [Page 10] RFC 4787 NAT UDP Unicast Requirements January 2007

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 [RFC3550] rule that RTP use
 even ports, and RTCP use odd ports.  RFC 3550 allows any port numbers
 to be used for RTP and RTCP if the two numbers are specified
 separately; for example, using [RFC3605].  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.
 REQ-4:  It is RECOMMENDED that a NAT have a "Port parity
    preservation" behavior of "Yes".
 Justification:  This is to avoid breaking peer-to-peer applications
    that do not explicitly and separately specify RTP and RTCP port
    numbers and that follow the RFC 3550 rule to decrement an odd RTP
    port to make it even.  The same considerations apply, as per the
    IP address pooling requirement.

4.2.3. Port Contiguity

 Some NATs attempt to preserve the port contiguity rule of RTCP=RTP+1.
 These NATs do things like sequential assignment or port reservation.
 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 problem with glare if many applications (or
 endpoints) are trying to open mappings simultaneously.  Port
 preservation is also problematic since it is wasteful, especially
 considering that a NAT cannot 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 follow rules to negotiate RTP and RTCP separately,
 and account for the very real possibility that the RTCP=RTP+1 rule

Audet & Jennings Best Current Practice [Page 11] RFC 4787 NAT UDP Unicast Requirements January 2007

 will be broken.  As this is an application requirement, it is outside
 the scope of this document.

4.3. Mapping Refresh

 NAT 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.
 REQ-5:  A NAT UDP mapping timer MUST NOT expire in less than two
    minutes, unless REQ-5a applies.
    a) For specific destination ports in the well-known port range
       (ports 0-1023), a NAT MAY have shorter UDP mapping timers that
       are specific to the IANA-registered application running over
       that specific destination port.
    b) The value of the NAT UDP mapping timer MAY be configurable.
    c) A default value of five minutes or more for the NAT UDP mapping
       timer is RECOMMENDED.
 Justification:  This requirement is to ensure that the timeout is
    long enough to avoid too-frequent timer refresh packets.
    a) Some UDP protocols using UDP use very short-lived connections.
       There can be very many such connections; keeping them all in a
       connections table could cause considerable load on the NAT.
       Having shorter timers for these specific applications is,
       therefore, an optimization technique.  It is important that the
       shorter timers applied to specific protocols be used sparingly,
       and only for protocols using well-known destination ports that
       are known to have a shorter timer, and that are known not to be
       used by any applications for other purposes.
    b) Configuration is desirable for adapting to specific networks
       and troubleshooting.
    c) This default is to avoid too-frequent timer refresh packets.
 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".

Audet & Jennings Best Current Practice [Page 12] RFC 4787 NAT UDP Unicast Requirements January 2007

 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".
 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".
 Justification:  Outbound refresh is necessary for allowing the client
    to keep the mapping alive.
    a) Inbound refresh may be useful for applications with no outgoing
       UDP traffic.  However, allowing inbound refresh may allow an
       external attacker or misbehaving application to keep a mapping
       alive indefinitely.  This may be a security risk.  Also, if the
       process is repeated with different ports, over time, it could
       use up all the ports on the NAT.

4.4. Conflicting Internal and External IP Address Spaces

 Many NATs, particularly consumer-level devices designed to be
 deployed by nontechnical users, routinely obtain their external IP
 address, default router, and other IP configuration information for
 their external interface dynamically from an external network, such
 as an upstream ISP.  The NAT, in turn, automatically sets up its own
 internal subnet in one of the private IP address spaces assigned to
 this purpose in [RFC1918], typically providing dynamic IP
 configuration services for hosts on this internal network.
 Auto-configuration of NATs and private networks can be problematic,
 however, if the NAT's external network is also in RFC 1918 private
 address space.  In a common scenario, an ISP places its customers
 behind a NAT and hands out private RFC 1918 addresses to them.  Some
 of these customers, in turn, deploy consumer-level NATs, which, in
 effect, act as "second-level" NATs, multiplexing their own private
 RFC 1918 IP subnets onto the single RFC 1918 IP address provided by
 the ISP.  There is no inherent guarantee, in this case, that the
 ISP's "intermediate" privately-addressed network and the customer's
 internal privately-addressed network will not use numerically
 identical or overlapping RFC 1918 IP subnets.  Furthermore, customers
 of consumer-level NATs cannot be expected to have the technical

Audet & Jennings Best Current Practice [Page 13] RFC 4787 NAT UDP Unicast Requirements January 2007

 knowledge to prevent this scenario from occurring by manually
 configuring their internal network with non-conflicting RFC 1918
 subnets.
 NAT vendors need to design their NATs to ensure that they function
 correctly and robustly even in such problematic scenarios.  One
 possible solution is for the NAT to ensure that whenever its external
 link is configured with an RFC 1918 private IP address, the NAT
 automatically selects a different, non-conflicting RFC 1918 IP subnet
 for its internal network.  A disadvantage of this solution is that,
 if the NAT's external interface is dynamically configured or re-
 configured after its internal network is already in use, then the NAT
 may have to renumber its entire internal network dynamically if it
 detects a conflict.
 An alternative solution is for the NAT to be designed so that it can
 translate and forward traffic correctly, even when its external and
 internal interfaces are configured with numerically overlapping IP
 subnets.  In this scenario, for example, if the NAT's external
 interface has been assigned an IP address P in RFC 1918 space, then
 there might also be an internal node I having the same RFC 1918
 private IP address P.  An IP packet with destination address P on the
 external network is directed at the NAT, whereas an IP packet with
 the same destination address P on the internal network is directed at
 node I.  The NAT therefore needs to maintain a clear operational
 distinction between "external IP addresses" and "internal IP
 addresses" to avoid confusing internal node I with its own external
 interface.  In general, the NAT needs to allow all internal nodes
 (including I) to communicate with all external nodes having public
 (non-RFC 1918) IP addresses, or having private IP addresses that do
 not conflict with the addresses used by its internal network.
 REQ-7:  A NAT device whose external IP interface can be configured
    dynamically MUST either (1) automatically ensure that its internal
    network uses IP addresses that do not conflict with its external
    network, or (2) be able to translate and forward traffic between
    all internal nodes and all external nodes whose IP addresses
    numerically conflict with the internal network.
 Justification:  If a NAT's external and internal interfaces are
    configured with overlapping IP subnets, then there is, of course,
    no way for an internal host with RFC 1918 IP address Q to initiate
    a direct communication session to an external node having the same
    RFC 1918 address Q, or to other external nodes with IP addresses
    that numerically conflict with the internal subnet.  Such nodes
    can still open communication sessions indirectly via NAT traversal
    techniques, however, with the help of a third-party server, such
    as a STUN server having a public, non-RFC 1918 IP address.  In

Audet & Jennings Best Current Practice [Page 14] RFC 4787 NAT UDP Unicast Requirements January 2007

    this case, nodes with conflicting private RFC 1918 addresses on
    opposite sides of the second-level NAT can communicate with each
    other via their respective temporary public endpoints on the main
    Internet, as long as their common, first-level NAT (e.g., the
    upstream ISP's NAT) supports hairpinning behavior, as described in
    Section 6.

5. Filtering Behavior

 This section describes various filtering behaviors observed in NATs.
 When an internal endpoint opens an outgoing 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.
    Endpoint-Independent Filtering:
       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.
    Address-Dependent Filtering:
       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:any 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.
    Address and Port-Dependent Filtering:
       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.

Audet & Jennings Best Current Practice [Page 15] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-8:  If application transparency is most important, it is
    RECOMMENDED that a NAT have an "Endpoint-Independent Filtering"
    behavior.  If a more stringent filtering behavior is most
    important, it is RECOMMENDED that a NAT have an "Address-Dependent
    Filtering" behavior.
    a) The filtering behavior MAY be an option configurable by the
       administrator of the NAT.
 Justification:  The recommendation to use Endpoint-Independent
    Filtering is aimed at maximizing application transparency; in
    particular, for applications that receive media simultaneously
    from multiple locations (e.g., gaming), or applications that use
    rendezvous techniques.  However, it is also possible that, in some
    circumstances, it may be preferable to have a more stringent
    filtering behavior.  Filtering independently of the external
    endpoint is not as secure: An unauthorized packet could get
    through 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 policy could interfere
    with applications that expect to receive UDP packets on more than
    one UDP port.  Using Endpoint-Independent Filtering or Address-
    Dependent Filtering instead of Address and Port-Dependent
    Filtering on a NAT (say, NAT-A) also has benefits when the other
    endpoint is behind a non-BEHAVE compliant NAT (say, NAT-B) that
    does not support REQ-1.  When the endpoints use ICE, if NAT-A uses
    Address and Port-Dependent Filtering, connectivity will require a
    UDP relay.  However, if NAT-A uses Endpoint-Independent Filtering
    or Address-Dependent Filtering, ICE will ultimately find
    connectivity without requiring a UDP relay.  Having the filtering
    behavior being an option configurable by the administrator of the
    NAT ensures that a NAT can be used in the widest variety of
    deployment scenarios.

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.

Audet & Jennings Best Current Practice [Page 16] RFC 4787 NAT UDP Unicast Requirements January 2007

   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 Behavior
 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 (X1:x1) or an external (X1':x1') source IP address and port.
 Therefore, the hairpinning NAT behavior can 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
 implementations that expect an external IP address and port.
 REQ-9:  A NAT MUST support "Hairpinning".
    a) A NAT Hairpinning behavior MUST be "External source IP address
       and port".
 Justification:  This requirement is to allow communications between
    two endpoints behind the same NAT when they are trying each
    other's external IP addresses.
    a) Using the external source IP address is necessary for
       applications with a restrictive policy of not accepting packets
       from IP addresses that differ from what is expected.

7. Application Level Gateways

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

Audet & Jennings Best Current Practice [Page 17] RFC 4787 NAT UDP Unicast Requirements January 2007

 Certain NATs have these ALGs turned on permanently, others have them
 turned on by default but allow them to be turned off, and others have
 them turned off by default but allow them be turned on.
 NAT ALGs may interfere with UNSAF methods or protocols that try to be
 NAT-aware and therefore must be used with extreme caution.
 REQ-10:  To eliminate interference with UNSAF NAT traversal
    mechanisms and allow integrity protection of UDP communications,
    NAT ALGs for UDP-based protocols SHOULD be turned off.  Future
    standards track specifications that define ALGs can update this to
    recommend the defaults for the ALGs that they define.
    a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
       the NAT administrator to enable or disable each ALG separately.
 Justification:  NAT ALGs may interfere with UNSAF methods.
    a) This requirement allows the user to enable those ALGs that are
       necessary to aid in the operation of some applications without
       enabling ALGs, which interfere with the operation of other
       applications.

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.
 Another example is that some NATs have an "Endpoint-Independent
 Mapping", combined with "Port Overloading", as long as two endpoints
 are not establishing sessions to the same external direction, but
 then switch their behavior to "Address and Port-Dependent Mapping"

Audet & Jennings Best Current Practice [Page 18] RFC 4787 NAT UDP Unicast Requirements January 2007

 without "Port Preservation" upon detection of these conflicting
 sessions establishments.
 Any NAT that changes the NAT Mapping or the Filtering behavior
 without configuration changes, 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 certainly possible that they exist.
 REQ-11:  A NAT MUST have deterministic behavior, i.e., it MUST NOT
    change the NAT translation (Section 4) or the Filtering
    (Section 5) Behavior at any point in time, or under any particular
    conditions.
 Justification:  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.

9. ICMP Destination Unreachable Behavior

 When a NAT sends a packet toward 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 mapping.  A NAT that performs the functions described
 in the paragraph above is referred to as "support ICMP Processing".
 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] and [RFC1435]), and
 traceroute.  Blocking any ICMP message is discouraged, and blocking
 ICMP Destination Unreachable is strongly discouraged.

Audet & Jennings Best Current Practice [Page 19] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-12:  Receipt of any sort of ICMP message MUST NOT terminate the
    NAT mapping.
    a) The NAT's default configuration SHOULD NOT filter ICMP messages
       based on their source IP address.
    b) It is RECOMMENDED that a NAT support ICMP Destination
       Unreachable messages.
 Justification:  This is easy to do and is used for many things
    including MTU discovery and rapid detection of error conditions,
    and has no negative consequences.

10. Fragmentation of Outgoing Packets

 When the MTU of the adjacent link is too small, fragmentation of
 packets going from the internal side to the external side of the NAT
 may occur.  This can occur if the NAT is doing Point-to-Point over
 Ethernet (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, or for other reasons.
 It is worth noting that many IP stacks do not use Path MTU Discovery
 with UDP packets.
 The packet could have its Don't Fragment bit set to 1 (DF=1) or 0
 (DF=0).
 REQ-13:  If the packet received on an internal IP address has DF=1,
    the NAT MUST send back an ICMP message "Fragmentation needed and
    DF set" to the host, as described in [RFC0792].
    a) If the packet has DF=0, the NAT MUST fragment the packet and
       SHOULD send the fragments in order.
 Justification:  This is as per RFC 792.
    a) This is the same function a router performs in a similar
       situation [RFC1812].

11. Receiving Fragmented Packets

 For a variety of reasons, a NAT may receive a fragmented packet.  The
 IP packet containing the header could arrive in any fragment,
 depending on network conditions, packet ordering, and the
 implementation of the IP stack that generated the fragments.

Audet & Jennings Best Current Practice [Page 20] RFC 4787 NAT UDP Unicast Requirements January 2007

 A NAT that is capable only of receiving fragments in order (that is,
 with the 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 fragments in or out of order and
 forwarding the individual fragments (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".
 REQ-14:  A NAT MUST support receiving in-order and out-of-order
    fragments, so it MUST have "Received Fragment Out of Order"
    behavior.
    a) A NAT's out-of-order fragment processing mechanism MUST be
       designed so that fragmentation-based DoS attacks do not
       compromise the NAT's ability to process in-order and
       unfragmented IP packets.
 Justification:  See Security Considerations.

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.  The requirements listed earlier in
 the document are consolidated here into a single section.
 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 [ICE].
 A NAT that supports all the mandatory requirements of this
 specification (i.e., the "MUST"), is "compliant with this
 specification".  A NAT that supports all the requirements of this
 specification (i.e., including the "RECOMMENDED") is "fully compliant
 with all the mandatory and recommended requirements of this
 specification".

Audet & Jennings Best Current Practice [Page 21] RFC 4787 NAT UDP Unicast Requirements January 2007

 REQ-1:  A NAT MUST have an "Endpoint-Independent Mapping" 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:  A NAT MUST NOT have a "Port assignment" behavior of "Port
    overloading".
    a) If the host's source port was in the range 0-1023, it is
       RECOMMENDED the NAT's source port 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 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 two
    minutes, unless REQ-5a applies.
    a) For specific destination ports in the well-known port range
       (ports 0-1023), a NAT MAY have shorter UDP mapping timers that
       are specific to the IANA-registered application running over
       that specific destination port.
    b) The value of the NAT UDP mapping timer MAY be configurable.
    c) A default value of five minutes or more 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".
 REQ-7  A NAT device whose external IP interface can be configured
    dynamically MUST either (1) Automatically ensure that its internal
    network uses IP addresses that do not conflict with its external
    network, or (2) Be able to translate and forward traffic between
    all internal nodes and all external nodes whose IP addresses
    numerically conflict with the internal network.
 REQ-8:  If application transparency is most important, it is
    RECOMMENDED that a NAT have "Endpoint-Independent Filtering"
    behavior.  If a more stringent filtering behavior is most
    important, it is RECOMMENDED that a NAT have "Address-Dependent
    Filtering" behavior.

Audet & Jennings Best Current Practice [Page 22] RFC 4787 NAT UDP Unicast Requirements January 2007

    a) The filtering behavior MAY be an option configurable by the
       administrator of the NAT.
 REQ-9:  A NAT MUST support "Hairpinning".
    a) A NAT Hairpinning behavior MUST be "External source IP address
       and port".
 REQ-10:  To eliminate interference with UNSAF NAT traversal
    mechanisms and allow integrity protection of UDP communications,
    NAT ALGs for UDP-based protocols SHOULD be turned off.  Future
    standards track specifications that define an ALG can update this
    to recommend the ALGs on which they define default.
    a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow
       the NAT administrator to enable or disable each ALG separately.
 REQ-11:  A NAT MUST have deterministic behavior, i.e., it MUST NOT
    change the NAT translation (Section 4) or the Filtering
    (Section 5) Behavior at any point in time, or under any particular
    conditions.
 REQ-12:  Receipt of any sort of ICMP message MUST NOT terminate the
    NAT mapping.
    a) The NAT's default configuration SHOULD NOT filter ICMP messages
       based on their source IP address.
    b) It is RECOMMENDED that a NAT support ICMP Destination
       Unreachable messages.
 REQ-13  If the packet received on an internal IP address has DF=1,
    the NAT MUST send back an ICMP message "Fragmentation needed and
    DF set" to the host, as described in [RFC0792].
    a) If the packet has DF=0, the NAT MUST fragment the packet and
       SHOULD send the fragments in order.
 REQ-14:  A NAT MUST support receiving in-order and out-of-order
    fragments, so it MUST have "Received Fragment Out of Order"
    behavior.
    a) A NAT's out-of-order fragment processing mechanism MUST be
       designed so that fragmentation-based DoS attacks do not
       compromise the NAT's ability to process in-order and
       unfragmented IP packets.

Audet & Jennings Best Current Practice [Page 23] RFC 4787 NAT UDP Unicast Requirements January 2007

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 document recommends that the timers for mapping be refreshed on
 outgoing packets (see REQ-6) 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 document recommends that the NAT filters be specific to the
 external IP address only (see REQ-8) and not to the external IP
 address and UDP 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.
 This document 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

Audet & Jennings Best Current Practice [Page 24] RFC 4787 NAT UDP Unicast Requirements January 2007

 Denial-of-Service (DoS) opportunity, if done incorrectly.
 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 [RFC3128] and [RFC1858].

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 [RFC3424].
 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.
 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 [RESULTS] 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.

Audet & Jennings Best Current Practice [Page 25] RFC 4787 NAT UDP Unicast Requirements January 2007

 Arch-5:  This work and the test results [RESULTS] provide a reference
          model for what any UNSAF proposal might encounter in
          deployed NATs.

15. Acknowledgments

 The editor would like to acknowledge Bryan Ford, Pyda Srisuresh, and
 Dan Kegel for their multiple contributions on peer-to-peer
 communications across a NAT.  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, Spencer Dawkins, Saikat Guha, Christian Huitema, Yutaka
 Takeda, Paul Hoffman, Lisa Dusseault, Pekka Savola, Peter Koch, Jari
 Arkko, and Alfred Hoenes for their contributions.

16. References

16.1. Normative References

 [RFC0768]     Postel, J., "User Datagram Protocol", STD 6, RFC 768,
               August 1980.
 [RFC0791]     Postel, J., "Internet Protocol", STD 5, RFC 791,
               September 1981.
 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

16.2. Informative References

 [RFC0792]     Postel, J., "Internet Control Message Protocol", STD 5,
               RFC 792, September 1981.
 [RFC1191]     Mogul, J. and S. Deering, "Path MTU discovery",
               RFC 1191, November 1990.
 [RFC1435]     Knowles, S., "IESG Advice from Experience with Path MTU
               Discovery", RFC 1435, March 1993.
 [RFC1812]     Baker, F., "Requirements for IP Version 4 Routers",
               RFC 1812, June 1995.
 [RFC1858]     Ziemba, G., Reed, D., and P. Traina, "Security
               Considerations for IP Fragment Filtering", RFC 1858,
               October 1995.

Audet & Jennings Best Current Practice [Page 26] RFC 4787 NAT UDP Unicast Requirements January 2007

 [RFC1918]     Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
               and E. Lear, "Address Allocation for Private
               Internets", BCP 5, RFC 1918, February 1996.
 [RFC2460]     Deering, S. and R. Hinden, "Internet Protocol, Version
               6 (IPv6) Specification", RFC 2460, December 1998.
 [RFC2623]     Eisler, M., "NFS Version 2 and Version 3 Security
               Issues and the NFS Protocol's Use of RPCSEC_GSS and
               Kerberos V5", RFC 2623, June 1999.
 [RFC2663]     Srisuresh, P. and M. Holdrege, "IP Network Address
               Translator (NAT) Terminology and Considerations",
               RFC 2663, August 1999.
 [RFC3022]     Srisuresh, P. and K. Egevang, "Traditional IP Network
               Address Translator (Traditional NAT)", RFC 3022,
               January 2001.
 [RFC3027]     Holdrege, M. and P. Srisuresh, "Protocol Complications
               with the IP Network Address Translator", RFC 3027,
               January 2001.
 [RFC3128]     Miller, I., "Protection Against a Variant of the Tiny
               Fragment Attack (RFC 1858)", RFC 3128, June 2001.
 [RFC3261]     Rosenberg, J., Schulzrinne, H., Camarillo, G.,
               Johnston, A., Peterson, J., Sparks, R., Handley, M.,
               and E. Schooler, "SIP: Session Initiation Protocol",
               RFC 3261, June 2002.
 [RFC3424]     Daigle, L. and IAB, "IAB Considerations for UNilateral
               Self-Address Fixing (UNSAF) Across Network Address
               Translation", RFC 3424, November 2002.
 [RFC3489]     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.
 [RFC3550]     Schulzrinne, H., Casner, S., Frederick, R., and V.
               Jacobson, "RTP: A Transport Protocol for Real-Time
               Applications", STD 64, RFC 3550, July 2003.
 [RFC3605]     Huitema, C., "Real Time Control Protocol (RTCP)
               attribute in Session Description Protocol (SDP)",
               RFC 3605, October 2003.

Audet & Jennings Best Current Practice [Page 27] RFC 4787 NAT UDP Unicast Requirements January 2007

 [RFC4380]     Huitema, C., "Teredo: Tunneling IPv6 over UDP through
               Network Address Translations (NATs)", RFC 4380,
               February 2006.
 [RFC3489bis]  Rosenberg, J., "Simple Traversal Underneath Network
               Address Translators (NAT) (STUN)", Work in Progress,
               October 2006.
 [ICE]         Rosenberg, J., "Interactive Connectivity Establishment
               (ICE): A Methodology for Network Address Translator
               (NAT) Traversal for Offer/Answer Protocols", Work
               in Progress, October 2006.
 [RESULTS]     Jennings, C., "NAT Classification Test Results", Work
               in Progress, October 2006.
 [TURN]        Rosenberg, J., "Obtaining Relay Addresses from Simple
               Traversal Underneath NAT (STUN)", Work in Progress,
               October 2006.
 [ITU.H323]    "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 2456
 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

Audet & Jennings Best Current Practice [Page 28] RFC 4787 NAT UDP Unicast Requirements January 2007

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Audet & Jennings Best Current Practice [Page 29]

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