Network Address Translation (NAT) Behavioral Requirements for Unicast UDP
RFC 4787

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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