IP Multicast Requirements for a Network Address Translator (NAT) and a Network Address Port Translator (NAPT)
RFC 5135

Network Working Group                                            D. Wing
Request for Comments: 5135                                     T. Eckert
BCP: 135                                             Cisco Systems, Inc.
Category: Best Current Practice                            February 2008

    IP Multicast Requirements for a Network Address Translator (NAT)
              and a Network Address Port Translator (NAPT)

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.

Abstract

   This document specifies requirements for a for a Network Address
   Translator (NAT) and a Network Address Port Translator (NAPT) that
   support Any Source IP Multicast or Source-Specific IP Multicast.  An
   IP multicast-capable NAT device that adheres to the requirements of
   this document can optimize the operation of IP multicast applications
   that are generally unaware of IP multicast NAT devices.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology Used in This Document  . . . . . . . . . . . . . .  2
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  NATing IP Multicast Data Packets . . . . . . . . . . . . .  5
       4.1.1.  Receiving Multicast Data Packets . . . . . . . . . . .  5
       4.1.2.  Sending Multicast Data Packets . . . . . . . . . . . .  5
     4.2.  IGMP Version Support . . . . . . . . . . . . . . . . . . .  6
       4.2.1.  IGMPv1 or IGMPv2 . . . . . . . . . . . . . . . . . . .  7
       4.2.2.  IGMPv3 . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Any Source Multicast Transmitters  . . . . . . . . . . . .  8
   5.  Requirements Summary . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Application Considerations  . . . . . . . . . . . . . 14

Wing & Eckert            Best Current Practice                  [Page 1]
RFC 5135             NAT IP Multicast Requirements         February 2008

1.  Introduction

   In order for IP multicast applications to function well over NATs,
   multicast UDP must work as seamlessly as unicast UDP.  However, NATs
   have little consistency in IP multicast operation, which results in
   inconsistent user experiences and failed IP multicast operation.

   This document targets requirements intended to enable correct
   operations of Any Source Multicast and Source-Specific Multicast in
   devices running Internet Group Management Protocol (IGMP) proxy
   routing and NAT and without applying NAT to IP multicast group
   addresses.  This profile of functionality is the expected best
   practice for residential access routers, small branch routers, or
   similar deployments.

   Most of the principles outlined in this document do also apply when
   using protocols other than IGMP, such as Protocol Independent
   Multicast - Sparse Mode (PIM-SM), or when performing NAT between
   multiple "inside" interfaces, but explicit consideration for these
   cases is outside the scope of this document.

   This document describes the behavior of a device that functions as a
   NAT for unicast flows and also forwards IP multicast traffic in
   either direction ('inside' to 'outside', or 'outside' to 'inside').
   This allows a host 'inside' the NAT to both receive multicast traffic
   and to source multicast traffic.  Hosts on the 'inside' interface(s)
   of a NAT indicate their interest in receiving an IP multicast flow by
   sending an IGMP message to their local interface.  An IP multicast-
   capable NAT will see that IGMP message (IGMPv1 [RFC1112], IGMPv2
   [RFC2236], IGMPv3 [RFC3376]), possibly perform some functions on that
   IGMP message, and forward it to its upstream router.  This causes the
   upstream router to send that IP multicast traffic to the NAT, which
   forwards it to those 'inside' segment(s) with host(s) that had
   previously sent IGMP messages for that IP multicast traffic.

   Out of scope of this document are PIM-SM [RFC4601] and IPv6
   [RFC2460].  The IGMP Proxy devices that are scoped in this document
   do not forward PIM-SM.  IPv6 is out of scope because NAT is not
   considered necessary with IPv6.

   This document is a companion document to "NAT Behavioral Requirements