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Copyright Notice

Copyright © The Internet Society (2006).

Abstract

This document places requirements on a Network Address Translator (NAT) and Network Address and Port Translator (NAPT) that supports IP multicast by implementing an IGMP proxy.

Requirements Language

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Table of Contents

1.  Problem Statement
2.  Introduction
    2.1.  Background
3.  NAPT Multicast Requirements
    3.1.  Keep NAT Binding Open
4.  Security Considerations
5.  IANA Considerations
6.  Acknowledgments
7.  Normative References
§  Author's Address
§  Intellectual Property and Copyright Statements

1. Problem Statement

For users to accept and enjoy multicast, multicast UDP must work as seamlessly as unicast UDP. However, today's equipment has little consistency in multicast operation which results in inconsistant user experiences and failed multicast operation.

2. Introduction

This document describes the behavior of a device providing multicast functions similar to [I-D.ietf-magma-igmp-proxy] (Fenner, B., He, H., Haberman, B., and H. Sandick, “IGMP/MLD-based Multicast Forwarding ('IGMP/MLD Proxying'),” April 2004.) and that additionally functions as a Network Address and Port Translator (NAPT), as described in section 4.1.2 of [RFC2663] (Srisuresh, P. and M. Holdrege, “IP Network Address Translator (NAT) Terminology and Considerations,” August 1999.).

Specifically out of scope of this document are:

• sending multicast traffic,
• PIM-SM (Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S., Handley, M., and V. Jacobson, “Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification,” June 1998.) [RFC2362],
• IPv6, and,
• IGMPv1.

Sending multicast traffic is out of scope because it requires NATting the source IP address of such transmitted multicast traffic. Similarly, PIM is used only between routers and the IGMP Proxy devices that are scoped in this document do not function as routers. IPv6 is out of scope because NAT is not considered necessary with IPv6. IGMPv1 is not significantly deployed on the Internet.

This document does not describe how to implement multicast, IGMPv2, or IGMPv3 in an IGMP Proxy device. Rather, it provides requirements for an IGMP Proxy device so that hosts behind the NAT can receive multicast traffic without any knowledge of the IGMP Proxy.

2.1. Background

As detailed in the Document Scope section, the primary functions of an IGMP proxy device are to collect IGMP traffic from one interface and relay it to another interface, and accept multicast traffic from thatinterface and route -- or replicate it -- to other interface(s).

When a NAPT isn't used, a host might be connected to the Internet in a configuration such as this:

             +-------------+
  +------+   |  DSL modem  |        +------------+
  | host +---+     or      +---//---+ WAN Router |
  +------+   | cable modem |        +------------+
             +-------------+

When an IGMP Proxy device is added to such a network, its behavior is identical towards the upstream (WAN) router. Specifically, when dealing with multicast, the IGMP Proxy has the same behavior towards the WAN as if it was a host.

  +------+  +------------+   +-------------+
  | host +--+            |   |  DSL modem  |        +------------+
  +------+  | IGMP Proxy +---+     or      +---//---+ WAN Router |
  +------+  |   (NAPT)   |   | cable modem |        +------------+
  | host +--+            |   +-------------+
  +------+  +------------+

This document is a companion document to "NAT/Firewall Behavioral Requirements" (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) [I-D.ietf-behave-nat-udp].

3. NAPT Multicast Requirements

All requirements of [I-D.ietf-magma-igmp-proxy] (Fenner, B., He, H., Haberman, B., and H. Sandick, “IGMP/MLD-based Multicast Forwarding ('IGMP/MLD Proxying'),” April 2004.) apply equally to NAPTs and are incorporated herein by reference.

Additionally, there are some requirements particular to NATs that implement IGMP proxies:

3.1. Keep NAT Binding Open

The NAT UDP requirements (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) [I-D.ietf-behave-nat-udp] document only requires that a NAT binding be kept open for inside-to-outside UDP flows. However, with multicast traffic, UDP traffic will only arrive outside-to-inside.

Hosts will periodically send IGMP Report messages to indicate continued interest in receiving the multicast traffic. As long as the IGMP Proxy sees a host is interested in receiving the flow, the NAT MUST continue to receive multicast traffic from the WAN and send it to the interfaces with interested hosts.

Per IGMPv3, the default transmission interval for the periodic Membership Report is one second. Per IGMPv2, the default transmission interval for the periodic Unsolicited Report Interval is 10 seconds. If a host no longer sends its periodic messages within those timeframes, the NAT MAY consider the host no longer wants to receive the multicast traffic and can inform the upstream WAN router and close the NAT binding. However, it is suggested that the NAT wait until 3 missing unsolicited reports (to account for packet loss on the LAN, especially wireless LANs), or that the NAT first query the host using IGMPv2 or IGMPv3.

4. Security Considerations

Compliance with this specification does not increase security risks beyond those already discussed in the Security Considerations section of IGMPv3 (Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, “Internet Group Management Protocol, Version 3,” October 2002.) [RFC3376].

5. IANA Considerations

This document does not require any IANA registrations.

6. Acknowledgments

Thanks to Bryan McLaughlin and Yiqun Cai for their assistance in writing this document.

7. Normative References

Author's Address

Intellectual Property Statement

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Copyright Statement

Copyright © The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

Acknowledgment

Funding for the RFC Editor function is currently provided by the Internet Society.