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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 Internet Group Management Protocol (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.  NAPT Inbound Refresh Behavior
    3.2.  Extend Mapping Refresh
4.  Security Considerations
5.  IANA Considerations
6.  Acknowledgments
7.  References
    7.1.  Normative References
    7.2.  Informational 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, NATs have 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 proxy functions as described in [RFC4605] (Fenner, B., He, H., Haberman, B., and H. Sandick, “Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying"),” August 2006.) 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 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 (Deering, S., “Host extensions for IP multicasting,” August 1989.) [RFC1112]. 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 NAPT is not considered necessary with IPv6. IGMPv1 is not significantly deployed on the Internet.

This document does not describe how to implement multicast, IGMPv2 (Fenner, W., “Internet Group Management Protocol, Version 2,” November 1997.) [RFC2236], or IGMPv3 (Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, “Internet Group Management Protocol, Version 3,” October 2002.) [RFC3376] in an IGMP Proxy device. Rather, it provides requirements for an IGMP Proxy device so that hosts behind the NAPT can receive multicast traffic without any knowledge of the IGMP Proxy.

2.1. Background

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 that interface 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 Behavioral Requirements for Unicast UDP" (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) [I-D.ietf-behave-nat-udp].

3. NAPT Multicast Requirements

The NAPTed hosts will periodically send IGMP Report messages to indicate continued interest in receiving the multicast traffic. Per IGMPv3 (Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Thyagarajan, “Internet Group Management Protocol, Version 3,” October 2002.) [RFC3376], the default transmission interval for the periodic Membership Report is one second. Per IGMPv2 (Fenner, W., “Internet Group Management Protocol, Version 2,” November 1997.) [RFC2236], the default transmission interval for the periodic Unsolicited Report Interval is 10 seconds. If a NAPTed host no longer sends its periodic messages within those timeframes, the NAPT device MAY consider the host no longer wants to receive the multicast traffic and can inform the upstream WAN router and close the NAPT mapping. However, it is RECOMMENDED that the NAPT wait until 3 missing unsolicited reports (to account for packet loss on the LAN, especially wireless LANs), or that the NAPT first query the host using IGMPv2 or IGMPv3.

In addition to the above requirements, the NAPT device MUST also:

• follow the RFC2119 requirements of [RFC4605] (Fenner, B., He, H., Haberman, B., and H. Sandick, “Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying"),” August 2006.), and;
• have "NAT Inbound Refresh Behavior", as described in [I-D.ietf-behave-nat-udp] (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) (see Section 3.1 (NAPT Inbound Refresh Behavior)), and;
• extend the Mapping Refresh Timer, as described in [I-D.ietf-behave-nat-udp] (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.), to 60 minutes for UDP packets originating from hosts receiving any multicast stream that are sent to UDP ports above 1023 (see Section 3.2 (Extend Mapping Refresh)).

3.1. NAPT Inbound Refresh Behavior

Multicast traffic arrives only outside-to-inside. Thus, a NAPT needs to also meet REQ-6a of NAT UDP requirements (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) [I-D.ietf-behave-nat-udp] because multicast traffic typically only flows ouside-to-inside.

3.2. Extend Mapping Refresh

RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [RFC3550] uses the source transport address (source IP address and source UDP port), in addition to the the RTP/RTCP SSRC value, to identify session members. If a session member sees the same SSRC arrive from a different transport address, that session member will perform RTP collision detection (section 8.2 of [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.)). If a NAPT followed the requirements of [I-D.ietf-behave-nat-udp] (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) (and timed out a UDP session after 2 minutes of inactivity) and the multicast group is sufficiently large (approximately 300 members with a normal 50kbps audio RTP stream), the elapsed time between a NAPTed host sending its RTCP Receiver Reports would exceed 2 minutes, causing an unnecessary RTP collision detection to be performed by other session members.

To prevent this unnecessary RTP collision detection by other session members, the other session members need to see the same source transport address for the RTP and RTCP traffic from the NAPTed host. This requires the NAPT not assign a new UDP source port for that traffic. A NAPT is unable to associate a received multicast session with its unicasted RTCP Receiver Reports. Thus, this document requires the NAPT to extend its UDP mapping refresh timer. This requirement also facilitates other, non-RTP multicast applications.

This requirement applies to ports above 1023 because RTP and RTCP are only used on ports above 1023. Other, non-RTP multicast feedback protocols are also expected to use ports above 1023.

If a NAPT has exhausted its resources, the NAPT MAY time out a mapping before 60 minutes have elapsed. However, a NAPT is still required to follow the minimum mapping duration of [I-D.ietf-behave-nat-udp] (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” June 2006.) in order to comply with that specification.

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] and IGMP/MLD Proxying (Fenner, B., He, H., Haberman, B., and H. Sandick, “Internet Group Management Protocol (IGMP) / Multicast Listener Discovery (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying"),” August 2006.) [RFC4605].

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. References

7.1. Normative References

7.2. Informational References

Author's Address

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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.