Network Address Port Translator (NAPT) Any-Source Multicast Requirement
draft-ietf-behave-multicast-04

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Copyright Notice

Copyright © The Internet Society (2006).

Abstract

This document places a requirement on a Network Address Translator (NAT) and Network Address and Port Translator (NAPT) that supports any-source multicast.

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.  Extend Mapping Timer for ASM Traffic
4.  Considerations for Source-Specific Multicast (SSM)
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgments
8.  References
    8.1.  Normative References
    8.2.  Informational References
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Problem Statement

For users to accept and enjoy any-source 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

A multicast NAPT device that adheres to the requirements of this document can optimize the operation of any-source multicast applications that are generally unaware of multicast NAPT devices.

This document describes the behavior of a device providing any-source 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.) using ICMPv1 (Deering, S., “Host extensions for IP multicasting,” August 1989.) [RFC1112] or ICMPv2 (Fenner, W., “Internet Group Management Protocol, Version 2,” November 1997.) [RFC2236], 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], and IPv6. 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.

This document describes how an IGMP Proxy device can NAPT multicast traffic so that existing any-source multicast applications function without awareness the multicast traffic they send has been NAPTted.

2.1.  Background

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 |    +------------+
             +-------------+

The primary functions of an IGMP proxy device are to collect IGMP traffic from the 'inside' interface and relay it to the 'outside' interface, and accept multicast traffic from that 'outside' interface and route -- or replicate it -- to the 'inside' interface(s). Packets with a multicast destination IP address do not have their destination IP address changed by a NAPT. However, their source IP address and source UDP port is changed if the packet goes from an 'inside' interface of a NAPT to the 'outside' interface of a NAPT -- similar to the behavior of a a unicast packet.

  +----+   +-------------+
  |host+---+ +---------+ |  +-----------+
  +----+   | |Multicast| |  | DSL modem |    +------------+
           | |  Proxy  | +--+    or     +-//-+ WAN Router |
  inside   | +---------+ |  |cable modem|    +------------+
interfaces |             |  +-----------+
           |  +------+   |
  +----+   |  | NAPT |   |  outside
  |host+---+  +------+   | interfaces
  +----+   +-------------+
        IGMP Proxy NAPT Device

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

3.  Extend Mapping Timer for ASM Traffic

If a NAPTed host is receiving any multicasts stream, and that NAPTed host sends UDP traffic to the same multicast address the NAPTed host is receiving, the NAPT MUST have a UDP mapping timer of 60 minutes. 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 (REQ-5 of [I‑D.ietf‑behave‑nat‑udp] (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” October 2006.)).

Discussion: 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 merely followed the requirements of [I‑D.ietf‑behave‑nat‑udp] (Audet, F. and C. Jennings, “NAT Behavioral Requirements for Unicast UDP,” October 2006.) and timed out a UDP session after 2 minutes of inactivity and RTCP receiver reports are sent less often than every 2 minutes, RTP collision detection would be performed by other session members sharing the same SSRC, complicating diagnostic tools. This situation can occur, for example, with a multicast group of approximately 300 members with a normal 50kbps audio RTP stream.

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 to assign the same UDP source port for that RTCP traffic. This requirement also facilitates other, non-RTP multicast applications which may function similarly.

4.  Considerations for Source-Specific Multicast (SSM)

There are no special requirements on a NAPT when NAPTing Source-Specific Multicast (Holbrook, H., Cain, B., and B. Haberman, “Using Internet Group Management Protocol Version 3 (IGMPv3) and Multicast Listener Discovery Protocol Version 2 (MLDv2) for Source-Specific Multicast,” August 2006.) [RFC4604] traffic. This is because with SSM, the RTCP feedback traffic from a NAPTed host is sent to a unicast address and [I‑D.ietf‑avt‑rtcpssm] (Chesterfield, J., “RTCP Extensions for Single-Source Multicast Sessions with Unicast Feedback,” March 2006.) encourages SSM applications to not rely exclusively on transport address for collision detection.

5.  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].

6.  IANA Considerations

This document does not require any IANA registrations.

7.  Acknowledgments

Thanks to Yiqun Cai, Stephen Casner, Marcus Maranhao, Bryan McLaughlin, and Magnus Westerlund for their assistance in writing this document.

8.  References

8.1. Normative References

8.2. Informational References

Author's Address

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