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Versions: (draft-wing-behave-multicast) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5135

BEHAVE                                                           D. Wing
Internet-Draft                                             Cisco Systems
Expires: March 18, 2007                               September 14, 2006


  Multicast Requirements for a Network Address Port Translator (NAPT)
                     draft-ietf-behave-multicast-03

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Copyright Notice

   Copyright (C) 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



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   document are to be interpreted as described in RFC 2119 [RFC2119].


Table of Contents

   1.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     2.1.  Background  . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  NAPT Multicast Requirements . . . . . . . . . . . . . . . . . . 4
     3.1.  NAPT Inbound Refresh Behavior . . . . . . . . . . . . . . . 4
     3.2.  Extend Mapping Refresh  . . . . . . . . . . . . . . . . . . 5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     7.2.  Informational References  . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8
































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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] and that additionally
   functions as a Network Address and Port Translator (NAPT), as
   described in section 4.1.2 of [RFC2663].

   Specifically out of scope of this document are PIM-SM [RFC2362],
   IPv6, and IGMPv1 [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
   [RFC2236], or IGMPv3 [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 |        +------------+
                +-------------+








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   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" [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 [RFC3376], the default transmission interval for the periodic
   Membership Report is one second.  Per IGMPv2 [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:

   o  follow the RFC2119 requirements of [RFC4605], and;
   o  have "NAT Inbound Refresh Behavior", as described in [I-D.ietf-
      behave-nat-udp] (see Section 3.1), and;
   o  extend the Mapping Refresh Timer, as described in [I-D.ietf-
      behave-nat-udp], 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).

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 [I-D.ietf-behave-nat-udp]
   because multicast traffic typically only flows ouside-to-inside.





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3.2.  Extend Mapping Refresh

   RTP [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]).  If a
   NAPT followed the requirements of [I-D.ietf-behave-nat-udp] (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] 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 [RFC3376] and IGMP/MLD Proxying [RFC4605].


5.  IANA Considerations

   This document does not require any IANA registrations.


6.  Acknowledgments




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


7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [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")", RFC 4605, August 2006.

   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, November 1997.

   [I-D.ietf-behave-nat-udp]
              Audet, F. and C. Jennings, "NAT Behavioral Requirements
              for Unicast UDP", draft-ietf-behave-nat-udp-07 (work in
              progress), June 2006.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

7.2.  Informational References

   [RFC2362]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
              S., Handley, M., and V. Jacobson, "Protocol Independent
              Multicast-Sparse Mode (PIM-SM): Protocol Specification",
              RFC 2362, June 1998.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.






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Author's Address

   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: dwing@cisco.com










































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Acknowledgment

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




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