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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 Working Group                                             D. Wing
Internet-Draft                                                 T. Eckert
Intended status:  Best Current                       Cisco Systems, Inc.
Practice                                                   June 20, 2007
Expires:  December 22, 2007


Multicast Requirements for a Network Address (and port) Translator (NAT)
                     draft-ietf-behave-multicast-07

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 22, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies requirements for a Network Address (and port)
   Translator (NAT) that supports any source multicast or source
   specific IP multicast.  A multicast-capable NAT device that adheres
   to the requirements of this document can optimize the operation of
   multicast applications that are generally unaware of multicast NAT
   devices.




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Table of Contents

   1.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Conventions Used in this Document  . . . . . . . . . . . . . .  5
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  NATting of IP Multicast Packets  . . . . . . . . . . . . .  5
     4.2.  IGMP Versions  . . . . . . . . . . . . . . . . . . . . . .  6
       4.2.1.  IGMPv1 or IGMPv2 . . . . . . . . . . . . . . . . . . .  7
       4.2.2.  IGMPv3 . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Any Source Multicast Transmitters  . . . . . . . . . . . .  8
     4.4.  Transport Protocol Support . . . . . . . . . . . . . . . .  9
   5.  Requirements Summary . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informational References . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15





























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1.  Problem Statement

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


2.  Introduction

   This document describes the requirements of an IP multicast-capable
   NAT.  These requirements allow existing UDP any source IP multicast
   [RFC1112] applications or source specific IP multicast [RFC4607]
   applications to function without awareness of the multicast-capable
   NAT device.  Additionally, non-UDP IP multicast applications can be
   received.

   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').
   Hosts on the 'inside' interface(s) of a NAT indicate their interest
   in receiving a multicast flow by sending an IGMP message to their
   local interface.  A 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
   multicast traffic to the NAT, which forwards it to those inside
   segment(s) with host(s) that had previously sent IGMP messages for
   that 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
   for Unicast UDP" [RFC4787].

2.1.  Background












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   When a NAT 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 |    +------------+
                            +-------------+

               Figure 1: Network without NATting IGMP Proxy

   If instead of a single host as shown in Figure 1, one or more LANs
   with potentially multiple hosts are to be connected, with the same
   type of service termination on the DSL or cable modem, a NAT device
   is added as shown in Figure 2.  This device in general perform
   routing and NAT functions such that it does look like a single host
   towards the DSL/cable modem.


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

                 Figure 2: Network with NATing IGMP Proxy

   In IP multicast, IGMP is the protocol used by hosts, such as the one
   shown in Figure 1.  For the NAT device in Figure 2 to look like the
   single host for IP multicast services towards the DSL/cable modem and
   to forward IP multicast traffic from and to the multiple hosts in the
   picture, it needs to perform so called "IGMP Proxying" [RFC4605] --
   but within the context of also performing NAT.  NAT is not covered by
   [RFC4605].  Adding NAT to IGMP proxying does not need to change the
   processing of the IGMP messages as defined in RFC4605:

      IGMP messages are never logically forwarded by the IGMP proxying
      device, but rather sourced or received by it.  In general, receipt
      of IGMP messages by the device updated IGMP state maintained by
      the device and either those changes or timers trigger the sending
      of IGMP messages.  "Forwarding" of IGMP protocol messages may thus
      only happen implicitly by implementation optimizations that create



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      shortcuts in this machinery.

   This specifically means that IGMP protocol packets sent by the NAT
   device will always use IP address of th interface (inside or outside)
   to which they are sent, but because those packets are logically
   "sourced" and not "forwarded" , NAT does not have any impact into
   this.

   Adding NAT to IGMP proxying does change the processing of IP
   multicast data packets forwarded across the IGMP proxying device as
   described in the following sections.  These changes do actually
   simplify the ability to deploy IGMP proxying over a device that does
   NOT perform NAT.

   With an IGMP Proxy NAT Device, IP multicast data traffic sourced from
   hosts on the inside is NATed such that it will look like being
   sourced from a directly connected host to the WAN router, thus
   eliminating all non-standard PIM-SM concerns/configurations described
   in section 3.2 of [RFC4605].


3.  Conventions Used in this Document

   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 [RFC2119].

   In this document, the term "NAT" applies to both Network Address and
   Port Translator (NAPT) as well as a NAT that does not translate
   ports.

   The term 'inside' refers to the interface(s) on a NAT which contain
   hosts that wish to send or receive multicast traffic.  The term
   'outside' refers to the interface(s) the NAT forwards IGMP membership
   messages to, and where the NAT routes multicast traffic that
   originates from hosts on its 'inside' interface.


4.  Requirements

4.1.  NATting of IP Multicast Packets

   Unlike unicast flows, packets with a multicast destination IP address
   do not have their destination IP address or destination port changed
   by a NAT.  However, their source IP address (and source UDP port, in
   some cases with a NAPT) is changed if the packet goes from an
   'inside' interface of a NAT to the 'outside' interface of a NAT --
   similar to the behavior of a unicast packet across those same



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   interfaces.

   REQ-1:  For IP multicast packets that are forward to a host(s) on its
           inside interface(s), a NAT MUST NOT modify the destination IP
           address or destination port of the packets.

      Note:  If a NAT were to violate this requirement and modify the
      destination IP or port addresses, the NAT would also need to
      modify session announcements (e.g., electronic program guides,
      SAP) and session establishment and control (e.g., SIP, RTSP)
      messages.  Such modification is not considered a best practice.

      Note:  This behavior is required for UDP, but has a useful side-
      effect that it permits other, non-UDP multicast protocols across a
      NAT (e.g., PGM [RFC3208], RSVP [RFC2750]).

   The following requirement is normal NAT behavior for unicast packets,
   as described in [RFC4787], and provides support for multicast senders
   behind the NAT:

   REQ-2:  A NAT MUST modify the source IP address of packets that
           arrive from an 'inside' interface towards the 'outside'
           interface so that those packets use the NAT's public IP
           address(es).

           a:  If the NAT also performs port translation (that is, it is
               a NAPT), the NAT MUST also create a mapping to allow
               responses to that multicast packet to be received by the
               appropriate host.  For any source multicast, also see
               Section 4.3.  For source specific multicast, also see
               Section 4.2.2.

           b:  To support learning their public transport address, the
               NAT MUST have "Endpoint-Independent Mapping" behavior
               (REQ-1 of [RFC4787]) no matter if the destination IP
               address is a unicast address or a multicast address.

4.2.  IGMP Versions

   REQ-3:  A NAT MAY support IGMPv1 (although IGMPv1 is considered
           obsolete).

   REQ-4:  A NAT MUST support IGMPv2.

   REQ-5:  A NAT SHOULD support IGMPv3.






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4.2.1.  IGMPv1 or IGMPv2

   For IGMPv1 and IGMPv2, a NAT can successfully operate by merely
   forwarding IGMP membership reports and queries between the interested
   hosts (on its internal interface) towards its external interface.

   REQ-6:  If a NAT supports IGMPv1 and/or IGMPv2 (but not IGMPv3), the
           NAT MAY simply receive IGMP membership reports on the inside
           interface, NAT them, and relay the IGMP membership report,
           and do the same function in the opposite direction to the
           IGMP listeners.  That is, the NAT does not need to do any
           aggregation of IGMP messages.

           a:  However, it is RECOMMENDED that such a NAT implement
               IGMP/MLD Proxying [RFC4605], because IGMP aggregation
               provides a useful optimization.

4.2.2.  IGMPv3

   When a IGMPv3 proxying device receives an IGMP membership on an
   inside interface, it creates its own IGMP proxying membership state
   and its own IGMP forwarding table.  It then creates an independent
   IGMP membership report on its outside interface reporting the
   multicast groups/channels -- but there is no direct relationship or
   "forwarding" of IGMP membership reports or queries across the
   interfaces.  The NAT device will subsequently receive a multicast
   data packet on the outside ('public') interface and forward the
   multicast packet to inside ('internal') interfaces based on its IGMP
   forwarding table.

   By performing NAT on IGMPv3 membership reports, the membership
   reports appear to originate from a single IGMPv3 reporter instead of
   different reporters.  Because IGMPv3 has different types of
   membership reports differentiating between status (IS_INCLUDE,
   IS_EXCLUDE) and change indication (e.g., TO_INCLUDE, TO_EXCLUDE), if
   a NAT were to interleave reports from two or more reporters (joining
   and leaving the same groups) the NAT would create a sequence of
   packets that are not compliant with an IGMPv3 reporter [RFC3376].
   For this reason, the following requirements are specified:

   REQ-7:  If a NAT supports IGMPv3, the NAT MUST:

           a:  implement IGMP/MLD Proxying [RFC4605].  Such compliance
               causes the NAT to aggregate the IGMPv3 membership reports
               and report only the aggregated information upstream, and;






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           b:  support any source multicast listeners and transmitters
               (Section 4.3), and;

           c:  support source specific multicast listeners and
               transmitters ([RFC4604], section 4.2 of [RFC4607]).

   Failure to implement IGMP aggregation ([RFC4605]) will cause
   undesired temporary blackholing of multicast traffic.  For example,
   consider two hosts behind the same NAT.  If one host is joining a
   session at the same time another is leaving the session, and the NAT
   were to merely relay the join and leave upstream, the session will be
   terminated, and the join and leave announcements would not comply
   with section 5 of [RFC3376].

   Primarily due to NATs functioning as IGMP proxies with multiple
   receivers behind the NAT, multicast applications are encouraged to
   use identifiers, rather than IP addresses and UDP ports, to identify
   specific multicast receivers (e.g., [I-D.ietf-avt-rtcpssm] encourages
   SSM applications to not rely exclusively on transport addresses for
   collision detection).  As compared to any source multicast, the use
   of such receiver identifiers removes the need for the NAT to have
   long mapping timers; instead, the timers in [RFC4787] are used when a
   host transmits to a source specific IP multicast address.

      Note:  SSM requires listeners to know the SSM channel (S,G), which
      is comprised of the IP source address (S) and the multicast group
      (G).  An SSM sender needs to communicate its IP address in its SSM
      session establishment message (e.g., SDP).  When the SSM sender is
      behind a NAT and the SSM receiver(s) are on the other side of that
      NAT, the SSM sender will need to determine its IP source address
      relevant to the SSM receivers; generally, this will be the public
      IP address of the NAT.  This public address needs to be included
      in the SSM session establishment message (e.g., SDP) so that
      listeners on the public side of the NAT can receive the SSM
      channel.

      If there are SSM listeners on both the public and private side of
      the NAT, it may be valuable to consider using ICE
      [I-D.ietf-mmusic-ice] in the session advertisement; the full scope
      of the interaction between SSM and ICE is beyond the scope of this
      document

4.3.  Any Source Multicast Transmitters

   Any source multicast (ASM) uses the IP addresses in the 224/8 through
   231/8, and 233/8 through 239/8 range [IANA-ALLOC].

   When a host both receives an ASM stream and sends traffic into it,



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   using RTP [RFC3550], there is a potential problem if a NAT merely
   followed the requirements of [RFC4787].  The problem is that RTP uses
   the source transport address (source IP address and source UDP port)
   and 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 NAT merely followed the
   requirements of [RFC4787] 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
   and potentially interfering with jitter buffer algorithms.  This
   situation can occur, for example, with a multicast group of
   approximately 300 members with a normal 50kbps audio RTP stream.

   REQ-8:  If a host on the inside interface of a NAT belongs to an any
           source multicast host group and the host sends a UDP packet
           to the same group, the NAT SHOULD have a UDP mapping timer of
           60 minutes for that mapping.

           a:  This UDP mapping SHOULD be destroyed when the host leaves
               that host group.  The NAT is aware of this through
               receipt of an IGMP message from the host.

           b:  If a NAT has exhausted its resources, the NAT MAY time
               out that mapping before 60 minutes have elapsed, but this
               is discouraged.  Note that even in a situation with
               resource exhaustion, a NAT is still required to follow
               the minimum mapping duration of 2 minutes (REQ-5 of
               [RFC4787]).

4.4.  Transport Protocol Support

   REQ-9:   A NAT MUST support transport of multicast UDP with both
            multicast receivers and with multicast transmitters on the
            'inside' interface(s) of the NAT.

   REQ-10:  A NAT SHOULD support transport of multicast non-UDP
            protocols (e.g., PGM [RFC3208], RSVP [RFC2750]) with
            multicast receivers on the 'inside' interface(s) of the NAT.


5.  Requirements Summary

   This section summarizes the requirements; if there is a difference in
   this summary and the text in the main body of the document, the main
   body takes precedence.




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   REQ-1:   For IP multicast packets that are forward to a host(s) on
            its inside interface(s), a NAT MUST NOT modify the
            destination IP address or destination port of the packets.

   REQ-2:   a NAT MUST modify the source IP address of packets that
            arrive from an 'inside' interface towards the 'outside'
            interface so that those packets use the NAT's public IP
            address(es).

            a:  If the NAT also performs port translation (that is, it
                is a NAPT), the NAT MUST also create a mapping to allow
                responses to that multicast packet to be received by the
                appropriate host.  For any source multicast, also see
                Section 4.3.  For source specific multicast, also see
                Section 4.2.2.

            b:  To support learning their public transport address, the
                NAT MUST have "Endpoint-Independent Mapping" behavior
                (REQ-1 of [RFC4787]) no matter if the destination IP
                address is a unicast address or a multicast address.

   REQ-3:   A NAT MAY support IGMPv1 (although IGMPv1 is considered
            obsolete).

   REQ-4:   A NAT MUST support IGMPv2.

   REQ-5:   A NAT SHOULD support IGMPv3.

   REQ-6:   If a NAT supports IGMPv1 and/or IGMPv2 (but not IGMPv3), the
            NAT MAY simply receive IGMP membership reports on the inside
            interface, NAT them, and relay the IGMP membership report,
            and do the same function in the opposite direction to the
            IGMP listeners.  That is, the NAT does not need to do any
            aggregation of IGMP messages.

            a:  However, it is RECOMMENDED that such a NAT implement
                IGMP/MLD Proxying [RFC4605], because IGMP aggregation
                provides a useful optimization.

   REQ-7:   If a NAT supports IGMPv3, the NAT MUST:

            a:  implement [RFC4605].  Such compliance causes the NAT to
                aggregate the IGMPv3 membership reports and report only
                the aggregated information upstream, and;







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            b:  support any source multicast listeners and transmitters
                (Section 4.3), and;

            c:  support source specific multicast listeners and
                transmitters ([RFC4604], section 4.2 of [RFC4607]).

   REQ-8:   If a host on the inside interface of a NAT belongs to an any
            source multicast host group and the host sends a UDP packet
            to the same group, the NAT SHOULD have a UDP mapping timer
            of 60 minutes for that mapping.

            a:  This UDP mapping SHOULD be destroyed when the host
                leaves that host group.  The NAT is aware of this
                through receipt of an IGMP message from the host.

            b:  If a NAT has exhausted its resources, the NAT MAY time
                out that mapping before 60 minutes have elapsed, but
                this is discouraged.  Note that even in a situation with
                resource exhaustion, a NAT is still required to follow
                the minimum mapping duration of 2 minutes (REQ-5 of
                [RFC4787]).

   REQ-9:   A NAT MUST support transport of multicast UDP with both
            multicast receivers and multicast transmitters on the
            'inside' interface(s) of the NAT.

   REQ-10:  A NAT SHOULD support transport of multicast non-UDP
            protocols (e.g., PGM [RFC3208], RSVP [RFC2750]) with
            multicast receivers on the 'inside' interface(s) of the NAT.


6.  Security Considerations

   The Security Considerations sections of IGMPv3 [RFC3376] and IGMP
   Proxying [RFC4605] apply to a device complying with this document.

   When a host is using RTP and participating in an any source multicast
   session, the host's periodic RTCP receiver reports cause the NAT to
   create a mapping.  When the group size is less than approximately
   300, the RTCP reports are sent frequently enough that a NAT's mapping
   will always be kept open.  When the group size is larger than
   approximately 300, the RTCP reports are sent less frequently.  The
   recommendation in Section 4.3 causes the NAT mapping to be kept open
   for the duration of the host's participation in that multicast
   session no matter the size of the multicast host or periodicity of
   the host's RTCP transmissions.





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7.  IANA Considerations

   This document does not require any IANA registrations.


8.  Acknowledgments

   Thanks to Yiqun Cai, Stephen Casner, Remi Denis-Courmont, Alfred
   Hines, Prashant Jhingran, Albert Manfredi, Marcus Maranhao, Bryan
   McLaughlin, Pekka Savola, and Magnus Westerlund for their assistance
   in writing this document.


9.  References

9.1.  Normative References

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

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

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

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

   [RFC4604]  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", RFC 4604, August 2006.

   [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.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.







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9.2.  Informational References

   [I-D.ietf-avt-rtcpssm]
              Chesterfield, J., "RTCP Extensions for Single-Source
              Multicast Sessions with Unicast Feedback",
              draft-ietf-avt-rtcpssm-13 (work in progress), March 2007.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-16 (work in progress), June 2007.

   [IANA-ALLOC]
              Internet Assigned Numbers Authority, "Internet Multicast
              Addresses",
              <http://www.iana.org/assignments/multicast-addresses>.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2750]  Herzog, S., "RSVP Extensions for Policy Control",
              RFC 2750, January 2000.

   [RFC3208]  Speakman, T., Crowcroft, J., Gemmell, J., Farinacci, D.,
              Lin, S., Leshchiner, D., Luby, M., Montgomery, T., Rizzo,
              L., Tweedly, A., Bhaskar, N., Edmonstone, R.,
              Sumanasekera, R., and L. Vicisano, "PGM Reliable Transport
              Protocol Specification", RFC 3208, December 2001.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.












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Authors' Addresses

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

   Email:  dwing@cisco.com


   Toerless Eckert
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  eckert@cisco.com

































Wing & Eckert           Expires December 22, 2007              [Page 14]

Internet-Draft         NAT Multicast Requirements              June 2007


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