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Versions: 00 01 02 03                                                   
Internet Engineering Task Force                                 MAGMA WG
INTERNET-DRAFT                                     Isidor Kouvelas/Cisco
draft-ietf-magma-mrdssm-01.txt                           3 November 2002
                                                       Expires: May 2003

              Multicast Router Discovery SSM Range Option

Status of this Document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

This document is a product of the IETF MAGMA WG.  Comments should be
addressed to the authors, or the WG's mailing list at magma@ietf.org.


     This document defines the Multicast Router Discovery option
     for advertising the configured IPv4 Source Specific Multicast
     destination address range.

Kouvelas                                                        [Page 1]

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1.  Introduction

     With current multicast deployment in the Internet, different
multicast routing protocols coexist and operate under separate parts of
the multicast address space [1]. Multicast routers are consistently
configured with information that maps specific multicast destination
address ranges to multicast routing protocols. Part of this
configuration describes the subset of the address space that is used by
source-specific multicast (SSM) [2].

     There are currently two requirements for a router to advertise its
configured SSM range on its attached links:

o On links with multiple multicast routers, advertisement of the
  configured SSM range by each router can be used to discover miss-

o IP systems with multicast sources or receivers can use the
  advertisements to learn the SSM group range with which the network is

     This document defines an optional extension for the IPv4 Multicast
Router Discovery protocol [3] which can be used to advertise the SSM
range.  Note that the SSM range for IPv6 is well defined and a mechanism
to allow additional ranges to operate in SSM mode on a per-link bases is
not required.

2.  SSM Range Option Format

     The SSM Range option SHOULD be included in all Multicast Router
Advertisement messages [3]. It contains the list of multicast
destination address ranges that are configured to operate under Source
Specific Multicast on this router.  The format of the option is as

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|    Type=X     |   Length=var  |  Mask-Len-1   |    Prefix-1 ...
|  Mask-Len-2   |   Prefix-2 ...
|                            ...                                |

Kouvelas                                            Section 2.  [Page 2]

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Type The type value of the Multicast Router Advertisement SSM Range
     option is X (TBD by IANA).

     The length of the SSM Range Discovery option is variable and
     depends on the number of destination ranges present in the option
     as well as the sizes of the ranges.

     The mask length for the nth address range.

     The multicast destination address prefix for the nth range present
     in this option. The size of the prefix field is variable and
     depends on the number of significant bits in the prefix (specified
     in the corresponding Mask-Len field). The field is padded by enough
     trailing bits to make the end of the field fall on an octet
     boundary. Note that the value of the trailing bits is irrelevant.

3.  Notes on Option Processing

     Routers originating Multicast Router Advertisement messages SHOULD
NOT include more than one SSM Range Discovery option in each message.
Systems with a multicast capable IP host stack that receive a Multicast
Router Advertisement message with more than one SSM Range Discovery
options MUST only use the prefixes from the last SSM Range Discovery
option in the message as the active SSM range.

     A router receiving a Multicast Router Advertisement message with an
SSM Range Discovery Option MUST compare the contents of the option with
the multicast address ranges in the local SSM configuration and signal
any differences to the administrator in a rate-limited manner.

4.  Security Considerations

     Multicast Router Advertisement messages are IGMP messages sent to
the All-Systems multicast group ( which is not forwarded by
routers. Only rogue systems on a connected link can masquerade as
multicast routers. Such rogue systems can include the SSM Range
Discovery option in their messages and cause the SSM range mapping to be

Kouvelas                                            Section 4.  [Page 3]

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incorrectly set by hosts on the link. The next Multicast Router
Advertisement from a real valid router on the link will restore the
correct mapping. This spec mandates that routers log the reception of
inconsistent range advertisements which makes it easier to detect rogue

5.  IANA Considerations

     This document introduces the new SSM Range Option for the Multicast
Router Discovery protocol. This option requires a new MRD type value to
be assigned by IANA.

6.  Acknowledgments

The author would like to thank Bill Fenner and Dave Thaler for their
contribution to this document.

7.  Authors' Addresses

     Isidor Kouvelas
     Cisco Systems
     170 W. Tasman Drive
     San Jose, CA 95134

8.  References

[1] Z. Albanna, K. Almeroth, D. Meyer, M. Schipper, "IANA Guidelines for
     IPv4 Multicast Address Assignments", RFC 3171 (BCP 51), August

[2] H. Holbrook, B. Cain, "Source-Specific Multicast for IP", work in
     progress, <draft-ietf-ssm-arch-00.txt>, 21 November 2001.

[3] S. Biswas, B. Haberman, "IGMP Multicast Router Discovery", Work In
     Progress, <draft-ietf-idmr-igmp-mrdisc-??.txt>, 2002.

Kouvelas                                            Section 8.  [Page 4]

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Kouvelas                                            Section 8.  [Page 5]