INTERNET-DRAFT                                           David Meyer
draft-ietf-mboned-admin-ip-space-01.txt         University of Oregon
Category: Informational                                November
Category:Best Current Practice                         December 1996
Expire in six months

                  Administratively Scoped IP Multicast

Status of this Memo

   This document provides information specifies an Internet Best Current Practice for the
   Internet Community.  It
   does not define a standard of any kind. Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

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   This document defines the "administratively scoped IP multicast
   space" to be  the range to . In addition,
   it describes a simple set of semantics for the implementation of Admin-
   Administratively Scoped IP Multicast.

   This memo is a product of the MBONE Deployment Working Group (MBONED)
   in the Operational Requirements area of the Internet Engineering Task
   Force. Submit comments to <> or the author.


   Much of this memo is taken from "Administratively Scoped IP Multi-
   Multicast", Van Jacobson and Steve Deering, presented at the 30th
   IETF, Toronto, Canada, 25 July 1994. Mark Handley and Dave Thaler
   also made insightful comments on the orignal draft.


   Most current IP multicast implementations achieve some level of scop-
   ing by using the TTL field in the IP header. Typical MBONE (Multicast
   Backbone) usage has been to engineer TTL thresholds that confine
   traffic to some administratively defined topological region.  The
   basic forwarding rule for interfaces with configured TTL thresholds
   is that for a packet is not forwarded across the interface unless its
   remaining TTL greater than the threshold.

   TTL scoping has been used to control the distribution of multicast
   traffic with the objective of easing stress on scarce resources
   (e.g., bandwidth), or to achieve some kind of improved privacy or
   scaling properties.  In addition, the TTL is also used in its tradi-
   tional role to limit datagram lifetime. Given these often conflicting
   roles, TTL scoping has proven difficult to implement reliably, and
   the resulting schemes have often been complex and difficult to under-

   On the other hand, by using administratively scoped IP multicast, one
   can achieve locally scoped multicast with simple, clear semantics.
   The key properties of any implementation of administratively scoped
   IP multicast are that (i). packets addressed to administratively
   scoped multicast addresses do not cross configured administrative
   boundaries, and (ii). administratively scoped multicast addresses are
   locally assigned, and hence are not guaranteed required to be unique across
   administrative boundaries. These properties are sufficient to imple-
   ment administrative scoping.

Allocation of the Administratively Scoped IP Multicast Address Space

   IANA should allocate the  range to to be
   the "Administratively Scoped IP Multicast" address space.


   In order to support administratively scoped IP multicast, a router
   should support the configuration of scoped IP multicast boundaries.
   Such a router, called a boundary router, does not forward packets
   matching its boundary definition in either direction across its
   border (the bi-directional check prevents problems with multicaccess
   networks).  In addition, a boundary router always prunes the boundary
   for dense-mode groups, or doesn't accept joins for sparse-mode groups
   [PIMSM] in the administratively scoped range.

Structure of the IPv4 Administratively Scoped Multicast Space

   The structure of the IP version 4 administratively scoped multicast
   space is loosely based on the IP Version 6 Multicast Addresses
   [RFC1884] assignments, and is partitioned into the following scope

           organization-local scope
           site-local scope      

   The other two scope classes of interest, link-local scope and global
   scope, already exist to some extent in IP version 4 multicast space.
   In particular, the link-local scope is The existing
   global scope allocations are currently somewhat more granular, and

          ST Multicast Groups
          Multimedia Conference Calls
                    SAPv1 Announcements
                    SAPv0 Announcements (deprecated)
        SAP Dynamic Assignments
      DIS transient groups
        VMTP transient groups

   for current multicast address assignments.

Topological Requirements for Administrative Boundaries

   An administratively scoped IP multicast region is defined to be a
   topological region in which there are one or more boundary routers
   with common boundary definitions. Such a router is said to be a boun-
   dary for scoped addresses in the range defined in its configuration.

   Network administrators may configure a scope region whenever local
   multicast scope is required. In addition, an administrator may con-
   figure overlapping scope regions (networks can be in multiple scope
   regions) where convenient, with the only limitations being that a
   scope region must be connected (there must be a path between any two
   nodes within a scope region that doesn't leave that region), and con-
   vex (i.e., no path between any two points in the region can cross a
   region boun-
   dary). boundary).

Example: DVMRP

   DVMRP [DVMRP] implementations could be extended to support a boundary
   attribute in the interface configuration [ASMA]. The boundary
   attribute attri-
   bute that includes a prefix and mask, and has the semantics that
   packets matching the prefix and mask do not not pass the boundary. As
   mentioned above, the implementation would also prune the boundary.

Security Considerations

   While security considerations are not explicitly discussed in this
   memo, it is important to note that a boundary router as described
   here should not be considered to provide any kind of firewall func-


      [ASMA]    V. Jacobson,  S. Deering, "Administratively Scoped IP
                Multicast", , presented at the 30th IETF, Toronto,
                Canada, 25 July 1994.

      [DVMRP]   T. Pusateri, "Distance Vector Multicast Routing
                Protocol", draft-ietf-idmr-dvmrp-v3-03, September,

      [RFC1884] R. Hinden. et. al., "IP Version 6 Addressing
                Architecture", RFC1884, December 1995.

      [PIMSM]   Estrin, D, et. al., "Protocol Independent Multicast-Sparse Multicast
                Sparse Mode (PIM-SM): Protocol Specification",
      , October, 1996.

Author's Address

   David Meyer
   Advanced Network Technology Center
   University of Oregon
   1225 Kincaid St.
   Eugene, OR 97403

   phone:  +1 541.346.1747