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MBONED Working Group                                         David Meyer
Internet Draft                                      University of Oregon
Category                                           Best Current Practice

draft-ietf-mboned-admin-ip-space-03.txt                        June 1997

                  Administratively Scoped IP Multicast

1. Status of this Memo

   This document is an Internet-Draft. 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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

2. Abstract

   This document defines the "administratively scoped IPv4 multicast
   space" to be the range to In addition, it
   describes a simple set of semantics for the implementation of
   Administratively Scoped IP Multicast. Finally, it provides a mapping
   between the IPv6 multicast address classes [RFC1884] and IPv4
   multicast address classes.

   This memo is a product of the MBONE Deployment Working Group (MBONED)
   in the Operations and Management Area of the Internet Engineering
   Task Force. Submit comments to <mboned@ns.uoregon.edu> or the author.

David Meyer                                             FORMFEED[Page 1]

Internet Draft  draft-ietf-mboned-admin-ip-space-03.txt        June 1997

3. Acknowledgments

   Much of this memo is taken from "Administratively Scoped IP
   Multicast", Van Jacobson and Steve Deering, presented at the 30th
   IETF, Toronto, Canada, 25 July 1994. Steve Casner, Mark Handley and
   Dave Thaler have also provided insightful comments on earlier
   versions of this document.

4. Introduction

   Most current IP multicast implementations achieve some level of
   scoping 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 a packet is not forwarded across the interface
   unless its remaining TTL is 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
   traditional 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 understand.

   A more serious architectural problem concerns the interaction of TTL
   scoping with broadcast and prune protocols (e.g., DVMRP [DVMRP]). The
   particular problem is that in many common cases, TTL scoping can
   prevent pruning from being effective. Consider the case in which a
   packet has either had its TTL expire or failed a TTL threshold. The
   router which discards the packet will not be capable of pruning any
   upstream sources, and thus will sink all multicast traffic (whether
   or not there are downstream receivers). Note that while it might seem
   possible to send prunes upstream from the point at which a packet is
   discarded, this strategy can result in legitimate traffic being
   discarded, since subsequent packets could take a different path and
   arrive at the same point with a larger TTL.

   On the other hand, administratively scoped IP multicast can provide
   clear and simple semantics for scoped IP multicast. The key
   properties 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 required to be unique across administrative boundaries.

David Meyer                                             FORMFEED[Page 2]

Internet Draft  draft-ietf-mboned-admin-ip-space-03.txt        June 1997

5. Definition of the Administratively Scoped IPv4 Multicast Space

   The administratively scoped IPv4 multicast address space is defined
   to be the range to

6. Discussion

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

7. The Structure of the Administratively Scoped Multicast Space

   The structure of the IP version 4 administratively scoped multicast
   space is loosely based on the IP Version 6 Addressing Architecture
   described in RFC 1884 [RFC1884]. This document defines two important
   scopes: the IPv4 Local Scope and IPv4 Organization Local Scope. These
   scopes are described below.

7.1. The IPv4 Local Scope -- is defined to be the IPv4 Local Scope.  The Local
   Scope is the minimal enclosing scope, and hence is not further
   divisible. Although the exact extent of a Local Scope is site
   dependent, locally scoped regions must obey certain topological
   constraints. In particular, a Local Scope must not span any other
   scope boundary. Further, a Local Scope must be completely contained
   within or equal to any larger scope. In the event that scope regions
   overlap in area, the area of overlap must be in its own local scope.
   This implies that any scope boundary is also a boundary for the Local
   Scope. The more general topological requirements for administratively
   scoped regions are discussed below.

7.1.1. Expansion of the IPv4 Local Scope

   The IPv4 Local Scope space grows "downward". As such, the IPv4 Local
   Scope may grow downward from into the reserved ranges and However, these ranges should not
   be utilized until the space is no longer sufficient.

David Meyer                                             FORMFEED[Page 3]

Internet Draft  draft-ietf-mboned-admin-ip-space-03.txt        June 1997

7.2. The IPv4 Organization Local Scope -- is defined to be the IPv4 Organization Local Scope,
   and is the space from which an organization should allocate sub-
   ranges when defining scopes for private use.

7.2.1. Expansion of the IPv4 Organization Local Scope

   The ranges, and are
   unassigned and available for expansion of this space.  These ranges
   should be left unassigned until the space is no longer
   sufficient. This is to allow for the possibility that future
   revisions of this document may define additional scopes on a scale
   larger than organizations.

7.3. Other IPv4 Scopes of Interest

   The other two scope classes of interest, statically assigned link-
   local scope and global scope already exist in IPv4 multicast space.
   The statically assigned link-local scope is The
   existing static global scope allocations are somewhat more granular,
   and include

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

   See [RFC1700] for current multicast address assignments (this list
   can also be found, possibly in a more current form, on

David Meyer                                             FORMFEED[Page 4]

Internet Draft  draft-ietf-mboned-admin-ip-space-03.txt        June 1997

8. 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
   boundary for scoped addresses in the range defined in its

   Network administrators may configure a scope region whenever
   constrained multicast scope is required. In addition, an
   administrator may configure 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 convex (i.e., no path between any two points
   in the region can cross a region boundary).

   Finally, note that any scope boundary is a boundary for the Local
   Scope. This implies that packets sent to groups covered by must not be forwarded across any link for which a
   scoped boundary is defined.

9. Partitioning of the Administratively Scoped Multicast Space

   The following table outlines the partitioning of the IPv4 multicast
   space, and gives the mapping from IPv4 multicast prefixes to IPv6
   SCOP values:

   IPv6 SCOP         RFC 1884 Description             IPv4 Prefix
      0                  reserved
      1                  node-local scope
      2                  link-local scope   
      3                  (unassigned)       
      4                  (unassigned)
      5                  site-local scope
      6                  (unassigned)
      7                  (unassigned)
      8                  organization-local scope
      A                  (unassigned)
      B                  (unassigned)
      C                  (unassigned)
      D                  (unassigned)
      E                  global scope       
      F                  reserved

David Meyer                                             FORMFEED[Page 5]

Internet Draft  draft-ietf-mboned-admin-ip-space-03.txt        June 1997


10. 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

11. References

      [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.txt,
                September, 1996.

      [PIMDM]   Deering, S, et. al., "Protocol Independent Multicast
                Version 2, Dense Mode Specification",
                draft-ietf-idmr-pim-dm-05.txt, April, 1997.

      [PIMSM]   Estrin, D, et. al., "Protocol Independent Multicast
                Sparse Mode (PIM-SM): Protocol Specification",
                draft-ietf-idmr-PIM-SM-spec-10.ps, March, 1997.

      [RFC1700] J. Reynolds, "ASSIGNED NUMBERS", RFC1700, October,

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

David Meyer                                             FORMFEED[Page 6]

Internet Draft  draft-ietf-mboned-admin-ip-space-03.txt        June 1997

12. Author's Address

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

   phone:  +1 541.346.1747
   email:  meyer@antc.uoregon.edu

David Meyer                                             FORMFEED[Page 7]

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