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INTERNET-DRAFT David Meyer
draft-ietf-mboned-admin-ip-space-01.txt University of Oregon
Category:Best Current Practice December 1996
Administratively Scoped IP Multicast
Status of this Memo
This document specifies an Internet Best Current Practice for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Internet Drafts
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).
Abstract
This document defines the "administratively scoped IP multicast
space" to be the range 239.0.0.0 to 239.255.255.255 . In addition,
it describes a simple set of semantics for the implementation of
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 <mboned@ns.uoregon.edu> or the author.
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INTERNET-DRAFT Administratively Scoped IP Multicast December 1996
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. Mark Handley and Dave Thaler
also made insightful comments on the orignal draft.
Introduction
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-
stand.
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 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 239.0.0.0 to 239.255.255.255 to be
the "Administratively Scoped IP Multicast" address space.
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Discussion
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
classes:
unassigned 239.0.0.0/10
unassigned 239.64.0.0/10
organization-local scope 239.128.0.0/10
site-local scope 239.192.0.0/10
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 224.0.0.0/24. The existing
global scope allocations are currently somewhat more granular, and
include
224.1.0.0-224.1.255.255 ST Multicast Groups
224.2.0.0-224.2.127.253 Multimedia Conference Calls
224.2.127.254 SAPv1 Announcements
224.2.127.255 SAPv0 Announcements (deprecated)
224.2.128.0-224.2.255.255 SAP Dynamic Assignments
224.252.0.0-224.255.255.255 DIS transient groups
232.0.0.0-232.255.255.255 VMTP transient groups
See ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses
for current multicast address assignments.
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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 boundary).
Example: DVMRP
DVMRP [DVMRP] implementations could be extended to support a boundary
attribute in the interface configuration [ASMA]. The boundary 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-
tionality.
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, September,
1996.
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[RFC1884] R. Hinden. et. al., "IP Version 6 Addressing
Architecture", RFC1884, December 1995.
[PIMSM] Estrin, D, et. al., "Protocol Independent Multicast
Sparse Mode (PIM-SM): Protocol Specification",
draft-ietf-idmr-PIM-SM-spec-09.ps, 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
email: meyer@antc.uoregon.edu
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