Internet Draft
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.
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ftp.isi.edu (US West Coast).
2. Abstract
This document defines the "administratively scoped IPv4 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. 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 or the author.
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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.
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5. Definition of the Administratively Scoped IPv4 Multicast Space
The administratively scoped IPv4 multicast address space is defined
to be the range 239.0.0.0 to 239.255.255.255.
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 -- 239.255.0.0/16
239.255.0.0/16 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 239.255.0.0/16 into the reserved ranges
239.254.0.0/16 and 239.253.0.0/16. However, these ranges should not
be utilized until the 239.255.0.0/16 space is no longer sufficient.
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7.2. The IPv4 Organization Local Scope -- 239.192.0.0/14
239.192.0.0/14 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 239.0.0.0/10, 239.64.0.0/10 and 239.128.0.0/10 are
unassigned and available for expansion of this space. These ranges
should be left unassigned until the 239.192.0.0/14 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 224.0.0.0/24. The
existing static global scope allocations are 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 [RFC1700] for current multicast address assignments (this list
can also be found, possibly in a more current form, on
ftp://ftp.isi.edu/in-notes/iana/assignments/multicast-addresses).
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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
configuration.
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
239.255.0.0/16 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 224.0.0.0/24
3 (unassigned) 239.255.0.0/16
4 (unassigned)
5 site-local scope
6 (unassigned)
7 (unassigned)
8 organization-local scope 239.192.0.0/14
A (unassigned)
B (unassigned)
C (unassigned)
D (unassigned)
E global scope 224.0.1.0-238.255.255.255
F reserved
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(unassigned) 239.0.0.0/10
(unassigned) 239.64.0.0/10
(unassigned) 239.128.0.0/10
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
functionality.
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,
1994.
[RFC1884] R. Hinden. et. al., "IP Version 6 Addressing
Architecture", RFC1884, December 1995.
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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
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