INTERNET-DRAFT David Meyer
draft-ietf-mboned-admin-ip-space-02.txt University of Oregon
Category:Best Current Practice April 1997
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.
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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 Operational Requirements area of the Internet Engineering Task
Force. Submit comments to <mboned@ns.uoregon.edu> or the author.
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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 also provided insightful comments on earlier versions of
this 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 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.
A more serious architectural problem with TTL scoping is that, in
many cases, it can prevent pruning from being effective. Consider the
case in which a packet either has its TTL expire or does not meet a
TTL threshold. The point (e.g., tunnel, interface) at which the
packet fails the TTL check will not be capable of pruning upstream
and hence will sink all traffic, independent of whether there are
downstream group members. Note that without somehow associating prune
state and TTL, this problem will persist. For example, while it might
seem possible to send a prune upstream from the point where the
packet is discarded, this strategy could prevent legitimate traffic
from being forwarded (subsequent packets could take a different path
and wind up at the same point with a larger TTL). However, if a prune
had been sent, the packet may not be forwarded on interfaces that it
should have been.
On the other hand, by using administratively scoped IP multicast, one
can achieve locally scoped multicast with simple, clear semantics.
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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 IPv4 Multicast Address Space
IANA should allocate the range 239.0.0.0 to 239.255.255.255 to be
the "Administratively Scoped IPv4 Multicast" address space.
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 multi-access
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 Administratively Scoped Multicast Space
The structure of the IP version 4 administratively scoped multicast
space is based on the IP Version 6 Addressing Architecture described
in RFC 1884. The following table outlines the partitioning of the
IPv4 multicast space, and gives the mapping to IPv6 SCOP values
[RFC1884].
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
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4 (unassigned) 239.254.0.0/16
5 site-local scope 239.253.0.0/16
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
(unassigned) 239.0.0.0/10
(unassigned) 239.64.0.0/10
(unassigned) 239.128.0.0/10
The IPv4 Local Scope -- 239.255.0.0/16
239.255.0.0/16 is the IPv4 Local Scope. While how local is the Local
Scope is site dependent, locally scoped regions must obey certain
topological constraints. In particular, a Local Scope must not span
any other boundary. That is, it must be completely contained within,
or equal to, any larger scope. In the event that two scope regions
overlap in area, the area that overlaps must be in it's own local
scope. This also means that any scope boundary is also a boundary for
the Local Scope. The more general topological requirements for admin-
istratively scoped regions are discussed below.
Other IPv4 Scopes of Interest
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The other two scope classes of interest, statically assigned link-
local scope and global scope already exist to some extent in IP ver-
sion 4 multicast space. In particular, the statically assigned 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.
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). Finally, as mentioned above, an important con-
straint on the configuration of local scopes is that the local scope
must not span any other boundary.
Finally, note that any scope boundary is a boundary for the Local
Scope. This implies that packets sent to groups in the 239.255/16
range must not be forwarded across any link with any scoped boundary
defined. That is, setting a boundary on a link for any prefix must
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also set a boundary on that link for the local scope prefix.
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.
[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-10.ps, March, 1996.
Author's Address
David Meyer
Advanced Network Technology Center
University of Oregon
1225 Kincaid St.
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Eugene, OR 97403
phone: +1 541.346.1747
email: meyer@antc.uoregon.edu
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