RFC2365 - Administratively Scoped IP Multicast

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　　Network Working Group D. Meyer
Request for Comments: 2365 University of Oregon
BCP: 23 July 1998
Category: Best Current Practice

Administratively Scoped IP Multicast

Status of this Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

1. 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 <mboned@ns.uoregon.edu> or the author.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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 RFC1884 [RFC1884]. This document defines two important
scopes: the IPv4 Local Scope and IPv4 Organization Local Scope. These
scopes are described below.

6.1. 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.

6.1.1. 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.

6.2. 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.

6.2.1. 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.

6.3. 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).

7. 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). However, it is important
to note that if administratively scoped areas intersect
topologically, then the outer scope must consist of its address space
minus the address spaces of any intersecting scopes. This requirement
prevents the problem that would arise when a path between two points
in a convex region crosses the boundary of an intersecting region.
For this reason, it is recommended that administrative scopes that
intersect topologically should not intersect in address range.

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.

8. 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 RFC1884 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
(unassigned) 239.0.0.0/10
(unassigned) 239.64.0.0/10
(unassigned) 239.128.0.0/10

9. Structure and Use of a Scoped Region

The high order /24 in every scoped region is reserved for relative
assignments. A relative assignment is an integer offset from highest
address in the scope and represents a 32-bit address (for IPv4). For
example, in the Local Scope defined above, 239.255.255.0/24 is
reserved for relative allocations. The de-facto relative assignment
"0", (i.e., 239.255.255.255 in the Local Scope) currently exists for
SAP [SAP]. The next relative assignment, "1", corresponds to the
address 239.255.255.254 in the Local Scope. The rest of a scoped
region below the reserved /24 is available for dynamic assignment
(presumably by an address allocation protocol).

In is important to note that a scope discovery protocol [MZAP] will
have to be developed to make practical use of scopes other than the
Local Scope. In addition, since any use of any administratively
scoped region, including the Local Scope, requires dynamically
assigned addressing, an Address Allocation Protocol (AAP) will need
to be developed to make administrative scoping generally useful.

9.1. Relative Assignment Guidelines

Requests for relative assignments should be directed to the IANA. The
IANA will be advised by an area expert when making relative address
assignments. The area expert will be appointed by the relevant Area
Director.

In general, relative addresses will be used only for bootstrapping to
dynamic address assignments from within the scope. As such, relative
assignments should only be made to those services that cannot use a
dynamic address assignment protocol to find the address used by that
service within the desired scope, such as a dynamic address
assignment service itself.

10. Security Considerations

It is recommended that organizations using the administratively
scoped IP Multicast addresses not rely on them to prevent sensitive
data from being transmitted outside the organization. Should a
multicast router on an administrative boundary be mis-configured,
have a bug in the administrative scoping code, or have other problems
that would cause that router to forward an administratively scoped IP
multicast packet outside of the proper scope, the organizations data
would leave its intended transmission region.

Organizations using administratively scoped IP Multicasting to
transmit sensitive data should use some confidentiality mechanism
(e.g. encryption) to protect that data. In the case of many existing
video-conferencing applications (e.g. vat), encryption is available
as an application feature and merely needs to be enabled (and
appropriate cryptographic keys securely distributed). For many other
applications, the use of the IP Encapsulating Security Payload (ESP)
[RFC-1825, RFC-1827] can provide IP-layer confidentiality though
encryption.

Within the context of an administratively scoped IP multicast group,
the use of manual key distribution might well be feasible. While
dynamic key management for IP Security is a research area at the time
this note is written, it is expected that the IETF will be extending
the ISAKMP key management protocol to support scalable multicast key
distribution in the future.

It is important to note that the "boundary router" described in this
note is not necessarily providing any kind of firewall capability.

11. References

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

[DVMRP] Pusateri, T., "Distance Vector Multicast Routing Protocol",
Work in Progress.

[MZAP] Handley, M., "Multicast-Scope Zone Announcement Protocol
(MZAP)", Work in Progress.

[PIMDM] Deering, S, et. al., "Protocol Independent Multicast
Version 2, Dense Mode Specification", Work in Progress.

[PIMSM] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M., Jacobson, V., Liu, C., Sharma, P., and L.
Wei, "Protocol Independent Multicast Sparse Mode (PIM-SM):
Protocol Specification", RFC2362, June 1998.

[RFC1700] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.

[RFC1884] Hinden. R., and S. Deering, "IP Version 6 Addressing
Architecture", RFC1884, December 1995.

[SAP] Handley, M., "SAP: Session Announcement Protocol", Work in
Progress.

12. Author's Address

David Meyer
Cisco Systems
San Jose, CA

EMail: dmm@cisco.com

13. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
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