Network Working Group Dave Thaler
Internet-Draft Microsoft
Expires: May 2002 12 November 2001
Unicast-Prefix-based IPv4 Multicast Addresses
<draft-thaler-ipv4-uni-based-mcast-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
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This specification defines an extension to the multicast
addressing architecture of the IP Version 4 protocol. The
extension presented in this document allows for unicast-prefix-
based allocation of multicast addresses. By delegating multicast
addresses at the same time as unicast prefixes, network operators
will be able to identify their multicast addresses without needing
to run an inter-domain allocation protocol.
1. Introduction
RFC 2770 [GLOP] defined an experimental allocation mechanism in
233/8 whereby an Autonomous System (AS) number is embedded in the
middle 16 bits of an IPv4 multicast address, resulting in 256
multicast addresses per AS. Advantages of this mechanism include
the ability to get multicast address space without an inter-domain
multicast address allocation protocol, and the ease of determining
the AS of the owner of an address for debugging and auditing
purposes.
Some disadvantages of GLOP include:
o only 256 addresses are automatically available per AS, and
obtaining any more requires administrative effort.
o when an AS covers multiple sites or organizations,
administration of the multicast address space within an AS
must be handled by other mechanisms, such as manual
administrative effort or MADCAP [MADCAP].
o during debugging, identifying the AS does not immediately
identify the owning organization, when an AS covers multiple
organizations.
More recently, a mechanism [V6UPBM] has been developed for IPv6
which provides a multicast range to every IPv6 subnet, which is at
a much finer granularity than an AS. As a result, the latter two
disadvantages above are avoided (and the first disadvantage does
not apply to IPv6 due to the extended size of the address space).
Two significant advantages of providing multicast space to every
subnet (rather than just to an entire AS) are that:
o multicast address allocation within the range need only be
coordinated within the subnet (e.g., via ZMAAP [ZMAAP]), and
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hence can be done with zero configuration.
o bidirectional shared tree routing protocols may easily locate
the direction to the root by doing a route lookup on a
unicast address derived from the multicast group address.
This draft specifies a mechanism similar to [V6UPBM], whereby a
range of IPv4 multicast address space is provided to most IPv4
subnets. A resulting advantage over GLOP is that the mechanisms
in IPv4 and IPv6 become more similar.
2. Address Space
IANA should assign a /8 for this mechanism (e.g., the 225/8 which
was previously leased to MASC). The remaining 24 bits will be
used as follows:
Bits: | 8 | Unicast Prefix Length | 24 - Unicast Prefix Length |
+-----+-----------------------+----------------------------+
Value: | 225 | Unicast Prefix | Group ID |
+-----+-----------------------+----------------------------+
For subnets with a /24 or shorter prefix, the unicast prefix of
the subnet is appended to the common /8. Any remaining bits may
be locally assigned by hosts within the link (e.g., using manual
configuration, or ZMAAP). Individual subnets with a prefix length
longer than 24 do not receive any multicast address space from
this mechanism; in such cases, MADCAP may be used.
Compared to GLOP, an AS will receive more address space via this
mechanism if it has more than a /16 for unicast space. An AS will
receive less address space than it does from GLOP if it has less
than a /16.
3. Security Considerations
Since dynamic assignment does not cross domain boundaries, the
same well known intra-domain security techniques can be applied as
with GLOP. Furthermore, the approach described here may have the
effect of reduced exposure to denial of space attacks based on
dynamic allocation, since the area of dynamic allocation is
reduced from an entire AS to only within individual subnets.
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4. Author's Address
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
5. References
[GLOP]
Meyer, D., and P. Lothberg, "GLOP Addressing in 233/8", RFC
2770, February 2000.
[MADCAP]
Hanna, S, Patel, B., and M. Shah, "Multicast Address Dynamic
Client Allocation Protocol (MADCAP)", RFC 2730, December
1999.
[V6UPBM]
Haberman, B., and D. Thaler, "Unicast-Prefix-based IPv6
Multicast Addresses", draft-ietf-ipngwg-uni-based-
mcast-03.txt, October 2001.
[ZMAAP]
Catrina, O., Thaler, D., Aboba, B., and E. Guttman, "Zeroconf
Multicast Address Allocation Protocol (ZMAAP)", draft-ietf-
zeroconf-zmaap-02.txt, October 2001.
6. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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works. However, this document itself may not be modified in any
way, such as by removing the copyright notice or references to the
Internet Society or other Internet organizations, except as needed
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for the purpose of developing Internet standards in which case the
procedures for copyrights defined in the Internet Standards
process must be followed, or as required to translate it into
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The limited permissions granted above are perpetual and will not
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