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Network Working Group                                  Dave Thaler
Internet-Draft                                           Microsoft
Expires: April 2005                               October 18, 2004





          Unicast-Prefix-based IPv4 Multicast Addresses
         <draft-ietf-mboned-ipv4-uni-based-mcast-02.txt>





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Copyright Notice

Copyright (C) The Internet Society (2004).  All Rights Reserved.






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Abstract

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 3180 [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    there is work in progress [AS4B] on expanding the size of an
     AS number to 4 bytes, and GLOP cannot work with such AS's.

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.

o    only 256 addresses are automatically available per AS, and
     obtaining any more requires administrative effort.

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 first three
disadvantages above are avoided (and the last disadvantage does
not apply to IPv6 due to the extended size of the address space).






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Another advantage of providing multicast space to every subnet
(rather than just to an entire AS) is that multicast address
allocation within the range need only be coordinated within the
subnet.

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.

This document proposes an experimental method of statically
allocating multicast addresses with global scope. As described in
section 4, this experiment will last for a period of one year, but
may be extended.


2.  Address Space

(RFC-editor: replace TBD below with IANA-assigned value, and
delete this note.)

A multicast address with the prefix TBD/8 indicates that the
address is a Unicast-Based Multicast (UBM) address.   The
remaining 24 bits can be used as follows:

Bits:  |  8  | Unicast Prefix Length | 24 - Unicast Prefix Length |
       +-----+-----------------------+----------------------------+
Value: | TBD | 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).  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.

The owner of a UBM address can be determined by taking the
multicast address, shifting it left by 8 bits, and identifying the
owner of the address space covering the resulting unicast address.





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3.  Security Considerations

The same well known intra-domain security techniques can be
applied as with GLOP.  Furthermore, when dynamic allocation is
used within a prefix, the approach described here may have the
effect of reduced exposure to denial of space attacks, since the
topological area within which nodes compete for addresses within
the same prefix is reduced from an entire AS to only within an
individual subnet.


4.  IANA Considerations

IANA should assign a /8 in the IPv4 multicast address space for
this purpose.

This assignment should timeout one year after the assignment is
made. The assignment may be renewed at that time.


5.  Author's Address

     Dave Thaler
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA  98052-6399
     Phone: +1 425 703 8835
     EMail: dthaler@microsoft.com


6.  Informative References

[AS4B]
     Vohra, Q. and E. Chen, "BGP support for four-octet AS number
     space", draft-ietf-idr-as4bytes-08.txt, Work in progress,
     March 2004.

[GLOP]
     Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", RFC
     3180, September 2001.

[MADCAP]
     Hanna, S, Patel, B. and M. Shah, "Multicast Address Dynamic
     Client Allocation Protocol (MADCAP)", RFC 2730, December
     1999.





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[V6UPBM]
     Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
     Multicast Addresses", RFC 3306, August 2002.


7.  Full Copyright Statement   Copyright (C) The Internet Society
(2004).  This document is subject to the rights, licenses and
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