Network Working Group                                          D. Thaler
Internet-Draft                                                 Microsoft
Expires: September 5, 2007                                 March 4, 2007


             Unicast-Prefix-based IPv4 Multicast Addresses
             draft-ietf-mboned-ipv4-uni-based-mcast-03.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Address Space . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Informative References  . . . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 5
   Intellectual Property and Copyright Statements  . . . . . . . . . . 6










































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.  Introduction

   RFC 3180 [RFC3180] defined an experimental allocation mechanism
   (called "GLOP") 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
      [RFC2730].
   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 [RFC3306] 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).

   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 [RFC3306], 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 Section 4, this experiment will last for a period of one
   year, but may be extended.


.  Address Space

   (RFC-editor: replace TBD below with IANA-assigned value, and delete



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   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, another mechanism must 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.


.  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.


.  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.






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.  Informative References

   [AS4B]     Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
              Number Space", February 2007, <http://www.ietf.org/
              internet-drafts/draft-ietf-idr-as4bytes-13.txt>.

   [RFC2730]  Hanna, S., Patel, B., and M. Shah, "Multicast Address
              Dynamic Client Allocation Protocol (MADCAP)", RFC 2730,
              December 1999.

   [RFC3180]  Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8",
              BCP 53, RFC 3180, September 2001.

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, August 2002.


Author's Address

   Dave Thaler
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Phone: +1 425 703 8835
   Email: dthaler@microsoft.com
























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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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