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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1. 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.
2. 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.
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.
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5. 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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