Internet Engineering Task Force (IETF) D. Thaler
Request for Comments: 6034 Microsoft
Category: Standards Track October 2010
ISSN: 2070-1721
Unicast-Prefix-Based IPv4 Multicast Addresses
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 assignment 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6034.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Address Space . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . . 5
1. Introduction
RFC 3180 [RFC3180] defines an 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 that was assigned the address for debugging and auditing
purposes.
Some disadvantages of GLOP include:
o RFC 4893 [RFC4893] expands the size of an AS number to 4 bytes,
and GLOP cannot work with 4-byte AS numbers.
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 the
Multicast Address Dynamic Client Allocation Protocol (MADCAP)
[RFC2730].
o During debugging, identifying the AS does not immediately identify
the correct 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 that
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 a subnet, rather
than just to an entire AS, is that multicast address assignments
within the range need only be coordinated within the subnet.
This document specifies a mechanism similar to [RFC3306], whereby a
range of global IPv4 multicast address space is provided to each
organization that has unicast address space. A resulting advantage
over GLOP is that the mechanisms in IPv4 and IPv6 become more
similar.
This document does not obsolete or update RFC 3180, as the mechanism
described in RFC 3180 is still required for organizations with prefix
allocations more specific than /24. Organizations using RFC 3180
allocations may continue to do so. In fact, it is conceivable that
an organization might use both RFC 3180 allocations and the
allocation method described in this document.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Address Space
A multicast address with the prefix 234/8 indicates that the address
is a Unicast-Based Multicast (UBM) address. The remaining 24 bits
are used as follows:
Bits: | 0 thru 7 | 8 thru N | N+1 thru 31 |
+-------+--------------------+-----------------------------+
Value: | 234 | Unicast Prefix | Group ID |
+-------+--------------------+-----------------------------+
For organizations with a /24 or shorter prefix, the unicast prefix of
the organization is appended to the common /8. Any remaining bits
may be assigned by any mechanism the organization wishes.
For example, an organization that has a /16 prefix assigned might
choose to assign multicast addresses manually from the /24 multicast
prefix derived from the above method. Alternatively, the
organization might choose to delegate the use of multicast addresses
to individual subnets that have a /24 or shorter unicast prefix, or
it might choose some other method.
Organizations 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.
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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 organization that is assigned a UBM address can be determined by
taking the multicast address, shifting it left by 8 bits, and
identifying who has been assigned the address space covering the
resulting unicast address.
The embedded unicast prefix MUST be a global unicast prefix (i.e., no
loopback, multicast, link-local, or private-use IP address space).
In addition, since global unicast addresses are not permanently
assigned, UBM addresses MUST NOT be hard-coded in applications.
4. Examples
The following are a few examples of the structure of unicast-prefix-
based multicast addresses.
o Consider an organization that has been assigned the global unicast
address space 192.0.2.0/24. This means that organization can use
the global multicast address 234.192.0.2 without coordinating with
any other entity. Someone who sees this multicast address and
wants to find who is using it can mentally shift the address left
by 8 bits to get 192.0.2.0, and can then look up who has been
assigned unicast address space that includes that address.
o Consider an organization that has been assigned a larger address
space, x.y.0.0/16. This organization can use the global multicast
address space 234.x.y.0/24 without coordinating with any other
entity, and can assign addresses within this space by any
mechanism the organization wishes. Someone who sees a multicast
address (say) 234.x.y.10 and wants to find who is using it can
mentally shift the address left by 8 bits to get x.y.10.0, and can
then look up who has been assigned unicast address space that
includes that address.
5. 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-service 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 organization
or an even smaller area.
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6. IANA Considerations
IANA has assigned a /8 in the global IPv4 multicast address space for
this purpose.
7. Acknowledgments
This document was updated based on feedback from the MBoneD working
group. In particular, Tim Chown, Toerless Eckert, Prashant Jhingran,
Peter Koch, John Linn, Dave Meyer, Pekka Savola, Greg Shepherd, and
Stig Venaas provided valuable suggestions on the text.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[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.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, May 2007.
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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