GLOP Addressing in 233/8
How every network with an AS number gets its own block of fixed multicast addresses, with no sign-up step.
Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", BCP 53, RFC 3180, DOI 10.17487/RFC3180, September 2001, <https://www.rfc-editor.org/info/rfc3180>.
@misc{rfc3180,
series = {Request for Comments},
number = 3180,
howpublished = {RFC 3180},
publisher = {RFC Editor},
doi = {10.17487/RFC3180},
url = {https://www.rfc-editor.org/info/rfc3180},
author = {D. Meyer and P. Lothberg},
title = {{GLOP Addressing in 233/8}},
pagetotal = 5,
year = 2001,
month = sep,
} RFC text
Contents
Network Working Group D. Meyer
Request for Comments: 3180 P. Lothberg
Obsoletes: 2770 Sprint
BCP: 53 September 2001
Category: Best Current Practice
GLOP Addressing in 233/8
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document defines the policy for the use of 233/8 for statically
assigned multicast addresses.
1. Introduction
It is envisioned that the primary use of this space will be many-to-
many applications. This allocation is in addition to those described
on [IANA] (e.g., [RFC2365]). The IANA has allocated 223/8 as per RFC
2770 [RFC2770]. This document obsoletes RFC 2770.
2. Problem Statement
Multicast addresses have traditionally been allocated by a dynamic
mechanism such as SDR [RFC2974]. However, many current multicast
deployment models are not amenable to dynamic allocation. For
example, many content aggregators require group addresses that are
fixed on a time scale that is not amenable to allocation by a
mechanism such as described in [RFC2974]. Perhaps more seriously,
since there is not general consensus by providers, content
aggregators, or application writers as to the allocation mechanism,
the Internet is left without a coherent multicast address allocation
scheme.
The MALLOC working group has created a specific strategy for global
multicast address allocation [RFC2730, RFC2909]. However, this
approach has not been widely implemented or deployed. This document
proposes a solution for a subset of the problem, namely, those cases
not covered by Source Specific Multicast.
3. Address Space
The IANA has allocated 223/8 as per RFC 2770 [RFC2770]. RFC 2770
describes the administration of the middle two octets of 233/8 in a
manner similar to that described in RFC 1797:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 233 | 16 bits AS | local bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1. Example
Consider, for example, AS 5662. Written in binary, left padded with
0s, we get 0001011000011110. Mapping the high order octet to the
second octet of the address, and the low order octet to the third
octet, we get 233.22.30/24.
4. Allocation
As mentioned above, the allocation proposed here follows the RFC 1797
(case 1) allocation scheme, modified as follows: the high-order octet
has the value 233, and the next 16 bits are a previously assigned
Autonomous System number (AS), as registered by a network registry
and listed in the RWhois database system. This allows a single /24
per AS.
As was the case with RFC 1797, using the AS number in this way allows
automatic assignment of a single /24 to each service provider and
does not require an additional registration step.
4.1. Private AS Space
The part of 233/8 that is mapped to the private AS space [RFC1930] is
assigned to the IRRs [RFC3138].
5. Large AS Numbers
It is important to note that this approach will work only for two
octet AS numbers. In particular, it does not work for any AS number
extension scheme.
6. Security Considerations
The approach described here may have the effect of reduced exposure
to denial-of-service attacks based on dynamic allocation. Further,
since dynamic assignment does not cross domain boundaries, well-known
intra-domain security techniques can be applied.
7. IANA Considerations
The IANA has assigned 233/8 for this purpose.
8. Acknowledgments
This proposal originated with Peter Lothberg's idea that we use the
same allocation (AS based) as described in RFC 1797. Randy Bush and
Mark Handley contributed many insightful comments, and Pete and
Natalie Whiting contributed greatly to the readability of this
document.
9. References
[IANA] http://www.iana.org/numbers.html
[RFC1797] IANA, "Class A Subnet Experiment", RFC 1797, April 1995.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
RFC 1930, March 1996.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", RFC
2365, July 1998.
[RFC2374] Hinden, R., O'Dell, M. and S. Deering, "An IPv6
Aggregatable Global Unicast Address Format", RFC 2374, July
1998.
[RFC2730] Hanna, S., Patel, B. and M. Shah, "Multicast Address
Dynamic Client Allocation Protocol (MADCAP)", RFC 2730,
December 1999.
[RFC2770] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", RFC
2770, February 2000.
[RFC2909] Radoslavov, P., Estrin, D., Govindan, R., Handley, M.,
Kumar, S. and D. Thaler, "The Multicast Address-Set Claim
(MASC) Protocol", RFC 2909, September 2000.
[RFC2974] Handley, M., Perkins, C. and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3138] Meyer, D., "Extended Assignments in 233/8", RFC 3138, June
2001.
10. Authors' Addresses
David Meyer
Sprint
VARESA0104
12502 Sunrise Valley Drive
Reston VA, 20196
EMail: dmm@sprint.net
Peter Lothberg
Sprint
VARESA0104
12502 Sunrise Valley Drive
Reston VA, 20196
EMail: roll@sprint.net
11. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative 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 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 languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§2Why Fixed Multicast Addresses
RFC 3180 sets the policy for using 233/8 for statically assigned multicast addresses. The authors expect the space to be used mainly by many-to-many applications, in addition to the ranges already described by IANA, such as the administratively scoped ones in RFC 2365.
Multicast addresses had traditionally been allocated dynamically, for example with the Session Announcement Protocol (RFC 2974). The RFC says that many deployment models do not suit this: content aggregators need group addresses that are fixed on a time scale dynamic allocation does not offer, and providers, aggregators and application writers had no consensus on one allocation mechanism. The MALLOC working group had defined a global strategy (RFC 2730, RFC 2909), but it had not been widely implemented or deployed. This document addresses a subset of the problem: the cases not covered by Source Specific Multicast.
Some groups need a mailbox address that stays the same for a long time, and the old way of handing out addresses on the fly did not work for them. This gives them a steady address instead.
§3Address Layout and Allocation
The scheme follows the case 1 allocation of RFC 1797, modified as follows. The high-order octet is 233, and the next 16 bits are a previously assigned Autonomous System (AS) number, as registered by a network registry and listed in the RWhois database system. The remaining 8 bits are local. That allows a single /24 per AS.
Because the AS number is used directly, each service provider gets a /24 automatically and no additional registration step is needed.
For example, AS 5662 is 0001011000011110 in binary, left padded with zeros. The high-order octet becomes the second octet of the address and the low-order octet becomes the third, giving 233.22.30/24. The part of 233/8 that maps to the private AS space (RFC 1930) is assigned to the IRRs (RFC 3138).
Your network's number is turned into part of the address, like a school number becoming the street name. So everyone can work out their own street without asking.
§5The Two-Octet AS Limit
The RFC states that this approach works only for two-octet AS numbers. In particular, it does not work for any AS number extension scheme, because there are only 16 bits in the address to hold the AS number.
The street name only has room for a short number. A school with a much longer number would not fit on the sign.
§6Security and IANA Notes
The RFC says the approach may reduce exposure to denial-of-service attacks that are based on dynamic allocation. Since dynamic assignment does not cross domain boundaries here, well-known intra-domain security techniques can be applied.
For IANA, the document records that IANA has assigned 233/8 for this purpose. It replaces the earlier description in RFC 2770, which RFC 3180 obsoletes.
Because each network only hands out addresses inside its own street, it is easier to guard against troublemakers.