RFC 6034

RFC 6034 is a Standards Track document from the IETF, written by D. Thaler of Microsoft, dated October 2010. It doesn't obsolete anything and doesn't update anything (RFC 3180 stays as is, bestie 🎀).

Unicast-Prefix-Based IPv4 Multicast Addresses ✨

Internet Engineering Task Force (IETF) · Author: D. Thaler (Microsoft) · Category: Standards Track · Date: October 2010 · Obsoletes: nothing · Updates: nothing 💖

Abstract 🦋

okay so this spec extends the multicast addressing architecture of IP Version 4 🌸 It lets you hand out multicast addresses alongside unicast prefixes, so network operators can tell which multicast addresses are theirs without running an inter-domain allocation protocol. no cap, that's so clean 💅

Status of This Memo 🧚

This is an Internet Standards Track document 🔥

It's a product of the IETF, it represents the consensus of the IETF community, got public review, and was approved for publication by the IESG. More info on Internet Standards is in Section 2 of RFC 5741. For the current status, errata, and feedback, check http://www.rfc-editor.org/info/rfc6034 💜

Copyright Notice 🎀

Copyright (c) 2010 IETF Trust and 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) as in effect on the publication date. Read those carefully, bestie, because they spell out your rights and restrictions. Code Components pulled from this document must include Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and come without warranty as described in that license 🙈

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 🌸

okay so RFC 3180 [RFC3180] defines a scheme called "GLOP" inside 233/8. It embeds an Autonomous System (AS) number in the middle 16 bits of an IPv4 multicast address, which gives you 256 multicast addresses per AS. The perks are that you get multicast space without an inter-domain multicast address allocation protocol, and it's easy to figure out which AS got an address, which helps with debugging and auditing 🔍

GLOP has some downsides though 😭

• RFC 4893 [RFC4893] expands AS numbers to 4 bytes, and GLOP cannot work with 4-byte AS numbers.
• When an AS covers multiple sites or organizations, you have to manage multicast space inside the AS some other way, like manual admin work or the Multicast Address Dynamic Client Allocation Protocol (MADCAP) [RFC2730].
• While debugging, knowing the AS doesn't immediately tell you the right organization when an AS covers several organizations.
• Only 256 addresses come automatically per AS, and getting more takes admin effort.

more recently, [RFC3306] came out for IPv6 and gives every IPv6 subnet a multicast range. That's way finer than an AS, so the first three problems above don't happen there, and the last one doesn't apply to IPv6 because the address space is huge 🧚

Giving multicast space to a subnet instead of a whole AS has another perk: address assignments inside that range only need coordinating within the subnet ✨

this document specifies something similar to [RFC3306], where each organization with unicast address space gets a range of global IPv4 multicast address space. Compared to GLOP, the IPv4 and IPv6 mechanisms end up looking more alike, which is a vibe 💅

This document does not obsolete or update RFC 3180. The mechanism in RFC 3180 is still required for organizations with prefix allocations more specific than /24, and organizations already using RFC 3180 allocations may keep doing so. It's even conceivable that an organization might use both RFC 3180 allocations and the method 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 a Unicast-Based Multicast (UBM) address. The remaining 24 bits are laid out like this:

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 organization's unicast prefix gets appended to the common /8. Any remaining bits may be assigned by whatever mechanism the organization wants 🎀

For example, an organization with a /16 prefix might choose to assign multicast addresses manually from the /24 multicast prefix derived from the method above. Or it might delegate multicast addresses to individual subnets that have a /24 or shorter unicast prefix, or it might pick some other method entirely 🌈

Organizations with a prefix length longer than 24 don't get any multicast space from this mechanism, so another mechanism MUST be used for them 💀

Compared to GLOP, an AS gets more address space through this mechanism if it has more than a /16 of unicast space, and less if it has less than a /16 🍓

To find out which organization a UBM address belongs to, take the multicast address, shift it left by 8 bits, and check who has been assigned the address space covering the resulting unicast address 🔍

The embedded unicast prefix MUST be a global unicast prefix (no loopback, multicast, link-local, or private-use IP address space). Also, since global unicast addresses aren't permanently assigned, UBM addresses MUST NOT be hard-coded in applications 🚫

4. Examples 🧚

Here are a few examples of what unicast-prefix-based multicast addresses look like 👑

• Say an organization was assigned the global unicast address space 192.0.2.0/24. That means it can use the global multicast address 234.192.0.2 without coordinating with anyone else. If you see that multicast address and want to know who's using it, mentally shift it left by 8 bits to get 192.0.2.0, then look up who was assigned unicast address space that includes that address 🕵️
• Say an organization has a bigger block, x.y.0.0/16. It can use the global multicast address space 234.x.y.0/24 without coordinating with anyone else, and it can assign addresses inside that space by any mechanism it likes. If you see a multicast address like 234.x.y.10 and want to find its user, shift it left by 8 bits to get x.y.10.0, then look up who was assigned unicast address space that includes that address 💖

5. Security Considerations 🔥

The same well-known intra-domain security techniques that work with GLOP can be applied here too. Also, when dynamic allocation is used within a prefix, this approach may reduce exposure to denial-of-service attacks, because the topological area where nodes compete for addresses within the same prefix shrinks from an entire AS down to just one organization or an even smaller area. That's a big glow-up 💅

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. Tim Chown, Toerless Eckert, Prashant Jhingran, Peter Koch, John Linn, Dave Meyer, Pekka Savola, Greg Shepherd, and Stig Venaas gave valuable suggestions on the text, and we love them for it 💜

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

That's the whole spec, bestie 🌈✨ periodt 💖