IANA Guidelines for IPv4 Multicast Address Assignments
How IANA carves up and hands out the IPv4 multicast address space, block by block.
Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for IPv4 Multicast Address Assignments", BCP 51, RFC 5771, DOI 10.17487/RFC5771, March 2010, <https://www.rfc-editor.org/info/rfc5771>.
@misc{rfc5771,
series = {Request for Comments},
number = 5771,
howpublished = {RFC 5771},
publisher = {RFC Editor},
doi = {10.17487/RFC5771},
url = {https://www.rfc-editor.org/info/rfc5771},
author = {M. Cotton and L. Vegoda and D. Meyer},
title = {{IANA Guidelines for IPv4 Multicast Address Assignments}},
pagetotal = 11,
year = 2010,
month = mar,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Cotton
Request for Comments: 5771 L. Vegoda
BCP: 51 ICANN
Updates: 2780 D. Meyer
Obsoletes: 3138, 3171 March 2010
Category: Best Current Practice
ISSN: 2070-1721
IANA Guidelines for IPv4 Multicast Address Assignments
Abstract
This document provides guidance for the Internet Assigned Numbers
Authority (IANA) in assigning IPv4 multicast addresses. It obsoletes
RFC 3171 and RFC 3138 and updates RFC 2780.
Status of This Memo
This memo documents an Internet Best Current Practice.
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
BCPs 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/rfc5771.
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
carefully, as they describe your rights and restrictions with respect
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.
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Definition of Current Assignment Practice .......................3
4. Local Network Control Block (224.0.0/24) ........................4
4.1. Assignment Guidelines ......................................4
5. Internetwork Control Block (224.0.1/24) .........................5
5.1. Assignment Guidelines ......................................5
6. AD-HOC Blocks (I, II, and III) ..................................5
6.1. Assignment Guidelines ......................................5
7. SDP/SAP Block (224.2/16) ........................................5
7.1. Assignment Guidelines ......................................5
8. Source-Specific Multicast Block (232/8) .........................6
8.1. Assignment Guidelines ......................................6
9. GLOP Block (233/8) ..............................................6
9.1. Assignment Guidelines ......................................6
9.2. AD-HOC Block III ...........................................6
10. Administratively Scoped Block (239/8) ..........................7
10.1. Assignment Guidelines .....................................7
10.1.1. Relative Offsets ...................................7
11. Application Form ...............................................7
11.1. Size of Assignments of IPv4 Multicast Addresses ...........7
12. Annual Review ..................................................8
12.1. Address Reclamation .......................................8
12.2. Positive Renewal ..........................................8
13. Use of IANA Reserved Addresses .................................8
14. IANA Considerations ............................................8
15. Security Considerations ........................................9
16. Acknowledgments ................................................9
17. References .....................................................9
17.1. Normative References ......................................9
17.2. Informative References ....................................9
1. Introduction
The Internet Assigned Numbers Authority (IANA) (www.iana.org) is
charged with allocating parameter values for fields in protocols that
have been designed, created, or are maintained by the Internet
Engineering Task Force (IETF). RFC 2780 [RFC2780] provides the IANA
guidance in the assignment of parameters for fields in newly
developed protocols. This memo expands on section 4.4.2 of RFC 2780
and attempts to codify existing IANA practice used in the assignment
of IPv4 multicast addresses.
This document is a revision of RFC 3171 [RFC3171], which it
obsoletes. It also obsoletes RFC 3138 [RFC3138] and updates
[RFC2780].
The terms "Specification Required", "Expert Review", "IESG Approval",
"IETF Review", and "Standards Action", are used in this memo to refer
to the processes described in [RFC5226].
In general, due to the relatively small size of the IPv4 multicast
address space, further assignment of IPv4 multicast address space is
recommended only in limited circumstances. Specifically, the IANA
should only assign addresses in those cases where:
- the dynamic selection Session Description Protocol/Session
Announcement Protocol (SDP/SAP);
- GLOP (not an acronym);
- Source-Specific Multicast (SSM); or
- Administratively Scoped address spaces cannot be used.
The guidelines described below are reflected in [IANA-protocols].
Network operators should also be aware of the availability of IPv6
multicast addresses and consider using them where feasible.
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 BCP 14, RFC 2119
[RFC2119].
The word "allocation" designates a block of addresses managed by a
registry for the purpose of making assignments and allocations. The
word "assignment" designates a block of addresses, or a single
address, registered to an end-user for use on a specific network or
set of networks.
3. Definition of Current Assignment Practice
Unlike IPv4 unicast address assignment, where blocks of addresses are
delegated to Regional Internet Registries (RIRs), IPv4 multicast
addresses are assigned directly by the IANA. Current registration
groups appear as follows [IANA]:
Address Range Size Designation
------------- ---- -----------
224.0.0.0 - 224.0.0.255 (/24) Local Network Control Block
224.0.1.0 - 224.0.1.255 (/24) Internetwork Control Block
224.0.2.0 - 224.0.255.255 (65024) AD-HOC Block I
224.1.0.0 - 224.1.255.255 (/16) RESERVED
224.2.0.0 - 224.2.255.255 (/16) SDP/SAP Block
224.3.0.0 - 224.4.255.255 (2 /16s) AD-HOC Block II
224.5.0.0 - 224.255.255.255 (251 /16s) RESERVED
225.0.0.0 - 231.255.255.255 (7 /8s) RESERVED
232.0.0.0 - 232.255.255.255 (/8) Source-Specific Multicast Block
233.0.0.0 - 233.251.255.255 (16515072) GLOP Block
233.252.0.0 - 233.255.255.255 (/14) AD-HOC Block III
234.0.0.0 - 238.255.255.255 (5 /8s) RESERVED
239.0.0.0 - 239.255.255.255 (/8) Administratively Scoped Block
The IANA generally assigns addresses from the Local Network Control,
Internetwork Control and AD-HOC blocks. Assignment guidelines for
each of these blocks, as well as for the Source-Specific Multicast,
GLOP, and Administratively Scoped blocks, are described below.
4. Local Network Control Block (224.0.0/24)
Addresses in the Local Network Control Block are used for protocol
control traffic that is not forwarded off link. Examples of this
type of use include OSPFIGP All Routers (224.0.0.5) [RFC2328].
4.1. Assignment Guidelines
Pursuant to section 4.4.2 of [RFC2780], assignments from the Local
Network Control Block follow an Expert Review, IESG Approval, or
Standards Action process. See IANA [IANA] for the current set of
assignments.
5. Internetwork Control Block (224.0.1/24)
Addresses in the Internetwork Control Block are used for protocol
control traffic that MAY be forwarded through the Internet. Examples
include 224.0.1.1 (Network Time Protocol (NTP) [RFC4330]) and
224.0.1.68 (mdhcpdiscover [RFC2730]).
5.1. Assignment Guidelines
Pursuant to section 4.4.2 of [RFC2780], assignments from the
Internetwork Control Block follow an Expert Review, IESG Approval, or
Standards Action process. See IANA [IANA] for the current set of
assignments.
6. AD-HOC Blocks (I, II, and III)
Addresses in the AD-HOC blocks (including 224.0.2.0 - 224.0.255.255,
224.3.0.0 - 224.4.255.255, and 233.252.0.0 - 233.255.255.255) were
traditionally used for assignments for those applications that don't
fit in either the Local or Internetwork Control blocks. These
addresses MAY be globally routed and are typically used by
applications that require small blocks of addressing (e.g., less than
a /24 ). Future assignments of blocks of addresses that do not fit
in the Local Network or Internetwork Control blocks will be made in
AD-HOC Block III.
6.1. Assignment Guidelines
In general, the IANA SHOULD NOT assign addresses in the AD-HOC
blocks. However, the IANA MAY, under special circumstances, assign
addresses from these blocks. Pursuant to section 4.4.2 of [RFC2780],
assignments from the AD-HOC blocks follow an Expert Review, IESG
Approval, or Standards Action process. See [IANA] for the current
set of assignments.
7. SDP/SAP Block (224.2/16)
Addresses in the SDP/SAP Block are used by applications that receive
addresses through the Session Announcement Protocol [RFC2974] for use
via applications like the session directory tool (such as [SDR]).
7.1. Assignment Guidelines
Since addresses in the SDP/SAP Block are chosen randomly from the
range of addresses not already in use [RFC2974], no IANA assignment
policy is required. Note that while no additional IANA assignment is
required, addresses in the SDP/SAP Block are explicitly for use by
SDP/SAP and MUST NOT be used for other purposes.
8. Source-Specific Multicast Block (232/8)
SSM [RFC4607] is an extension of IP Multicast in which traffic is
forwarded to receivers from only those multicast sources for which
the receivers have explicitly expressed interest and is primarily
targeted at one-to-many (broadcast) applications. Note that this
block was initially assigned to the Versatile Message Transaction
Protocol (VMTP) transient groups [IANA].
8.1. Assignment Guidelines
Because the SSM model essentially makes the entire multicast address
space local to the host, no IANA assignment policy is required.
Note, however, that while no additional IANA assignment is required,
addresses in the Source-Specific Multicast Block are explicitly for
use by SSM and MUST NOT be used for other purposes.
9. GLOP Block (233/8)
Addresses in the GLOP Block are globally-scoped, statically-assigned
addresses. The assignment is made, for a domain with a 16-bit
Autonomous System Number (ASN), by mapping a domain's autonomous
system number, expressed in octets as X.Y, into the middle two octets
of the GLOP Block, yielding an assignment of 233.X.Y.0/24. The
mapping and assignment is defined in [RFC3180]. Domains with a
32-bit ASN MAY apply for space in AD-HOC Block III, or consider using
IPv6 multicast addresses.
9.1. Assignment Guidelines
Because addresses in the GLOP Block are algorithmically pre-assigned,
no IANA assignment policy is required.
9.2. AD-HOC Block III
[RFC3138] delegated to the RIRs the assignment of the GLOP sub-block
(233.252.0.0 - 233.255.255.255) mapped by the private Autonomous
System (AS) space (64512-65534) and the IANA reserved ASN 65535
[RFC1930]. This space was known as Extended GLOP (EGLOP). RFC 3138
should not have asked the RIRs to develop policies for the EGLOP
space because [RFC2860] reserves that to the IETF. It is important
to make this space available for use by network operators, and it is
therefore appropriate to obsolete RFC 3138 and classify this address
range as available for AD-HOC assignment as per the guidelines in
section 6.
The first /24 in this range, 233.252.0.0/24, is assigned as "MCAST-
TEST-NET" for use in documentation and example code. 233.252.0.0/24
SHOULD be used in conjunction with the [RFC2606] domain names
example.com or example.net in vendor and protocol documentation.
Addresses within 233.252.0.0/24 MUST NOT appear on the public
Internet.
10. Administratively Scoped Block (239/8)
Addresses in the Administratively Scoped Block are for local use
within a domain and are described in [RFC2365].
10.1. Assignment Guidelines
Since addresses in this block are local to a domain, no IANA
assignment policy is required.
10.1.1. Relative Offsets
The relative offsets [RFC2365] are used to ensure that a service can
be located independent of the extent of the enclosing scope (see
[RFC3180] for details). Since there are only 256 such offsets, the
IANA should only assign a relative offset to a protocol that provides
an infrastructure supporting service. Examples of such services
include the Session Announcement Protocol [RFC2974]. Pursuant to
section 4.4.2 of [RFC2780], assignments of relative offsets follow an
Expert Review, IESG Approval, or Standards Action process. See
[IANA] for the current set of assignments.
11. Application Form
Requests for multicast address assignments can be submitted through
the application form on the IANA web site at [IANA-registration]. It
is important to submit sufficient detail to allow the IESG designated
expert to review the application. If the details given in the
request are not clear, or further information is needed, the IESG
designated expert may request additional information before assigning
an address.
11.1. Size of Assignments of IPv4 Multicast Addresses
Occasionally, more than one multicast address is required. In these
cases, multiple addresses are available in AD-HOC Block III. Where
there is a requirement for a very large number of addresses, the
assignment will be staged. The additional stages will only be made
after the complete use of the initial assignment(s).
A separate document describing the policy governing assignment of
addresses in the AD-HOC blocks I, II, and III will be developed and
published. The format, location, and content has not yet been
decided and so these will be documented in a future version of this
document.
12. Annual Review
Given the dynamic nature of IPv4 multicast and its associated
infrastructure, and the previously undocumented IPv4 multicast
address assignment guidelines, the IANA should conduct an annual
review of currently assigned addresses.
12.1. Address Reclamation
During the review described above, addresses that were mis-assigned
should, where possible, be reclaimed or reassigned.
The IANA should also review assignments in the AD-HOC, "DIS Transient
Groups", and ST Multicast Groups [RFC1819] blocks and reclaim those
addresses that are not in use on the global Internet (i.e., those
applications that can use SSM, GLOP, or Administratively Scoped
addressing, or are not globally routed).
12.2. Positive Renewal
It is occasionally appropriate to make temporary assignments that can
be renewed as necessary. In cases where this happens the registrant
needs to positively request an extension to the temporary assignment
or the addresses assigned. When the IANA has not received a request
to renew the registration of a temporary assignment within 30 days of
the expiry of the assignment, it MUST be removed from the multicast
registry.
Addresses returned to the IANA when a temporary assignment ends MUST
NOT be assigned to anyone other than the last registrant for at least
one calendar year.
13. Use of IANA Reserved Addresses
Applications MUST NOT use addressing in the IANA reserved blocks.
14. IANA Considerations
IANA has updated its IPv4 multicast request and assignment procedures
to reflect this document.
15. Security Considerations
The assignment guidelines described in this document do not alter the
security properties of either the Any Source or Source-Specific
Multicast service models.
16. Acknowledgments
The authors would like to thank Joe St. Sauver, John Meylor, Randy
Bush, Thomas Narten, Marshall Eubanks, Zaid Albanna (co-author of RFC
3171), Kevin Almeroth (co-author of RFC 3171), Pekka Savola, and
Alfred Hoenes for their constructive feedback and comments.
17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226, May
2008.
17.2. Informative References
[IANA] IANA, "IANA Protocol Registries", <http://www.iana.org/>.
[IANA-protocols]
IANA, "IANA Protocol Registries",
<http://www.iana.org/protocols>.
[IANA-registration]
IANA, "IANA Protocol Registration Forms",
<http://www.iana.org/protocols/apply>.
[RFC1819] Delgrossi, L., Ed., and L. Berger, Ed., "Internet Stream
Protocol Version 2 (ST2) Protocol Specification - Version
ST2+", RFC 1819, August 1995.
[RFC1930] Hawkinson, J. and T. Bates, "Guidelines for creation,
selection, and registration of an Autonomous System (AS)",
BCP 6, RFC 1930, March 1996.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
[RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
RFC 2365, July 1998.
[RFC2606] Eastlake 3rd, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.
[RFC2730] Hanna, S., Patel, B., and M. Shah, "Multicast Address
Dynamic Client Allocation Protocol (MADCAP)", RFC 2730,
December 1999.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers", BCP
37, RFC 2780, March 2000.
[RFC2860] Carpenter, B., Baker, F., and M. Roberts, "Memorandum of
Understanding Concerning the Technical Work of the Internet
Assigned Numbers Authority", RFC 2860, June 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.
[RFC3171] Albanna, Z., Almeroth, K., Meyer, D., and M. Schipper,
"IANA Guidelines for IPv4 Multicast Address Assignments",
BCP 51, RFC 3171, August 2001.
[RFC3180] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", BCP
53, RFC 3180, September 2001.
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4
for IPv4, IPv6 and OSI", RFC 4330, January 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[SDR] University College London / ISI, "Session Directory Tool",
<http://www-mice.cs.ucl.ac.uk/multimedia/software/sdr/>.
Authors' Addresses
Michelle Cotton
Internet Corporation for Assigned Names and Numbers
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
United States of America
Phone: +310-823-9358
EMail: michelle.cotton@icann.org
URI: http://www.iana.org/
Leo Vegoda
Internet Corporation for Assigned Names and Numbers
4676 Admiralty Way, Suite 330
Marina del Rey, CA 90292
United States of America
Phone: +310-823-9358
EMail: leo.vegoda@icann.org
URI: http://www.iana.org/
David Meyer
EMail: dmm@1-4-5.net
AI ELI5
On this page
§1Why Guidelines Are Needed
The Internet Assigned Numbers Authority (IANA) allocates parameter values for protocols made or maintained by the IETF. RFC 2780 gives it guidance for newly developed protocols. This memo expands on section 4.4.2 of that RFC and tries to codify what IANA already does when it assigns IPv4 multicast addresses.
Because the IPv4 multicast address space is relatively small, the RFC recommends further assignment only in limited circumstances. IANA should assign addresses only where dynamic selection through SDP/SAP, GLOP, Source-Specific Multicast (SSM) or Administratively Scoped address spaces cannot be used. Network operators are also told to consider IPv6 multicast addresses where feasible.
The document is a revision of RFC 3171, which it obsoletes. It also obsoletes RFC 3138 and updates RFC 2780.
There are only so many group addresses, so the keeper should only give out a new one when nothing else will do.
§2The Registration Groups
Unlike IPv4 unicast, where blocks go to Regional Internet Registries, IPv4 multicast addresses are assigned directly by IANA. The RFC lists the current registration groups, from 224.0.0.0 up to 239.255.255.255. Some ranges are marked RESERVED, and applications MUST NOT use addressing in the IANA reserved blocks.
IANA generally assigns addresses from the Local Network Control, Internetwork Control and AD-HOC blocks. The other blocks (SSM, GLOP and Administratively Scoped) mostly manage themselves, as the later sections explain.
The RFC also defines its words: an allocation is a block of addresses managed by a registry for making assignments, and an assignment is a block or single address registered to an end user for a specific network or set of networks.
The whole list of group addresses is split into labelled neighbourhoods. Some are in use, some are saved for later, and nobody may use the saved ones.
- 224.0.0.0 - 224.0.0.255Local Network Control — Control traffic not forwarded off link.
- 224.0.1.0 - 224.0.1.255Internetwork Control — Control traffic that MAY be forwarded through the Internet.
- 224.0.2.0 - 224.0.255.255AD-HOC Block I — Applications that fit neither control block.
- 224.2.0.0 - 224.2.255.255SDP/SAP — Addresses chosen by SAP applications; no IANA policy.
- 224.3.0.0 - 224.4.255.255AD-HOC Block II — Applications that fit neither control block.
- 232.0.0.0 - 232.255.255.255Source-Specific Multicast — Reserved for SSM; no IANA policy.
- 233.0.0.0 - 233.251.255.255GLOP — Derived from a 16-bit AS number.
- 233.252.0.0 - 233.255.255.255AD-HOC Block III — Future assignments that fit no other block.
- 239.0.0.0 - 239.255.255.255Administratively Scoped — Local use within a domain; no IANA policy.
§3Control and AD-HOC Blocks
The Local Network Control Block (224.0.0/24) is for protocol control traffic that is not forwarded off link. One example is OSPFIGP All Routers (224.0.0.5). The Internetwork Control Block (224.0.1/24) is for control traffic that MAY be forwarded through the Internet, such as 224.0.1.1 for NTP. Assignments in both follow an Expert Review, IESG Approval or Standards Action process.
The AD-HOC blocks I, II and III were traditionally used for applications that fit neither control block. These addresses MAY be globally routed and suit applications needing small blocks, less than a /24. IANA SHOULD NOT assign from them in general, but MAY do so under special circumstances, using the same review processes. Future assignments that do not fit the control blocks will come from AD-HOC Block III.
Some addresses are for talk that stays on your street, some for talk that may travel across town, and a leftover pile for odd cases that the keeper hands out only reluctantly.
§4SDP/SAP, SSM, GLOP and Scoped Blocks
The SDP/SAP Block (224.2/16) is used by applications that get addresses through the Session Announcement Protocol. Addresses are chosen randomly from the range not already in use, so no IANA assignment policy is required, but the block MUST NOT be used for other purposes.
The Source-Specific Multicast Block (232/8) is for SSM, an extension of IP Multicast in which traffic is forwarded to receivers only from sources they explicitly asked for. No assignment policy is needed, and the block MUST NOT be used for other purposes. The block was initially assigned to Versatile Message Transaction Protocol (VMTP) transient groups.
The GLOP Block (233/8) is globally scoped and statically assigned. A domain with a 16-bit Autonomous System Number written as X.Y gets 233.X.Y.0/24, so no IANA policy is needed. Domains with a 32-bit ASN MAY apply for AD-HOC Block III space or consider IPv6 multicast.
The Administratively Scoped Block (239/8) is for local use within a domain and needs no IANA policy. Only 256 relative offsets exist, so IANA should assign one only to a protocol that provides an infrastructure supporting service, such as the Session Announcement Protocol.
Some neighbourhoods run themselves: programs pick their own address, your network number gives you one, or your company keeps its own private set. The keeper does not need to be asked.
§5Retiring Extended GLOP
RFC 3138 had delegated to the RIRs the assignment of the part of GLOP from 233.252.0.0 to 233.255.255.255, mapped from private AS numbers 64512-65534 and the reserved ASN 65535. This space was known as Extended GLOP (EGLOP).
The RFC says RFC 3138 should not have asked the RIRs to develop policies for it, because RFC 2860 reserves that to the IETF. Making the space available to operators is important, so RFC 3138 is obsoleted and the range is classed as AD-HOC Block III.
The first /24, 233.252.0.0/24, is assigned as MCAST-TEST-NET for documentation and example code. It SHOULD be used with the domain names example.com or example.net, and addresses in it MUST NOT appear on the public Internet.
An old rule let the wrong group hand out a set of addresses. This document takes that back and reserves a few for practice examples in manuals.
§6Applying, Reviewing and Reclaiming
Requests are submitted through the application form on the IANA web site. They need enough detail for the IESG designated expert to review them, and the expert may ask for more information before assigning an address.
When several addresses are needed, they come from AD-HOC Block III. For very large requests the assignment is staged, and later stages happen only after the earlier ones are completely used. The RFC says a separate document on AD-HOC assignment policy will be developed, with format and location not yet decided.
IANA should conduct an annual review of assigned addresses. Mis-assigned addresses should be reclaimed or reassigned where possible, and assignments in the AD-HOC, DIS Transient Groups and ST Multicast Groups blocks should be reviewed to reclaim addresses not in use on the global Internet.
Temporary assignments can be renewed, but the registrant must positively ask. If IANA has no renewal request within 30 days of expiry, the assignment MUST be removed from the registry. Returned addresses MUST NOT go to anyone but the last registrant for at least one calendar year.
You fill in a form, someone checks it, and once a year the keeper tidies up. If you forget to renew a loan, you lose the address, but nobody else can borrow it for a year.
§7Security Considerations
The RFC states that its assignment guidelines do not alter the security properties of either the Any Source or the Source-Specific Multicast service models. It adds nothing else on the subject. Its IANA Considerations section says only that IANA has updated its IPv4 multicast request and assignment procedures to reflect the document.
This is only about how addresses are handed out, so it does not make multicast any safer or riskier.