MLD Source Address: Source Address Selection for the Multicast Listener Discovery (MLD) Protocol
Lets IPv6 nodes send MLD Reports from the unspecified address while they are still checking their own link-local address.
Haberman, B., "Source Address Selection for the Multicast Listener Discovery (MLD) Protocol", RFC 3590, DOI 10.17487/RFC3590, September 2003, <https://www.rfc-editor.org/info/rfc3590>.
@misc{rfc3590,
series = {Request for Comments},
number = 3590,
howpublished = {RFC 3590},
publisher = {RFC Editor},
doi = {10.17487/RFC3590},
url = {https://www.rfc-editor.org/info/rfc3590},
author = {B. Haberman},
title = {{Source Address Selection for the Multicast Listener Discovery (MLD) Protocol}},
pagetotal = 6,
year = 2003,
month = sep,
} RFC text
Contents
Network Working Group B. Haberman
Request for Comments: 3590 Caspian Networks
Updates: 2710 September 2003
Category: Standards Track
Source Address Selection for the
Multicast Listener Discovery (MLD) Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
It has come to light that there is an issue with the selection of a
suitable IPv6 source address for Multicast Listener Discovery (MLD)
messages when a node is performing stateless address
autoconfiguration. This document is intended to clarify the rules on
selecting an IPv6 address to use for MLD messages.
1. Introduction
The original specification of the Multicast Listener Discovery
Protocol (MLD) for IPv6 [RFC 2710] mandates the use of a link-local
IPv6 source address for the transmission of MLD messages. In
addition, MLD also requires nodes to send MLD Report messages when
joining any IPv6 multicast group (except the All-Nodes address and
addresses of scope less than 2).
These MLD requirements conflict with the use of IPv6 multicast within
the Neighbor Discovery Protocol [RFC 2461]. For stateless
autoconfiguration, as defined in [RFC 2462], a node is required to
join several IPv6 multicast groups in order to perform Duplicate
Address Detection prior to its use. Since the only address the node
has is tentative, and cannot be used for communication, it does not
have a suitable address to utilize as a source address.
This document will clarify the IPv6 source address selection rules
for use with MLD when no link-local addresses are available.
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 [RFC 2119].
3. Justification
In [RFC 2710], Section 3 requires that all MLD messages be sent with
a valid link-local IPv6 source address. However, a node in the
process of performing duplicate address detection for its link-local
(LL) address will not have one available to use as a source address.
For this reason, this document allows the unspecified address to be
used as a source address for MLD messages being used during duplicate
address detection.
The discrepancies in the rules defined in [RFC 2710] and [RFC 2462]
has led to implementation issues. Several IPv6 implementations skip
sending MLD Report messages during duplicate address detection
because they have no valid link-local address. This leads to
operational problems when a node is attached to switches that perform
MLD snooping. In this scenario, duplicate address detection (DAD)
will complete successfully and collisions can occur once the address
is put into use because switches may not have forwarded the DAD
messages to all nodes on the link as required. This document fixes
this problem by specifying that MLD reports are to be sent using an
unspecified source address prior to DAD being started in order to
ensure that messages sent to LL multicast addresses (e.g., including
MLD) are forwarded to all appropriate nodes as required.
4. MLD Source Address Selection Guidelines
An MLD speaking node is required to choose a suitable IPv6 source
address for all MLD messages (Report, Done, and Query).
MLD Query messages MUST be sent with a valid link-local address as
the IPv6 source address. If a node (router or host) receives a query
message with an IPv6 source address set to the unspecified address
(::), it MUST silently discard the message and SHOULD log a warning.
MLD Report and Done messages are sent with a link-local address as
the IPv6 source address, if a valid address is available on the
interface. If a valid link-local address is not available (e.g., one
has not been configured), the message is sent with the unspecified
address (::) as the IPv6 source address.
Once a valid link-local address is available, a node SHOULD generate
new MLD Report messages for all multicast addresses joined on the
interface.
Routers receiving an MLD Report or Done message with the unspecified
address as the IPv6 source address MUST silently discard the packet
without taking any action on the packets contents.
Snooping switches MUST manage multicast forwarding state based on MLD
Report and Done messages sent with the unspecified address as the
IPv6 source address.
5. Source Address Selection Implications
In RFC 2710, MLD Report and Done messages are required to have an
IPv6 source address that is link-local. This memo augments that rule
by allowing these messages to contain the unspecified address (::) as
the source address.
The behavior of RFC 2710 implementations, when receiving a message
with a source address of ::, is dependent upon how the implementation
treats the unspecified address. That is, these messages will be
dropped if the implementation does not consider the unspecified
address to be link-local in scope.
As the unspecified address is only used when there is no link-local
address, RFC 2710 implementations discarding these packets will have
no affect on the packet's sender as the use should only be for
joining the link-local solicited-node multicast group [RFC 2462].
There is an implication to senders with respect to joining other
multicast groups prior to the activation of a link-local address.
The dropping of Reports using the unspecified address as a source
address could cause a lack of multicast traffic that is expected by
the node. This black hole will be temporary until the node can send
a Report with a valid link-local address.
6. Security Considerations
General security issues related to MLD are discussed in [RFC 2710].
For hosts and routers, all received MLD messages from an unspecified
source address are silently discarded. This is the required behavior
from [RFC 2710] and is not changed by this document. Thus, the
changes have no new security impacts.
In the case of snooping switches, multicast forwarding state will be
maintained based on Report and Done messages sent with the
unspecified address as the source address. However, the security
vulnerabilities in this scenario are similar to those describing
forged messages in the security considerations section of [RFC 2710].
7. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2710] Deering, S., Fenner, W. and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
1999.
8.2. Informative References
[RFC 2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC 2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
9. Author's Address
Brian Haberman
Caspian Networks
753 Bridgewater Drive
Sykesville, MD 21784
Phone: +1-410-552-1421
EMail: brian@innovationslab.net
10. Full Copyright Statement
Copyright (C) The Internet Society (2003). 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
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1The Conflict Between MLD and Autoconfiguration
The original Multicast Listener Discovery specification (RFC 2710) says that MLD messages must use a link-local IPv6 source address. It also requires a node to send an MLD Report when it joins any IPv6 multicast group, except the All-Nodes address and addresses of scope less than 2.
That clashes with stateless address autoconfiguration (RFC 2462). To perform duplicate address detection, a node has to join several multicast groups, but its only address is tentative and cannot be used for communication. It therefore has no suitable source address. RFC 3590 clarifies the source address rules for MLD when no link-local address is available.
MLD says to write your return address on every note, but a new computer is still checking whether its address is free. It has nothing to write.
§2Why the Rules Needed Fixing
Section 3 of RFC 2710 requires all MLD messages to carry a valid link-local source address. A node in the middle of duplicate address detection (DAD) for its link-local address does not have one. The RFC therefore allows the unspecified address as the source for MLD messages used during DAD.
The mismatch had caused real implementation problems. Several IPv6 implementations skipped sending MLD Reports during DAD because they had no valid link-local address. With switches that perform MLD snooping, DAD could then finish successfully while the switches had not forwarded the DAD messages to all nodes on the link. Address collisions could then occur once the address was in use.
If a switch never hears you are interested in a group, it will not pass you those messages. Then you cannot hear if someone else already has your address.
§3Source Address Selection Rules
An MLD node must pick a source address for all MLD messages: Report, Done and Query. The rules differ by message.
- Query: MUST be sent with a valid link-local source address. A node (router or host) that receives a Query with source
::MUST silently discard it and SHOULD log a warning. - Report and Done: use a link-local source if a valid one is available on the interface. If not, for example because none has been configured, the message is sent with the unspecified address
::.
Once a valid link-local address is available, a node SHOULD generate new MLD Reports for all multicast addresses joined on the interface. Routers that receive a Report or Done with source :: MUST silently discard the packet without acting on its contents. Snooping switches MUST manage multicast forwarding state based on such Reports and Done messages.
A question to the group needs a real return address. A plain I am here or I am leaving note may use a blank one, but routers ignore those, and the switches in the middle pay attention.
| Message | Source address | On receiving from :: |
|---|---|---|
| Query | MUST be a valid link-local address | Discard silently, SHOULD log a warning |
| Report THIS RFC | Link-local if available, otherwise :: | Routers discard silently; snooping switches track state |
| Done THIS RFC | Link-local if available, otherwise :: | Routers discard silently; snooping switches track state |
§4Effects on Existing RFC 2710 Implementations
This memo adds to the RFC 2710 rule by allowing Report and Done messages to carry ::. How an RFC 2710 implementation reacts to such a message depends on how it treats the unspecified address. If it does not count the unspecified address as link-local in scope, it will drop the packets.
That is usually harmless, because the unspecified address is used only when there is no link-local address, so it should only be for joining the link-local solicited-node multicast group. A sender that joins other multicast groups before its link-local address is active could suffer a temporary black hole, since Reports from :: may be dropped. The gap lasts until the node can send a Report with a valid link-local address.
Some older computers ignore notes with a blank return address. That is fine for this job, and anything missed is fixed as soon as the computer gets a real address.
§5Security Considerations
The RFC points to RFC 2710 for the general security issues of MLD. For hosts and routers, all received MLD messages from an unspecified source address are silently discarded, which is the required behavior from RFC 2710 and is not changed here. The RFC concludes that the changes have no new security impacts.
Snooping switches, however, now keep multicast forwarding state based on Report and Done messages sent from the unspecified address. The RFC says the vulnerabilities in that case are similar to those for forged messages, which RFC 2710 describes in its security considerations section.
Nothing new is risky for computers and routers, since they ignore blank-address notes. The switches do act on them, so someone could fake a note, much like before.