MAGMA Working Group B. Haberman
draft-ietf-magma-mld-source-03.txt Caspian Networks
Expires April 2003 October 2002
Source Address Selection for Multicast
Listener Discovery Protocol (RFC 2710)
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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It has come to light that there is an issue with the selection of a
suitable IPv6 source address for Multicast Listener Discovery
messages when a node is performing stateless address
autoconfiguration. This memo is intended to clarify the rules on
selecting an IPv6 address to use for MLD messages.
The original specification of the Multicast Listener Discovery
Protocol[RFC 2710] mandated 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
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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.
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].
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 router 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 MUST be sent with a valid link-local
address as the IPv6 source address. If a valid link-local address is
not available, the message MUST be sent with the unspecified address
(::) as the IPv6 source address.
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
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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 termporary until the node can send
a Report with a valid link-local address.
General security issues related to MLD are discussed in [RFC 2710].
The ability to send MLD messages with the unspecified address can
lead to on-link abuse that is harder to trace. However, due to the
expected behavior of RFC 2710 implementations to drop these messages
and the additional rule that Query messages with the unspecified
address be dropped, the impact of such messages will not affect
multicast capabilities on the link.
[RFC 2710] Deering, S., Fenner, W., Haberman, B., "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, October
[RFC 2461] Narten, T., Nordmark, E., Simpson, W., "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
[RFC 2462] Thomson, S., Narten, T., "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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