Internet DRAFT - draft-ietf-magma-mld-source


MAGMA Working Group                                          B. Haberman
draft-ietf-magma-mld-source-07.txt                      Caspian Networks
Expires December 2003                                          June 2003

                 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.

   This document updates RFC 2710.


   The original specification of the Multicast Listener Discovery
   Protocol[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

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   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.


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].


   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
   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 discrepencies 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 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.

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

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   (::), 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

   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.

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.

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INTERNET DRAFT                                                 June 2003

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].


   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., Haberman, B., "Multicast
                 Listener Discovery (MLD) for IPv6", RFC 2710, October

   Informative References

      [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.

Author's Address

   Brian Haberman
   Caspian Networks
   One Park Drive
   Suite 300
   Research Triangle Park, NC  27709
   Phone: +1-919-949-4828

Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

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INTERNET DRAFT                                                 June 2003

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Haberman                                                        [Page 5]