Source Address Selection for the Multicast Listener Discovery (MLD) Protocol
draft-ietf-magma-mld-source-07

Versions: 00 01 02 03 04 05 06 07 rfc3590                               
MAGMA Working Group                                          B. Haberman
draft-ietf-magma-mld-source-05.txt                      Caspian Networks
Expires August 2003                                        February 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.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   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.

Introduction

   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.

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].

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
   address will not have one available to use as a source address.  For
   this reason, this document provides an alternative IPv6 source
   address for MLD messages being used during duplicate address
   detection.

   In addition, Sections 5 and 6 of [RFC 2710] mandates that a node
   receiving an MLD Report message verify that the IPv6 source address
   is a link-local address.  This document relaxes this rule in order to
   support the alternative IPv6 source address in use 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.  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 occur once the
   address is put into use.  These rules will allow implementations to
   correctly implement duplicate address detection and operate in the
   described environment.

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



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   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.

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.

Security Considerations

   General security issues related to MLD are discussed in [RFC 2710].

   The ability to send MLD messages with the unspecified address can



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   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.

References

   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
                 1999.

   Informative References

      [RFC 2461] Narten, T., Nordmark, E., Simpson, W., "Neighbor
                 Discovery for IP Version 6 (IPv6)", RFC 2461, December
                 1998.

      [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
   EMail: bkhabs@nc.rr.com

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Haberman                                                        [Page 5]