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rfc:rfc3590

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.

Haberman Standards Track [Page 1] RFC 3590 Source Address Selection for MLD Protocol September 2003

 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.

Haberman Standards Track [Page 2] RFC 3590 Source Address Selection for MLD Protocol September 2003

 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.

Haberman Standards Track [Page 3] RFC 3590 Source Address Selection for MLD Protocol September 2003

 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.

Haberman Standards Track [Page 4] RFC 3590 Source Address Selection for MLD Protocol September 2003

9. Author's Address

 Brian Haberman
 Caspian Networks
 753 Bridgewater Drive
 Sykesville, MD  21784
 Phone: +1-410-552-1421
 EMail: brian@innovationslab.net

Haberman Standards Track [Page 5] RFC 3590 Source Address Selection for MLD Protocol September 2003

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
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Haberman Standards Track [Page 6]

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