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Network Working Group                                        B. HabermanRequest for Comments: 3590                              Caspian NetworksUpdates:2710                                             September 2003Category: Standards TrackSource Address Selection for theMulticast Listener Discovery (MLD) ProtocolStatus 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   InRFC 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 ofRFC 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 inBCP-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.  References8.1.  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 20039.  Author's Address   Brian Haberman   Caspian Networks   753 Bridgewater Drive   Sykesville, MD  21784   Phone: +1-410-552-1421   EMail: brian@innovationslab.netHaberman                    Standards Track                     [Page 5]

RFC 3590       Source Address Selection for MLD Protocol  September 200310.  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]