RFC 3590

RFC 3590 🎀 Source Address Selection for the Multicast Listener Discovery (MLD) Protocol 💖

Written by Brian Haberman of Caspian Networks 🧚 Category: Standards Track ✨ Date: September 2003 🌸 Updates: 2710 (it tweaks [RFC 2710] 💅) Obsoletes: nothing listed in the header 🦋

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Status of This Memo ✨

okay so this doc is an Internet standards track protocol for the Internet community, and it's basically asking everyone to talk about it and suggest improvements 🫶 For the current standardization state and status, check the "Internet Official Protocol Standards" (STD 1) list. Distribution is unlimited, so share away 💜

Copyright Notice 🌈

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

Abstract 💅

so here's the tea: there's an issue with picking the right IPv6 source address for Multicast Listener Discovery (MLD) messages when a node is doing stateless address autoconfiguration 😭 This document is here to clear up the rules for choosing which IPv6 address goes in MLD messages ✨

1. Introduction 🌸

the original MLD spec for IPv6 [RFC 2710] says MLD messages have to use a link-local IPv6 source address 🔗 On top of that, nodes also have to send MLD Report messages whenever they join any IPv6 multicast group, except the All-Nodes address and addresses with scope less than 2 🙈

these MLD rules clash with how IPv6 multicast works in the Neighbor Discovery Protocol [RFC 2461]. With stateless autoconfiguration, as described in [RFC 2462], a node has to join several IPv6 multicast groups so it can run Duplicate Address Detection before using its address 😵 The problem is that the only address it has is still tentative and can't be used for communication, so it has no proper address to use as a source address 💀

this document is going to spell out the IPv6 source address selection rules for 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 says all MLD messages have to be sent with a valid link-local IPv6 source address. But a node that's in the middle of duplicate address detection for its link-local (LL) address doesn't have one to use yet 😭 So this document lets the unspecified address be used as the source address for MLD messages sent during duplicate address detection 🫶

the rules in [RFC 2710] and [RFC 2462] don't quite match, and that has caused implementation problems 🙃 Several IPv6 implementations skip sending MLD Report messages during duplicate address detection because they don't have a valid link-local address. That becomes a real headache when a node is connected to switches that do MLD snooping 🍓 In that setup, duplicate address detection can finish successfully, but collisions can still happen once the address is in use, because the switches might not have forwarded the DAD messages to every node on the link as required 💀 This document fixes it by saying MLD reports get sent with an unspecified source address before DAD starts, so messages sent to LL multicast addresses (including MLD, for example) reach all the 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 whose IPv6 source address is 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 isn't available (for example, one hasn't 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 that receive 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 packet's 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 adds to that rule by letting these messages carry the unspecified address (::) as the source address. 💅

How an RFC 2710 implementation reacts to a message with a source address of :: depends on how that implementation treats the unspecified address. In other words, these messages get dropped if the implementation doesn't consider the unspecified address to be link-local in scope. 🤔

Since the unspecified address is only used when there's no link-local address, RFC 2710 implementations that discard these packets won't affect the sender at all, because the unspecified address should only be used for joining the link-local solicited-node multicast group [RFC 2462]. 🧚

There's also an implication for senders who join other multicast groups before a link-local address is active. Dropping Reports that use the unspecified address as the source could mean the node stops getting multicast traffic it expects 🕳️ That black hole is only temporary, though, because the node can send a Report with a valid link-local address once it has one ✨

6. Security Considerations 🔐

General security issues for MLD are covered in [RFC 2710]. 🫶

For hosts and routers, every received MLD message from an unspecified source address is silently discarded. That's the behavior required by [RFC 2710], and this document doesn't change it, so these changes don't add any new security impacts. 🛡️

For snooping switches, multicast forwarding state gets maintained based on Report and Done messages sent with the unspecified address as the source address. The security vulnerabilities in this case are similar to the forged-message issues described in the security considerations section of [RFC 2710]. 😱

7. Intellectual Property Statement 📜

The IETF doesn't take a position on whether any intellectual property or other rights claimed for the technology in this document are valid or in scope, or whether any license under those rights is available. It also doesn't claim to have checked for any such rights. Info on the IETF's procedures for rights in standards-track and standards-related documents can be found in BCP-11. Copies of rights claims that were submitted for publication, along with any licensing assurances, or the results of attempts to get a general license or permission for implementers or users of this spec, can be obtained from the IETF Secretariat. 💼

The IETF invites anyone interested to tell it about any copyrights, patents, patent applications, or other proprietary rights that might cover technology required to practice this standard. Send that info 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.

9. Author's Address 🧚

Brian Haberman
Caspian Networks
753 Bridgewater Drive
Sykesville, MD  21784

Phone: +1-410-552-1421
EMail: brian@innovationslab.net

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. 🫶