



Network Working Group                                        N. Karstens
Internet-Draft                                                    Garmin
Intended status: Standards Track                            D. Farinacci
Expires: 1 October 2026                                      lispers.net
                                                              M. McBride
                                                               Futurewei
                                                           30 March 2026


       Zero-Configuration Assignment of IPv6 Multicast Addresses
               draft-ietf-pim-ipv6-zeroconf-assignment-08

Abstract

   Describes a zero-configuration protocol for dynamically assigning
   IPv6 multicast addresses.  Applications randomly assign multicast
   group IDs from a specified range and prevent collisions by using
   Multicast DNS (mDNS) to publish records under a new "eth-addr.arpa"
   domain.  This protocol satisfies all of the criteria listed in draft-
   ietf-pim-zeroconf-mcast-addr-alloc-ps.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 1 October 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components



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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Veto Records  . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Use on Networks with Multiple Subnets . . . . . . . . . . . .   4
   4.  Evaluation of Solution  . . . . . . . . . . . . . . . . . . .   5
     4.1.  Notes on Collision Detection  . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Domain Name Reservation Considerations  . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [I-D.ietf-pim-zeroconf-mcast-addr-alloc-ps] includes a problem
   statement and requirements for a zero-configuration method for
   dynamically assigning multicast addresses.  This document describes a
   process that fulfills these requirements by having applications
   randomly assign IPv6 multicast group IDs from a specified range and
   using mDNS [RFC6762] to prevent collisions.

   Note that DNS-based Service Discovery (DNS-SD) [RFC6763] uses several
   different DNS record types, published using either Unicast or
   Multicast DNS, to facilitate service discovery.  This document uses a
   single DNS record type (PTR), published using Multicast DNS, to
   coordinate IPv6 multicast address assignment in a zero-configuration
   environment.  The DNS records in this protocol may be published
   alongside records for other domain name services, such as DNS-SD, or
   they may be published alone. mDNS is used in favor of a new protocol
   with the expectation that functionality for address assignment can be
   achieved using existing mDNS implementations.

   This protocol is well-suited for networks that rely on IPv6 multicast
   and already deploy mDNS.







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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Procedure

   When an application is preparing to transmit a multicast stream, it
   first generates a random group ID in the range 0x90000000-0x9FFFFFFF,
   which IANA is REQUESTED to assign from the "Dynamic Multicast Group
   IDs" registry (see Section 5).  It combines this with the Interface
   Identifier (IID) of the intended source address for the multicast
   stream to generate a link-scoped IPv6 multicast address [RFC4489].
   The application then calculates the multicast Ethernet address that
   will be used to transmit the data ([RFC2464], Section 7) and uses
   that to construct a string like a reverse-mapping domain, using a new
   "eth-addr.arpa" domain.

   For example, given a source address of fe80::a12:34ff:fe56:7890, the
   IPv6 multicast address may be ff32:00ff:a12:34ff:fe56:7890:9abc:def0,
   the group ID 9abc:def0, the multicast Ethernet address
   33:33:9A:BC:DE:F0, and the resulting string is
   "0.f.e.d.c.b.a.9.3.3.3.3.eth-addr.arpa".

   The application then uses the mDNS probing algorithm described in
   [RFC6762], Section 8.1 to continuously query for a PTR record with
   the same name as the generated string.  If the probing algorithm
   completes without any conflict, then the application begins
   advertising its own unique PTR record using that name.  The PTRDNAME
   field consists of a unique application identifier, in the form of a
   DNS label, followed by the device's host name (for example,
   "application.example.local.").  Integrating a unique identifier in
   this manner allows for multiple applications to be on the same host.
   Note that A/AAAA records may also be published for this host name
   ("example.local."), though this is not a requirement for this design.

   Because this protocol is focused specifically on allocating IPv6
   multicast addresses, records MUST be published using the IPv6
   multicast address for mDNS.  In order to be compatible with existing
   mDNS implementations, records MAY also be published using the IPv4
   multicast address for mDNS.

   Once the PTR record is advertised, the host may then begin
   transmitting multicast data using the generated address.




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   The application shall retain the group ID value and use it the next
   time the multicast stream is transmitted.  This allows the network to
   quickly settle on a configuration that will never have another
   collision as long as the network is unchanged.

   If a conflict is detected at any point, then the application stops
   transmitting that multicast stream and starts the process over using
   a different group ID.  As before, this new group ID is also retained,
   overwriting any group ID previously saved for this multicast stream.

   The host network stack may optionally monitor the network for traffic
   that uses the same destination multicast Ethernet address, but a
   different destination multicast IPv6 address.  If this is detected,
   then the application responds the same as a collision.

   While intended primarily for allocating IPv6 multicast addresses on
   the same subnet (link-local scope), the same technique could also
   apply to a larger network as long as mDNS traffic is routed between
   subnets (for any scope excluding global scope).

2.1.  Veto Records

   [I-D.ietf-pim-zeroconf-mcast-addr-alloc-ps] describes collisions
   occurring in the network infrastructure.  When an infrastructure
   component detects a collision it cannot resolve, it triggers a
   conflict with the application by publishing a veto record.  A veto
   record is a unique PTR record using the string generated for the
   address as its name and the PTRDNAME field set to the string "veto",
   formatted as a DNS label.  The veto record is published without
   probing.

   Applications respond to the conflict the same as to a collision.  The
   application retains its new group ID, so the same conflict is not
   repeated in the future.

3.  Use on Networks with Multiple Subnets

   The protocol can be extended across multiple subnets if PTR records
   are distributed between subnets (for example, by using an mDNS
   reflector or the Discovery Proxy described in [RFC8766]).  Note that
   the stream MUST use a unicast-prefix based IPv6 multicast address
   ([RFC3306]), as the link-scoped IPv6 multicast address used above is
   scoped to the local link.

   The protocol's reliance on cooperating hosts (see Section 6) makes it
   unsuited for use on the global Internet.  [RFC8815] recommends
   Source-Specific Multicast in this environment.




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4.  Evaluation of Solution

   [I-D.ietf-pim-zeroconf-mcast-addr-alloc-ps] contains a list of
   criteria to evaluate potential solutions.  The following is an
   analysis of how this protocol satisfies the requirements listed in
   that document:

   1.  [REQ-1] Unique Address Assignment: Use of the protocol results in
       a unique address being assigned to the multicast group at both
       the network and link layers.

   2.  [REQ-2] Resilience to Single Points of Failure: The protocol uses
       mDNS, which uses a peer-to-peer communication model and so has no
       single points of failure, as long as network connectivity is not
       interrupted.

   3.  [REQ-3] Zero User Configuration: The protocol operates without
       requiring user or administrator configuration.

   4.  [REQ-4] Coexistence with Multicast Address Allocation Solutions:
       This document assigns a range of group IDs from the "Dynamic
       Multicast Group IDs" registry for use with the protocol, which
       allows the protocol to coexist with other multicast IP address
       allocation solutions (see
       [I-D.ietf-pim-updt-ipv6-dyn-mcast-addr-grp-id]).

   5.  [REQ-5] Single-Subnet Operation: The protocol supports operation
       within a single IPv6 subnet.

   6.  [REQ-6] No External Connectivity: The protocol does not require
       Internet access or connectivity to external infrastructure.

   7.  [REQ-7] Supports Multiple Host Applications: Incorporating a
       unique application identifier into the PTR record's PTRDNAME
       field allows the protocol to support multiple applications on the
       same host, each operating independently of the others.

   8.  [REQ-8] Collision Detection and Resolution: The protocol includes
       a mechanism to detect and resolve multicast address collisions at
       both the network and link layers.  See Section 4.1 for additional
       analysis.

   The following is an analysis of how this protocol satisfies the
   desirable characteristics listed in
   [I-D.ietf-pim-zeroconf-mcast-addr-alloc-ps]:

   1.  [CONS-1] Multi-Subnet Support: The protocol can operate across
       multiple subnets (see Section 3).



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   2.  [CONS-2] Standards Compatibility: The protocol uses existing
       protocols without requiring any changes.

   3.  [CONS-3] Cross-Platform Availability: The protocol uses mDNS,
       which is implemented on many host platforms and operating
       systems.

   4.  [CONS-4] Minimal Dependency on Manufacturing Data: The protocol
       does not rely on pre-loaded configuration or device-specific
       manufacturing data.

   5.  [CONS-5] Low Overhead: The protocol uses mDNS, which is designed
       to minimize the volume and frequency of network traffic generated
       during normal operation.

   6.  [CONS-6] Advertisement: This document does not prescribe any
       mechanism for advertising the IPv6 address used for the multicast
       stream.  However, because the protocol already uses mDNS, a
       natural option for doing so would be to use DNS-SD ([RFC6763]) to
       advertise a "_udp" service and publish the address used for the
       multicast stream in a TXT record.

   7.  [CONS-7] Network Topology: Veto records ensure that the protocol
       works regardless of the underlying topology and adjacencies.

4.1.  Notes on Collision Detection

   However, because mDNS is designed to be a low-bandwidth protocol, it
   can take a signficant amount of time to detect a record collision
   after a network partition is repaired.  This is not a concern on
   networks where all multicast streams are established before any
   likely partition event because all group IDs will have been selected
   and stored for future use.

   It is a greater concern on networks where multicast streams may be
   established at any time.  Deployments on these networks may consider
   engaging a detection mechanism and prompting hosts to send
   unsolicited mDNS response messages when the partition is repaired.

5.  IANA Considerations

   IANA should allocate a block of group IDs from the "Dynamic Multicast
   Group IDs" registry in the "IPv6 Multicast Address Space" registry
   group that was created by
   [I-D.ietf-pim-updt-ipv6-dyn-mcast-addr-grp-id].  The range of this
   block should be 0x90000000-0x9FFFFFFF and the description should be
   the title of this document.




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   The domain "eth-addr.arpa" should be registered in the .arpa registry
   (https://www.iana.org/domains/arpa).  The usage should indicate that
   it is for mapping Ethernet addresses to local domain names and it
   should reference this document.

   The special-use domain "9.3.3.3.3.eth-addr.arpa." should be
   registered in the "Special-Use Domain Names" registry
   (https://www.iana.org/assignments/special-use-domain-names).  This
   domain should not be delegated.

5.1.  Domain Name Reservation Considerations

   [RFC6761], Section 5 includes a list of questions that must be
   answered when reserving a new Special-Use Domain Name.  The answers
   to these questions for the "9.3.3.3.3.eth-addr.arpa" Special-Use
   Domain, and any names falling within this domain, are as follows:

   1.  Users are free to use these names as they would for any other
       reverse-mapping domain.  Because this mapping is closely-related
       to the local network, users SHOULD be aware that these names are
       likely to yield different results on different networks.

   2.  Application software SHOULD recognize these names as a reverse-
       mapping domain and MAY associate them with the protocol described
       in this document.

   3.  Name resolution APIs and libraries SHOULD recognize these names
       as being used exclusively with mDNS, and so SHOULD NOT send
       queries for these names to their configured unicast DNS
       server(s).

   4.  Caching DNS servers SHOULD recognize these names as being used
       exclusively with mDNS, SHOULD NOT attempt to look up records
       associated with these names, and SHOULD respond to any query with
       NXDOMAIN.

   5.  Authoritative DNS servers SHOULD recognize these names as being
       used exclusively with mDNS and SHOULD respond to any query with
       NXDOMAIN.

   6.  DNS server operators MUST NOT configure an authoritative DNS
       server to answer queries for these names.

   7.  DNS Registries/Registrars MUST NOT register 9.3.3.3.3.eth-
       addr.arpa names.






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6.  Security Considerations

   This algorithm only works in environments where all hosts are
   cooperating.  Malicious hosts could deny service by repeatedly
   triggering address conflicts.

7.  Acknowledgement

   Special thanks to the National Marine Electronics Association for
   their contributions in developing marine industry standards and their
   support for this work.

   Thanks also to the members of the PIM working group for their early
   brainstorming sessions and review of this draft, and to Esko Dijk for
   his review of the draft.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
              <https://www.rfc-editor.org/info/rfc2464>.

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, DOI 10.17487/RFC3306,
              August 2002, <https://www.rfc-editor.org/info/rfc3306>.

   [RFC4489]  Park, J., Shin, M., and H. Kim, "A Method for Generating
              Link-Scoped IPv6 Multicast Addresses", RFC 4489,
              DOI 10.17487/RFC4489, April 2006,
              <https://www.rfc-editor.org/info/rfc4489>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.






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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

   [I-D.ietf-pim-updt-ipv6-dyn-mcast-addr-grp-id]
              Karstens, N., Farinacci, D., and M. McBride, "Updates to
              Dynamic IPv6 Multicast Address Group IDs", Work in
              Progress, Internet-Draft, draft-ietf-pim-updt-ipv6-dyn-
              mcast-addr-grp-id-13, 10 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pim-
              updt-ipv6-dyn-mcast-addr-grp-id-13>.

   [I-D.ietf-pim-zeroconf-mcast-addr-alloc-ps]
              Karstens, N., Farinacci, D., and M. McBride, "Zeroconf
              Multicast Address Allocation Problem Statement and
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-pim-zeroconf-mcast-addr-alloc-ps-13, 17 February
              2026, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pim-zeroconf-mcast-addr-alloc-ps-13>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC8766]  Cheshire, S., "Discovery Proxy for Multicast DNS-Based
              Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
              2020, <https://www.rfc-editor.org/info/rfc8766>.

   [RFC8815]  Abrahamsson, M., Chown, T., Giuliano, L., and T. Eckert,
              "Deprecating Any-Source Multicast (ASM) for Interdomain
              Multicast", BCP 229, RFC 8815, DOI 10.17487/RFC8815,
              August 2020, <https://www.rfc-editor.org/info/rfc8815>.

Authors' Addresses

   Nate Karstens
   Garmin International, Inc.
   1200 E. 151st St.
   Olathe, KS 66062-3426
   United States of America
   Email: nate.karstens@gmail.com








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   Dino Farinacci
   lispers.net
   San Jose, CA
   United States of America
   Email: farinacci@gmail.com


   Mike McBride
   Futurewei
   United States of America
   Email: michael.mcbride@futurewei.com








































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