



Internet Engineering Task Force (IETF)                        T. Winters
Request for Comments: 9818                                       QA Cafe
Updates: 7084                                                  July 2025
Category: Informational                                                 
ISSN: 2070-1721


  DHCPv6 Prefix Delegation on IPv6 Customer Edge (CE) Routers in LANs

Abstract

   This document defines requirements for IPv6 Customer Edge (CE)
   routers to support DHCPv6 Prefix Delegation for distributing
   available prefixes to LAN devices that were delegated to an IPv6 CE
   router.  This document updates RFC 7084.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9818.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Terminology
   4.  IPv6 End-User Network Architecture
   5.  Requirements
     5.1.  LAN Prefix Delegation Requirements (LPD)
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

   This document describes guidelines for DHCPv6 Prefix Delegation in
   IPv6 Customer Edge (CE) routers [RFC7084] in order to properly
   utilize the IPv6 prefixes delegated by service providers.  Many
   service providers assign larger address blocks than /64 to CE
   routers, as recommended in [RFC6177].  If an IPv6 CE router does not
   support the Identity Association for Prefix Delegation (IA_PD) Prefix
   Option (Section 21.21 of [RFC8415]) on the LAN, it will not be able
   to assign any prefixes beyond its local interfaces, limiting the
   usefulness of assigning prefixes larger than /64 by the operator.
   Supporting IA_PD on the LAN interfaces of a CE router will allow
   those unused prefixes to be distributed into a network.  Note that
   efforts such as those of the Stub Networking Auto Configuration
   (SNAC) Working Group depend on IPv6 prefixes being properly
   distributed in the LAN.

   Two models, hierarchical prefix and flat, were proposed in the past
   for prefix sub-delegation beyond an IPv6 CE router.  Hierarchical
   prefix delegation requires an IPv6 CE router to sub-delegate IPv6
   prefixes based on a set of rules.  If more than one router uses
   hierarchical prefix delegation, an IPv6 prefix tree is created.  When
   no routing protocol is enabled to discover the network topology, it
   is possible to have an unbalanced prefix delegation tree, which leads
   to running out of prefixes.  More information on hierarchical prefix
   delegation can be found, e.g., in Section 8.5 of CableLabs IPv6
   eRouter specification [eRouter].  A flat prefix delegation requires
   the router to be provisioned with the initial prefix and to assign
   /64 prefixes to all other prefix requests from routers in the LAN-
   facing interface.  The default configuration of CE routers is
   designed to be a flat model to support zero-configuration networking.

   This document does not cover dealing with multi-prefix networks with
   more than one provider.  Due to the complexity of a solution that
   would require routing, provisioning, and policy, this is out of scope
   of this document.

2.  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.

   This document uses these keywords not strictly for the purpose of
   interoperability, but rather for the purpose of establishing
   industry-common baseline functionality.  As such, the document points
   to several other specifications to provide additional guidance to
   implementers regarding any protocol implementation required to
   produce a successful CE router that interoperates successfully with a
   particular subset of currently deployed and planned common IPv6
   access networks.

3.  Terminology

   The document makes use of the following terms, some of which are from
   Section 2 of [RFC8200].

   IPv6 node:  A device that implements IPv6.

   IPv6 router:  An IPv6 node that forwards IPv6 packets not explicitly
      addressed to itself.

   IPv6 host:  An IPv6 node that is not a router.

   ULA:  Unique Local Address, as defined in [RFC4193].

   GUA:  Global Unicast Address, as defined in [RFC4291].

4.  IPv6 End-User Network Architecture

   The end-user network for IPv6 contains stub networks.  Figure 1
   illustrates the model topology.

                          +-----------+
                          |  Service  |
                          |  Provider |
                          |   Router  |
                          +-----+-----+
                                |
                                |
                                |  Customer
                                |  Internet Connection
                                |
                          +-----v-----+
                          |   IPv6    |
                          |    CE     |
                          |  Router   |
                          +-----+-----+
                                |
                         +------+-------+
                         |              |
                         |              |
                     +---+----+   +-----+-----+
                     |  IPv6  |   |           |
                     |  Host  |   |   Router  |
                     |        |   |           |
                     +--------+   +-----+-----+
                                        |
                                        |
                                    +---+----+
                                    |  IPv6  |
                                    |  Host  |
                                    |        |
                                    +--------+

                  Figure 1: Example IPv6 End-User Topology

5.  Requirements

   IPv6 CE routers distribute configuration information obtained during
   WAN interface provisioning to LAN-facing IPv6 hosts and routers.  A
   CE router that is compliant with [RFC7084] would only provide IPv6
   hosts with configuration information.  This document allows for
   addressing and routing of IPv6 prefixes to both hosts and routers.
   These requirements are in addition to the ones in Section 4.3 of
   [RFC7084].

5.1.  LAN Prefix Delegation Requirements (LPD)

   LPD-1:   Each IPv6 CE router MUST support IPv6 prefix assignment
            according to Section 13.3 of [RFC8415] (Identity Association
            for Prefix Delegation (IA_PD) option) on its LAN
            interface(s).

   LPD-2:   Each IPv6 CE routers MUST assign a prefix from the delegated
            prefix as specified by L-2 in Section 4.3 of [RFC7084].  If
            insufficient prefixes are available, the IPv6 CE router MUST
            log a system management error.

   LPD-3:   The prefix assigned to a link MUST NOT change in the absence
            of a local policy or a topology change.

   LPD-4:   After LAN link prefix assignments, the IPv6 CE router MUST
            keep the remaining IPv6 prefixes available to other routers
            via Prefix Delegation.

   LPD-5:   IPv6 CE routers MUST maintain a local routing table that is
            dynamically updated with leases and the associated next hops
            as they are delegated to clients.  Absent explicit
            filtering, packets with destination addresses in a delegated
            prefix MUST be forwarded to that prefix regardless of which
            interface they are received on.  When a delegated prefix is
            released or expires, the associated route MUST be removed
            from the IPv6 CE router's routing table.  A delegated prefix
            expires when the valid lifetime assigned in the IA_PD
            expires without being renewed.  When a prefix is released or
            expires, it MUST be returned the pool of available prefixes.

   LPD-6:   By default, the IPv6 CE router filtering rules MUST allow
            forwarding of packets with an outer IPv6 header containing a
            source address belonging to delegated prefixes, along with
            reciprocal packets from the same flow, following the
            recommendations of [RFC6092].  This updates WPD-5 of
            [RFC7084] to not drop packets from prefixes that have been
            delegated.  IPv6 CE routers MUST continue to drop packets,
            including destination address, that are not assigned to the
            LAN or delegated.

   LPD-7:   The IPv6 CE routers MUST provision IA_PD prefixes with a
            prefix-length of 64 on the LAN-facing interface unless
            configured to use a different prefix-length by the CE router
            administrator.  The prefix-length of 64 is used as that is
            the current prefix-length supported by SLAAC [RFC4862].  For
            hierarchical prefix delegation, a prefix-length shorter than
            64 may be configured.

   LPD-8:   IPv6 CE routers configured to generate a ULA prefix as
            defined in ULA-1 of Section 4.3 of [RFC7084] MUST continue
            to provision available GUA IPv6 prefixes.

   LPD-9:   If an IPv6 CE router is provisioning both a ULA and GUA via
            prefix delegation, the GUA SHOULD appear first in the DHCPv6
            packets.

   LPD-10:  IPv6 CE routers MUST NOT delegate prefixes via DHCPv6 on the
            LAN using lifetimes that exceed the remaining lifetimes of
            the corresponding prefixes learned on the WAN.

6.  Security Considerations

   This document does not add any new security considerations beyond
   those mentioned in Section 4 of [RFC8213], Section 22 of [RFC8415],
   and Section 6 of [RFC6092].

7.  IANA Considerations

   This document has no IANA actions.

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>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,
              <https://www.rfc-editor.org/info/rfc7084>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8213]  Volz, B. and Y. Pal, "Security of Messages Exchanged
              between Servers and Relay Agents", RFC 8213,
              DOI 10.17487/RFC8213, August 2017,
              <https://www.rfc-editor.org/info/rfc8213>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

8.2.  Informative References

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,
              <https://www.rfc-editor.org/info/rfc6092>.

   [RFC6177]  Narten, T., Huston, G., and L. Roberts, "IPv6 Address
              Assignment to End Sites", BCP 157, RFC 6177,
              DOI 10.17487/RFC6177, March 2011,
              <https://www.rfc-editor.org/info/rfc6177>.

   [eRouter]  CableLabs, "IPv4 and IPv6 eRouter Specification", Data-
              Over-Cable Service Interface Specifications, Version I22,
              May 2024,
              <https://www.cablelabs.com/specifications/CM-SP-eRouter>.

Acknowledgements

   Thanks to the following people for their guidance and feedback:
   Marion Dillon, Erik Auerswald, Esko Dijk, Tim Carlin, Richard
   Patterson, Ted Lemon, Michael Richardson, Martin Huneki, Gabor
   Lencse, Ole Troan, Brian Carpenter, David Farmer, Kyle Rose, Mohamed
   Boucadair, Tim Chown, Ron Bonica, and Erica Johnson.

Author's Address

   Timothy Winters
   QA Cafe
   100 Main Street, Suite #212
   Dover, NH 03820
   United States of America
   Email: tim@qacafe.com
