RFC 9818

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

📅 published July 2025, from ietf

RFC 9818: DHCPv6 Prefix Delegation on IPv6 Customer Edge (CE) Routers in LANs 🎀

Okay bestie, this one is by T. Winters from QA Cafe, published July 2025 as an Informational doc (so it's a guidance thing, not a Standards Track spec 💅). It's an IETF product with ISSN 2070-1721, and it updates RFC 7084 🦋. The vibe is "how do we get CE routers to hand out the big prefixes the ISP already gave them, and actually use them." Let's go! 💖

Abstract ✨

Okay so the main point: this doc sets out requirements for IPv6 Customer Edge (CE) routers so they can support DHCPv6 Prefix Delegation. The goal is handing out the prefixes they were delegated to LAN devices. It also updates RFC 7084 🫶.

Status of This Memo 📜

This one is not an Internet Standards Track spec. It's published just for information 🌸

It's an IETF product and it reflects the IETF community's consensus. It got public review and the Internet Engineering Steering Group (IESG) approved it for publication. Not every IESG-approved doc is a candidate for any level of Internet Standard, so check Section 2 of RFC 7841 for that. 🍓

For the current status, any errata, and how to send feedback, go to https://www.rfc-editor.org/info/rfc9818.

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

This doc is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they stand on the publication date. Please read those carefully because they spell out your rights and restrictions for this doc. Code Components pulled out of this doc have to include the Revised BSD License text described in Section 4.e of the Trust Legal Provisions, and they come with no warranty, as the Revised BSD License says. 💜

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 🌈

Okay so this doc is all about how CE routers [RFC7084] should do DHCPv6 Prefix Delegation, so they can actually make use of the IPv6 prefixes service providers hand them. Lots of providers give CE routers blocks bigger than /64, as [RFC6177] recommends 😭

Here's the catch. If a CE router doesn't support the Identity Association for Prefix Delegation (IA_PD) Prefix Option (Section 21.21 of [RFC8415]) on the LAN, it can't hand prefixes out past its own local interfaces. So that big block the operator gave it just sits there doing nothing 🙈 Supporting IA_PD on the LAN interfaces lets those unused prefixes get spread through the network. Also, efforts like the Stub Networking Auto Configuration (SNAC) Working Group depend on IPv6 prefixes being properly distributed in the LAN, so this matters 🔥

Two models have been proposed for sub-delegating prefixes past a CE router: hierarchical and flat. 🧚

With hierarchical prefix delegation, the CE router sub-delegates prefixes based on a set of rules. If more than one router does this, you get an IPv6 prefix tree. Without a routing protocol to discover the network topology, that tree can end up unbalanced, and then you run out of prefixes 💀 For more on hierarchical delegation, see Section 8.5 of the CableLabs IPv6 eRouter specification [eRouter].

With a flat model, the router gets provisioned with the initial prefix and gives out /64 prefixes to every other prefix request coming from routers on the LAN-facing interface. The default config for CE routers is flat, which makes zero-configuration networking work 🫶

One more thing: this doc does not cover multi-prefix networks with more than one provider. That needs routing, provisioning, and policy work, which is a whole complexity monster, so it's out of scope here 🚫

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.

Fun fact: this doc uses these keywords not strictly for interoperability, but to set an industry-common baseline. It also points to several other specs for extra guidance, covering any protocol implementation needed to build a CE router that works with a particular subset of currently deployed and planned common IPv6 access networks 💅

3. Terminology 📚

Here's the vocab for the doc. Some of these terms come 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 IPv6 end-user network is made up of stub networks. Figure 1 shows the model topology, and it's giving very clean diagram energy ✨

                      +-----------+
                      |  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 pass along the configuration info they got while provisioning the WAN interface to the LAN-facing IPv6 hosts and routers. A CE router that follows [RFC7084] only gives configuration info to IPv6 hosts. This doc lets you address and route IPv6 prefixes to both hosts and routers 👑

These requirements are on top of the ones in Section 4.3 of [RFC7084] 🫶

5.1. LAN Prefix Delegation Requirements (LPD) 💖

Here's the main event, the LPD requirements. Read them closely, bestie 🔍

  • 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 to 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. 💅

8.1. Normative References ✨

okay bestie, these are the must-reads this doc leans on 📚🫶

8.2. Informative References

these ones are the extra reading 🌸 good for the backstory and context ✨

Acknowledgements

big thank-yous to everyone who gave guidance and feedback 🫶💖 the whole squad:

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
📦 where this page comes from