Internet-Draft IPv6 CE Routers LAN Prefix Delegation February 2024
Winters Expires 30 August 2024 [Page]
Internet Engineering Task Force
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T. Winters
QA Cafe

IPv6 CE Routers LAN Prefix Delegation


This document defines requirements for IPv6 CE Routers to support DHCPv6 Prefix Delegation for redistributing any unused prefix(es) that were delegated to the IPv6 CE Router. This document updates RFC 7084.

Status of This Memo

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

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This Internet-Draft will expire on 30 August 2024.

Table of Contents

1. Introduction

This document defines DHCPv6 Prefix Delegation in IPv6 CE Routers ([RFC7084]) in order to properly utilize the IPv6 prefixes assigned by service providers. Many ISP will assign a prefix larger then /64 to the CE Router, as recommended in [RFC6177]. If an IPv6 CE Router doesn't support IA_PD on the LAN it will not be able to assign any prefixes beyond it's local interfaces. Limiting the usefulness of assigning prefixes larger than /64 by the operator. Supporting IA_PD on the LAN interfaces will allow for those unused prefixes to be distributed into a network. Work is being done in the areas such as Stub Networking Auto Configuration (SNAC) working group that depends on IPv6 addresses being properly distibuted.

Two models, hierarchical prefix and flat, have been proposed in the past for prefix sub-delegation beyond the IPv6 CE Router. Hierarchical prefix delegation requires an IPv6 CE Router to sub delegate IPv6 prefixes based on set of rules. If more then one router uses hierarchical prefix delegation, a IPv6 prefix tree is created. When no routing protocol is present to discover the network topology it's possible to have unbalanced prefix delegation tree which leads to running out of prefixes. For more information on heirachical prefix delegation is contained in Section 8.5 of CableLabs IPv6 eRouter Specifiction [eRouter]. A flat prefix delegation requires the router provisioned with the initial prefix then assign /64 prefixes to all other prefix request from routers downstream. This is the default configuration is designed to be the flat model to support zero configuration networking.

The Home Networking working group producted solutions for prefix delegation for home networks, including [RFC7695]. The Home Networking solutions haven't been deployed by ISPs or IPv6 CE Routers since the publication partially due to their complexity.

This document does not cover dealing with multi-provisioned networks with more than one provider. Due to complexity of a solution that will 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 also makes use of internal conceptual variables to describe protocol behavior and external variables that an implementation must allow system administrators to change. The specific variable names, how their values change, and how their settings influence protocol behavior are provided to demonstrate protocol behavior. An implementation is not required to have them in the exact form described here, as long as its external behavior is consistent with that described in this document.

3. Terminology

The following terminology is defined for this document.

4. IPv6 End-User Network Architecture

The end-user network that has IPv6 Customer Edge Routers with routers and hosts downstream. Figure 1 illustrates the model topology.

                     |  Service  |
                     |  Provider |
                     |   Router  |
                           |  Customer
                           |  Internet Connection
                     |   IPv6    |
                     |    CE     |
                     |  Router   |
                    |              |
                    |              |
                +---+----+   +-----+------+
                |  IPv6  |   |            |
                |  Host  |   |  Router    |
                |        |   |            |
                +--------+   +------------+
Figure 1: Example IPv6 End User Topology

5. Requirements

The IPv6 CE Router distributes configuration information obtained during WAN interface provisioning to IPv6 hosts and routers. Previously, a router based on [RFC7084] would only provide IPv6 hosts with individual addresses; this update allows for addressing and routing of IPv6 prefixes to both hosts and routers.

5.1. LAN Prefix Delegation Requirements


The IPv6 CE Router MUST support a DHCPv6 server capable of IPv6 prefix assignment according to [RFC8415] (Identity Association for Prefix Delegation (IA_PD) option).


The IPv6 CE Router MUST assign a prefix from the delegated prefix to each of its LAN links. If not enough addresses are available the IPv6 CE Router SHOULD log a system management error.


The prefix assigned to a link MUST NOT change in the absence of topology or configuration changes.


After LAN link prefix assignment the IPv6 CE Router MUST make the remaining IPv6 prefixes available to other routers via Prefix Delegation.


The IPv6 CE Router MUST install a route to the assigned IA_PD with a next-hop of the IPv6 node that was assigned the prefix. The IPv6 CE Router MUST remove the route when IA_PD lease expires.


By default, the IPv6 CE Router firewall 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]


The IPv6 CE Router SHOULD by default provision IA_PD IA prefixes with a prefix-length of 64.

6. Security Considerations

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

7. IANA Considerations

This document makes no request of IANA.

8. 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.

9. References

9.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Woodyatt, J., Ed., "Recommended Simple Security Capabilities in Customer Premises Equipment (CPE) for Providing Residential IPv6 Internet Service", RFC 6092, DOI 10.17487/RFC6092, , <>.
Narten, T., Huston, G., and L. Roberts, "IPv6 Address Assignment to End Sites", BCP 157, RFC 6177, DOI 10.17487/RFC6177, , <>.
Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic Requirements for IPv6 Customer Edge Routers", RFC 7084, DOI 10.17487/RFC7084, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Volz, B. and Y. Pal, "Security of Messages Exchanged between Servers and Relay Agents", RFC 8213, DOI 10.17487/RFC8213, , <>.
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, , <>.

9.2. Informative References

Pfister, P., Paterson, B., and J. Arkko, "Distributed Prefix Assignment Algorithm", RFC 7695, DOI 10.17487/RFC7695, , <>.
CableLabs, "IPv4 and IPv6 eRouter Specification Version I21", , <>.

Author's Address

Timothy Winters
QA Cafe
100 Main Street, Suite #212
Dover, NH 03820
United States of America