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OPS v6ops IPv6 Internet Protocol Version 6 DHCPv6 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.

Introduction This document describes guidelines for DHCPv6 Prefix Delegation in IPv6 Customer Edge (CE) routers 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 . If an IPv6 CE router does not support the Identity Association for Prefix Delegation (IA_PD) Prefix Option () 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 . 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.

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 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.

Terminology The document makes use of the following terms, some of which are from .

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 .

GUA:

Global Unicast Address, as defined in .

IPv6 End-User Network Architecture The end-user network for IPv6 contains stub networks. illustrates the model topology.

Example IPv6 End-User Topology

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 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 .

LAN Prefix Delegation Requirements (LPD)

1. Each IPv6 CE router MUST support IPv6 prefix assignment according to (Identity Association for Prefix Delegation (IA_PD) option) on its LAN interface(s).
2. Each IPv6 CE routers MUST assign a prefix from the delegated prefix as specified by L-2 in . If insufficient prefixes are available, the IPv6 CE router MUST log a system management error.
3. The prefix assigned to a link MUST NOT change in the absence of a local policy or a topology change.
4. After LAN link prefix assignments, the IPv6 CE router MUST keep the remaining IPv6 prefixes available to other routers via Prefix Delegation.
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.
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 . This updates WPD-5 of 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.
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 . For hierarchical prefix delegation, a prefix-length shorter than 64 may be configured.
8. IPv6 CE routers configured to generate a ULA prefix as defined in ULA-1 of MUST continue to provision available GUA IPv6 prefixes.
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.
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.

Security Considerations This document does not add any new security considerations beyond those mentioned in , , and .

IANA Considerations This document has no IANA actions.

References Normative References Informative References Data-Over-Cable Service Interface Specifications, Version I22

Acknowledgements Thanks to the following people for their guidance and feedback: , , , , , , , , , , , , , , , , and .