| Internet-Draft | IPv6 CE Routers LAN Prefix Delegation | April 2025 |
| Winters | Expires 23 October 2025 | [Page] |
This document defines requirements for IPv6 Customer Edge (CE) routers to support DHCPv6 Prefix Delegation for distributing unused prefixes that were delegated to a IPv6 CE router. This document updates RFC 7084.¶
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This document defines requirements for DHCPv6 Prefix Delegation in IPv6 Customer Edge (CE) routers ([RFC7084]) in order to properly utilize the IPv6 prefixes assigned 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 Stub Networking Auto Configuration (SNAC) Working Group that depends 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 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 unbalanced prefix delegation tree which leads to running out of prefixes. More information on heirarchical prefix delegation can be found, e.g., in Section 8.5 of CableLabs IPv6 eRouter Specifiction [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 Router supporting prefix delegation is designed to be a flat model to support zero configuration networking.¶
This document does not cover dealing with multi-provisioned networks with more than one provider. Due to complexity of a solution that would require routing, provisioning and policy, this is out of scope of this document.¶
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.¶
The following terminology is defined for this document.¶
The end-user network for IPv6 that is a stub network. Figure 1 illustrates the model topology.¶
+-----------+
| Service |
| Provider |
| Router |
+-----+-----+
|
|
| Customer
| Internet Connection
|
+-----v-----+
| IPv6 |
| CE |
| Router |
+-----+-----+
|
+------+-------+
| |
| |
+---+----+ +-----+------+
| IPv6 | | |
| Host | | Router |
| | | |
+--------+ +------------+
IPv6 CE router distribute configuration information obtained during WAN interface provisioning to LAN-facing IPv6 hosts and routers. An [RFC7084] compliant CE router would only provide IPv6 hosts with configuration information. This 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].¶
IPv6 CE routers MUST support IPv6 prefix assignment according to [RFC8415] (Identity Association for Prefix Delegation (IA_PD) option) on its LAN interface.¶
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 SHOULD log a system management error.¶
The prefix assigned to a link MUST NOT change in the absence of a local policy or a topology change.¶
After LAN link prefix assignments, the IPv6 CE router MUST make the remaining IPv6 prefixes available to other routers via Prefix Delegation.¶
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. When a delegated prefix is released or expires, the associated route MUST be removed from the IPv6 CE router's routing table.¶
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 destination address that is not assigned to the LAN or delegated.¶
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 or 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 then 64 may be configured.¶
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.¶
If an IPv6 CE router is provisioning both ULA and GUA via prefix delegation, the GUA SHOULD appear first in the packet.¶
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.¶
This document does not add any new security considerations beyond those mentioned in Section 4 of [RFC8213], Section 22 of [RFC8415] and [RFC6092].¶
This document makes no request of IANA.¶
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.¶