rfc-8319

Support for Adjustable Maximum Router Lifetimes per Link

S. Krishnan, J. Korhonen, S. Chakrabarti, E. Nordmark, A. Yourtchenko

date2018-02 streamIETF areaint wg6man statusPROPOSED STANDARD pages7 canonicalhttps://www.rfc-editor.org/rfc/rfc8319 doi10.17487/RFC8319

The IPv6 Neighbor Discovery protocol specifies the maximum time allowed between sending unsolicited multicast Router Advertisements (RAs) from a router interface as well as the maximum router lifetime. It also allows the limits to be overridden by documents that are specific to the link layer. This document allows for overriding these values on a per-link basis. This document specifies updates to the IPv6 Neighbor Discovery Protocol (RFC 4861) to increase the maximum time allowed between sending unsolicited multicast RAs from a router interface as well as to increase the maximum router lifetime.

updates

rfc-4861 — Neighbor Discovery for IP version 6 (IPv6)

Extracted elements (7)

design-rationale §1

On some data link layers, multicast is not natively supported, making any reduction of multicast traffic highly beneficial. The fixed protocol constants in RFC 4861 made it difficult to relax multicast timers for Neighbor Discovery, motivating per-link overrides of MaxRtrAdvInterval and router lifetime.

ip, multicast

design-rationale §3

The relationship between AdvDefaultLifetime and MaxRtrAdvInterval is expressed as the ratio K = AdvDefaultLifetime / MaxRtrAdvInterval, representing the number of RAs guaranteed before router lifetime expiry. With 1% packet loss, K=2 yields 99.99% reliability and K=3 yields 99.9999% reliability; the document leaves the exact choice to each link's use case rather than mandating a specific K.

interoperability-note §5

Legacy hosts on a link with updated routers may have issues with a Router Lifetime exceeding 9000 seconds. While tested general-purpose operating system implementations appear to handle values above 9000, RFC 4861 already requires receivers to handle any value, so non-compliant implementations may incorrectly reject such RAs.

normative-requirement §3 MUST

AdvDefaultLifetime MUST be set to a value greater than or equal to the selected MaxRtrAdvInterval. If it is smaller, the router lifetime is guaranteed to expire before the next Router Advertisement can be sent, causing an outage.

protocol-element §4

RFC 4861 Section 4.2 is updated to raise the maximum Router Lifetime field value from 9000 seconds to 65535 seconds, enabling longer-lived router entries on links where RAs are infrequent.

protocol-element §4

RFC 4861 Section 6.2.1 is updated to raise the maximum value of the AdvDefaultLifetime router configuration variable from 9000 seconds to 65535 seconds, consistent with the raised Router Lifetime ceiling.

security-consideration §6

On links where Router Advertisements are infrequent, a rogue router sending an unsolicited RA has greatly increased detrimental effect, since hosts may rely on a single RA for a very long router lifetime. RA-Guard (RFC 6105) and SEcure Neighbor Discovery (SEND, RFC 3971) are recommended mitigations.

ip, security