ドラフト Using Subnet-Specific Link-Local Addresses draft-link-v6ops-gulla を読んでみた！

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datatracker.ietf.or

Jen Linkova さんの新しいドラフト。

リンクローカルアドレスをRAを広告するサブネットごとに作った方が、サブネットが変わった時に便利だよねって話。

JenさんはGoogleの社内ネットワークの運営をしていて、社内ネットワークでクライアントにIPv4アドレスを使わなくてすむようにIPv6 Mostly Networkを提案しRFCへ。

IPv6 Mostly Networkのための二つのRFC：

RFC 8781 Discovering PREF64 in Router Advertisements 1 RFC
RFC 8925 IPv6-Only Preferred Option for DHCPv4

社内ネットワークでIPv6 Mostlyを進めていく中で、たくさんIPv6の問題点を見つけそれにどんどん解決策を作っていく。Dual StackだとIPv4にフォールバックされる関係で観測されなかった（issueをだれも上げなかった)問題がIPv6 Mostlyになると可視化され解決する必要が出てくる。

今回はIPv6でVLAN間をクライアントが移動する時に起きる問題（リナンバリング）

スライドこちらから。

https://www.ipv6.org.uk/wp-content/uploads/2023/11/13_IPv6-Mostly-Office_-JenLinkova_UK-IPv6-Council-2023.pdf

This document recommends that the link-local address the router sends the router advertisement from should depend on the network prefix(es) assigned to the router interface. As a result, Router Advertisements containing different sets of PIOs are sent from different link-local addresses. That allows the hosts to select the source address from the prefix advertized by the reachable router. As a result the host would be able to recover from the renyumbering events much faster.

新しいプレフィックスをRAで広告するようになったらルータの送信元Link-local addressを変えれば賢くない？って提案 (informational)

RFC6724 Default Address Selectionルール5.5ではNextHopが広告したプレフィックスをソースアドレスとして使う必要がある。

ルータがサブネットの変更と共に送信元のLink-local addressを変えることで、このルール5.5に従うホストは速やかにリナンバリングに対応することができる。

4.1. Default Address Selection Rule 5.5 and Renumbering
Rule 5.5 of the Default Source Address Selection ([RFC6724]) requires the host to prefer addresses in a prefix advertised by the next-hop. It allows the multihomed host to select the source address correctly: when two routers advertize different prefixes, the host wull be sending packets with source address from a given prefix to the router the prefix was received from.

In case of renumbering if both old and new prefixes are advertized by the same router (received from a router with the same link-local address), then Rule 5.5 doesn't help selecting the correct (working) source address. However if the subnet change also leads to the default router address change, then a host implementing Rule 5.5 could recover from the renumbering quickly:

The host is connected to a network A, receives an RA from the router (link-local address LLA_A) with a PIO containg pref_a, forms IPv6 addresses from that prefix using SLAAC.

The host attachment changes from network A to network B. The host doesn’t detect the network change and doesn’t clear the IPv6 stack.

The host receives an RA from the router (link-local address LLA_B) with a new PIO for pref_b and forms new addresses from that prefix.

Link-local address LLA_A is not reachable anymore, as the host changes the network attachement point. Neighbor Unreachability Detection ([RFC4861]) detects it and removes LLA_A from the list of default routers.

The host is using LLA_B as a next-hop for outgoing traffic, so addresses from the pref_b are selected, and addresses from pref_a are not used.