Thomas, >>> Can someone explain to me the rationale for mandating 4191 in 6204? >>> What was the scenario that was envisioned that necessitates 4191? > >> it was the only way we found to keep support for ULA prefixes. the >> scenario is if you have a home CPE with ULA enabled, but no upstream >> IPv6 connectivity. if that router advertises itself as a router for >> default, as opposed to router for ULA, host implementations that >> don't do 3484bis, and don't handle ICMPs, will contribute to IPv6 >> breakage. e.g. 75 second time outs per TCP connection. > > Seems to me, the real issue is the following (from RFC 6204): > > G-4: By default, an IPv6 CE router that has no default router(s) on > its WAN interface MUST NOT advertise itself as an IPv6 default > router on its LAN interfaces. That is, the "Router Lifetime" > field is set to zero in all Router Advertisement messages it > originates [RFC4861]. > > G-5: By default, if the IPv6 CE router is an advertising router and > loses its IPv6 default router(s) on the WAN interface, it MUST > explicitly invalidate itself as an IPv6 default router on each > of its advertising interfaces by immediately transmitting one > or more Router Advertisement messages with the "Router > Lifetime" field set to zero [RFC4861]. > > That is, a CPE router with multiple LAN interfaces is not allowed to > advertise itself as a default router (when it loses internet > connectivity), when that is arguably a fine thing for it to do. > > I have to wonder whether RFC 6204 has gotten this wrong. It is very > CPE centric, assuming it is the only router on the home network. It > has conflated the notion of a router being fine as a default router > for the hosts attached to the same link (i.e., via RAs) and the the > more traditional router notion of default route. > > I.e., if the CPE router has multiple LAN interfaces, each with a ULA > (or other prefix) assigned to it, advertising itself as a default > router seems like a perfectly reasonable thing to do.
indeed, but unfortunately that causes IPv6 breakage. this is not at all CPE centric, from the CPE perspective advertising itself as a default router is obviously not an issue. the problem is host implementations that have a ULA address and a default route (but no connectivity to the IPv6 Internet). these hosts will choose IPv6 first when connecting to dual-stack destinations, this leads to multi-minute time outs. we had some very extensive discussions on this exact point during the last call of RFC6104. I hope we do not have to rehash those here. cheers, Ole -------------------------------------------------------------------- IETF IPv6 working group mailing list ipv6@ietf.org Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6 --------------------------------------------------------------------
