[This issue spans some WGs but it originates from here I believe.]
We've had a small discussion in NEMO about multiple default routers on a link, and whether these DRs are allowed to advertise different prefix sets or not.
Think of this small network:
ISP A ISP B \ / Ra Rb | | -+-----+-----+- | H
Further assume that Ra advertises prefix Pa and Rb advertises prefix Pb. Host H will have two DRs, hear two prefixes and build addresses out of the both.
What I'm asking for is whether this scenario was ever considered "broken" unless there is some form of coordination between Ra and Rb. I seem to recall that there was a discussion on ipng/IPv6 on exactly this long ago but on the other hand I can't find it.
The reason for calling this scenario broken is that H must be careful with what source address to use when sending to either default router (and that is a requirement we can't put on a legacy IPv6 host). One can assume that both ISPs perform ingress filtering. If H contructs a packet with a source address Sa from Pa and:
- sends it to Ra: - the packet will be routed successfully
- but if it was to send it to Rb instead: - (1) ISP B would drop it due to ingress filtering, or - (2) Rb would have to forward it to Ra, or - (3) Rb would have to tunnel it to ISP A, or - (4) Rb would have to send a redirect to H, or - (5) Rb or ISP B would generate an ICMP error about bad source address
Alternatives 2-5 are typically described in draft-huitema-multi6-hosts, but they either:
- require some form of coordination between the routers and their ISPs, or they
- require that the host H implement some additional mapping between prefixes/source addresses and default routers.
To H there is no difference between Ra and Rb. Both claim that they are default routers. Now both should be able to forward any traffic from H. But unless Ra and Rb are coordinated in some of the ways described above or that host H implement _new_ requirements in draft-huitema-multi6-hosts (which standard IPv6 hosts do not), then host H will have to take a chance on what default router to use (last heard? first heard?). If H happened to select Ra as default router, packets with source Sa will go through, but no packets with source Sb. And default router selection will not happen again until Ra becomes unreachable.
So the question is: am I correct to regard this scenario as broken and say it should not be encouraged? I also think that nobody plans to build fixed networks like this, just for these reasons. However, people may want to build NEMO-type of networks just like this, as it is convenient when mobile routers are rather independent, they come and go, and each connects to one ISP and each only advertises a prefix from its ISP. Awkwardly, then it is more important than ever that the mobile routers are cooperating otherwise legacy IPv6 hosts won't be able to get plain connectivity.
/Mattias
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