On Tue, 9 Mar 2004, Mattias Pettersson wrote:
> Hi,
>
> [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.
I think this is not broken at all. The host should select the correct
prefix according to the source address selection rules. I tried this
scenario approximately 3 years ago on a KAME stack and it was working
correctly.
Best Regards,
Janos Mohacsi
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