Re: [v6ops] Question on multi-homed nodes and address/route selection

[Gert Doering <[e-mail]>]

Hi,

On Fri, Mar 03, 2017 at 04:53:15PM +0000, Simon Hobson wrote:
> Where there are multiple routers (eg separate routers provided
> by each ISP), then my *assumption* would be that the IPv6 stack
> would forward packets to the router which advertised the prefix
> containing the IP address the host/service has chosen to use for
> the outbound connection.

That only works if these routers are actually directly adjacent to
the host making the decision.  In the more generalized homenet model,
routers could be meshed in any ways, and the routers injecting a 
/48 into "the homenet mesh" could all be talking through an intermediate
router to the host - so the host would see a RA with (say) 3x /64 coming
from the same router.

But this is easily solved by source-dependent routing - "forward packets
with a source address from ISP A out via the line to ISP A, and so on
for ISP B, C, ...".

The source address selection part is much harder, and to my knowlege,
still mostly unsolved.  6724 has a list of rules that are not really
helping your use case ("multiple ISPs that have different characteristics,
and some of them might not even be working right in this moment") - 
especially, source-address *failover* is not something I've seen work
("I tried source A, and it did not work, so maybe ISP A has a problem
reaching target Z.  Let's try source address B, via ISP B -> Z").

> In general we would frown upon this setup since it doesn't work well in the IPv4 world - in the general case it means manual configuration on nodes to select the egress router and is inflexible.

The homenet architecture puts this nicely on the routers, and that part
really works quite well ("built a test setup, liked the result").

Gert Doering
        -- NetMaster
-- 
have you enabled IPv6 on something today...?

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