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Re: Source address selection in IPv6 multihomed multi-addressed sites
On vrijdag, jul 18, 2003, at 07:57 Europe/Amsterdam, Masataka Ohta
wrote:
However, to enable source address filtering to discard packets with
source addresses not belonging to an ISP, it is useful to enable a
host, not some intelligent intermediate router, select a source
address compatible with an outgoing ISP. For that purpose, intra
domain routing protocols or something like that should maintain
routing table entries with not only preference values of an external
routes, but also proper prefixes to be selected for source
addresses,
if the entries are chosen by a host.
It should be noted that it is already doable with the current OSPF
spec.
Hm, how would that work? In BGP you could see the next hop AS number
and map this to a source address, but in OSPF there is no obvious way
to do this. (Although I'm sure a non-obvious way can be created.)
However, I certainly wouldn't want hosts to interact with OSPF as this
is a somewhat fragile protocol. In RIP you can simply ignore what hosts
have to say and in BGP you can filter it, but in OSPF as-is you can
only hope the host don't send any information that screws up the
routing table.
And then there is still the problem of how useful this information is.
Even today with BGP is is fairly common that BGP selects a very bad
route because A and F can either communicate through B and C or through
D and E. As far as BGP is concernet the paths are equally preferable: A
- B - C - F or A - D - E - F. What BGP doesn't know is that the
interconnect between B and C is 500 km away while the interconnect
between D and E is on another continent. Obviously this problem is only
going to get worse as the routing table becomes smaller. That's why I
think that there will always be upward pressure for the routing table
size.