On Fri, Nov 20, 2009 at 1:23 PM, John G. Scudder <[email protected]> wrote:
> It looks to me as though your level 4A FIB aggregation algorithm has a
> bug... at least if you agree that "may create forwarding loops" is a bug.
> [...]
> Packets to 10/8 other than the listed prefixes will loop (e.g. to
> 10.64.1.1). Of course, they have to reach A or B first but it's not
> difficult to imagine a scenario in which they could.
Hi John,
As I understand it, that's a feature, not a bug. It's present in both
level 3 and level 4. Routing to process:unreachable is partially
abandoned in order to get additional FIB aggregation. Packets to those
addresses continue until they hit a router that happens to route them
to process:unreachable or until the TTL expires routing them to
process:timeexceeded.
Caveats about smurf-like attack vectors understood.
Regards,
Bill Herrin
>
> This problem arises from the case where you randomly pick a next hop. If
> two adjacent routers do this, they may end up picking each other. I haven't
> worked this through for the 4B algorithm, but considering that it has a
> similar random selection step, I would guess the same problem could arise.
>
> This leads me to reflect that my biggest concern with the draft as it stands
> is that the correctness condition as stated ("after longest match lookup, if
> a destination address has a non-NULL next-hop in the un-aggregated FIB, it
> should have the same next-hop in the aggregated FIB") is a tad questionable.
> IMO the base correctness condition is "has black-box identical behavior to
> a router not using FIB compression", which is stronger than what's stated.
> You'd need to drop the "has a non-NULL next-hop" caveat. This seems to be
> what your Level 1 and 2 algorithms provide (or anyway, are intended to
> provide). I think you need to state clearly what the correctness conditions
> are for the other levels. (And given a clear statement of the correctness
> conditions, proof of correctness would be very desirable!)
>
> Regards,
>
> --John
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>
--
William D. Herrin ................ [email protected] [email protected]
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004
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