Re: [lisp] Comments on draft-fuller-lisp-alt-04

[Vince Fuller <[e-mail]>]

> > 5. What should an ALT router do when it receives a packet whose
> > destination EID is unreachable (either because of the failure of an ALT
> > session or because the corresponding EID block has not been allocated) ?
> >  Should it drop the packet or send back an ICMP destination unreachable
> > over the normal topology towards the source RLOC ?
> 
> There are actually multiple cases here.
> 
> If an ALT router receives a Map-Request or Data Probe that matches an EID
> Prefix that it knows does not exist (e.g. if it is the aggregator for
> 10.1.0.0/16, the prefix 10.1.254.0/24 is not assigned, and it receives
> a Map-Request or Data Probe destined to 10.1.254.1), it returns a Negative
> Map-Reply for the shortest covering prefix that is not a LISP EID. A
> Negative Map-Reply is a Map-Reply with zero RLOCs. In this example, the
> EID prefix in the Map-Reply could be anything from 10.1.254.1/32 to
> 10.1.0.0/16 and would depend on what, if any, assignments had been made from
> 10.1.0.0/16.
> 
> The case where an EID prefix does exist in the LISP database but has no
> reachable RLOCs is, in some sense, more interesting. Dino - what do is the
> correct behavior for an aggregating ALT router in this case?

In a separate discussion, Dino pointed-out that an ALT router may be running
vanilla BGP and GRE with no knowledge at all of LISP. This may, in fact, be
the common case. If so, then the ALT router will not know anything about the
Map-Request - it will only know that the destination IP address (the EID) is
unreachable and will simply drop the packet. Not clear whether it should even
return an ICMP unreachable.

I'm going to have additional discussions with my co-authors to determine
what, if any, special processing an ALT router, either one that is
configured to run LISP or one that isn't, should do in this case.

Thanks for pointing it out.

	--Vince