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Re: [RRG] Stretch in case of LISP+ALT



Although I always cautioned when ANY hierarchical routing was accused to cause a stretch ratio =3, I would be curious to hear from stretch experienced researchers what they think about the stretch ratio in case of LISP/ALT.


Does it matter what the stretch ratio is for Map-Requests? Are you thinking about Data Probes and the effect on packet delivery performance?

( I cautioned because of a different and btw really virtual topology in mind; LISP/ALT employs a real and not virtual topology of tunnels).


Please elborate. The LISP+ALT topology is supported by a set of routes advertised and learned in another BGP process reflecting the addressing and configuration of a tunnel topology.

It's "virtual" (even though this term is way overloaded) because two eBGP speakers don't have a direct physical link between them in most cases. The have a GRE tunnel between them and the eBGP peering addresses are the addresses configured on the tunnel (and are not the tunnel endpoint addresses which are out of RLOC space).

Could a relationship between stretch and mapping churn exist like:

The less ETRs, the less mapping churn, but the greater the stretch ratio!


Well in my mind the number of ETRs is relative to the number of connections a site has to the Internet. You can't and probably don't want to reduce that because it's necessary for the site's multihoming ability.

The mapping churn will be dominate on how long you keep mappings cached.

Resp. a smaller stretch value can only be achieved by more ETRs and more EID-RLOC propagation, i.e. more update churn, i.e. less solving the scalability problem ?


Smaller stretch can occur if you can map the tunnel topology closer to the physical topology.

Dino


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