draft-kiesel-alto-xdom-disc-02 has this (Sect 3.2, Endpoint Cost Service):
If an ALTO client wants to query the Endpoint Cost Service (see Section
11.5 of RFC 7285 [RFC7285]) for the costs from source address X to
destination address(es) Y (and Z), it has to invoke the crossdomain ALTO
server discovery procedure with parameter X. The result will be the IRD
URI of the ALTO server to query for the costs from X to Y (and Z).
My first comment was why do cross-domain ALTO server discover on X, rather
than Y or Z?
Then I realized there is a good reason. I believe most ECS queries will be
either 1->N or N->1. That is, costs from many sources to one destination
(client selecting server from which to download a file) or one source to
many dests (client selecting server for upload).
So if the query is 1->N or N->1, do cross-domain discovery on the one
address. That is probably the address of the real client - the one which
needs ALTO guidance. So use the ALTO server which has the best data for
that address.
- Wendy Roome
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