I'd make an even stronger statement: a client MUST NOT compare ordinal
costs between different response messages, even from the same service. A
client MAY compare one ordinal value against another ordinal in the same
response, but that's all.
For example, suppose a client fetches a full ordinal cost map twice in
close succession. The ordinal values in the second response may be
completely different from those in the first response, even if there was
no change in underlying network performance.
Put it another way: When a client requests an ordinal cost mode, the ALTO
Server MAY apply a non-linear order-preserving transformation to the
underlying numerical costs, and that transform MAY vary from request to
request.
IHMO, I thought the reason for defining ordinal costs was to allow an ALTO
server to obfuscate the underlying numerical costs. So if the server was
really paranoid, and will to expend the cycles, it might very well use a
different obfuscation algorithm for each request.
- Wendy Roome
>Date: Thu, 16 May 2013 21:51:30 +0100
>From: Ben Niven-Jenkins <[email protected]>
>Subject: Re: [alto] Equivalence of various identifiers from an ALTO
> Server
>
>(BTW I'm not sure you can compare a filtered costmap against a full cost
>map if the costs are ordinal)
>
>Ben
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