Re: [alto] Interop test


Y. Richard Yang
 



On Saturday, May 30, 2015, Wendy Roome <w.roome@...> wrote:
I thought RFC 7285 required order-consistency between numerical & ordinal modes for the same metric. But I cannot find that requirement. Too bad! I would have added that if I realized it wasn't there.

I remembered that the removal of the consistency requirement was a conscious decision, not an omission, to reduce the load of consistency checking. I buy your point below of a server supporting only one, in the sense of providing the maximum allowed by the privacy concern.

Regarding supporting both in the interop, I will be happy to wait a bit to hear others' opinions.

Richard

But if we take off our lawyer hats, and look at RFC 7285 as an agreement between friends, rather than a formal contract between adversaries, then I think it is reasonable to say that if the ordinal mode costs do not preserve the order of the numerical costs, then that server is wrong.

Similarly, a router is allowed to drop packets, and I do not think there is any formal requirement that it cannot drop *every* packet. So theoretically you could glue eight jacks to a block of hardwood and market it as a router. Maybe you could avoid getting charged for fraud. Just don't expect anyone to buy more than one! :-)

And looking at it from a real-world perspective, the whole concept is irrelevant. The only reason for defining ordinal mode is to allow a server to hide the numerical costs. For example, if the numerical costs to three PIDs are 10, 11 and 100, a client can deduce that the middle pid is close. If the costs are 10, 99 and 100, a client can deduce that the middle pid is distant. If the ordinal costs are 1,2,3, a client cannot deduce anything other than 1 is better than 2 is better than 3.

So if a server offers numerical costs, there is no advantage for it to also offer ordinal mode costs.

And if numerical costs are available, there is no advantage to a client to use ordinal costs. Maybe if the client could assume the ordinal costs are 1,2,3,… -- but the client cannot.

So in practice, no server will offer both numerical & ordinal mode for the same metric.

That means a formal compliance test should only require one mode or the other, but not both. However, keep the interop simple, unless someone objects, I suggest we require both modes. 

- Wendy

From: "Y. Richard Yang" <yry@...>
Date: Fri, May 29, 2015 at 12:24
To: Wendy Roome <w.roome@...>
Cc: "alto@..." <alto@...>, "Bertz, Lyle T [CTO]" <Lyle.T.Bertz@...>, Hans Seidel <hseidel@...>, "alto-dev@..." <alto-dev@...>
Subject: Re: [alto] Interop test

 
and it can verify that ordinal cost values are consistent with the order of the known numerical values.

This is a reasonable validation. An issue is that RFC7285 specifies only that "An ALTO server MUST support at least one of the following modes:  numerical and ordinal." I believe that RFC7285 chose to not specify the consistency between the two modes. Hence, this is beyond compliance, right? Hence, I feel that separating RFC7285-conforming and beyond can be helpful.


--
Richard

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