Re: [alto] AD review of draft-ietf-alto-performance-metrics-15

Martin Duke <martin.h.duke@gmail.com> Mon, 05 April 2021 15:03 UTC

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From: Martin Duke <martin.h.duke@gmail.com>
Date: Mon, 5 Apr 2021 08:03:07 -0700
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To: "Y. Richard Yang" <yry@cs.yale.edu>
Cc: IETF ALTO <alto@ietf.org>, Brian Trammell <ietf@trammell.ch>
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Subject: Re: [alto] AD review of draft-ietf-alto-performance-metrics-15
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Hi Richard,

Where needed, responses inline.

On Fri, Apr 2, 2021 at 6:18 PM Y. Richard Yang <yry@cs.yale.edu> wrote:

> - Sec 2.1. The cost-source model is conceptually sound, but the
>> justification for it seems underexplained. What exactly is a client going
>> to do with this information? What different behaviors would a client
>> execute if the context was e.g. "sla" instead of "nominal?" To the extent
>> the parameters are not machine readable, like links to webpages, are we
>> really expecting this information to be presented to the humans behind ALTO
>> clients?
>>
>
> Good comment. This is a point which the WG indeed discussed multiple
> times. The decision was that providing the source will give the server the
> ability to indicate the source, and the client knows more about the
> context. Some operational aspects are discussed in Sec. 5.1. There were
> some discussions on potential normative specifications on the client-side
> and/or the server-side, but the final decision is that ALTO provides
> non-normative information. For example, the basic cost values are
> non-normative, and only best-effort information. There can be out-of-ALTO
> control loops, for example, business contracts, and the cost-source
> supports indication of such information channel.
>
> The comment on clarifying whether the info will be for humans behind is a
> great one. One use case setting discussed is that the link can provide
> machine-readable specifications such as a machine-readable language
> specifying the measurement parameters, but this is considered out of scope.
> Will you recommend that we include more some more elaboration in the
> operational considerations section (i.e., extending Sec. 5.1)?
>

If it were me, I'd put some of it in the intro and some motivation in 2.1,
and maybe go through some miscellaneous considerations in 5. But that's
less important that it be there somewhere.

If the intent is that it be machine-readable, then there are several places
where this standard is going to need more standardization (i.e. precise
definition of text fields).

But zooming out: I understand that the point is that "the client knows more
about the context," which is pretty much what 5.1 says. But I don't
understand if the "client" is the user or a user agent, and what either one
would actually do with the information. Would the application execute a
policy based on the source? Why would it use a latency that came from an
sla, but not from measurement? etc.



>
>> - Sec 2.1 I am confused about the meaning of the "sla" cost-source. Does
>> this refer to an SLA the ALTO client has with the network? Between the
>> target IP and the network? Or something else? If the first, does this link
>> to client authentication in some way? If the second, what are the privacy
>> implications of exposing these SLAs?
>>
>
> It is target IP and the network. Here is some text in the current version
> on the authentication and privacy aspects (Sec. 6):
> "Indeed, TE
>    performance is a highly sensitive ISP information; therefore, sharing
>    TE metric values in numerical mode requires full mutual confidence
>    between the entities managing the ALTO server and the ALTO client.
>    ALTO servers will most likely distribute numerical TE performance to
>    ALTO clients under strict and formal mutual trust agreements.  On the
>    other hand, ALTO clients must be cognizant on the risks attached to
>    such information that they would have acquired outside formal
>    conditions of mutual trust."
>
> Will this be OK?
>

That privacy information is alright, but exposing the details of
third-party SLAs deserves special attention. But to follow up your answer:
if the client has a better SLA than the target, this won't show up in the
metrics at all?


>
> - Sec 2.1. Related to the above, the text suggests that any cost-source
>> expressed as "import" could also be expressed as "estimation". Why would
>> the server do this? The text should say, or perhaps it would be
>> conceptually cleaner if "estimation" and "import" were mutually exclusive
>> sources by definition.
>>
>
> In the early WG discussion, they were considered separate, and then the
> agreement was that import is a special case of estimation, with more
> specific dependency tracking. Consider data provenance of how the ALTO data
> are computed. Estimation means that the server does not want to indicate
> the specific details, and the important gives a precise indication of the
> exact protocols.
>

OK, I now understand that "import" implies a specific set of parameters. I
can't understand what value this distinction has, but that just circles
around to me not understanding the cost-source information at all.



> - Sec 5.4.1: "...the ALTO server may provide the client with the validity
> period of the exposed metric values."
>
> Shouldn't there be a standard format for this? Or are you implying the use
> of cost-calendar?
>

Good catch. The decision of the WG at the time was to use HTTP whenever
possible. For example, the freshness is indicated by HTTP timestamp (see
Sec. 5.2); by consistency, then, we should use HTTP Expires. We can add
this. Agree?

Sure.

Martin