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

Martin Duke <> Tue, 30 March 2021 00:30 UTC

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From: Martin Duke <>
Date: Mon, 29 Mar 2021 17:30:12 -0700
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Subject: Re: [alto] AD review of draft-ietf-alto-performance-metrics-15
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One small correction: I'm jumping the gun on the author policy; 6 is
probably OK for now.

On Mon, Mar 29, 2021 at 11:33 AM Martin Duke <>

> Hello authors,
> Thank you very much for writing this draft. It is clearly a useful
> extension to ALTO and is quite clearly written, even to someone who is not
> a practitioner. I have numerous comments/questions and a few nits.
> These points are all invitations to discussion, rather than commands about
> what to change, as I've missed much of the WG deliberations that led to
> this text.
> - There are six authors. Having more than 5 editors/authors listed on the
> front page requires strong justification and chair/AD approval. See the RFC
> Editor statement [1]. You are encouraged to designate a few editors for the
> front page and list as many authors as desired at the end.
> - 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?
> - 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?
> - 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.
> - Sec 3. I would prefer it if the parameters field in each of these
> definitions was a bit more strict. This relates to my confusion about
> machine-readable vs. human readable data; if this is meant to be
> machine-readable, then e.g. Sec 3.4.4 should be more specific in spelling
> out that the IGP protocols should be in a format with the RFC number, for
> instance. If it's to be human readable for a purpose I don't understand,
> then these looser definitions are probably OK.
> - Sec 3.4 Unlike the other metrics, I have no idea what a client is to do
> with the hop count metric, since clients don't care about hop count. Hops
> only affect users through delay and loss rate, which is present in other
> metrics. Is the intent here to provide a proxy for delay when direct delay
> information is not available? If so, we should say so.
> - Sec 5.3. I suggest a reword.
> OLD:
> To address this issue, the only defined "routingcost" metric can be
>    only "estimation".
> NEW:
> To address this issue, if the "routingcost" metric contains a cost-context
> field, it MUST be "estimation."
> What should clients do if it's not "estimation?" Can they use it or reject
> the metric
> as malformed?
> - 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?
> - Sec 5.4.2: I don't understand what this section is saying. Can the
> server provide new metrics not in the spec? Or is it saying that the server
> can take singletons about link one-way delays and compose path one-way and
> two-way delays, for example?
> - Sec 1. An initial sentence introducing ALTO at the beginning would be
> helpful, e.g.
> "ALTO [RFC 7285] provides a means for client to identify the most
> efficient information source when multiple copies of such information
> exist, by quering path information from an HTTP server."
> - Sec 2. The second paragraph is a little hard to read. Suggestion:
> OLD:
> On the other hand, to be able to use coarse-grained information such
>    as routing system information (e.g., [RFC8571 <>]), which may not
>    provide fine-grained information such as (iii) Method of Measurement
>    or Calculation and (vi) Measurement Timing, this document provides
>    context information to indicate the source of information and hence
>    available metric details.
> NEW:
>   This document specifies context information to indicate the metric
> source, which can allow clients to obtain fine-grained information like
> (ii) Method of Measurement or Calculation and (vi) Measurement Timing."
> - Sec 2.1 In Fig. 1, please expand "NBI" on first use.
> - Sec 3.1.3 Expand "PID" on first use.
> - Sec 3.1.4 s/recommended/RECOMMENDED
> - Sec 3.4 s/metric hopcount/hopcount metric
> - Sec 4.1.3 would this be correct: s/give the throughput/give the maximum
> throughput
> - Sec 6. s/is a highly sensitive/is highly sensitive
> Thanks
> Martin
> [1]