Telechat Review of draft-ietf-alto-multi-cost-08
review-ietf-alto-multi-cost-08-artart-telechat-thomson-2017-04-05-00

Document: draft-ietf-alto-multi-cost-08
Date: 2017-04-06
Reviewer: Martin Thomson

This document describes how ALTO can be used to acquire cost maps with multiple
cost metric instead of a single metric.

The document very carefully deals with backwards compatibility with existing
ALTO servers and clients.  I don't anticipate many issues arising from the
deployment of this protocol.

I started reading -07, but finished with -08.  I checked that the issues I raise
still exist, but I'm not infallible.  Apologies if I get something wrong.

Minor issues: I've identified a few issues that are more than nits, these are
marked "IMPORTANT" below.


The abstract includes considerable justificiation.  An abstract only needs to
describe the *what*, not the *why*.  Thus, what is there could be simplified
considerably, e.g.,

   This document defines a new method for retrieving multiple cost metrics in a
   single request for an ALTO filtered cost map.  It also defines improvements
   to the constraints that can be used for filtered cost maps.

Section 1

The introduction uses a bunch of odd terms.  Some of these are recognizable from
the ALTO specification, but some of the jargon seems unnecessary.  In
particular, "Internet View", "Provider Network region" and "Vector costs".  All
of which I think that I understand, but they make the doc hard to follow.

Generally, I found the introduction quite hard to follow, both for that reason
and structurally.  The introduction could be a lot shorter and more concise:

1. ALTO defines multiple cost types (and more are being defined).
2. Clients sometimes consume multiple cost types.
3. Requesting multiple cost types at the same time is more efficient (for
   several reasons).
4. This document defines how to do that.
5. Separately, when multiple cost types are present, more sophisticated
   filtering can improve efficiency further.
6. This document defines how to do that too.

Section 2

There are several items in the list here that are not used: Application Client,
Network Service Provider, maybe more.  Please check and remove those that don't apply.

The RFC 7285 section reference thing is unnecessary.

This document doesn't cite RFC 2119, but it uses the keywords.

Section 3.1

The example shows an empty cost-type, but the schema you define allows it to be
absent.  You REALLY need to pick one.  I don't believe that this is a
compatibility issue: once you have determined that a client supports multi-cost,
then you can do anything you like, just be clear about it.

Section 3.2

I found the argument about the ease of writing a parser to be quite
unconvincing.  However, a new media type that is largely the same as the
existing media type won't necessarily result in code duplication.

Just say what it is you expect to happen and don't try to be apologetic about
it.  What you have appears to be a workable design.

Section 3.5

This section is confusing.  You only need to say that you are not altering full
cost map resources in any way and that clients need to use filtered cost maps if
they want multiple costs at the same time.  (Obviously you could have, but
creating multiple resources with the full combinatorial mess caused by combining
many cost types is unwieldy.)  At a minimum, the second paragraph here can be
removed.

Section 3.6.2

IMPORTANT: You don't define what happens when a client provides "or-constraints"
and "constraints" at the same time.  There are several valid options, but you
need to choose.

Section 3.6.3

It is probably worth explicitly noting that if "testable-cost-types" does not
include values from "multi-cost-types", then those types can't be included in
"constraints"/"or-constraints".

Please explain the default value for the index for the
"constraints"/"or-constraints" express in this section in addition to where it
is hidden in a note later in the document.

Section 3.6.5

Uppercase for "must not" in the second paragraph.  (The "may" later in the
paragraph might be better as "can".)

In the example, the resource named "filtered-multicost-map" is provided for
legacy reasons only.  Why bother including "max-cost-types" and
"cost-type-names" at all when "filtered-cost-map-extended" includes all that and
more?

Section 4.1.1

The definition of the schema here (and later) actually redefines the object
completely.  I found that confusing initially.  It would be good to identify the
*changes* from the base specification somehow.

Can testable-cost-type-names be present and empty if cost-constraints is false?
The first part of the definition permits that, the second forbids it.

Section 4.1.2

The redefinition of PIDFilter is unnecessary.

IMPORTANT: pids is optional in RFC 7285.  Why the change?

I find the redefinition of the optionality of cost-type to be worthy of special
note.

In the definition of "or-constraints", you use a "database query" where words
would suffice.

Section 4.1.3

I find the choice of value for "cost-type" to be problematic.  It is a string in
the base protocol, so changing to an empty object is likely to cause more issues
than simply omitting it.

Section 4.2 contains mismatched braces/parens for section references.

Section 4.2.2

IMPORTANT: This provides a definition for ReqFilteredCostMap that is very
different to that in the base specification.  I think that this should have been
ReqEndpointCostMap.

As before, repeating the definition of EndpointFilter is unnecessary.

Section 4.2.3 - see comment on 4.1.3

Section 5

I would be more comfortable if the examples used obviously-spurious metrics
(e.g., "cattle-head-count", "smell", "shoe-size", etc...) than these metrics
that are pretty plausible.  More so when you claim that they are widely valued,
which implies some sort of validity.

It should be relatively easy to populate Content-Length now that the examples
are "final".

Section 5.1

You have unmatched braces in "meta" due to the comment.

Section 5.2

Do you want to show one of the examples as having no cost values at all across
all the cost types?