[Extra] Barry Leiba's Discuss on draft-ietf-extra-imap-fetch-preview-03: (with DISCUSS and COMMENT)

Barry Leiba via Datatracker <noreply@ietf.org> Mon, 08 April 2019 03:25 UTC

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Subject: [Extra] Barry Leiba's Discuss on draft-ietf-extra-imap-fetch-preview-03: (with DISCUSS and COMMENT)
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Barry Leiba has entered the following ballot position for
draft-ietf-extra-imap-fetch-preview-03: Discuss

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DISCUSS:
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— Section 3.1 —

I don’t understand “the client’s priority decision”: what decision is that? 
And what’s the point of giving the server a list of algorithms here, given that
they all have to be ones that are supported by the server?  Won’t the server
always have to use the first one in the list?  If not, please add some text
explaining what the server does.

— Section 3.2 —

   If the preview is not available, the server MUST return NIL as the
   PREVIEW response.  A NIL response indicates to the client that
   preview information MAY become available in a future PREVIEW FETCH
   request.  Note that this is semantically different than returning a
   zero-length string, which indicates an empty preview.

I think the MUST here is hard to follow, because the text doesn’t make a clear
enough distinction between “preview is not available” and “an empty preview”. 
Can you expand the text a bit to explain the distinction more clearly, as this
is a protocol requirement?  Also, as I noted in response to Meral’s Gen-ART
review it would be good to be clear how encrypted messages should be handled in
this regard.

— Section 4.1 —

   The preview text MUST be treated as text/plain MIME data by the
   client.

I think this requires a normative reference to RFC 2046.

— Section 5.1 —

The way you have LAZY working isn’t really consistent with the IMAP protocol
model.  In that model, the client would not have to ask for the preview twice,
one with LAZY and one without.  Instead, with LAZY, the server would return
FETCH PREVIEW responses when it could — perhaps some in the first set of FETCH
responses, and some, where the PREVIEW part was missing before, in unsolicited
FETCH responses when the preview became available.  That way, the server has
the responsibility of setting off a separate task to generate the previews, and
to send them to the client when it has them (at which point it either saves the
for future FETCHes or doesn’t).

As it’s written here, the client has to open a separate IMAP session with the
server and ask a second time for the previews it’s missing — a separate session
to avoid blocking other action on the main session.  And if the server has spun
off a task to preemptively generate them because the client asked once (a good
practice, given the description here) it has to retain them for some indefinite
period waiting for the client to ask again.

Why was this not done with the first mechanism?

— Section 7 —

As was mentioned in Ben’s review, either the ABNF for “capability” is in error
(it should not include “preview-mod-ext”) or the description needs to be
significantly beefed up.  I’m guessing that the intent is that PREVIEW=
capabilities include both algorithms and modifiers, that PREVIEW=FUZZY is
required, that the presence of any preview algorithm implies PREVIEW=LAZY such
that the latter not only need not be specified, but is not permitted to be.  So
we might have “PREVIEW=FUZZY PREVIEW=FURRY PREVIEW=SLEEPY”, which would mean we
support the algorithms FUZZY and FURRY, and the modifiers LAZY and SLEEPY.  Is
that correct?

That seems somewhat obtuse to me, overloading the PREVIEW= capability and
inviting confusion.

— Section 8 —

It seems like a bad idea to have to keep the IMAP Capabilities registry in sync
with the two new registries: as it stands, when you add a new algorithm you
have to add it to the Preview Algorithms registry, and also add a corresponding
entry in the Capabilities registry... and similarly for a modifier, if I have
that right above.

Why not follow the model of AUTH= and RIGHTS=, and just reserve the PREVIEW=
capability in the registry, allowing it to apply to entries from the two new
registries?  That avoids inconsistencies in registrations if we later add
algorithms or modifiers.


----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

— Section 3.1 —

Nit: Please change “alternately” to “alternatively”.

   These algorithms MUST be one
   of the algorithms identified as supported in the PREVIEW capability
   responses.

There’s a number-agreement problem here.

NEW
   All algorithms in the list MUST have been included in the
   list of algorithms identified as supported in the PREVIEW capability
   responses.
END

— Section 3.2 —

   This relaxed requirement permits a
   server to offer previews as an option without requiring potentially
   burdensome storage and/or processing requirements to guarantee
   immutability for a use case that does not require this strictness.

That’s sensible, but can you include some text giving an example of a situation
where the preview might change?  Given that the messages themselves are
immutable, why would applying the same algorithm to the same text give
different results?

— Section 4.1 —

   The server SHOULD limit the length of the preview text to 200 preview
   characters.  This length should provide sufficient data to generally
   support both various languages (and their different average word
   lengths) and different client display size requirements.

   The server MUST NOT output preview text longer than 256 preview
   characters.

The text here should make it clear, because many implementers do not understand
the difference, that these refer to *characters*, not *bytes*, and that 200 or
256 characters can possibly be much longer than 256 bytes.  I worry that an
implementer might allocate a buffer of 256 bytes, thinking that’s enough, and
have it overflowed.

   The server SHOULD remove any formatting markup that exists in the
   original text.

This is OK as it is, but perhaps a bit more specific than necessary.  I think
the sense is that the server is meant to do its best to render the preview as
plain text, because that’s what the client will treat it as.  As such, I would
fold this into the earlier paragraph that talks about no transfer encoding, and
maybe say it something like this:

   The generated string will be treated by the client as plain text, so
   the server should do its best to provide a meaningful plain text string.
   The generated string MUST NOT be content transfer encoded and MUST be
   encoded in UTF-8 [RFC3629].  For purposes of this section, a "preview
   character" is defined as a single UCS character encoded in UTF-8.  The
   server SHOULD also remove any formatting markup, and do what other
   processing might be useful in rendering the preview as plain text.