Subject: [Gen-art] Gen-ART Last Call Review of draft-melnikov-imap-search-res-06.txt

From: Spencer Dawkins <[email-masked]>
Date: Mon, 31 Dec 2007 16:10:50 -0600

Hi, Alexey,

I have been selected as the General Area Review Team (Gen-ART)
reviewer for this draft (for background on Gen-ART, please see
http://www.alvestrand.no/ietf/gen/art/gen-art-FAQ.html).

Please resolve these comments along with any other Last Call comments
you may receive.

Thanks, and Happy New Year,

Spencer

Document: draft-melnikov-imap-search-res-06.txt
Reviewer: Spencer Dawkins
Review Date:  2007-12-31
IETF LC End Date: 2008-01-12
IESG Telechat date:

Summary: This document is on the right path for publication as a proposed 
standard, but has some open issues, described in the review.

Comments:

I used three prefixes - "Spencer (protocol)", "Spencer (readability)", and
"Spencer (nit)" in my review. The nits are included as a convenience for
anyone editing the document.

The "protocol" comments are most significant, and there are only a few of
these.

Abstract

   Many IMAP clients use the result of a SEARCH command as the input to
   perform another operation, for example fetching the found messages,
   deleting them or copying them to another mailbox.

   This can be achieved using standard IMAP operations described in RFC
   3501, however this would be suboptimal: the server will send the list
   of found messages to the client, after that the client will have to
   parse the list, reformat it and send it back to the server.  The
   client can't pipeline the SEARCH command with the subsequent command.

Spencer (readability) - in this paragraph, there are three ways that this 
extension improves normal operation - for bandwidth (because the list isn't 
sent in both directions when it doesn't have to be), for processing (since 
the list isn't parsed and reformatted when it doesn't have to be), and for 
serialization (since the two commands can now be pipelined). In the body of 
the document, a fourth improvement is also mentioned ("server 
optimization"), but isn't described at all. The two lists should be the 
same, and I wouldn't mind seeing some description of "server optimization" 
in the body of the document.

   This document proposes an IMAP extension that allows a client to tell
   a server to use the result of a SEARCH (or UID SEARCH) command as an
   input to any subsequent command.

2.   Introduction and Overview

Spencer (readability) - This section combines introduction/overview (not
normative) and protocol operation (normative). I'd suggest separating the
two. I'd also suggest using some of the abstract text as an actual
introduction - this section assumes that you've read the abstract, I think,
and not everyone will automatically do that.

   The SEARCH result reference extension defines a new SEARCH result
   option [IMAPABNF] "SAVE" that tells the server to remember the result
   of the SEARCH or UID SEARCH command (as well as any command based on
   SEARCH, e.g. SORT and THREAD [SORT]) and store it in an internal
   variable that we will reference as the "search result variable". The
   client can use the "$" marker to reference the content of this
   internal variable. The "$" marker can be used instead of message (or
   UID) sequence in order to indicate that the server should substitute

Spencer (nit) - "message" looks like a noun until you get past "(or UID)"
and discover it's an adjective. Suggest s/message (or UID) sequence/message
sequence or UID sequence/.

   it with the list of messages from the search result variable.  Thus
   the client can use the result of the latest remembered SEARCH command
   as a parameter to another command.  The search result marker has
   several advantages:
    * it avoids wasted bandwidth and associated delay;
    * it allows the client to pipeline a SEARCH [IMAP4] command with a
      subsequent FETCH/STORE/COPY/SEARCH [IMAP4] or UID EXPUNGE
   [UIDPLUS] command;
    * the client doesn't need to spend time reformatting the
      result of a SEARCH command into a message set used in
      the subsequent command;
    * it allows the server to perform optimizations.

Spencer (readability) - in this list, there are FOUR advantages, not three 
as above... can you give any information about what optimizations a server 
can perform because this extension is being used?

   In absence of any other SEARCH result option, the SAVE result option
   also suppresses any SEARCH response that would have been otherwise
   returned by the SEARCH command.

Spencer (readability) - somewhere around here, the actual protocol
specification section starts... :-)

   Upon successful completion of a SELECT or an EXAMINE command (after
   the tagged OK response), the current search result variable is reset
   to the empty sequence.

Spencer (protocol) - I'd like to better understand why this design choice
was made. This statement seems to conflict with text in the next paragraph, 
so I'm not exactly sure what's going on here.

   A successful SEARCH command with the SAVE result option sets the
   value of the search result variable to the list of messages found in
   the SEARCH command. For example, if no messages were found, the
   search result variable will contain the empty list.  A SEARCH command
   that caused the server to return BAD tagged response, a SEARCH
   command with no SAVE result option that caused the server to return
   NO tagged response, or a successful SEARCH command with no SAVE
   result option MUST NOT change the search result variable.  A SEARCH

Spencer (protocol) - I'm now confused. The previous paragraph says "reset to
the empty sequence" upon successful completion of a SELECT command, but this
is saying that "a successful SEARCH command with no SAVE result option MUST
NOT change the search result variable" - what am I missing? (I'm actually OK
with the "no messages" and "BAD tagged response" exceptions, but the 
successful case seems to contradict the previous paragraph)

   command with the SAVE result option that caused the server to return
   NO tagged response sets the value of the search result variable to
   the empty sequence.

   When a message listed in the search result variable is EXPUNGEd, it
   is automatically removed from the list.  Implementors are reminded
   that if the server stores the list as a list of message numbers, it
   MUST automatically adjust them when notifying the client about

Spencer (readability) - is the meaning of "automatically adjust them"
obvious to everyone but me? Do you just remove the message number from the
list, or renumber the list, or ... ?

   expunged messages.

   Note that even if the "$" marker contains the empty list of messages,
   it must be treated by all commands accepting message sets as
   parameters, as a valid, but non matching list of messages. For

Spencer (readability) - this is correct as written, but hard to parse.
Perhaps "treated as a valid but non matching list of messages, by all
commands that accept message sets as parameters"?

   example, the "FETCH $" command would return tagged OK response and no
   FETCH responses.  See also the Example # 5 below.

 2.1.   Examples

   The client can also pipeline the two commands:

   Example 2:
               C: A282 SEARCH RETURN (SAVE) FLAGGED SINCE 1-Feb-1994
                  NOT FROM "Smith"
               C: A283 FETCH $ (UID INTERNALDATE FLAGS RFC822.HEADER)
               S: A282 OK SEARCH completed
               S: * 2 FETCH (UID 14 ...
               S: * 84 FETCH (UID 100 ...
               S: * 882 FETCH (UID 1115 ...
               S: A283 OK completed

Spencer (protocol) - I'm not seeing any description of how pipelined
commands deal with first-command failure - is this relevant? (does the
server execute the second command if the first command generates a
BAD-tagged response? etc)

   2) The following example demonstrates that the result of one SEARCH
      command can be used to subset the result of another SEARCH
      command:

   Example 3:
               C: A300 SEARCH RETURN (SAVE) SINCE 1-Jan-2004
                   NOT FROM "Smith"
               S: A300 OK SEARCH completed
               C: A301 UID SEARCH UID $ SMALLER 4096
               S: * SEARCH 17 900 901
               S: A301 OK completed

   Note that the second command in Example 3 can be replaced with:
               C: A301 UID SEARCH $ SMALLER 4096
   and the result of the command would be the same.

Spencer (readability) - I didn't quite follow this. I think what confused me
is the introductory text - isn't this example demonstrating "that the result
of one SEARCH command can be used as input to a second SEARCH command"?
"Subset" as written seemed to say that the first result was taking an action
("subset"), but it's just input to the second SEARCH...

 2.2.   Multiple Commands in Progress

Spencer (readability) - could you insert a sentence that introduces Example
7 and explains what you illustrate in this example? (Is it only what the
//comment says?) (same for Example 8)

        Example 7:
                    C: G282 SEARCH RETURN (SAVE) KEYWORD $Junk
                    C: G283 SEARCH RETURN (ALL) SINCE 28-Oct-2006
                        FROM "Eric"
                    //The server can execute the two SEARCH commands
                    //in any order, as the don't have any dependency.

Spencer (nit) - s/the/they/
                    //Note that the second command is making use of
                    //the [ESEARCH] extension.
                    S: * ESEARCH (TAG "G283") ALL 3:15,27,29:103
                    S: G283 OK SEARCH completed
                    S: G282 OK SEARCH completed

        Example 8:
                    C: H282 SEARCH RETURN (SAVE) KEYWORD $Junk
                    C: H283 SEARCH RETURN (SAVE) SINCE 28-Oct-2006
                        FROM "Eric"
                    //The result of the second SEARCH overrides the
                    //result of the first.
                    S: H283 OK SEARCH completed
                    S: H282 OK SEARCH completed

 2.3.   Interaction with ESEARCH extension

        When the SAVE result option is combined with the MIN or MAX
        [ESEARCH] result option, and none of the other ESEARCH result
        options are present, the corresponding MIN/MAX is returned (if
        the search result is not empty) but the "$" marker would contain
        a single message as returned in the MIN/MAX return item. If the
        SAVE result option is combined with both MIN and MAX result
        options, and none of the other ESEARCH result options are
        present, the "$" marker would contain one or two messages as
        returned in the MIN/MAX return items.  If the SAVE result option
        is combined with ALL and/or COUNT result option, the "$" marker
        would always contain all messages found by the SEARCH or UID
        SEARCH command.  (Note that the last rule might affect ESEARCH
        implementations that optimize how COUNT result is constructed.)

Spencer (readability) - is it obvious to everyone except me *how* the last
rule might affect these implementations?

Spencer (nit) - the previous paragraph is pretty dense. It might be more
readable as a bulleted list (one bullet per case).

   3.   Formal Syntax

4.   Security Considerations

   This extension requires the server to keep additional state, that may
   be used to simplify Deny of Service attacks. In order to minimize
   damage from such attacks server implementations MAY limit the number
   of saved searches they allow across all connections at any given time
   and return the tagged NO response to a SEARCH RETURN (SAVE) command
   when this limit is exceeded.

Spencer (readability) - I'm guessing there is no way for a client to
discover this is the reason for the tagged NO response? but it might be nice
to say so explicitly. Just curious - is this what an IMAP server would do
today, when it detects a DoS attack (by an attacker who's just working 
harder)? If so, it might be nice to point that out, too.

5.  IANA Considerations

   This document defines the "X-DRAFT-I05-SEARCHRES" <<Fix upon
   publication>> IMAP capability.  IANA is requested to add it to the
   IMAP4 Capabilities Registry, which is currently located at:

      http://www.iana.org/assignments/imap4-capabilities

Spencer (nit) - IANA has suggested "search-res" as the permanent name for 
this capability. You might add it here, just for completeness. 

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