Network Working Group M. Slusarz
Internet-Draft Open-Xchange Inc.
Intended status: Standards Track April 16, 2018
Expires: October 18, 2018
IMAP4 Extension: Message Snippet Generation
draft-slusarz-imap-fetch-snippet-00
Abstract
This document specifies an IMAP protocol extension which allows a
client to request that a server provide an abbreviated representation
of a message (a snippet of text) that can be used by a client to
provide a useful contextual preview of the message contents.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 18, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. FETCH Data Item . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Command . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Response . . . . . . . . . . . . . . . . . . . . . . . . 4
4. SNIPPET Algorithms . . . . . . . . . . . . . . . . . . . . . 4
4.1. FUZZY . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. SNIPPET Priority Modifiers . . . . . . . . . . . . . . . . . 5
5.1. LAZY . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Change History (To be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Many modern mail clients display small extracts of the body text as
an aid to allow a user to quickly decide whether they are interested
in viewing the full message contents. Mail clients implementing the
Internet Message Access Protocol (IMAP; RFC 3501 [RFC3501]) would
benefit from a standardized, consistent way to generate these brief
previews of messages (a "snippet").
Generation of snippets on the server has several benefits. First, it
allows consistent representation of snippets across all clients.
This standardized display can reduce user confusion when using
multiple clients, as abbreviated message representations in clients
will show identical message details.
Second, server-side snippet generation is more efficient. A client-
based algorithm needs to issue, at a minimum, a FETCH BODYSTRUCTURE
command in order to determine which MIME [RFC2045] body part(s)
should be represented in the snippet. Subsequently, at least one
FETCH BODY command may be needed to retrieve body data used in
snippet generation. These FETCH commands cannot be pipelined since
the BODYSTRUCTURE query must be parsed on the client before the list
of parts to be retrieved via the BODY command(s) can be determined.
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Additionally, it may be difficult to predict the amount of body data
that must be retrieved to adequately represent the part via a
snippet, therefore requiring inefficient fetching of excessive data
in order to account for this uncertainty. For example, a snippet
algorithm to display data contained in a text/html [RFC2854] part
will likely strip the markup tags to obtain textual content.
However, without fetching the entire content of the part, there is no
way to guarantee that sufficient non-tag content will exist unless
either 1) the entire part is retrieved or 2) an additional partial
FETCH is executed when the client determines that it does not possess
sufficient data from a previous partial FETCH to display an adequate
representation of the snippet.
Finally, server generation allows caching in a centralized location.
Using server generated snippets allows snippets to be generated
globally once per message, and then cached indefinitely. Retrieval
of message data may be expensive within a server, for example, so a
server can be configured to reduce its storage retrieval load by pre-
generating snippet data.
A server that supports the SNIPPET extension indicates this with one
or more capability names consisting of "SNIPPET=" followed by a
supported snippet algorithm name. This format provides for future
upwards-compatible extensions and/or the ability to use locally-
defined snippet algorithms.
2. Conventions Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
"User" is used to refer to a human user, whereas "client" refers to
the software being run by the user.
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
3. FETCH Data Item
3.1. Command
To retrieve a snippet for a message, the "SNIPPET" FETCH attribute is
used when issuing a FETCH command.
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If no algorithm identifier is provided, the server decides which of
its built-in algorithms to use to generate the snippet text.
Alternately, the client may explicitly indicate which algorithm(s)
should be used in a parenthesized list after the SNIPPET attribute
containing the name of the algorithm. These algorithms MUST be one
of the algorithms identified as supported in the SNIPPET capability
responses. If a client requests an algorithm that is unsupported,
the server MUST return a tagged BAD response.
The order of the algorithms in the parenthesized list (from left to
right) defines the client's priority decision. Duplicate algorithms
in the list SHOULD be ignored. For purposes of duplicate detection,
priority modifiers (Section 5) should be ignored. A server MUST
honor a client's algorithm priority decision.
3.2. Response
The algorithm used by the server to generate the snippet is returned
preceding the snippet string.
The server returns a variable-length string that is the generated
snippet for that message.
A server SHOULD strive to generate the same string for a given
message for each request. However, since snippets are understood to
be a representation of the message data and not a canonical view of
its contents, a client MUST NOT assume that a message snippet is
immutable for a given message. This relaxed requirement permits a
server to offer snippets 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.
If the snippet is not available, the server MUST return NIL as the
SNIPPET response. A NIL response indicates to the client that
snippet information MAY become available in a future SNIPPET FETCH
request.
4. SNIPPET Algorithms
4.1. FUZZY
The FUZZY algorithm directs the server to use any internal algorithm
it desires, subject to the below limitations, to generate a textual
snippet for a message.
The FUZZY algorithm MUST be implemented by any server that supports
the SNIPPET extension.
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The generated string MUST NOT be content transfer encoded and MUST be
encoded in UTF-8 [RFC3629].
The snippet text MUST be treated as text/plain MIME data by the
client.
The server SHOULD limit the length of the snippet text to 100
characters. The server MUST NOT output snippet text longer than 200
characters.
The server SHOULD remove any formatting markup that exists in the
original text.
If the FUZZY algorithm generates a snippet that is not based on the
body content of the message and the LANGUAGE [RFC5255] extension is
supported by the server, the snippet text SHOULD be generated
according to the the language rules that apply to human-readable
text.
5. SNIPPET Priority Modifiers
5.1. LAZY
The LAZY modifier directs the server to return the snippet
representation only if that data can be returned without undue delay
to the client.
This modifier allows a client to inform the server that snippet data
is nice-to-have, but the server SHOULD NOT block the return of other
FETCH information at the expense of generating the snippet data.
For example, a client displaying the initial mailbox listing to a
user may want to display snippet information associated with messages
in that listing. However, this information is secondary to providing
the mailbox listing, with message details, and the client is willing
to load any unavailable snippets in the background and display them
as they are provided by the server. In this case, the client would
use the LAZY modifier to the desired algorithm(s) to direct the
server to only return pre-generated snippet data so that retrieval of
the other FETCH information is not blocked by possibly expensive
snippet generation.
The LAZY modifier MUST be implemented by any server that supports the
SNIPPET extension.
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6. Examples
Example 1: Requesting FETCH without explicit algorithm selection
C: A1 CAPABILITY
S: * CAPABILITY IMAP4rev1 SNIPPET=FUZZY
S: A1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: A2 FETCH 1 (RFC822.SIZE SNIPPET)
S: * 1 FETCH (RFC822.SIZE 20000 SNIPPET (FUZZY {61}
S: This is the first line of text from the first text part.
S: ))
S: A2 OK FETCH complete.
Example 2: Requesting FETCH with explicit algorithm selection
C: B1 CAPABILITY
S: * CAPABILITY IMAP4rev1 SNIPPET=FUZZY
S: B1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: B2 FETCH 1 (RFC822.SIZE SNIPPET (FUZZY))
S: * 1 FETCH (RFC822.SIZE 20000 SNIPPET (FUZZY {61}
S: This is the first line of text from the first text part.
S: ))
S: B2 OK FETCH complete.
Example 3: Requesting FETCH with invalid explicit algorithm selection
C: C1 CAPABILITY
S: * CAPABILITY IMAP4rev1 SNIPPET=FUZZY
S: C1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: C2 FETCH 1 (RFC822.SIZE SNIPPET (X-SNIPPET-ALGO))
S: C2 BAD FETCH contains invalid snippet algorithm name.
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Example 4: Use explicit algorithm priority selection, with LAZY
modifier, to obtain snippets during initial mailbox listing if
readily available; otherwise, load snippets in background
C: D1 CAPABILITY
S: * CAPABILITY IMAP4rev1 SNIPPET=FUZZY
S: D1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: D2 FETCH 1:3 (ENVELOPE SNIPPET (LAZY=FUZZY))
S: * 1 FETCH (ENVELOPE ("Wed, 25 Oct 2017 15:03:11 +0000" [...])
SNIPPET (FUZZY {61}
S: This is the first line of text from the first text part.
S: ))
S: * 2 FETCH (SNIPPET (FUZZY "") ENVELOPE
("Thu, 26 Oct 2017 12:17:23 +0000" [...]))
S: * 3 FETCH (ENVELOPE ("Fri, 27 Oct 2017 22:19:21 +0000" [...])
SNIPPET (FUZZY NIL))
S: D2 OK FETCH completed.
[...Client knows that message 2 has a snippet that is empty;
therefore, client only needs to request message 3 snippet again
(e.g. in background)...]
C: D3 FETCH 3 (SNIPPET (FUZZY))
S: * 3 FETCH (SNIPPET (FUZZY {25}
S: First sentence of mail 3.
S: ))
S: D3 OK Fetch completed.
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Example 5: Retrieve snippet information for search results within a
single mailbox. Use SEARCHRES [RFC5182] extension to save a round-
trip.
C: E1 CAPABILITY
S: * CAPABILITY IMAP4rev1 SNIPPET=FUZZY SEARCHRES
S: E1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: E2 SEARCH RETURN (SAVE) FROM "FOO"
C: E3 FETCH $ (UID SNIPPET (LAZY=FUZZY))
S: E2 OK SEARCH completed.
S: * 5 FETCH (UID 13 SNIPPET (FUZZY {9}
S: Snippet!
S: ))
S: * 9 FETCH (UID 23 SNIPPET (FUZZY NIL))
S: E3 OK FETCH completed.
[...Retrieve message 9 snippet in background...]
C: E4 UID FETCH 23 (SNIPPET (FUZZY))
S: * 9 FETCH (SNIPPET (FUZZY {17}
S: Another snippet!
S: ))
S: E4 OK FETCH completed.
7. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in ABNF [RFC5234]. It includes definitions
from IMAP [RFC3501].
capability =/ "SNIPPET=FUZZY"
fetch-att =/ "SNIPPET" [SP "(" snippet-alg-fetch *(SP
snippet-alg-fetch) ")"]
msg-att-dynamic =/ "SNIPPET" SP "(" snippet-alg SP nstring ")"
snippet-alg = "FUZZY" / snippet-alg-ext
snippet-alg-ext = atom ; New algorithms MUST be registered with
; IANA
snippet-alg-fetch = snippet-alg / snippet-mod "=" snippet-alg
snippet-mod = "LAZY" / snippet-mod-ext
snippet-mod-ext = atom ; New priority modifiers MUST be
; registered with IANA
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8. Acknowledgements
The author would like to thank the following people for their
comments and contributions to this document: Stephan Bosch, Teemu
Huovila, Jeff Sipek, Timo Sirainen, Steffen Templin, and Aki Tuomi.
9. IANA Considerations
IMAP4 [RFC3501] capabilities are registered by publishing a standards
track or IESG-approved experimental RFC. The registry is currently
located at:
http://www.iana.org/assignments/imap-capabilities
This document requests that IANA adds the "SNIPPET=FUZZY" capability
to the IMAP4 [RFC3501] capabilities registry.
10. Security Considerations
There are no known additional security issues with this extension
beyond those described in the base protocol described in IMAP4
[RFC3501].
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<https://www.rfc-editor.org/info/rfc3501>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5255] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
DOI 10.17487/RFC5255, June 2008,
<https://www.rfc-editor.org/info/rfc5255>.
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11.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media
Type", RFC 2854, DOI 10.17487/RFC2854, June 2000,
<https://www.rfc-editor.org/info/rfc2854>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last
SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March
2008, <https://www.rfc-editor.org/info/rfc5182>.
Appendix A. Change History (To be removed by RFC Editor before
publication)
Changes from draft-slusarz-imap-fetch-snippet-00:
o TODO
Author's Address
Michael Slusarz
Open-Xchange Inc.
Denver, Colorado
US
Email: michael.slusarz@open-xchange.com
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