EXTRA M. Slusarz
Internet-Draft Open-Xchange Inc.
Intended status: Standards Track August 3, 2018
Expires: February 4, 2019

IMAP4 Extension: Message Snippet Generation


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.

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Table of Contents

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 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 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.

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 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.

"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.

As with all IMAP extension documents, the case used in writing IMAP protocol elements herein is chosen for editorial clarity, and implementations must pay attention to the numbered rules at the beginning of [RFC3501] Section 9.

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.

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 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.

The generated string MUST NOT be content transfer encoded and MUST be encoded in UTF-8.

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 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.

6. Examples

Example 1: Requesting FETCH without explicit algorithm selection

  S: A1 OK Capability command completed.
  [...a mailbox is SELECTed...]
  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

  S: B1 OK Capability command completed.
  [...a mailbox is SELECTed...]
  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

  S: C1 OK Capability command completed.
  [...a mailbox is SELECTed...]
  S: C2 BAD FETCH contains invalid snippet algorithm name.

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

  S: D1 OK Capability command completed.
  [...a mailbox is SELECTed...]
  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: ))
     ("Thu, 26 Oct 2017 12:17:23 +0000" [...]))
  S: * 3 FETCH (ENVELOPE ("Fri, 27 Oct 2017 22:19:21 +0000" [...])
  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)...]
  S: First sentence of mail 3.
  S: ))
  S: D3 OK Fetch completed.

Example 5: Retrieve snippet information for search results within a single mailbox. Use SEARCHRES extension to save a round-trip.

  S: E1 OK Capability command completed.
  [...a mailbox is SELECTed...]
  S: E2 OK SEARCH completed.
  S: Snippet!
  S: ))
  S: E3 OK FETCH completed.
  [...Retrieve message 9 snippet in background...]
  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. It includes definitions from IMAP.

  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

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 capabilities are registered by publishing a standards track or IESG-approved experimental RFC. The registry is currently located at:

This document requests that IANA adds the "SNIPPET=FUZZY" capability to the IMAP4 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.

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.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5255] Newman, C., Gulbrandsen, A. and A. Melnikov, "Internet Message Access Protocol Internationalization", RFC 5255, DOI 10.17487/RFC5255, June 2008.

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.
[RFC2854] Connolly, D. and L. Masinter, "The 'text/html' Media Type", RFC 2854, DOI 10.17487/RFC2854, June 2000.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003.
[RFC5182] Melnikov, A., "IMAP Extension for Referencing the Last SEARCH Result", RFC 5182, DOI 10.17487/RFC5182, March 2008.

Appendix A. Change History (To be removed by RFC Editor before publication)

Changes from draft-slusarz-imap-fetch-snippet-00:

Author's Address

Michael Slusarz Open-Xchange Inc. Denver, Colorado US EMail: michael.slusarz@open-xchange.com