EXTRA M. Slusarz
Internet-Draft Open-Xchange Inc.
Intended status: Standards Track February 16, 2019
Expires: August 20, 2019

IMAP4 Extension: Message Preview Generation


This document specifies an Internet Message Access Protocol (IMAP) protocol extension allowing a client to request a server-generated abbreviated representation of message data useful as a contextual preview of the entire message.

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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.

Generation of a preview on the server has several benefits. First, it allows consistent representation of previews across all clients. This standardized display can reduce user confusion when using multiple clients, as abbreviated message representations in clients will show identical message contents.

Second, server-side preview 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 preview. Subsequently, at least one FETCH BODY command may be needed to retrieve body data used in preview 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 preview, therefore requiring inefficient fetching of excessive data in order to account for this uncertainty. For example, a preview 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 preview.

Finally, server generation allows caching in a centralized location. Using server generated previews allows global generation 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 preview data.

A server indicates support for this extension by listing one or more capability names consisting of "PREVIEW=" followed by a supported preview algorithm name. This format provides for future upwards-compatible extensions and/or the ability to use locally-defined preview algorithms and priority modifiers.

2. Conventions Used In This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC8174] when, and only when, they appear in all capitals, as shown here.

"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 preview for a message, the "PREVIEW" 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 preview text.

Alternately, the client may explicitly indicate which algorithm(s) should be used in a parenthesized list after the PREVIEW attribute containing the name of the algorithm. These algorithms MUST be one of the algorithms identified as supported in the PREVIEW 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 preview is returned preceding the preview string.

The server returns a variable-length string that is the generated preview for that message.

Example: Retrieving preview information in a SELECTed mailbox

  S: * 1 FETCH (PREVIEW (FUZZY "Preview text!"))
  S: A1 OK FETCH complete.

A server SHOULD strive to generate the same string for a given message for each request. However, since previews 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 preview is immutable for a given message. 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.

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.

4. PREVIEW 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 preview for a message.

The FUZZY algorithm MUST be implemented by any server that supports the PREVIEW extension.

The generated string MUST NOT be content transfer encoded and MUST be encoded in UTF-8. For purposes of this section, a "preview character" is defined as a single UCS character encoded in UTF-8.

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

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

If the FUZZY algorithm generates a preview that is not based on the body content of the message and the LANGUAGE extension is supported by the server, the preview text SHOULD be generated according to the the language rules that apply to human-readable text. For example, a message that consists of a single image MIME part has no human-readable text to generate preview information from. Instead, the server may wish to output a description that the message contains an image and describe some attributes of the image, such as image format, size, and filename. This descriptive text is not a product of the message body itself but is rather auto-generated data by the server, and should thus use the rules defined for human-generated text described in the LANGUAGE extension (if supported on the server).

5. PREVIEW Priority Modifiers

5.1. LAZY

The LAZY modifier directs the server to return the preview representation only if that data can be returned without undue delay to the client.

This modifier allows a client to inform the server that preview data is nice-to-have, but the server SHOULD NOT block the return of other FETCH information at the expense of generating the preview data.

For example, a client displaying the initial mailbox listing to a user may want to display preview 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 previews in the background and display them as they are provided by the server. In this case, the client would apply the LAZY modifier to the desired algorithm(s) to direct the server to only return pre-generated preview data so that retrieval of the other FETCH information is not blocked by possibly expensive preview generation.

Generally, the LAZY modifier will only be used once per mailbox load during the initial listing. If preview information is not available during this initial FETCH, the expectation is that a second non-LAZY FETCH will take place after mailbox listing activities are complete. Thus, a maximum of 2 PREVIEW FETCH queries should occur for any message in a selected mailbox. A client SHOULD NOT continually issue LAZY PREVIEW FETCH commands in a selected mailbox as the server is under no requirement to return preview information for this command, which could lead to an unnecessary waste of system and network resources. See Example 4 in the Examples section for how this can be implemented.

The LAZY modifier MUST be implemented by any server that supports the PREVIEW 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 5647 PREVIEW (FUZZY {200}
  S: Lorem ipsum dolor sit amet, consectetur adipiscing elit.
  S: Curabitur aliquam turpis et ante dictum, et pulvinar dui congue.
  S: Maecenas hendrerit, lorem non imperdiet pellentesque, nulla
  S: ligula nullam
  S: ))
  S: A2 OK FETCH complete.

Example 2: Requesting FETCH with explicit algorithm selection.

  S: * 1 FETCH (RFC822.SIZE 91377 PREVIEW (FUZZY {53}
  S: Preview text generated from message body text data.
  S: ))
  S: B1 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 preview algorithm name.

Example 4: Use explicit algorithm priority selection, with LAZY modifier, to obtain previews during initial mailbox listing if readily available; otherwise, load previews in background.

  S: * 1 FETCH (ENVELOPE ("Wed, 25 Oct 2017 15:03:11 +0000" [...])
     PREVIEW (FUZZY "Preview text for message 1."))
     ("Thu, 26 Oct 2017 12:17:23 +0000" [...]))
  S: * 3 FETCH (ENVELOPE ("Fri, 27 Oct 2017 22:19:21 +0000" [...])
  S: D1 OK FETCH completed.
  [...Client knows that message 2 has a preview that is empty;
      therefore, client only needs to request message 3 preview again
      (e.g. in background)...]
  S: Message data from message 3.
  S: ))
  S: D2 OK Fetch completed.

Example 5: Retrieve preview 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: * 5 FETCH (UID 13 PREVIEW (FUZZY "Preview!"))
  S: E3 OK FETCH completed.
  [...Retrieve message 9 preview in background...]
  S: * 9 FETCH (UID 23 PREVIEW (FUZZY "Another preview!"))
  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        =/ "PREVIEW=" (preview-alg / preview-mod-ext)

  fetch-att         =/ "PREVIEW" [SP "(" preview-alg-fetch *(SP
                       preview-alg-fetch) ")"]

  msg-att-dynamic   =/ "PREVIEW" SP "(" preview-alg SP nstring ")"

  preview-alg       =  "FUZZY" / preview-alg-ext

  preview-alg-ext   =  preview-atom  ; New algorithm names SHOULD be
                                     ; registered with IANA and MUST
                                     ; conform with the
                                     ; recommendations described in
                                     ; RFC 6648, Section 3

  preview-alg-fetch =  preview-alg / preview-mod "=" preview-alg

  preview-atom      =  1*<any ATOM-CHAR except "=">

  preview-mod       =  "LAZY" / preview-mod-ext

  preview-mod-ext   =  preview-atom  ; New priority modifier names
                                     ; SHOULD be registered with IANA
                                     ; and MUST conform with the
                                     ; recommendations described in
                                     ; RFC 6648, Section 3

8. 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 "PREVIEW=FUZZY" capability to the IMAP4 capabilities registry.

This document requests that IANA create a new PREVIEW algorithms registry, which registers preview algorithms by publishing a standards track or IESG-approved experimental RFC. This document constitutes registration of the FUZZY algorithm in that registry.

This document requests that IANA create a new PREVIEW priority modifiers registry, which registers preview priority modifiers by publishing a standards track or IESG-approved experimental RFC. This document constitutes registration of the LAZY priority modifier in that registry.

9. Security Considerations

Implementation of this extension might enable denial-of-service attacks against server resources, due to excessive memory or CPU usage during preview generation or increased storage usage if preview results are stored on the server after generation. Servers MAY limit the resources that preview generation uses.

10. References

10.1. 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.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 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.
[RFC6648] Saint-Andre, P., Crocker, D. and M. Nottingham, "Deprecating the "X-" Prefix and Similar Constructs in Application Protocols", BCP 178, RFC 6648, DOI 10.17487/RFC6648, June 2012.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

10.2. 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.
[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:

Changes from draft-ietf-extra-imap-fetch-snippet-00:

Changes from draft-ietf-extra-imap-fetch-preview-00:

Changes from draft-ietf-extra-imap-fetch-preview-01:

Changes from draft-ietf-extra-imap-fetch-preview-02:


The author would like to thank the following people for their comments and contributions to this document: Stephan Bosch, Bron Gondwana, Teemu Huovila, Steffen Lehmann, Alexey Melnikov, Chris Newman, Jeff Sipek, Timo Sirainen, Steffen Templin, and Aki Tuomi.

Author's Address

Michael M. Slusarz Open-Xchange Inc. 530 Lytton Avenue Palo Alto, California 94301 US EMail: michael.slusarz@open-xchange.com