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IMAP4 Extension: Message Preview Generation

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8970.
Author Michael Slusarz
Last updated 2019-07-01
Replaces draft-ietf-extra-imap-fetch-snippet
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Bron Gondwana
Shepherd write-up Show Last changed 2019-01-03
IESG IESG state Became RFC 8970 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES. Needs 8 more YES or NO OBJECTION positions to pass.
Responsible AD Alexey Melnikov
Send notices to Bron Gondwana <>
IANA IANA review state Version Changed - Review Needed
EXTRA                                                         M. Slusarz
Internet-Draft                                         Open-Xchange Inc.
Intended status: Standards Track                            July 1, 2019
Expires: January 2, 2020

              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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 2, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used In This Document . . . . . . . . . . . . . .   3
   3.  FETCH Data Item . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Command . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Response  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  PREVIEW Algorithms  . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  FUZZY . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  PREVIEW Priority Modifiers  . . . . . . . . . . . . . . . . .   6
     5.1.  LAZY  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Appendix A.  Change History (To be removed by RFC Editor before
                publication) . . . . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

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 [RFC3501] would benefit from a
   standardized, consistent way to generate these brief previews of

   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 [RFC2045] 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.

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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
   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 [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 preview.

   Finally, server generation allows caching in a centralized location.
   Using server-generated previews allows global generation once per
   message, and then cached for the retention period of the source
   message.  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.

2.  Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [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

   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.

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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 list is provided, the server decides which of its
   built-in algorithms to use to generate the preview text.

   The client may explicitly indicate which algorithm(s) should be used
   to generate the preview list via a parenthesized list of algorithm
   names output after the PREVIEW attribute.  Unsupported algorithms in
   the list MUST be ignored.  If the list contains no supported
   algorithms, the server MUST return a tagged BAD response to the FETCH

   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.

   A server should return preview data for the first algorithm that
   returns non-empty preview text.  Empty preview text is defined as
   either the NIL response or an empty (zero length) string.  If no
   algorithm produces non-empty preview text, the server should respond
   with the preview data generated by the final algorithm in the list.

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

     C: A1 FETCH 1 (PREVIEW)
     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

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   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.
   For example, the underlying IMAP server may change for an account due
   to a software upgrade; account state information may be retained in
   the migration, but the new server may support a different PREVIEW
   generation algorithm.  Thus, message state may remain the same but
   the FETCH PREVIEW response may change, if that data was not part of
   the data migration.

   It is possible that preview text is not available now, but might be
   available later -- perhaps the server's preview-generating resources
   are overloaded, there is a server-imposed timeout during preview
   generation, or there is some transient issue with fetching the
   message body.  In such cases, the server will return NIL as the
   preview response, and the client can try to retrieve the preview

   On the other hand, it is possible that the server has determined that
   no meaningful preview text can be generated for a particular message,
   and that decision won't change later.  Examples of this involve
   encrypted messages, content types the server does not support
   previews of, and other situations where the server is not able to
   extract information for a preview.  In such cases, the server will
   return a zero-length string.  Clients should not send another FETCH
   for a preview for such messages.

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 preview text MUST be treated as text/plain [RFC2046] media type
   data by the client.

   The generated string MUST NOT be content transfer encoded and MUST be
   encoded in UTF-8 [RFC3629].  The server SHOULD remove any formatting
   markup and do what other processing might be useful in rendering the
   preview as plain text.

   For purposes of this section, a "preview character" is defined as a
   single UCS character encoded in UTF-8.  Note: a single preview

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   character may compromise multiple octets, so any buffers implemented
   to conform to the string limitations identified in this document
   should be sized to prevent possible overflow errors.

   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

   If the FUZZY algorithm generates a preview that is not based on the
   body content of the message and the LANGUAGE [RFC5255] extension is
   supported by the server, the preview text SHOULD be generated
   according to 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 from which to generate preview information.
   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.

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   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

   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.

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   Example 3: Requesting FETCH with invalid explicit algorithm

     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: * 3 FETCH (PREVIEW (FUZZY {30}
     S: Message data from message 3.
     S: ))
     S: D2 OK Fetch completed.

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   Example 5: Retrieve preview information for search results within a
   single mailbox.  Use SEARCHRES [RFC5182] extension to save a round-

     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 [RFC5234].  It includes definitions
   from IMAP [RFC3501].

     capability        =/ "PREVIEW=" preview-alg

     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
                                        ; [RFC6648], Section 3

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

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

     preview-mod       =  "LAZY"

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8.  IANA Considerations

   IMAP4 [RFC3501] 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=" capability to
   the IMAP4 [RFC3501] capabilities registry.

   This document requests that IANA create a new "IMAP FETCH PREVIEW
   Algorithms" registry, which registers preview algorithms by IETF
   Review [RFC8126].  An assignment consists of the algorithm name (as
   defined by the preview-alg-ext ABNF entry) and the document that
   defines the algorithm.  This document constitutes registration of the
   FUZZY algorithm 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.  In order to mitigate
   such attacks, servers SHOULD log the client authentication identity
   on FETCH PREVIEW operations in order to facilitate tracking of
   abusive clients.

   Just as the messages they summarize, preview data may contain
   sensitive information.  If generated preview data is stored on the
   server, e.g. for caching purposes, these previews MUST be protected
   with equivalent authorization and confidentiality controls as the
   source message.

10.  References

10.1.  Normative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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              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., Ed. 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,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [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, <>.

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

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

   o  Added standardized language to Section 2 regarding IMAP ABNF

   o  Changed draft name to draft-ietf-extra-imap-fetch-snippet-##

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

   o  Changed nomenclature from "snippet" to "preview"

   o  Changed draft name to draft-ietf-extra-imap-fetch-preview-##

   o  Update to RFC 8174 boilerplate

   o  Updated length requirements for PREVIEW=FUZZY

   o  Added preview-atom ABNF to limit use of "=" character

   o  UTF-8 is a normative reference

   o  Clarify that characters for purpose of length limitations are
      defined as UCS characters as encoded by UTF-8

   o  Fix some incorrect literal lengths in examples

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

   o  Updated postal address

   o  Added example to FETCH response section

   o  Added example on how LANGUAGE extension may influence preview

   o  Added recommendation that only one LAZY FETCH be executed for a
      message per mailbox

   o  Added request to create algorithm and modifier registries

   o  Added requirement that algorithm and modifier names conform to RFC

   o  Added DoS attack info to security considerations

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   o  Distinguish between NIL response and zero-length string

   o  Don't use deprecated "X-" convention in example

   o  Spelling and nits

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

   o  Fix capability ABNF

   o  Removed CAPABILITY string for examples where it did not add
      valuable context

   o  Altered preview data in examples to cover a variety of potential
      server return scenarios

   o  Added "SHOULD be registered" language to algorithm names and
      priority modifiers

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

   o  Move Acknowledgments to un-numbered appendix

   o  Improved abstract text

   o  Consistently use "priority modifiers" instead of "modifiers"

   o  Update example to conform with RFC 3501 UID FETCH requirements

   Changes from draft-ietf-extra-imap-fetch-preview-03:

   o  Remove preview modifier registry request

   o  Improve instructions for registration of algorithms

   o  Add storage information to security considerations

   o  Clarify parsing of algorithm list in FETCH command

   o  Clarify difference between NIL response and zero-length string

   o  Add normative reference for text/plain

   o  Add warning regarding buffers and multiple octet preview

   o  Clarify how to handle preview data return when using an explicit
      algorithm list

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   o  Various editorial fixes

   Changes from draft-ietf-extra-imap-fetch-preview-04:

   o  Make clear that preview caching is tied to retention period of the
      source message

   Changes from draft-ietf-extra-imap-fetch-preview-05:

   o  Clarify "zero-length string" preview data vs. NIL preview data

   o  MIME data -> media type

   o  Capability registration should not include the algorithm name

   o  Give example of how PREVIEW data might change over time


   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, Barry Leiba, 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


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