1. RFC 8970
Internet Engineering Task Force (IETF)                        M. Slusarz
Request for Comments: 8970                             Open-Xchange Inc.
Category: Standards Track                                  December 2020
ISSN: 2070-1721

              IMAP4 Extension: Message Preview Generation


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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions Used in This Document
   3.  FETCH Data Item
     3.1.  Command
     3.2.  Response
     3.3.  Preview Text Format
   4.  LAZY Priority Modifier
     4.1.  LAZY
     4.2.  Client Implementation Advice
   5.  Examples
   6.  Formal Syntax
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Author's Address

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 textual previews
   of messages.

   Generation of a preview on the server has several benefits.  First,
   it allows consistent representation of previews across all clients.
   While different clients might generate quite different preview text,
   having common preview text generated by the server can give a more
   consistent user experience to those who use multiple clients.

   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.

   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 that preview can be 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 via the "PREVIEW"
   capability name.

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 Section 9 of [RFC3501].

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.

3.2.  Response

   The server returns a variable-length string that is the generated
   preview for that message.  This string is intended to be viewed by
   the user as a contextual preview of the entire message and is not
   intended to be interpreted in any way by the client software.

   Example: Retrieving preview information in a SELECTed mailbox.

     C: A1 FETCH 1 (PREVIEW)
     S: * 1 FETCH (PREVIEW "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 an approximation 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.
   For example, the underlying IMAP server may change due to a system
   software upgrade; an account's state information may be retained in
   the migration, but the new server may generate different preview text
   than the old server.

   It is possible that the server has determined that no meaningful
   preview text can be generated for a particular message.  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
   MUST return a zero-length string.  Clients SHOULD NOT send another
   FETCH for a preview for such messages.  (As discussed previously,
   preview data is not immutable, so there is chance that at some point
   in the future the server would be able to generate meaningful text.
   However, this scenario is expected to be rare, so a client should not
   continually send out requests to try to detect this infrequent

   If the LAZY modifier (Section 4.1) is used, the server MAY return NIL
   for the preview response, indicating that preview generation could
   not be completed without causing undue delay.  A server MUST NOT
   return NIL to a FETCH PREVIEW request made without the LAZY modifier.

3.3.  Preview Text Format

   The generated 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 whatever processing might be useful in rendering the
   preview as plain text.

   For purposes of this section, a "preview character" is defined as a
   single Universal Character Set (UCS) character encoded in UTF-8.
   Note: a single preview character may comprise 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 diverse client display size requirements.

   The server MUST NOT output preview text longer than 256 preview

   If the preview is not generated based on the body content of the
   message, and the LANGUAGE extension [RFC5255] 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; it should thus use the rules defined for human-readable text
   described in the LANGUAGE extension (if supported on the server).

4.  LAZY Priority Modifier

4.1.  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.

   If this modifier is used, and the server is unable to return preview
   data without undue delay, the server MUST return NIL as the preview

   The LAZY modifier MUST be implemented by any server that supports the
   PREVIEW extension.

4.2.  Client Implementation Advice

   Upon opening a mailbox, a client generally performs a FETCH of
   message details in order to create a listing to present to the user
   (e.g., ENVELOPE data).  Using this extension, a client may want to
   additionally display preview information as part of this listing.
   Quickly providing the base mailbox listing with basic message details
   is the primary goal of this command as this is required to allow the
   user to begin interacting with the mailbox.  Preview data is likely
   to be of secondary importance; it provides useful context, but it is
   not necessary to perform message actions.  A client can load
   unavailable previews in the background and display them
   asynchronously to the user as the preview data is provided by the

   In this scenario, the client would add the PREVIEW data item, with
   the LAZY modifier, to the list of FETCH items needed to generate the
   mailbox listing.  This allows the server to advantageously return
   preview data without blocking the primary goal of quickly returning
   the basic message details used to generate the mailbox listing.

   Once this initial FETCH is complete, the client can then issue FETCH
   requests, without the LAZY modifier, to load the PREVIEW data item
   for the messages in which preview data was not returned.  It is
   RECOMMENDED that these FETCH requests be issued in small batches,
   e.g., 50 messages per FETCH command, since preview generation may be
   expensive and a single large request may exceed server resource

   See Example 2 in Section 5 for an implementation of this strategy.

   A client SHOULD NOT continually issue FETCH PREVIEW requests with the
   LAZY modifier 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.

5.  Examples

   Example 1: Requesting preview without LAZY modifier.

     S: A1 OK Capability command completed.
     [...a mailbox is SELECTed...]
     S: * 1 FETCH (RFC822.SIZE 5647 PREVIEW {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 preview with LAZY modifier, to obtain previews
   during initial mailbox listing if readily available; otherwise, load
   previews in background.

     S: * 1 FETCH (ENVELOPE ("Wed, 23 Sep 2020 15:03:11 +0000" [...])
        PREVIEW "Preview text for message 1.")
        ("Thu, 24 Sep 2020 12:17:23 +0000" [...]))
     S: * 3 FETCH (ENVELOPE ("Fri, 25 Sep 2020 09:13:45 +0000" [...])
        PREVIEW NIL)
     S: * 4 FETCH (ENVELOPE ("Sat, 26 Sep 2020 07:11:18 +0000" [...])
        PREVIEW NIL)
     S: B1 OK FETCH completed.
     [...Client has preview for message 1 and knows that message 2 has
         a preview that is empty; only need to request preview of
         messages 3 & 4 (e.g., in background)...]
     C: B2 FETCH 3:4 (PREVIEW)
     S: * 3 FETCH (PREVIEW {30}
     S: Message data from message 3.
     S: )
     S: * 4 FETCH (PREVIEW "Message 4 preview")
     S: B2 OK Fetch completed.

   Example 3: Requesting preview for search results within a single
   mailbox.  Use the SEARCHRES extension [RFC5182] to save a round-trip.

     S: C1 OK Capability command completed.
     [...a mailbox is SELECTed...]
     S: C2 OK SEARCH completed.
     S: * 5 FETCH (UID 13 PREVIEW "Preview!")
     S: * 9 FETCH (UID 23 PREVIEW NIL)
     S: C3 OK FETCH completed.
     [...Retrieve message 9 preview in background...]
     S: * 9 FETCH (UID 23 PREVIEW "Another preview!")
     S: C4 OK FETCH completed.

6.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) as described in [RFC5234].  It includes definitions from
   IMAP [RFC3501].

     capability        =/ "PREVIEW"

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

     msg-att-dynamic   =/ "PREVIEW" SP nstring

     preview-mod       =  "LAZY"

7.  IANA Considerations

   IMAP [RFC3501] capabilities are registered by publishing a Standards
   Track or IESG-approved Experimental RFC in the "IMAP Capabilities"
   registry located at <http://www.iana.org/assignments/imap-

   IANA has added the "PREVIEW" capability to this registry.

8.  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.  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.

   Servers MAY limit the resources that preview generation uses.  Such
   resource limitations might, in an extreme example, cause a server to
   return a preview that is the empty string for a message that
   otherwise would have had a non-empty preview.  However, it is
   recommended that at least some preview text be provided in this
   situation, even if the quality of the preview is degraded.

   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.

9.  References

9.1.  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,

              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, <https://www.rfc-editor.org/info/rfc3629>.

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  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, <https://www.rfc-editor.org/info/rfc5182>.


   The author would like to thank the following people for their
   comments and contributions to this document: Stephan Bosch, Bron
   Gondwana, Teemu Huovila, Neil Jenkins, Steffen Lehmann, Barry Leiba,
   Alexey Melnikov, Chris Newman, Pete Resnick, 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
   United States of America

   Email: michael.slusarz@open-xchange.com
  1. RFC 8970