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
Abstract
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
https://www.rfc-editor.org/info/rfc8970.
Copyright Notice
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document authors. All rights reserved.
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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
Acknowledgments
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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
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 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
occurrence.)
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 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 diverse client display size requirements.
The server MUST NOT output preview text longer than 256 preview
characters.
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
response.
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
server.
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
limits.
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.
C: A1 CAPABILITY
S: * CAPABILITY IMAP4rev1 PREVIEW
S: A1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: A2 FETCH 1 (RFC822.SIZE PREVIEW)
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.
C: B1 FETCH 1:4 (ENVELOPE PREVIEW (LAZY))
S: * 1 FETCH (ENVELOPE ("Wed, 23 Sep 2020 15:03:11 +0000" [...])
PREVIEW "Preview text for message 1.")
S: * 2 FETCH (PREVIEW "" ENVELOPE
("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.
C: C1 CAPABILITY
S: * CAPABILITY IMAP4rev1 PREVIEW SEARCHRES
S: C1 OK Capability command completed.
[...a mailbox is SELECTed...]
C: C2 SEARCH RETURN (SAVE) FROM "FOO"
C: C3 FETCH $ (UID PREVIEW (LAZY))
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...]
C: C4 UID FETCH 23 (PREVIEW)
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-
capabilities>.
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,
<https://www.rfc-editor.org/info/rfc2046>.
[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>.
[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,
<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>.
[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,
<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>.
[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>.
Acknowledgments
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