RFC 5957 PROPOSED STANDARD IMAP · EMAIL · SORTING
IMAP SORT display: Display-Based Address Sorting for the IMAP4 SORT Extension
IMAP sort criteria that order messages by the name people see in From and To, not the bare address.
RFC EDITOR
Karp, D., "Display-Based Address Sorting for the IMAP4 SORT Extension", RFC 5957, DOI 10.17487/RFC5957, July 2010, <https://www.rfc-editor.org/info/rfc5957>.
BIBTEX
@misc{rfc5957,
series = {Request for Comments},
number = 5957,
howpublished = {RFC 5957},
publisher = {RFC Editor},
doi = {10.17487/RFC5957},
url = {https://www.rfc-editor.org/info/rfc5957},
author = {D. Karp},
title = {{Display-Based Address Sorting for the IMAP4 SORT Extension}},
pagetotal = 5,
year = 2010,
month = jul,
} LINEAGE · FROM THE RFC INDEXOpen full graph →
RFC text
Contents
Internet Engineering Task Force (IETF) D. Karp
Request for Comments: 5957 Zimbra
Updates: 5256 July 2010
Category: Standards Track
ISSN: 2070-1721
Display-Based Address Sorting for the IMAP4 SORT Extension
Abstract
This document describes an IMAP protocol extension enabling server-
side message sorting on the commonly displayed portion of the From
and To header fields.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5957.
Copyright Notice
Copyright (c) 2010 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
(http://trustee.ietf.org/license-info) 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.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. DISPLAY Sort Value for an Address ...............................2
4. The DISPLAYFROM and DISPLAYTO Sort Criteria .....................3
5. Formal Syntax ...................................................3
6. Security Considerations .........................................3
7. Internationalization Considerations .............................4
8. IANA Considerations .............................................4
9. Normative References ............................................4
1. Introduction
The [SORT] extension to the [IMAP] protocol provides a means for the
server-based sorting of messages. It defines a set of sort criteria
and the mechanism for determining the sort value of a message for
each such ordering.
The [SORT] FROM and TO orderings sort messages lexically on the
[IMAP] addr-mailbox of the first address in the message's From and To
headers, respectively. This document provides two alternative
orderings, DISPLAYFROM and DISPLAYTO, which sort messages based on
the first From or To address's [IMAP] addr-name (generally the same
as its [RFC5322] display-name), when present.
A server that supports the full [SORT] extension as well as both the
DISPLAYFROM and DISPLAYTO sort criteria indicates this by returning
"SORT=DISPLAY" in its CAPABILITY response.
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 [RFC2119].
3. DISPLAY Sort Value for an Address
For the purposes of the sort criteria defined in this document, the
sort value for an [IMAP] address structure is defined as follows:
o If the address structure's [IMAP] addr-name is non-NIL, apply the
procedure from [RFC5255], Section 4.6. (That is, decode any
[RFC2047] encoded-words and convert the resulting character string
into a charset valid for the currently active [RFC4790] collation,
with a default of UTF-8.) If the resulting octet string is not
the empty string, use it as the sort value for the address.
o Otherwise, if the address structure's [IMAP] addr-mailbox and
[IMAP] addr-host are both non-NIL, the sort value for the address
is addr-mailbox@addr-host.
o Otherwise, if the address structure's [IMAP] addr-mailbox is non-
NIL, the sort value for the address is its addr-mailbox.
o If none of the above conditions are met, the sort value for the
address is the empty string.
4. The DISPLAYFROM and DISPLAYTO Sort Criteria
This document introduces two new [SORT] sort criteria, DISPLAYFROM
and DISPLAYTO. A message's sort value under these orderings MUST be
derived as follows:
A "derived-addr" value is created from the [IMAP] envelope structure
resulting from a FETCH ENVELOPE on the message. For DISPLAYFROM, the
derived-addr value is the [IMAP] env-from value. For DISPLAYTO, the
derived-addr value is the [IMAP] env-to value.
o If the derived-addr value is NIL, the message's sort value is the
empty string.
o Otherwise, the message's sort value is the DISPLAY sort value of
the first [IMAP] address in the derived-addr value.
5. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. [IMAP] defines the
non-terminal "capability", and [SORT] defines "sort-key".
capability =/ "SORT=DISPLAY"
sort-key =/ "DISPLAYFROM" / "DISPLAYTO"
6. Security Considerations
This document defines an additional IMAP4 capability. As such, it
does not change the underlying security considerations of [IMAP].
The author believes that no new security issues are introduced with
this additional IMAP4 capability.
7. Internationalization Considerations
DISPLAYFROM and DISPLAYTO are string-based sort criteria. As stated
in [SORT], the active [RFC4790] collation as per [RFC5255] MUST be
used when sorting such strings.
The DISPLAYFROM and DISPLAYTO orderings sort on the full decoded
[IMAP] addr-name, when present. They do not attempt to parse this
string in a locale- or language-dependent manner in order to
determine and sort on some semantically meaningful substring such as
the surname.
8. IANA Considerations
[IMAP] capabilities are registered by publishing a Standards Track or
IESG-approved Experimental RFC. This document constitutes
registration of the SORT=DISPLAY capability in the [IMAP]
capabilities registry.
9. Normative References
[IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4790] Newman, C., Duerst, M., and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790, March
2007.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC5255] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
June 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[SORT] Crispin, M. and K. Murchison, "Internet Message Access
Protocol - SORT and THREAD Extensions", RFC 5256, June
2008.
Author's Address
Dan Karp
Zimbra
3401 Hillview Avenue
Palo Alto, CA 94304
USA
EMail: dkarp@zimbra.com
URI: http://www.zimbra.com
AI ELI5
On this page
PROBLEMThe IMAP SORT extension sorts FROM and TO on the bare mailbox part of the first address, which is not what users see in their message lists.
MECHANISMTwo new sort criteria, DISPLAYFROM and DISPLAYTO, sort on the address's display name when present, falling back to the mailbox address; servers advertise SORT=DISPLAY.
WHY IT MATTERSIt updates RFC 5256 and adds one capability to the IMAP registry.
§1Introduction
The SORT extension (RFC 5256) to IMAP (RFC 3501) lets the server sort messages. Its FROM and TO orderings sort lexically on the addr-mailbox of the first address in the From and To headers.
This document adds DISPLAYFROM and DISPLAYTO, which sort on the first address's addr-name (generally the display-name) when present. A server supporting full SORT and both criteria returns SORT=DISPLAY in its CAPABILITY response.
Mail used to be sorted by the hidden address. Now it can be sorted by the name you actually see.
§2How the Sort Value Is Chosen
For an address, the sort value comes from the first rule that applies. If addr-name is non-NIL, decode any RFC 2047 encoded-words and convert the result into a charset valid for the active collation (default UTF-8), as in RFC 5255; if that is not empty, use it. Otherwise, if mailbox and host are both non-NIL, use addr-mailbox@addr-host. Otherwise, if only the mailbox is non-NIL, use it. Otherwise the value is the empty string.
For a message, take the envelope from FETCH ENVELOPE. DISPLAYFROM uses env-from and DISPLAYTO uses env-to. If that value is NIL the sort value is the empty string; otherwise it is the sort value of the first address in it.
Use the person's name if there is one; if not, use their email address; if there is nothing, use blank.
- 1addr-name — Decoded display name, if non-NIL and not empty.
- 2mailbox@host — If addr-mailbox and addr-host are both non-NIL.
- 3mailbox — If only addr-mailbox is non-NIL.
- 4empty string — If none of the above apply.
§3Syntax and Internationalization
The ABNF adds SORT=DISPLAY to capability and DISPLAYFROM and DISPLAYTO to sort-key.
Both criteria are string-based, so the active collation MUST be used. They sort on the full decoded addr-name and do not try to pick out a meaningful part such as the surname.
The sorting looks at the whole name as written, not just the last name.
§4Security and IANA
The extension only adds an IMAP capability, so it does not change the underlying security considerations of IMAP. The author believes no new security issues are introduced.
IANA registration: this document registers the SORT=DISPLAY capability in the IMAP capabilities registry.
It is only a new way to sort, so it adds no new risk, and the new name goes on the official list.
§5References
[IMAP]RFC 3501Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.March 2003[SORT]RFC 5256Crispin, M. and K. Murchison, "Internet Message Access Protocol - SORT and THREAD Extensions", RFC 5256, June 2008.June 2008[RFC5255]RFC 5255Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet Message Access Protocol Internationalization", RFC 5255, June 2008.June 2008[RFC5322]RFC 5322Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008.October 2008