Internet Engineering Task Force (IETF) D. Karp
Request for Comments: 5957 Zimbra
Updates: 5256 July 2010
Category: Standards Track
Display-Based Address Sorting for the IMAP4 SORT Extension
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
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
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RFC 5957 July 2010
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
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.
Karp IMAP4 Display-Based Address Sorting [Page 2]
RFC 5957 July 2010
o Otherwise, if the address structure's [IMAP] addr-mailbox and
[IMAP] addr-host are both non-NIL, the sort value for the address
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
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.
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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
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]
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
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
[RFC5255] Newman, C., Gulbrandsen, A., and A. Melnikov, "Internet
Message Access Protocol Internationalization", RFC 5255,
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
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[SORT] Crispin, M. and K. Murchison, "Internet Message Access
Protocol - SORT and THREAD Extensions", RFC 5256, June
3401 Hillview Avenue
Palo Alto, CA 94304
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