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Display-Based Address Sorting for the IMAP4 SORT Extension
RFC 5957

Document Type RFC - Proposed Standard (July 2010)
Updates RFC 5256
Author Dan Karp
Last updated 2022-03-26
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Alexey Melnikov
Send notices to (None)
RFC 5957
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


   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 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
   ( 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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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

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",
   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.

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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
      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.

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RFC 5957                                                       July 2010

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

              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,
              June 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

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RFC 5957                                                       July 2010

   [SORT]     Crispin, M. and K. Murchison, "Internet Message Access
              Protocol - SORT and THREAD Extensions", RFC 5256, June

Author's Address

   Dan Karp
   3401 Hillview Avenue
   Palo Alto, CA  94304


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