Display-based Address Sorting for the IMAP4 SORT Extension
draft-ietf-morg-sortdisplay-01

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Abstract

This document describes an IMAP protocol extension enabling server-side message sorting based on the commonly-displayed portion of the From and To header fields.

Table of Contents

1.  Conventions Used in This Document
2.  Introduction
3.  DISPLAY Sort Value for a Mailbox
4.  The DISPLAYFROM and DISPLAYTO Sort Keys
5.  Formal Syntax
6.  Security Considerations
7.  Internationalization Considerations
8.  IANA Considerations
9.  Normative References
§  Author's Address

1.  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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Introduction

The [SORT] (Crispin, M. and K. Murchison, “Internet Message Access Protocol - SORT and THREAD Extensions,” June 2008.) extension to the [IMAP] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) protocol provides a means for 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] (Crispin, M. and K. Murchison, “Internet Message Access Protocol - SORT and THREAD Extensions,” June 2008.) extension's FROM ordering sorts messages lexically on the [IMAP] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) addr-part of the first mailbox in the From header. Likewise, its TO ordering sorts on the [IMAP] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) addr-part of the first address in the message's To header. This document provides two alternate orderings, DISPLAYFROM and DISPLAYTO, which sort messages lexically based on the first From or To address's [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) display-name, when present.

A server that supports the full [SORT] (Crispin, M. and K. Murchison, “Internet Message Access Protocol - SORT and THREAD Extensions,” June 2008.) extension as well as the DISPLAYFROM and DISPLAYTO sort criteria indicates this by returning "SORT=DISPLAY" in its CAPABILITY response.

3.  DISPLAY Sort Value for a Mailbox

For the purposes of the sort keys defined in this document, the sort value for an [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) mailbox is defined as follows:

• If the mailbox contains an [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) display-name, replace each instance of [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) CFWS in the display-name with a single space, trim leading and trailing whitespace, and decode any quoted-strings and [RFC2047] (Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.) encoded-words. If the resulting string is not the empty string, use it as the sort value for the mailbox.
• Otherwise, completely remove all [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) FWS and CFWS from the mailbox's [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) addr-spec and use the resulting string as the mailbox's sort value.

4.  The DISPLAYFROM and DISPLAYTO Sort Keys

This document introduces two new [SORT] (Crispin, M. and K. Murchison, “Internet Message Access Protocol - SORT and THREAD Extensions,” June 2008.) sort-keys, DISPLAYFROM and DISPLAYTO.

A message's sort value under the DISPLAYFROM ordering MUST be derived as follows:

• If the message's From header is absent or if the header contains no [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) mailboxes, the message's sort value is the empty string.
• Otherwise, the message's sort value is the DISPLAY sort value of the first mailbox in its From header.

A message's sort value under the DISPLAYTO ordering MUST be derived as follows:

• If the message's To header is absent or if the header contains no [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) addresses, the message's sort value is the empty string.
• If the first address in the message's To header is an [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) group, the message's sort value is the group's display-name.
• Otherwise, the message's sort value is the DISPLAY sort value of the first [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) mailbox in its To header.

5.  Formal Syntax

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [RFC4234] (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.). [IMAP] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) defines the non-terminal "capability" and [SORT] (Crispin, M. and K. Murchison, “Internet Message Access Protocol - SORT and THREAD Extensions,” June 2008.) 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] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.). 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] (Crispin, M. and K. Murchison, “Internet Message Access Protocol - SORT and THREAD Extensions,” June 2008.), the collations mandated by I18NLEVEL=1 or I18NLEVEL=2 from [RFC5255] (Newman, C., Gulbrandsen, A., and A. Melnikov, “Internet Message Access Protocol Internationalization,” June 2008.) MUST be followed when sorting such strings.

The DISPLAYFROM and DISPLAYTO orderings sort on the full decoded [RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) display-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] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) 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] (Crispin, M., “INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1,” March 2003.) capabilities registry.

9. Normative References

Author's Address