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Network Working Group M. Crispin
Request for Comments: 2061 University of Washington
Category: Informational December 1996
IMAP4 COMPATIBILITY WITH IMAP2BIS
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Introduction
The Internet Message Access Protocol (IMAP) has been through several
revisions and variants in its 10-year history. Many of these are
either extinct or extremely rare; in particular, several undocumented
variants and the variants described in RFC 1064, RFC 1176, and RFC
1203 fall into this category.
One variant, IMAP2bis, is at the time of this writing very common and
has been widely distributed with the Pine mailer. Unfortunately,
there is no definite document describing IMAP2bis. This document is
intended to be read along with RFC 1176 and the most recent IMAP4
specification (RFC 2060) to assist implementors in creating an IMAP4
implementation to interoperate with implementations that conform to
earlier specifications. Nothing in this document is required by the
IMAP4 specification; implementors must decide for themselves whether
they want their implementation to fail if it encounters old software.
At the time of this writing, IMAP4 has been updated from the version
described in RFC 1730. An implementor who wishes to interoperate
with both RFC 1730 and RFC 2060 should refer to both documents.
This information is not complete; it reflects current knowledge of
server and client implementations as well as "folklore" acquired in
the evolution of the protocol. It is NOT a description of how to
interoperate with all variants of IMAP, but rather with the old
variant that is most likely to be encountered. For detailed
information on interoperating with other old variants, refer to RFC
1732.
IMAP4 client interoperability with IMAP2bis servers
A quick way to check whether a server implementation supports the
IMAP4 specification is to try the CAPABILITY command. An OK response
will indicate which variant(s) of IMAP4 are supported by the server.
Crispin Informational [Page 1]
RFC 2061 IMAP4 Compatibility December 1996 If the client does not find any of its known variant in the response, it should treat the server as IMAP2bis. A BAD response indicates an IMAP2bis or older server. Most IMAP4 facilities are in IMAP2bis. The following exceptions exist: CAPABILITY command The absense of this command indicates IMAP2bis (or older). AUTHENTICATE command. Use the LOGIN command. LSUB, SUBSCRIBE, and UNSUBSCRIBE commands No direct functional equivalent. IMAP2bis had a concept called "bboards" which is not in IMAP4. RFC 1176 supported these with the BBOARD and FIND BBOARDS commands. IMAP2bis augmented these with the FIND ALL.BBOARDS, SUBSCRIBE BBOARD, and UNSUBSCRIBE BBOARD commands. It is recommended that none of these commands be implemented in new software, including servers that support old clients. LIST command Use the command FIND ALL.MAILBOXES, which has a similar syn- tax and response to the FIND MAILBOXES command described in RFC 1176. The FIND MAILBOXES command is unlikely to produce useful information. * in a sequence Use the number of messages in the mailbox from the EXISTS unsolicited response. SEARCH extensions (character set, additional criteria) Reformulate the search request using only the RFC 1176 syn- tax. This may entail doing multiple searches to achieve the desired results. BODYSTRUCTURE fetch data item Use the non-extensible BODY data item. body sections HEADER, TEXT, MIME, HEADER.FIELDS, HEADER.FIELDS.NOT Use body section numbers only. BODY.PEEK[section] Use BODY[section] and manually clear the \Seen flag as necessary. Crispin Informational [Page 2]
RFC 2061 IMAP4 Compatibility December 1996 FLAGS.SILENT, +FLAGS.SILENT, and -FLAGS.SILENT store data items Use the corresponding non-SILENT versions and ignore the untagged FETCH responses which come back. UID fetch data item and the UID commands No functional equivalent. CLOSE command No functional equivalent. In IMAP2bis, the TRYCREATE special information token is sent as a separate unsolicited OK response instead of inside the NO response. IMAP2bis is ambiguous about whether or not flags or internal dates are preserved on COPY. It is impossible to know what behavior is supported by the server. IMAP4 server interoperability with IMAP2bis clients The only interoperability problem between an IMAP4 server and a well-written IMAP2bis client is an incompatibility with the use of "\" in quoted strings. This is best avoided by using literals instead of quoted strings if "\" or <"> is embedded in the string. Security Considerations Security issues are not discussed in this memo. Author's Address Mark R. Crispin Networks and Distributed Computing University of Washington 4545 15th Aveneue NE Seattle, WA 98105-4527 Phone: (206) 543-5762 EMail: MRC@CAC.Washington.EDU Crispin Informational [Page 3]