Network Working Group M. Crispin
Internet Draft: IMAP4 Compatibility University of Washington
Document: internet-drafts/draft-crispin-imap-compat-00.txt March 1996
IMAP4 COMPATIBILITY WITH IMAP2BIS
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
To learn the current status of any Internet-Draft, please check the
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(Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
Rim).
A revised version of this draft document will be submitted to the RFC
editor as an Informational RFC for the Internet Community.
Discussion and suggestions for improvement are requested, and should
be sent to imap@CAC.Washington.EDU. This document will expire before
30 September 1996. Distribution of this memo is unlimited.
This document is based upon RFC 1732, but with a focus toward
interoperability with IMAP2bis and not with other, extremely rare,
variants of IMAP.
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
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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 (currently an Internet Draft) 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 is being updated from the version
described in RFC 1730. An implementor who wishes to interoperate
with both RFC 1730 and the updated version 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.
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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.
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.
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BODY.PEEK[section]
Use BODY[section] and manually clear the \Seen flag as
necessary.
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.
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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
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