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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.

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 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

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