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.



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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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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: [email protected]












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