rfc4468.txt

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Network Working Group                                          C. Newman
Request for Comments: 4468                              Sun Microsystems
Updates: 3463                                                   May 2006
Category: Standards Track


                   Message Submission BURL Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The submission profile of Simple Mail Transfer Protocol (SMTP)
   provides a standard way for an email client to submit a complete
   message for delivery.  This specification extends the submission
   profile by adding a new BURL command that can be used to fetch
   submission data from an Internet Message Access Protocol (IMAP)
   server.  This permits a mail client to inject content from an IMAP
   server into the SMTP infrastructure without downloading it to the
   client and uploading it back to the server.





















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RFC 4468           Message Submission BURL Extension            May 2006


Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. BURL Submission Extension .......................................3
      3.1. SMTP Submission Extension Registration .....................3
      3.2. BURL Transaction ...........................................3
      3.3. The BURL IMAP Options ......................................4
      3.4. Examples ...................................................5
      3.5. Formal Syntax ..............................................6
   4. 8-Bit and Binary ................................................7
   5. Updates to RFC 3463 .............................................7
   6. Response Codes ..................................................7
   7. IANA Considerations .............................................9
   8. Security Considerations .........................................9
   9. References .....................................................11
      9.1. Normative References ......................................11
      9.2. Informative References ....................................12
   Appendix A.  Acknowledgements .....................................13

1.  Introduction

   This specification defines an extension to the standard Message
   Submission [RFC4409] protocol to permit data to be fetched from an
   IMAP server at message submission time.  This MAY be used in
   conjunction with the CHUNKING [RFC3030] mechanism so that chunks of
   the message can come from an external IMAP server.  This provides the
   ability to forward an email message without first downloading it to
   the client.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
   in this document are to be interpreted as defined in "Key words for
   use in RFCs to Indicate Requirement Levels" [RFC2119].

   The formal syntax uses the Augmented Backus-Naur Form (ABNF)
   [RFC4234] notation including the core rules defined in Appendix B of
   RFC 4234.












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3.  BURL Submission Extension

   This section defines the BURL submission extension.

3.1.  SMTP Submission Extension Registration

   1.  The name of this submission extension is "BURL".  This extends
       the Message Submission protocol on port 587 and MUST NOT be
       advertised by a regular SMTP [RFC2821] server on port 25 that
       acts as a relay for incoming mail from other SMTP relays.

   2.  The EHLO keyword value associated with the extension is "BURL".

   3.  The BURL EHLO keyword will have zero or more arguments.  The only
       argument defined at this time is the "imap" argument, which MUST
       be present in order to use IMAP URLs with BURL.  Clients MUST
       ignore other arguments after the BURL EHLO keyword unless they
       are defined by a subsequent IETF standards track specification.
       The arguments that appear after the BURL EHLO keyword may change
       subsequent to the use of SMTP AUTH [RFC2554], so a server that
       advertises BURL with no arguments prior to authentication
       indicates that BURL is supported but authentication is required
       to use it.

   4.  This extension adds the BURL SMTP verb.  This verb is used as a
       replacement for the DATA command and is only permitted during a
       mail transaction after at least one successful RCPT TO.

3.2.  BURL Transaction

   A simple BURL transaction will consist of MAIL FROM, one or more RCPT
   TO headers, and a BURL command with the "LAST" tag.  The BURL command
   will include an IMAP URL pointing to a fully formed message ready for
   injection into the SMTP infrastructure.  If PIPELINING [RFC2920] is
   advertised, the client MAY send the entire transaction in one round
   trip.  If no valid RCPT TO address is supplied, the BURL command will
   simply fail, and no resolution of the BURL URL argument will be
   performed.  If at least one valid RCPT TO address is supplied, then
   the BURL URL argument will be resolved before the server responds to
   the command.

   A more sophisticated BURL transaction MAY occur when the server also
   advertises CHUNKING [RFC3030].  In this case, the BURL and BDAT
   commands may be interleaved until one of them terminates the
   transaction with the "LAST" argument.  If PIPELINING [RFC2920] is
   also advertised, then the client may pipeline the entire transaction
   in one round-trip.  However, it MUST wait for the results of the
   "LAST" BDAT or BURL command prior to initiating a new transaction.



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   The BURL command directs the server to fetch the data object to which
   the URL refers and include it in the message.  If the URL fetch
   fails, the server will fail the entire transaction.

3.3.  The BURL IMAP Options

   When "imap" is present in the space-separated list of arguments
   following the BURL EHLO keyword, it indicates that the BURL command
   supports the URLAUTH [RFC4467] extended form of IMAP URLs [RFC2192]
   and that the submit server is configured with the necessary
   credentials to resolve "urlauth=submit+" IMAP URLs for the submit
   server's domain.

   Subsequent to a successful SMTP AUTH command, the submission server
   MAY indicate a prearranged trust relationship with a specific IMAP
   server by including a BURL EHLO keyword argument of the form
   "imap://imap.example.com".  In this case, the submission server will
   permit a regular IMAP URL referring to messages or parts of messages
   on imap.example.com that the user who authenticated to the submit
   server can access.  Note that this form does not imply that the
   submit server supports URLAUTH URLs; the submit server must advertise
   both "imap" and "imap://imap.example.com" to indicate support for
   both extended and non-extended URL forms.

   When the submit server connects to the IMAP server, it acts as an
   IMAP client and thus is subject to both the mandatory-to-implement
   IMAP capabilities in Section 6.1.1 of RFC 3501, and the security
   considerations in Section 11 of RFC 3501.  Specifically, this
   requires that the submit server implement a configuration that uses
   STARTTLS followed by SASL PLAIN [SASL-PLAIN] to authenticate to the
   IMAP server.

   When the submit server resolves a URLAUTH IMAP URL, it uses submit
   server credentials when authenticating to the IMAP server.  The
   authentication identity and password used for submit credentials MUST
   be configurable.  The string "submit" is suggested as a default value
   for the authentication identity, with no default for the password.
   Typically, the authorization identity is empty in this case; thus the
   IMAP server will derive the authorization identity from the
   authentication identity.  If the IMAP URL uses the "submit+" access
   identifier prefix, the submit server MUST refuse the BURL command
   unless the userid in the URL's <access> token matches the submit
   client's authorization identity.

   When the submit server resolves a regular IMAP URL, it uses the
   submit client's authorization identity when authenticating to the
   IMAP server.  If both the submit client and the submit server's
   embedded IMAP client use SASL PLAIN (or the equivalent), the submit



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   server SHOULD forward the client's credentials if and only if the
   submit server knows that the IMAP server is in the same
   administrative domain.  If the submit server supports SASL mechanisms
   other than PLAIN, it MUST implement a configuration in which the
   submit server's embedded IMAP client uses STARTTLS and SASL PLAIN
   with the submit server's authentication identity and password (for
   the respective IMAP server) and the submit client's authorization
   identity.

3.4.  Examples

   In examples, "C:" and "S:" indicate lines sent by the client and
   server, respectively.  If a single "C:" or "S:" label applies to
   multiple lines, then the line breaks between those lines are for
   editorial clarity only and are not part of the actual protocol
   exchange.

   Two successful submissions (without and with pipelining) follow:

   <SSL/TLS encryption layer negotiated>
   C: EHLO potter.example.com
   S: 250-owlry.example.com
   S: 250-8BITMIME
   S: 250-BURL imap
   S: 250-AUTH PLAIN
   S: 250-DSN
   S: 250 ENHANCEDSTATUSCODES
   C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=
   S: 235 2.7.0 PLAIN authentication successful.
   C: MAIL FROM:<[e-mail]>
   S: 250 2.5.0 Address Ok.
   C: RCPT TO:<[e-mail]>
   S: 250 2.1.5 [e-mail] OK.
   C: BURL imap://[e-mail]/outbox
           ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
           :internal:91354a473744909de610943775f92038 LAST
   S: 250 2.5.0 Ok.

   <SSL/TLS encryption layer negotiated>
   C: EHLO potter.example.com
   S: 250-owlry.example.com
   S: 250-8BITMIME
   S: 250-PIPELINING
   S: 250-BURL imap
   S: 250-AUTH PLAIN
   S: 250-DSN
   S: 250 ENHANCEDSTATUSCODES
   C: AUTH PLAIN aGFycnkAaGFycnkAYWNjaW8=



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   C: MAIL FROM:<[e-mail]>
   C: RCPT TO:<[e-mail]>
   C: BURL imap://[e-mail]/outbox
           ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
           :internal:91354a473744909de610943775f92038 LAST
   S: 235 2.7.0 PLAIN authentication successful.
   S: 250 2.5.0 Address Ok.
   S: 250 2.1.5 [e-mail] OK.
   S: 250 2.5.0 Ok.

   Note that PIPELINING of the AUTH command is only permitted if the
   selected mechanism can be completed in one round trip, a client
   initial response is provided, and no SASL security layer is
   negotiated.  This is possible for PLAIN and EXTERNAL, but not for
   most other SASL mechanisms.

   Some examples of failure cases:

   C: MAIL FROM:<[e-mail]>
   C: RCPT TO:<[e-mail]>
   C: BURL imap://[e-mail]/outbox
           ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
           :internal:91354a473744909de610943775f92038 LAST
   S: 250 2.5.0 Address Ok.
   S: 550 5.7.1 Relaying not allowed: [e-mail]
   S: 554 5.5.0 No recipients have been specified.

   C: MAIL FROM:<[e-mail]>
   C: RCPT TO:<[e-mail]>
   C: BURL imap://[e-mail]/outbox
           ;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
           :internal:71354a473744909de610943775f92038 LAST
   S: 250 2.5.0 Address Ok.
   S: 250 2.1.5 [e-mail] OK.
   S: 554 5.7.0 IMAP URL authorization failed

3.5.  Formal Syntax

   The following syntax specification inherits ABNF [RFC4234] and
   Uniform Resource Identifiers [RFC3986].

      burl-param  = "imap" / ("imap://" authority)
                  ; parameter to BURL EHLO keyword

      burl-cmd    = "BURL" SP absolute-URI [SP end-marker] CRLF

      end-marker  = "LAST"




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4.  8-Bit and Binary

   A submit server that advertises BURL MUST also advertise 8BITMIME
   [RFC1652] and perform the down conversion described in that
   specification on the resulting complete message if 8-bit data is
   received with the BURL command and passed to a 7-bit server.  If the
   URL argument to BURL refers to binary data, then the submit server
   MAY refuse the command or down convert as described in Binary SMTP
   [RFC3030].

   The Submit server MAY refuse to accept a BURL command or combination
   of BURL and BDAT commands that result in un-encoded 8-bit data in
   mail or MIME [RFC2045] headers.  Alternatively, the server MAY accept
   such data and down convert to MIME header encoding [RFC2047].

5.  Updates to RFC 3463

   SMTP or Submit servers that advertise ENHANCEDSTATUSCODES [RFC2034]
   use enhanced status codes defined in RFC 3463 [RFC3463].  The BURL
   extension introduces new error cases that that RFC did not consider.
   The following additional enhanced status codes are defined by this
   specification:

   X.6.6 Message content not available

      The message content could not be fetched from a remote system.
      This may be useful as a permanent or persistent temporary
      notification.

   X.7.8 Trust relationship required

      The submission server requires a configured trust relationship
      with a third-party server in order to access the message content.

6.  Response Codes

   This section includes example response codes to the BURL command.
   Other text may be used with the same response codes.  This list is
   not exhaustive, and BURL clients MUST tolerate any valid SMTP
   response code.  Most of these examples include the appropriate
   enhanced status code [RFC3463].

   554 5.5.0 No recipients have been specified

      This response code occurs when BURL is used (for example, with
      PIPELINING) and all RCPT TOs failed.





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