Network Working Group C. Newman
Internet-Draft Sun Microsystems
Updates: 3463 (if approved) July 12, 2004
Expires: January 10, 2005
Message Submission BURL Extension
draft-newman-lemonade-burl-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
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 which 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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Table of Contents
1. Conventions Used in this Document . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. BURL Submission Extension . . . . . . . . . . . . . . . . . . 3
3.1 SMTP Submission Extension Registration . . . . . . . . . . 3
3.2 BURL Transaction . . . . . . . . . . . . . . . . . . . . . 4
3.3 The BURL IMAP Option . . . . . . . . . . . . . . . . . . . 4
3.4 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.5 Formal Syntax . . . . . . . . . . . . . . . . . . . . . . 6
4. 8-bit and Binary . . . . . . . . . . . . . . . . . . . . . . . 6
5. Updates to RFC 3463 . . . . . . . . . . . . . . . . . . . . . 7
6. Response Codes . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 10
9.2 Informative References . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11
A. Document History . . . . . . . . . . . . . . . . . . . . . . . 12
A.1 Changes from burl-00 . . . . . . . . . . . . . . . . . . . 12
A.2 Changes from compose-01 . . . . . . . . . . . . . . . . . 12
A.3 Changes from compose-00 . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. 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" [2].
The formal syntax use the Augmented Backus-Naur Form (ABNF) [4]
notation including the core rules defined in Appendix A of RFC 2234.
2. Introduction
This specification defines an extension to the standard Message
Submission [6] protocol to permit data to be fetched from an IMAP
server at message submission time. This MAY be used in conjunction
with the CHUNKING [10] 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.
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 [8] server on port 25 that acts as a
relay for incoming mail from other SMTP relays. Compliant
submission clients MUST attempt to use port 587 prior to falling
back to port 25, unless explicitly configured to do otherwise by
the user.
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 which appear after the BURL EHLO keyword may change
subsequent to the use of SMTP AUTH [7], so a server which
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 recipient.
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3.2 BURL Transaction
When a BURL-aware client connects to a submit server with the BURL
extension, it will first authenticate (using SMTP AUTH and perhaps
STARTTLS), and then can submit any number of messages with full
interoperability with important SMTP extensions such as delivery
status notifications [18].
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 [9] 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 occurs when the server also
advertises CHUNKING [10]. In this case, the BURL and BDAT commands
may be interleaved until one of them terminates the transaction with
the "LAST" argument. If PIPELINING [9] 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.
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 Option
When "imap" is present in the space-separated list of arguments
following the BURL EHLO keyword, that indicates the BURL command
supports the URLAUTH [13] extended form of IMAP URLs [3].
Subsequent to a successful SMTP AUTH command, the submission server
MAY indicate a pre-arranged 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 to mailboxes on imap.example.com which the user who
authenticated to the submit server can access.
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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:<harry@gryffindor.example.com>
S: 250 2.5.0 Address Ok.
C: RCPT TO:<ron@gryffindor.example.com>
S: 250 2.1.5 ron@gryffindor.example.com OK.
C: BURL imap://harry@gryffindor.example.com/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=
C: MAIL FROM:<harry@gryffindor.example.com>
C: RCPT TO:<ron@gryffindor.example.com>
C: BURL imap://harry@gryffindor.example.com/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 ron@gryffindor.example.com OK.
S: 250 2.5.0 Ok.
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Some example failure cases:
C: MAIL FROM:<harry@gryffindor.example.com>
C: RCPT TO:<malfoy@slitherin.example.com>
C: BURL imap://harry@gryffindor.example.com/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: malfoy@slitherin.example.com
S: 554 5.5.0 No recipients have been specified.
C: MAIL FROM:<harry@gryffindor.example.com>
C: RCPT TO:<ron@gryffindor.example.com>
C: BURL imap://harry@gryffindor.example.com/outbox
;uidvalidity=1078863300/;uid=25;urlauth=submit+harry
:internal:71354a473744909de610943775f92038 LAST
S: 250 2.5.0 Address Ok.
S: 250 2.1.5 ron@gryffindor.example.com OK.
S: 554 5.7.0 IMAP URL authorization failed
3.5 Formal Syntax
The following syntax specification inherits ABNF [4] and Uniform
Resource Identifiers [5].
burl-param = "imap" / ("imap://" authority)
; parameter to BURL EHLO keyword
burl-cmd = "BURL" SP absoluteURI [SP end-marker] CRLF
end-marker = "LAST"
4. 8-bit and Binary
A submit server which advertises BURL MUST also advertise 8BITMIME
[1] 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 [10].
The Submit server MAY refuse to accept a BURL command or combination
of BURL and BDAT commands which result in unencoded 8-bit data in
mail or MIME [16] headers. Alternatively, the server MAY accept such
data and down convert to MIME header encoding [17].
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5. Updates to RFC 3463
SMTP or Submit servers which advertise ENHANCEDSTATUSCODES [15] may
includes enhanced status codes defined in RFC 3463 [19]. The BURL
extension introduces new error cases which 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 [19].
554 5.5.0 No recipients have been specified
This response code occurs when BURL is used with PIPELINING and
all RCPT TOs failed.
503 5.5.0 Valid RCPT TO required before BURL
This response code is an alternative to the previous one when BURL
is used with PIPELINING and all RCPT TOs failed.
554 5.6.3 Conversion required but not supported
This response code occurs when the URL points to binary data and
the implementation does not support down conversion to base64.
This can also be used if the URL points to message data with 8-bit
content in headers and the server does not down convert such
content.
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554 5.3.4 Message too big for system
The message (subsequent to URL resolution) is larger than the
per-message size limit for this server.
554 5.7.8 URL resolution requires trust relationship
The submit server does not have a trust relationship with the IMAP
server specified in the URL argument to BURL.
552 5.2.2 Mailbox full
The recipient is local, the submit server supports direct delivery
and the recipient has exceeded his quota and any grace period for
delivery attempts.
554 5.6.6 IMAP URL resolution failed
The IMAP FETCHURL command returned an error or no data.
354 Waiting for additional BURL or BDAT commands
A BURL command without the "LAST" modifier was sent. The URL for
this BURL command was successfully resolved, but the content will
not be committed to persistent storage until the rest of the
message content is collected. For example, a Unix server may have
written the content to a queue file buffer, but not yet performed
an fsync() operation. If the server loses power, the content can
still be lost.
451 4.4.1 IMAP server unavailable
The connection to the IMAP server to resolve the URL failed.
250 2.5.0 Ok.
The URL was successfully resolved and the complete message data
has been committed to persistent storage.
250 2.6.4 MIME header conversion with loss performed
The URL pointed to message data which included mail or MIME
headers with 8-bit data. This data was converted to MIME header
encoding [17] but the submit server may not have correctly guessed
the unlabelled character set.
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7. IANA Considerations
When this is published as an RFC, the "BURL" SMTP extension as
described in Section 3 will be registered. This registration will be
marked for use by message submission [6] only in the registry.
8. Security Considerations
Modern SMTP submission servers often include content-based security
and denial-of-service defense mechanisms such as virus filtering,
size limits, server-generated signatures, spam filtering, etc.
Implementations of BURL should fetch the URL content prior to
application of such content-based mechanisms in order to preserve
their function.
Clients which generate unsolicited bulk email or email with viruses
could use this mechanism to compensate for a slow link between the
client and submit server. In particular, this mechanism would make
it feasible for a programmable cell phone or other device on a slow
link to become a significant source of unsolicited bulk email and/or
viruses. This makes it more important for submit server vendors
implementing BURL to have auditing and/or defenses against such
denial-of-service attacks including mandatory authentication, logging
which associates unique client identifiers with mail transactions,
limits on re-use of the same IMAP URL, rate limits, recipient count
limits and content filters.
Transfer of the URLAUTH [13] form of IMAP URLs in the clear can
expose the authorization token to network eavesdroppers.
Implementations which support such URLs can address this issue by
using a strong confidentiality protection mechanism. For example,
the SMTP STARTTLS [11] and the IMAP STARTTLS [12] extensions in
combination with a configuration setting which requires their use
with such IMAP URLs would address this concern.
Use of a pre-arranged trust relationship between a submit server and
a specific IMAP server introduces security considerations: a
compromise of the submit server should not automatically compromise
all accounts on the IMAP server so trust relationships involving
super-user proxy credentials are strongly discouraged. A system
which requires the submit server to authenticate to the IMAP server
with submit credentials and subsequently requires a URLAUTH URL to
fetch any content addresses this concern. A trusted third party
model for proxy credentials such as that provided by Kerberos5 [14]
would also suffice.
When a client uses SMTP STARTTLS to send a BURL command which
references non-public information there is a user expectation that
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the entire message content will be treated confidentially. To
address this expectation, the message submission server should use
STARTTLS or a mechanism providing equivalent data confidentiality
when fetching the content referenced by that URL.
A legitimate user of a submit server may try to compromise other
accounts on the server by providing an IMAP URLAUTH URL which points
to a server under that user's control which is designed to undermine
the security of the submit server. For this reason, the IMAP client
code which the submit server uses must be robust with respect to
arbitrary input sizes (including large IMAP literals) and arbitrary
delays from the IMAP server. Requiring a pre-arranged trust
relationship between a submit server and the IMAP server also
addresses this concern.
9. References
9.1 Normative References
[1] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D. Crocker,
"SMTP Service Extension for 8bit-MIMEtransport", RFC 1652, July
1994.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[5] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[6] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
December 1998.
[7] Myers, J., "SMTP Service Extension for Authentication", RFC
2554, March 1999.
[8] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
[9] Freed, N., "SMTP Service Extension for Command Pipelining", STD
60, RFC 2920, September 2000.
[10] Vaudreuil, G., "SMTP Service Extensions for Transmission of
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Large and Binary MIME Messages", RFC 3030, December 2000.
[11] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[12] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[13] Crispin, M. and C. Newman, "Internet Message Access Protocol
(IMAP) - URLAUTH Extension", draft-ietf-lemonade-urlauth-00
(work in progress), July 2004.
9.2 Informative References
[14] Kohl, J. and B. Neuman, "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[15] Freed, N., "SMTP Service Extension for Returning Enhanced Error
Codes", RFC 2034, October 1996.
[16] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[17] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
November 1996.
[18] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC 3461,
January 2003.
[19] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
January 2003.
Author's Address
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790
US
EMail: chris.newman@sun.com
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Appendix A. Document History
Note to RFC Editor: delete this section before publication as an RFC.
A.1 Changes from burl-00
o Enhanced security considerations section
o Updated URLAUTH reference
o Added updates to RFC 3463
o Added Response Codes section
A.2 Changes from compose-01
o Removed the conversion argument to BURL to simplify.
o Replace the conversion section with the simpler 8-bit and Binary
section.
o Removed the failhow argument to simplify and eliminate
race-condition which bothered people.
o Simplify specification to eliminate "composition" model and just
focus on BURL command.
o Make it clear that BURL can be used without the chunking
extension.
A.3 Changes from compose-00
o Added the end-marker "LAST", so this could be used without BDAT
and works with a pre-composed message.
o Changed "Message Composition" to "Message Submission with
Composition" in several places.
o Correct Spelling Errors
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Intellectual Property Statement
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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