Internet DRAFT - draft-melnikov-smtp-metadata
draft-melnikov-smtp-metadata
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Standards Track June 17, 2015
Expires: December 19, 2015
Simple Mail Transfer Protocol extension for relaying Metadata
draft-melnikov-smtp-metadata-01
Abstract
This memo defines an extension to the SMTP (Simple Mail Transfer
Protocol) service whereby message metadata (such as Trace header
fields, IMAP flags, Keying material, etc) can be transferred in
separate containers similar to BDAT (RFC 3030, SMTP CHUNKING)
command. This allows clean separation of transaction related state
from the message itself.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December 19, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Definition of the Metadata SMTP Extension . . . . . . . . . . 3
3.1. BMTD command . . . . . . . . . . . . . . . . . . . . . . 3
3.2. Initial List of Metadata Container types . . . . . . . . 4
3.3. Requirements on a Metadata Container type definition . . 5
4. Handling of messages received via SMTP . . . . . . . . . . . 5
4.1. Relay of messages to other conforming SMTP/LMTP servers . 5
4.2. Relay of messages to non-conforming SMTP/LMTP servers . . 6
4.3. Gatewaying a message into a foreign environment . . . . . 6
5. Use of METADATA with LMTP . . . . . . . . . . . . . . . . . . 6
6. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Deployment Considerations . . . . . . . . . . . . . . . . . . 8
8.1. Multiple MX records . . . . . . . . . . . . . . . . . . . 8
9. Open Issues/To Do . . . . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Security Considerations . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Background on Design Choices . . . . . . . . . . . . 11
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
This memo defines an extension to the SMTP (Simple Mail Transfer
Protocol) service whereby message metadata (such as Trace header
fields, IMAP flags, Keying material, etc) can be transferred in
separate containers similar to BDAT (RFC 3030, SMTP CHUNKING)
command. This allows clean separation of transaction related state
from the message itself.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] when they
appear in ALL CAPS. These words also appear in this document in
lower case as plain English words, absent their normative meanings.
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The formal syntax use the Augmented Backus-Naur Form (ABNF) [RFC5234]
notation including the core rules defined in Appendix B of RFC 5234
[RFC5234].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. Line breaks that do not start with a new "C:"
or "S:" exist for editorial reasons and are not a part of the
protocol.
3. Definition of the Metadata SMTP Extension
The Metadata SMTP service extension is defined as follows:
1. The textual name of this extension is "Metadata Transfer".
2. The EHLO keyword value associated with this extension is
"METADATA". Any server that advertises support for the
"METADATA" extension MUST also support SMTP CHUNKING (RFC 3030).
3. The EHLO keyword has no parameters
4. [[CREF1: Should BMTD be allowed before the DATA command? There
is no reason why not.]] A new SMTP verb, BMTD, is defined. The
BMTD verb takes one argument, which indicates the length, in
octets, of the binary metadata container that follows immediately
after the command. See Section 3.1 for the description of the
BMTD command and Section 6 for its syntax.
5. This extension doesn't add any new parameters to MAIL FROM or
RCPT TO commands.
6. The Metadata extension is valid for the submission service
[RFC6409] and LMTP [RFC2033].
3.1. BMTD command
After all MAIL and RCPT responses are collected and processed, the
message metadata is sent using a series of BMTD commands. The BMTD
command takes one required argument, the exact length of the metadata
segment ("container") in octets. The metadata is sent immediately
after the trailing <CR> <LF> of the BMTD command line. Once the
receiver-SMTP receives the specified number of octets, it will return
a 250 reply code.
BMTD commands MUST be sent before any BDAT [RFC3030] or BURL
[RFC4468] commands. If a server encounters BMTD command after BDAT/
BURL, it MUST respond with 503 "Bad sequence of commands" reply code.
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The state resulting from this error is indeterminate. A RSET command
MUST be sent to clear the transaction before continuing.
Each BMTD container starts with 2 octet container type, followed by
container type specific data. This means that the metadata segment
length can never be the value 1 (it can either be 0 or be equal or
greater than 2).
A 250 response MUST be sent to each successful BMTD data block
("chunk") within a mail transaction. If a failure occurs after a
BMTD command is received, the receiver-SMTP MUST accept and discard
the associated metadata and message data before sending the
appropriate 5XX or 4XX code. If a 5XX or 4XX code is received by the
sender-SMTP in response to a BMTD chunk, the transaction should be
considered failed and the sender- SMTP MUST NOT send any additional
BMTD segments. If the receiver- SMTP has declared support for
command pipelining [RFC2920], the receiver SMTP MUST be prepared to
accept and discard additional BDAT/BURL/BMTD chunks already in the
pipeline after the failed BMTD.
Note: An error on the receiver-SMTP such as disk full or imminent
shutdown can only be reported after the BMTD segment has been
received. It is therefore important to choose a reasonable chunk
size given the expected end-to-end bandwidth.
Note: Because the receiver-SMTP does not acknowledge the BMTD command
before the message data is sent, it is important to send the BMTD
only to systems that have declared their capability to accept BMTD
commands. Illegally sending a BMTD command and associated message
data to a non-METADATA capable system will result in the receiver-
SMTP parsing the associated message data as if it were a potentially
very long, ESMTP command line containing binary data.
More than one BMTD command can occur in a transaction. (However some
BMTD container types only allow for a single BMTD command with that
particular container type.) Any BMTD command MUST be followed by one
or more of BMTD/BDAT/BURL commands.
3.2. Initial List of Metadata Container types
Type 0: Trace header fields: Received, Return-Path, Authentication-
Results (RFC 7001), etc encoded as if they are a part of a message
header. Containers of this type can appear multiple types in a
transaction. MUST be supported by all compliant servers.
Type 1: IMAP Keywords [RFC3501] associated with the message (e.g.
$MDNSent, $Forwarded, \Answered). This is a space separated list of
IMAP keywords/flags. Container of this type MUST NOT appear more
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than once in a transaction. If the final LMTP delivers the message
to an IMAP capable mailstore, it MUST attempt setting the listed IMAP
keywords/flags on the message. Flags/keywords not supported by the
mailstore (or disallowed when a message is injected via LMTP) MUST be
ignored.
Keying material, a la Dark Mail. TBD if there is interest.
3.3. Requirements on a Metadata Container type definition
Each container type definition MUST specify if it can appear more
than once.
Unless specified by an extension mutually agreed by SMTP sender and
SMTP recipient, no container type can be defined as required (i.e.
appearing at least once in a SMTP transaction) or define how it can
be relayed to a non compliant MTA.
Each container type definition MUST describe how it is going to be
handled by the final MTA/LMTP server.
4. Handling of messages received via SMTP
This section describes how a conforming SMTP server should handle any
messages received via SMTP.
4.1. Relay of messages to other conforming SMTP/LMTP servers
The following rules govern the behavior of a conforming MTA (in the
role of an SMTP/LMTP client), when relaying a message which was
received via the SMTP protocol, to an SMTP/LMTP server that supports
the METADATA extension:
1. Instead of prepending trace fields to the message itself as
specified in RFC 5321, a relaying MTA SHOULD [[CREF2: Cross check
with RFC 5321 regarding insertion of Received header fields]]
insert a single BMTD container of type 0 (Trace fields)
containing its own trace header fields such as Received
[RFC5321], Authentication-Results [RFC7001], etc.
2. All other BMTD commands are relayed to conforming SMTP/LMTP
server in the order received. Intermediary servers SHOULD NOT
coalesce or reorder metadata containers of type 0 or any other
type that they understand. Intermediary servers MUST NOT
coalesce, reorder or drop metadata containers of any types that
they don't recognize.
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4.2. Relay of messages to non-conforming SMTP/LMTP servers
The following rules govern the behavior of a conforming MTA (in the
role of an SMTP/LMTP client), when relaying a message which was
received via the SMTP protocol, to an SMTP/LMTP server that does not
support the METADATA extension:
1. Data from each metadata container of type 0 (Trace fields) MUST
be extracted and prepended to the header of the message in the
order of BMTD commands.
2. All other BMTD chunks are discarded. [[CREF3: OPEN ISSUE. They
can also be converted to some magic header fields for logging and
debugging?]]
4.3. Gatewaying a message into a foreign environment
The following rules govern the behavior of a conforming MTA, when
gatewaying a message that was received via the SMTP protocol, into a
foreign (non-SMTP) environment:
1. If the destination environment is unable to provide an equivalent
of the BMTD command, the conforming MTA SHOULD behave as if it is
relaying to a non-conformant SMTP server (Section 4.2).
2. If the destination environment is capable of providing an
equivalent of the BMTD command, the conforming MTA SHOULD behave
as if it is relaying to a conformant SMTP server (Section 4.1),
converting any BMTD command to the equivalent in the destination
environment.
5. Use of METADATA with LMTP
An LMTP server can advertise support for the METADATA extension:
1. Data from containers of type 0 (Trace fields) is extracted (in
the order of the corresponding BMTD commands) and prepended to
the header of the message.
2. Handling of other container type is specific to the container
type.
3. Unsupported BMTD container types are discarded. [[CREF4: OPEN
ISSUE. They can also be converted to some magic header fields
for logging and debugging?]]
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6. Syntax
metadata-ehlo = "METADATA"
; Complies with the <ehlo-line> ABNF production from RFC 5321.
bmtd-cmd = "BMTD" SP chunk-size CR LF
chunk-size = 1*DIGIT
bmtd-container = container-type container-specific-data
container-type = <2 octets, extensible>
container-specific-data = <remaining container data>
DIGIT = <Defined in RFC 5234>
7. Example
The original submission (from MUA to MSA) might look like shown
below. Note that the example is also making use of the STARTTLS
[RFC3207], and DSN [RFC3461] SMTP extensions, even though there is no
requirement that these other extensions are to be supported when the
METADATA SMTP extension is implemented.
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S: 220 example.com SMTP server here
C: EHLO mua.example.com
S: 250-example.com
S: 250-STARTTLS
S: 250-AUTH SCRAM-SHA-1 DIGEST-MD5
S: 250-DSN
S: 250-CHUNKING
S: 250-ENHANCEDSTATUSCODES
S: 250 METADATA
C: AUTH SCRAM-SHA-1
[...authentication exchange...]
S: 235 2.7.0 Authentication successful
C: MAIL FROM:<eljefe@example.com> ENVID=QQ314159
S: 250 2.1.0 <eljefe@example.com> sender ok
C: RCPT TO:<topbanana@example.net>
S: 250 2.1.5 <topbanana@example.net> recipient ok
C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
ORCPT=rfc822;Dana@Ivory.example.net
S: 250 2.1.5 <Dana@Ivory.example.net> recipient ok
C: BMTD 40
C: <2 octets == type> ...
S: 250 2.1.0 message metadata accepted
C: BMTD 12
C: <2 octets == type 1>$Forwarded
S: 250 2.1.0 message metadata accepted
C: BDAT 86 LAST
C: To: Susan@random.com
C: From: Sam@random.com
C: Subject: This is a bodyless test message
S: 250 2.1.0 message accepted
C: QUIT
S: 221 2.0.0 goodbye
[[Need to fix byte counts/BMTD commands in the example]]
[[Add another example with PIPELINING]]
8. Deployment Considerations
8.1. Multiple MX records
If multiple DNS MX records are used to specify multiple servers for a
domain in section 5 of [RFC5321], it is strongly advised that all of
them support the METADATA extension . If one or more servers behave
differently in this respect, then it is strongly suggested that none
of the servers support the METADATA extension. Otherwise, unexpected
differences in message rejections can happen during temporary or
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permanent failures, which users might perceive as serious reliability
issues.
9. Open Issues/To Do
Document interaction with the SIZE extension. (Proposal: count each
BMTD chunk size against the SIZE limit)
Decide what should be allowed behaviour for handling of container
types unrecognized by intermediate server and final delivery agents.
10. IANA Considerations
This specification requests IANA to add the METADATA SMTP extension
to the "SMTP Service Extensions" registry (in
http://www.iana.org/assignments/mail-parameters). This extension is
suitable for the Submit port.
11. Security Considerations
TBD
12. References
12.1. Normative References
[RFC1870] Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10, RFC 1870,
November 1995.
[RFC2033] Myers, J., "Local Mail Transfer Protocol", RFC 2033,
October 1996.
[RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2920] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, September 2000.
[RFC3030] Vaudreuil, G., "SMTP Service Extensions for Transmission
of Large and Binary MIME Messages", RFC 3030, December
2000.
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[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)", RFC
3461, January 2003.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248, June 2008.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC7001] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 7001, September 2013.
12.2. Informative References
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009.
[RFC1845] Crocker, D. and N. Freed, "SMTP Service Extension for
Checkpoint/Restart", RFC 1845, September 1995.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
[RFC4468] Newman, C., "Message Submission BURL Extension", RFC 4468,
May 2006.
[RFC5228] Guenther, P. and T. Showalter, "Sieve: An Email Filtering
Language", RFC 5228, January 2008.
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Appendix A. Background on Design Choices
This Section provides some background on design choices made during
development of the METADATA SMTP extension.
Use of a new command like BDAT makes it very easy to send chunks of
binary data. Byte counted blobs are easy to parse and generate.
Appendix B. Acknowledgements
The idea suggested in this document is not new. John Klensin and
Paul Smith have suggested use of an SMTP extension for separating
metadata from the rest of email messages. Thank you to Chris Newman
for providing comments and suggesting how to make the extension
easier to implement. This document was also inspired by the Dark
Mail project.
This document cuts & pastes lots of text from RFC 3030.
Author's Address
Alexey Melnikov
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
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