Internet DRAFT - draft-melnikov-smtp-priority-tunneling
draft-melnikov-smtp-priority-tunneling
Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Informational K. Carlberg
Expires: January 31, 2013 G11
July 30, 2012
Tunneling of SMTP Message Transfer Priorities
draft-melnikov-smtp-priority-tunneling-04
Abstract
This memo defines a mechanism for tunneling of SMTP (Simple Mail
Transfer Protocol) Message Transfer Priority values through MTAs
(Message Transfer Agents) that don't support the MT-PRIORITY SMTP
extension.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 31, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 4
3. Handling of messages received via SMTP . . . . . . . . . . . . 4
3.1. Handling of the MT-PRIORITY parameter by the receiving
SMTP server . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Relay of messages to other conforming SMTP/LMTP servers . 5
3.3. Relay of messages to non-conforming SMTP/LMTP servers . . 5
3.4. Mailing lists and Aliases . . . . . . . . . . . . . . . . 5
3.5. Gatewaying a message into a foreign environment . . . . . 5
3.6. Interaction with DSN SMTP Extension . . . . . . . . . . . 5
4. Header field: MT-Priority . . . . . . . . . . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7.1. Modification of MT-Priority header field and DKIM . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This document is an experimental extension to the SMTP Message
Transfer Priorities extension [SMTP-PRIORITY]. It specifies
application layer tunneling of message priority, to convey the
priority of the messages through Message Transfer Agents (MTAs) that
do not support the Message Transfer Priorities extension. The
tunneling is done by adding a new message header field to Internet
message format specified in [RFC5322].
A number of other header fields are already in use, mostly in Mail
User Agents (MUAs), to convey meanings related to importance or
priority of messages. Examples of such header fields are Importance
[RFC2156], Priority [RFC2156] and X-Priority (undocumented).
Considering sometimes subtle and sometimes significant differences in
the meaning of these header fields and widely different syntax, this
document defines a new header field.
This document is motivated by 2 main deployment scenarios: (1) Mail
User Agent (MUA) talking to a non MT-PRIORITY aware Message
Submission Server (MSA), and (2) use of unextended MUA to talk to a
MT-PRIORITY aware MSA. These 2 use cases are discussed in more
details below.
The use case (1) is about a MT-PRIORITY capable MUA talking to a non
MT-PRIORITY capable MSA [RFC6409], which in turn is talking to a MT-
PRIORITY capable MTA [RFC5321]. Both MSA and MTA are within the same
Administrative and Management Domain (ADMD) and are on a fast
network, however some recipients are accessible via the MTA which is
talking over a slow link to the next MTA. Communications over that
slow link can benefit from use of MT-PRIORITY SMTP extension.
In the use case (2) a widely deployed client (such as a desktop
client) is talking to MT-PRIORITY capable MSA. The client might be
extendable via a plugin API provided by the client developers,
however existing APIs frequently allow easy manipulation of email
header fields, while not allowing for addition of SMTP protocol
features. In such a case installing a plugin to the client that can
set MT-Priority header field could provide easier and earlier
deployment of MT-PRIORITY SMTP extension in an organization without
requiring changes to desktop clients.
We note that the above use cases are not exhaustive and that other
use cases, variations of the above, may exist. The purpose of this
document is not to consider every scenario, but rather examples that
reinforce the need to consider a tunneling mechanism that can deal
with SMTP capable devices that do not support [SMTP-PRIORITY].
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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.
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.
This document uses the term "priority" specifically in relation to
the internal treatment of a message by the server: messages with
higher priorities may be given expedited handling, and those with
lower priorities may be handled only as resources become available.
3. Handling of messages received via SMTP
The subsections of this section update the corresponding subsections
of Section 4 of [SMTP-PRIORITY].
3.1. Handling of the MT-PRIORITY parameter by the receiving SMTP server
This specification inserts the following between steps 4 and 5 in
Section 4.1 of [SMTP-PRIORITY]:
4a. If the sending SMTP client hasn't specified the MT-PRIORITY
parameter to the MAIL FROM command, but the message has a single
syntactically valid MT-Priority header field (see Section 4),
then the value of this header field is the message priority.
4b. In absence of both the MT-PRIORITY MAIL FROM parameter and the
MT-Priority header field, other message header fields, such as
Priority [RFC2156] and X-Priority, MAY be used for determining
the priority under this "Priority Message Handling" SMTP
extension. But note that the Importance [RFC2156] header field
MUST NOT be used for determining the priority under this
"Priority Message Handling" SMTP extension, as it has different
semantics: the Importance header field is aimed at the user
recipient and not at the nodes responsible for transferring the
message.
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3.2. Relay of messages to other conforming SMTP/LMTP servers
This specification inserts the following between steps 1 and 2 in
Section 4.2 of [SMTP-PRIORITY].
1a. Note that rule 1 also applies to messages which didn't have any
priority explicitly specified using the MT-PRIORITY MAIL FROM
parameter or the MT-Priority header field.
3.3. Relay of messages to non-conforming SMTP/LMTP servers
This specification appends the following after step 1 in Section 4.3
of [SMTP-PRIORITY]:
2. The relaying MTA MUST first remove any and all existing MT-
Priority header fields from the message. (Please see Section 7
for additional considerations related to removal of the MT-
Priority header field.)
3. If the incoming message had a MT-PRIORITY parameter specified in
the MAIL FROM command *or* there was an MT-Priority header field
removed in the above step 2, then the relaying MTA MUST add its
own MT-Priority header field with the value determined by the
procedure in Section 3.1. Syntax of the MT-Priority header field
is specified in Section 4.
3.4. Mailing lists and Aliases
This specification makes no changes to Section 4.4 of
[SMTP-PRIORITY].
3.5. Gatewaying a message into a foreign environment
This specification inserts the following between steps 1 and 2 in
Section 4.5 of [SMTP-PRIORITY].
1a. Note that if the destination environment doesn't support
transport of arbitrary header field, the requirement in
Section 3.3 to add an MT-Priority header field doesn't apply.
3.6. Interaction with DSN SMTP Extension
This specification makes no changes to Section 4.6 of
[SMTP-PRIORITY].
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4. Header field: MT-Priority
Applicable protocol: mail [RFC5322]
Status: standard
Author/change controller: Alexey Melnikov / IESG (iesg@ietf.org) on
behalf of the IETF
Specification document(s): [[anchor7: this document]]
The MT-Priority header field conveys message transfer priority when
relaying a message through MTAs which don't support the MT-PRIORITY
SMTP extension.
ABNF for this header field is defined as follows:
priority-header-field = "MT-Priority:"
[CFWS] priority-value [CFWS] CRLF
where "priority-value" is defined in [SMTP-PRIORITY].
Example:
MT-Priority: -3
Example:
MT-Priority: 4 (ultra)
5. Example
An SMTP transaction with 2 recipients. 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 MT-PRIORITY SMTP extension is
implemented.
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S: 220 example.net SMTP server here
C: EHLO example.com
S: 250-example.net
S: 250-DSN
S: 250-STARTTLS
S: 250 MT-PRIORITY STANAG4406
C: STARTTLS
[...TLS negotiation...]
C: MAIL FROM:<eljefe@example.com> ENVID=QQ314159
MT-PRIORITY=3
S: 250 <eljefe@example.com> sender ok
C: RCPT TO:<topbanana@example.net>
S: 250 <topbanana@example.net> recipient ok
C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
ORCPT=rfc822;Dana@Ivory.example.net
S: 250 <Dana@Ivory.example.net> recipient ok
C: DATA
S: 354 okay, send message
C: (message goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
Here the receiving SMTP server supports the "STANAG4406" Priority
Assignment Policy [SMTP-PRIORITY] with 6 priority levels, so it will
use the priority value 4 internally (the next supported priority
higher or equal to 3) and will communicate the priority value 3 when
relaying it to the next hop (if necessary). When relaying the
message to the next hop which doesn't support the MT-PRIORITY SMTP
extension the transaction might look like this:
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S: 220 example.org SMTP server here
C: EHLO example.net
S: 250-example.org
S: 250-DSN
S: 250-STARTTLS
S: 250 SIZE
C: STARTTLS
[...TLS negotiation...]
C: MAIL FROM:<eljefe@example.com> ENVID=QQ314159
S: 250 <eljefe@example.com> sender ok
C: RCPT TO:<topbanana@example.net>
S: 250 <topbanana@example.net> recipient ok
C: RCPT TO:<Dana@Ivory.example.net> NOTIFY=SUCCESS,FAILURE
ORCPT=rfc822;Dana@Ivory.example.net
S: 250 <Dana@Ivory.example.net> recipient ok
C: DATA
S: 354 okay, send message
C: MT-Priority: 3
C: (the rest of the message goes here)
C: .
S: 250 message accepted
C: QUIT
S: 221 goodbye
6. IANA Considerations
IANA is requested to add the following list of header field names to
the "Permanent Message Header Field Names" registry (in
http://www.iana.org/assignments/message-headers/perm-headers.html):
Header field: MT-Priority
Applicable protocol: mail
Status: standard
Author/change controller: Alexey Melnikov / IESG (iesg@ietf.org) on
behalf of the IETF
Specification document(s): [[anchor9: this document]]
7. Security Considerations
This document allows a message priority to be tunneled through MTAs
which don't support the MT-PRIORITY SMTP extension by specifying how
it can be represented in the message itself (using the MT-Priority
header field). Thus it is important to ensure that an MTA receiving
a message containing the MT-Priority header field can trust that it
was set by an authorized agent. Use of technologies such as DKIM
[RFC6376] or S/MIME to sign the MT-Priority header field value can
enable a recipient to verify whether the specified priority value was
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generated by a trusted agent. In particular DKIM signing allows a
recipient to verify that the specified priority value was present
when the message was signed, and to verify who signed the message.
But note that the DKIM signer might not be the same agent that
generated the MT-Priority header field.
Message Submission Agents ought to only accept message transfer
priorities (whether by using the MT-PRIORITY parameter to the MAIL
FROM command or the MT-Priority header field in the message itself)
from users (or only certain groups of such users) who are
authenticated and authorized in some way that's acceptable to the
MSA. As part of this policy, they can also restrict maximum priority
values that different groups of users can request, and can override
the priority values specified by MUAs. Such MSAs are required to
replace any MT-Priority header field values that don't satisfy this
policy, when relaying to non MT-PRIORITY capable SMTP/LMTP servers.
See Section 7.1 for more details on on what might be consequences of
such changes.
Similarly, MTAs ought to only accept message transfer priorities
(whether by using the MT-PRIORITY parameter to the MAIL FROM command
or the MT-Priority header field in the message itself) from senders
(or only certain groups of such senders) who are authenticated and
authorized in some way that's acceptable to the MTA. As part of this
policy, they can also restrict maximum priority values that different
groups of senders can request, and can override the priority values
specified by them. Such MTAs are required to replace any MT-Priority
header field values that don't satisfy this policy, when relaying to
non MT-PRIORITY capable SMTP/LMTP servers. See Section 7.1 for more
details on on what might be consequences of such changes.
In the absence of the policy enforcement mentioned above an SMTP
server (whether an MSA or an MTA) implementing the MT-PRIORITY SMTP
extension might be susceptible to a Denial of Service attack. For
example, malicious clients (MUAs/MSAs/MTAs) can try to abuse this
feature by always requesting Priority 9.
To protect MT-Priority header field from modification or insertion,
MUAs, MSAs and MTAs inserting it into messages SHOULD use message
header protection mechanism such as DKIM [RFC6376]. But see
Section 7.1.
7.1. Modification of MT-Priority header field and DKIM
A MSA/MTA that receives a message with an MT-Priority header field
protected by DKIM, that wants to change the message priority due to
its policy is forced to choose between
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a. breaking DKIM signatures (by replacing the MT-Priority header
value),
b. leaving the message as is (and using the MT-PRIORITY MAIL FROM
parameter), relying on the fact that all downstream MTAs are
compliant with this specification,
c. rejecting the message.
Each of these choices is not perfect and work in a particular
situation, so these choices should be carefully considered during
implementation and deployment.
If the MSA/MTA decides to alter the message, it SHOULD re-sign the
message with DKIM.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP)
Service Extension for Delivery Status Notifications
(DSNs)", RFC 3461, January 2003.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol",
RFC 5321, October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format",
RFC 5322, October 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for
Mail", STD 72, RFC 6409, November 2011.
[SMTP-PRIORITY] Melnikov, A. and K. Carlberg, "Simple Mail Transfer
Protocol extension for Message Transfer Priorities",
draft-melnikov-smtp-priority-21 (work in progress),
2012.
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8.2. Informative References
[RFC2156] Kille, S., "MIXER (Mime Internet X.400 Enhanced
Relay): Mapping between X.400 and RFC 822/MIME",
RFC 2156, January 1998.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP
over Transport Layer Security", RFC 3207,
February 2002.
[RFC6376] Crocker, D., Hansen, T., and M. Kucherawy,
"DomainKeys Identified Mail (DKIM) Signatures",
RFC 6376, September 2011.
Appendix A. Acknowledgements
This document copies lots of text from
draft-schmeing-smtp-priorities-04.txt and
draft-schmeing-smtp-priorities-05.txt. So the authors of this
document would like to acknowledge contributions made by the authors
of draft-schmeing-smtp-priorities: Michael Schmeing and Jan-Wilhelm
Brendecke.
Many thanks for input provided by Steve Kille, David Wilson, John
Klensin, Dave Crocker, Graeme Lunt, Alessandro Vesely, Barry Leiba,
Bill McQuillan, Murray Kucherawy, SM, Glenn Parsons, Pete Resnick,
Chris Newman, Ned Freed, Claudio Allocchio, Martin Thomson, Joseph
Yee.
Special thanks to Barry Leiba for agreeing to shepherd this document.
Authors' Addresses
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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Ken Carlberg
G11
1601 Clarendon Blvd, #203
Arlington, VA 22209
USA
EMail: carlberg@g11.org.uk
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