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RFC 6758
Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 6758 Isode Ltd
Category: Informational K. Carlberg
ISSN: 2070-1721 G11
October 2012
Tunneling of SMTP Message Transfer Priorities
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 document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6758.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
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 ....4
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 the DSN SMTP Extension ....................5
4. Header Field: MT-Priority .......................................5
5. Example .........................................................6
6. IANA Considerations .............................................7
7. Security Considerations .........................................7
7.1. Modification of the MT-Priority Header Field and DKIM ......9
8. References ......................................................9
8.1. Normative References .......................................9
8.2. Informative References ....................................10
Appendix A. Acknowledgements ......................................11
1. Introduction
The SMTP Message Transfer Priorities extension [RFC6710] specifies a
mechanism to allow messages to be given a label to indicate
preferential handling, to enable mail handling nodes to take this
into account for onward processing. However, as with all SMTP
extensions, all SMTP Message Transfer Agents (MTAs) between the
source and the destination must support the extension in order for it
to be successfully used. This document describes an application-
layer tunneling of message priority, to convey the priority of the
messages through MTAs that do not support the Message Transfer
Priorities extension. The tunneling is done by adding a new message
header field to the Internet Message Format specified in [RFC5322].
A number of other header fields are already in use, mostly in Message
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) an MUA
talking to a non-MT-PRIORITY-aware Message Submission Agent (MSA),
and (2) the use of an unextended MUA to talk to an MT-PRIORITY-aware
MSA. These 2 use cases are discussed in more detail below.
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Use case (1) is about an MT-PRIORITY-capable MUA talking to a
non-MT-PRIORITY-capable MSA [RFC6409], which in turn is talking to an
MT-PRIORITY-capable MTA [RFC5321]. Both the MSA and MTA are within
the same ADministrative Management Domain (ADMD) and are on a fast
network; however, some recipients are accessible via the MTA that is
talking over a slow link to the next MTA. Communications over that
slow link can benefit from the use of the MT-PRIORITY SMTP extension.
In use case (2), a widely deployed client (such as a desktop client)
is talking to an MT-PRIORITY-capable MSA. The client might be
extendable via a plug-in 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 plug-in on the client that
can set the MT-Priority header field could provide easier and earlier
deployment of the 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 [RFC6710].
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 uses 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.
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3. Handling of Messages Received via SMTP
The subsections of this section update the corresponding subsections
of Section 4 of [RFC6710].
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 [RFC6710]:
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 the 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. Note, however, 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.
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 [RFC6710].
1a. Note that rule 1 also applies to messages that didn't have any
priority explicitly specified using the MT-PRIORITY MAIL FROM
parameter or the MT-Priority header field.
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3.3. Relay of Messages to Non-Conforming SMTP/LMTP Servers
This specification appends the following after step 1 in Section 4.3
of [RFC6710]:
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 an MT-PRIORITY parameter specified in
the MAIL FROM command *or* there was an MT-Priority header field
removed in step 2 above, then the relaying MTA MUST add its own
MT-Priority header field with the value determined by the
procedure in Section 3.1. The 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 [RFC6710].
3.5. Gatewaying a Message into a Foreign Environment
This specification inserts the following between steps 1 and 2 in
Section 4.5 of [RFC6710].
1a. Note that if the destination environment doesn't support the
transport of an arbitrary header field, the requirement in
Section 3.3 to add an MT-Priority header field doesn't apply.
3.6. Interaction with the DSN SMTP Extension
This specification makes no changes to Section 4.6 of [RFC6710].
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): RFC 6758
The MT-Priority header field conveys message transfer priority when
relaying a message through MTAs that don't support the MT-PRIORITY
SMTP extension.
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The 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 [RFC6710].
Example:
MT-Priority: -3
Example:
MT-Priority: 4 (ultra)
5. Example
Note that the following example of an SMTP transaction with 2
recipients is also making use of the STARTTLS [RFC3207] and Delivery
Status Notification (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.
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 [RFC6710] with 6 priority levels, so it will use
the priority value 4 internally (the next supported priority higher
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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 that doesn't support the MT-PRIORITY SMTP
extension, the transaction might look like this:
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 has added 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): RFC 6758
7. Security Considerations
This document allows a message priority to be tunneled through MTAs
that 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
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a message containing the MT-Priority header field can trust that it
was set by an authorized agent. The use of technologies such as
DomainKeys Identified Mail (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 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. Note, however, that the DKIM
signer might not be the same agent that generated the MT-Priority
header field.
MSAs 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. When relaying to non-MT-PRIORITY-capable SMTP/LMTP (Local Mail
Transfer Protocol) servers, such MSAs are required to replace any
MT-Priority header field values that don't satisfy this policy. See
Section 7.1 for more details on what the consequences of such changes
might be.
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. When relaying to non-MT-PRIORITY-capable SMTP/
LMTP servers, such MTAs are required to replace any MT-Priority
header field values that don't satisfy this policy. See Section 7.1
for more details on what the consequences of such changes might be.
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 the MT-Priority header field from modification or
insertion, MUAs, MSAs, and MTAs inserting it into messages SHOULD use
a message header protection mechanism such as DKIM [RFC6376];
however, see Section 7.1 for more information.
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7.1. Modification of the MT-Priority Header Field and DKIM
An MSA/MTA that receives a message with an MT-Priority header field
protected by DKIM and that wants to change the message priority due
to its policy is forced to choose between
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, and
c. rejecting the message.
None of these choices are perfect. They 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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, November 2011.
[RFC6710] Melnikov, A. and K. Carlberg, "Simple Mail Transfer
Protocol Extension for Message Transfer Priorities",
RFC 6710, August 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.
[SMTP-PRI-OLD]
Schmeing, M., Brendecke, J., and K. Carlberg, "SMTP
Service Extension for Priority Message Handling", Work
in Progress, August 2006.
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Appendix A. Acknowledgements
This document copies lots of text from "SMTP Service Extension for
Priority Message Handling" [SMTP-PRI-OLD]. Therefore, the authors of
this document would like to acknowledge contributions made by the
authors of that document: 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, and
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
Ken Carlberg
G11
1601 Clarendon Blvd, #203
Arlington, VA 22209
USA
EMail: carlberg@g11.org.uk
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