    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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