Simple Mail Transfer Protocol Extension for Message Transfer Priorities
RFC 6710

Internet Engineering Task Force (IETF)                       A. Melnikov
Request for Comments: 6710                                     Isode Ltd
Category: Standards Track                                    K. Carlberg
ISSN: 2070-1721                                                      G11
                                                             August 2012

Simple Mail Transfer Protocol Extension for Message Transfer Priorities

Abstract

   This memo defines an extension to the SMTP (Simple Mail Transfer
   Protocol) service whereby messages are given a label to indicate
   preferential handling, to enable mail handling nodes to take this
   information into account for onward processing.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6710.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Melnikov & Carlberg          Standards Track                    [Page 1]
RFC 6710        Message Transfer Priority SMTP Extension     August 2012

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Definition of the Priority SMTP Extension  . . . . . . . . . .  4
   4.  Handling of Messages Received via SMTP . . . . . . . . . . . .  5
     4.1.  Handling of the MT-PRIORITY Parameter by the Receiving
           SMTP Server  . . . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Relay of Messages to Other Conforming SMTP/LMTP Servers  .  6
     4.3.  Relay of Messages to Non-Conforming SMTP/LMTP Servers  . .  7
     4.4.  Mailing Lists and Aliases  . . . . . . . . . . . . . . . .  7
     4.5.  Gatewaying a Message into a Foreign Environment  . . . . .  7
     4.6.  Interaction with the DSN SMTP Extension  . . . . . . . . .  7
   5.  The Priority Service Extension . . . . . . . . . . . . . . . .  8
     5.1.  Expedited Transfer . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Timely Delivery  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Use of MT-PRIORITY with LMTP . . . . . . . . . . . . . . . . . 10
   7.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 14
     9.1.  Multiple MX Records  . . . . . . . . . . . . . . . . . . . 14
     9.2.  Priority Assignment Policies . . . . . . . . . . . . . . . 15
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Requirements on Priority Assignment Policy
           Registrations  . . . . . . . . . . . . . . . . . . . . . . 17
     10.2. Initial Priority Assignment Policy Registrations . . . . . 18
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
     12.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Priority Assignment Policy for Military Messaging . . 22
   Appendix B.  Priority Assignment Policy for MIXER  . . . . . . . . 23
   Appendix C.  Priority Assignment Policy for National Security
                / Emergency Preparedness (NS/EP)  . . . . . . . . . . 24
   Appendix D.  Possible Implementation Strategies  . . . . . . . . . 25
     D.1.  Probability  . . . . . . . . . . . . . . . . . . . . . . . 25
     D.2.  Preemption of Sessions or Transactions . . . . . . . . . . 25
     D.3.  Resource Allocation Models . . . . . . . . . . . . . . . . 26
   Appendix E.  Background on Design Choices  . . . . . . . . . . . . 26
   Appendix F.  Acknowledgements  . . . . . . . . . . . . . . . . . . 27