SMTP IPv6 to IPv4 Fallback: An Applicability Statement
draft-martin-smtp-ipv6-to-ipv4-fallback-00

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Network Working Group                                     F. Martin, Ed.
Internet-Draft                                                  LinkedIn
Updates: 5321 (if approved)                             A. Peterson, Ed.
Intended status: Standards Track                         Message Systems
Expires: May 18, 2014                                  November 14, 2013

         SMTP IPv6 to IPv4 Fallback: An Applicability Statement
               draft-martin-smtp-ipv6-to-ipv4-fallback-00

Abstract

   This Applicability Statement describes how Mail Transfer Agents
   (MTAs) can be encouraged to fall back to IPv4 when a message is
   refused over IPv6.

Status of This Memo

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   This Internet-Draft will expire on May 18, 2014.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
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Martin & Peterson         Expires May 18, 2014                  [Page 1]
Internet-Draft         SMTP IPv6 to IPv4 Fallback          November 2013

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Moving from IP Reputation to Domain Based Reputation  . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Indicating the sender-SMTP server to fall back to IPv4  . . .   3
     3.1.  With the service extension IPV6-IPV4-FALLBACK . . . . . .   3
     3.2.  Without service extension . . . . . . . . . . . . . . . .   4
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  SMTP Enhanced Status Codes Registry update  . . . . . . .   5
     5.2.  Simple Mail Transfer Protocol (SMTP) Service Extensions
           Registry update . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Examples and research  . . . . . . . . . . . . . . .   7
     A.1.  SMTP fall back from IPv6 to IPv4 without service
           extension . . . . . . . . . . . . . . . . . . . . . . . .   7
     A.2.  SMTP fall back from IPv6 to IPv4 with service extension .   8
     A.3.  Common Open Source MTA Design . . . . . . . . . . . . . .   9
       A.3.1.  MX RR configuration . . . . . . . . . . . . . . . . .  10
       A.3.2.  Rejecting Messages for Immediate Retry on IPv4  . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The Simple Mail Transfer Protocol (SMTP) is defined in [RFC5321].
   Section 5 of that document describes the process of host selection.
   SMTP clients in well known Mail Transfer Agent (MTA) software will
   retry a message using a different Mail Exchanger (MX) or network
   address if the message is temporarily rejected.  This document
   describes under which circumstances well known and widely deployed
   open source MTAs (and others) can be made to retry over IPv4 when an
   initial connection to IPv6 results in a temporary rejection.
   Furthermore, the purpose of this document is to record that behaviour
   and encourage its further adoption while at the same time providing
   for the future a well defined mechanism via a service extension with
   cooperating SMTP clients.  This behavior could be useful in, for
   instance, enforcing higher requirements for Simple Mail Transfer
   Protocol (SMTP) sessions over IPv6 than what exists on IPv4 without
   simply rejecting the message outright.

1.1.  Moving from IP Reputation to Domain Based Reputation

   IPv6 brings more IP addresses, which means building an IP-based
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