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SMTP Operational Experience in Mixed IPv4/v6 Environments
RFC 3974

Document type: RFC - Informational (January 2005; No errata)
Document stream: ISE
Last updated: 2013-03-02
Other versions: plain text, pdf, html

ISE State: (None)
Document shepherd: No shepherd assigned

IESG State: RFC 3974 (Informational)
Responsible AD: Bert Wijnen
Send notices to: <itojun@itojun.org>, <motonori@media.kyoto-u.ac.jp>, <bwijnen@lucent.com>

Network Working Group                                        M. Nakamura
Request for Comments: 3974                              Kyoto University
Category: Informational                                        J. Hagino
                                                 IIJ Research Laboratory
                                                            January 2005

       SMTP Operational Experience in Mixed IPv4/v6 Environments

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

IESG Note:

   The content of this RFC was at one time considered by the IETF, and
   therefore it may resemble a current IETF work in progress or a
   published IETF work.  This RFC is not a candidate for any level of
   Internet Standard.  The IETF disclaims any knowledge of the fitness
   of this RFC for any purpose, and in particular notes that the
   decision to publish is not based on IETF review for such things as
   security, congestion control, or inappropriate interaction with
   deployed protocols.  The RFC Editor has chosen to publish this
   document at its discretion.  Readers of this RFC should exercise
   caution in evaluating its value for implementation and deployment.

   This document contains a specific interpretation of the applicability
   of the MX processing algorithm in RFC 2821, Section 5, to dual-stack
   environments.  Implementors are cautioned that they must reference
   RFC 2821 for the full algorithm; this document is not to be
   considered a full restatement of RFC 2821, and, in case of ambiguity,
   RFC 2821 is authoritative.

Abstract

   This document discusses SMTP operational experiences in IPv4/v6 dual
   stack environments.  As IPv6-capable SMTP servers are deployed, it
   has become apparent that certain configurations of MX records are
   necessary for stable dual-stack (IPv4 and IPv6) SMTP operation.  This
   document clarifies the existing problems in the transition period
   between IPv4 SMTP and IPv6 SMTP.  It also defines operational
   requirements for stable IPv4/v6 SMTP operation.

Nakamura & Hagino            Informational                      [Page 1]
RFC 3974            SMTP in Dual Stack Environments         January 2005

   This document does not define any new protocol.

1.  Introduction

   Delivery of mail messages to the final mail drop is not always done
   by direct IP communication between the submitter and final receiver,
   and there may be some intermediate hosts that relay the messages.  So
   it is difficult to know at message submission (also at receiver side)
   that all intermediate relay hosts are properly configured.  It is not
   easy to configure all systems consistently since the DNS
   configuration used by mail message delivery systems is more complex
   than other Internet services.  During the transition period from IPv4
   to IPv6, more care should be applied to IPv4/v6 interoperability.

   This document talks about SMTP operational experiences in IPv4/v6
   dual stack environments.  As IPv6-capable SMTP servers are deployed,
   it has become apparent that certain configurations of MX records are
   necessary for stable dual-stack (IPv4 and IPv6) SMTP operation.

   This document does not discuss the problems encountered when the
   sending MTA and the receiving MTA have no common protocol (e.g., the
   sending MTA is IPv4-only while the receiving MTA is IPv6-only).  Such
   a situation can be resolved by making either side dual-stack or by
   making either side use a protocol translator (see Appendix A on
   issues with protocol translator).

2.  Basic DNS Resource Record Definitions for Mail Routing

   Mail messages on the Internet are typically delivered based on the
   Domain Name System [Mockapetris].  MX RRs are looked up in DNS to
   retrieve the names of hosts running MTAs associated with the domain
   part of the mail address.  DNS lookup uses IN class for both IPv4 and
   IPv6, and similarly IN MX records will be used for mail routing for
   both IPv4 and IPv6.  Hosts which have IPv6 connectivity and also want
   to have the mails delivered using IPv6 must define IPv6 addresses for
   the host name as well as IPv4 addresses [Thomson].

   An MX RR has two parameters, a preference value and the name of
   destination host.  The name of the destination host will be used to
   look up an IP address to initiate an SMTP connection [Partridge].

Nakamura & Hagino            Informational                      [Page 2]
RFC 3974            SMTP in Dual Stack Environments         January 2005

   For example, an IPv6-only site may have the following DNS

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