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SMTP target host selection in Mixed IPv4/IPv6 environments
draft-martin-smtp-target-host-selection-ipv4-ipv6-00

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Document Type This is an older version of an Internet-Draft whose latest revision is Expired
Authors Franck Martin , Alec Peterson
Last updated 2013-11-14
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draft-martin-smtp-target-host-selection-ipv4-ipv6-00
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 target host selection in Mixed IPv4/IPv6 environments
          draft-martin-smtp-target-host-selection-ipv4-ipv6-00

Abstract

   The Simple Mail Transfer Protocol (SMTP) is defined in [RFC5321].
   Section 5 of that document describes the process of host selection.
   While locating the target host in IPv4 is well defined, this process
   is unclear for sytems in IPv4 and IPv6 environemts.  This
   specification extends [RFC5321] to provide a standard way of
   selecting the target host in mixed environements.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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

Copyright Notice

   Copyright (c) 2013 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
     1.2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Locating the target host  . . . . . . . . . . . . . . . . . .   3
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .   4
     A.1.  Target list . . . . . . . . . . . . . . . . . . . . . . .   4
       A.1.1.  Example 1: dual stacked servers . . . . . . . . . . .   5
       A.1.2.  Example 2: single stacked servers . . . . . . . . . .   5
       A.1.3.  Example 3: single and dual stacked servers  . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Target host selection is discussed in [RFC5321] section 5 and some
   operational experience is described in [RFC3974].  However the
   process is unclear when selecting the target host in a mixed
   environment.  Several widely deployed open source MTA do not make
   this selection the same and sometimes do not find all the hosts.
   This specification addresses the issue of host selection in mixed
   environments.

1.1.  Requirements Language

   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 RFC 2119 [RFC2119].

1.2.  Rationale

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   [RFC5321] section 5.2 specifically discusses SMTP implementations
   taking into account 'local circumstances' when deciding which network
   addresses to use.  However, neither it nor [RFC3974] explore what
   'local circumstances' should be evaluated or how they should be
   evaluated.  The result has thus far been an inconsistent set of SMTP
   client implementations.  Providing a guideline for how SMTP client
   implementations should handle various dual-stack situations will
   improve the stability of the network while we operate in a dual-
   stack mode (which will likely be for an extended period of time).

2.  Locating the target host

   This part applies if one or more MX Resource Records (RR) [RFC1035]
   are found for a given name and if the sender-SMTP server is IPV4 and/
   or IPv6 capable.

   When a domain name associated with an MX RR is looked up and the
   associated data field obtained, the data field of that response MUST
   contain a domain name.  That domain name is queried for A RR if the
   sender-SMTP is IPv4 capable and is queried for AAAA RR if the sender-
   SMTP is IPv6 capable.  A list of IPv4 and IPv6 addresses of the SMTP
   servers to which the message should be directed is therefore built.

   When there are more than one MX RR associated to the given domain,
   the above process is done for each MX RR.  An ordered list of IPv4
   and IPv6 addresses is then established using the MX RR preference.
   Lower numbers are more preferred than higher ones.

   The sender-SMTP server first tries all the IP addresses (IPv4 and
   IPv6) with higher preference (lower number) before the IP addresses
   with lower preference (higher number).  The sender-SMTP server tries
   all the addresses up to a limit before deciding if the message is
   deliverable or not.  The sender-SMTP server tries at least two
   addresses in each IP familly it is enabled (2 IPv4 addresses if IPv4
   capable and 2 IPv6 addresses if IPv6 capable).

   For the same MX RR preference, when there is more than one IP address
   (IPv4/IPv6) to be tried, the sender-SMTP SHOULD maintain an internal
   sub preference list based on the rate of successful delivery of
   messages and based on the speed of such delivery.  If such internal
   sub preference list is not established or cannot be established then
   the sender-SMTP tries randomly any address within the same
   preference.  Using this method the sender-SMTP will not prefer IP
   addresses it cannot connect to, or IP addresses with more rejection
   or latency.  In some ways, this strategy is similar to the Happy
   Eveballs [RFC6555] strategy where web browsers initiate parralel
   connections to web servers using IPv4 and IPv6 and select the
   connection with better response.  It is however not similar, because

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   the sender-SMTP does not try to establish a connection to all the MX
   RR in parralel but remembers for a period of time which connection,
   for IP addresses with same MX RR preference, had better response.

3.  Acknowledgements

   Thanks to Murray Kucheraway for guidance in getting this draft out.

4.  IANA Considerations

   This section has no action requested of IANA.

   [RFC Editor: Please remove this section before publication.]

5.  Security Considerations

   Rogue sender-SMTP do not follow MX RR preferences, this existed over
   IPv4 and common defense can be used over IPv6

6.  References

6.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552, July
              2003.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

6.2.  Informative References

   [RFC3974]  Nakamura, M. and J. Hagino, "SMTP Operational Experience
              in Mixed IPv4/v6 Environments", RFC 3974, January 2005.

Appendix A.  Examples

A.1.  Target list

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A.1.1.  Example 1: dual stacked servers

   For example, a site with dual stacked servers may have the following
   DNS definitions:

      example.org.            IN MX   1  mx1.example.org.
                              IN MX   10 mx10.example.org.
      mx1.example.org.        IN AAAA 2001:db8:ffff::1
                              IN A    192.0.2.1
      mx10.example.org.       IN AAAA 2001:db8:ffff::2
                              IN A    192.0.2.2

   The ordered preference list will look initially:

      1  2001:db8:ffff::1
      1  192.0.2.1
      10 2001:db8:ffff::2
      10 192.0.2.2

A.1.2.  Example 2: single stacked servers

   For example, a site with single stacked servers may have the
   following DNS definitions:

      example.org.            IN MX   1  mx1-6.example.org.
                              IN MX   1  mx1.example.org
                              IN MX   10 mx10-6.example.org.
                              IN MX   10 mx10.example.org
      mx1-6.example.org.      IN AAAA 2001:db8:ffff::1
      mx1.example.org.        IN A    192.0.2.1
      mx10-6.example.org.     IN AAAA 2001:db8:ffff::2
      mx10.example.org        IN A    192.0.2.2

   The ordered preference list will look initially:

      1  2001:db8:ffff::1
      1  192.0.2.1
      10 2001:db8:ffff::2
      10 192.0.2.2

A.1.3.  Example 3: single and dual stacked servers

   For example, a site with a mix of single and dual stacked servers may
   have the following DNS definitions:

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      example.org.            IN MX   1  mx1.example.org.
                              IN MX   1  mx1-6.example.org
                              IN MX   1  mx2.example.org
                              IN MX   10 mx10.example.org.
      mx1.example.org.        IN A    192.0.2.1
      mx1-6.example.org.      IN AAAA 2001:db8:ffff::1
      mx2.example.org.        IN A    192.0.2.2
      mx10.example.org.       IN AAAA 2001:db8:ffff::2
      mx10.example.org        IN A    192.0.2.3

   The ordered preference list will look initially:

      1  192.0.2.1
      1  2001:db8:ffff::1
      1  192.0.2.2
      10 2001:db8:ffff::2
      10 192.0.2.3

Authors' Addresses

   Franck Martin (editor)
   LinkedIn
   Mountain View, CA
   US

   Email: fmartin@linkedin.com

   Alec Peterson (editor)
   Message Systems
   Columbia, MD
   US

   Email: alec@messagesystems.com

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