Internet DRAFT - draft-martin-smtp-target-host-selection-ipv4-ipv6
draft-martin-smtp-target-host-selection-ipv4-ipv6
Network Working Group F. Martin, Ed.
Internet-Draft LinkedIn
Updates: 5321 (if approved) A. Peterson, Ed.
Intended status: Standards Track Message Systems
Expires: June 08, 2014 December 05, 2013
SMTP target host selection in Mixed IPv4/IPv6 environments
draft-martin-smtp-target-host-selection-ipv4-ipv6-01
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 June 08, 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
Martin & Peterson Expires June 08, 2014 [Page 1]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
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 . . . . . . . . . . . . . . . . . 5
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 5
A.1. Target list . . . . . . . . . . . . . . . . . . . . . . . 5
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 . . . . . 6
A.1.4. Example 4: SMTP-Sender limiting the number of IP
tried per MX . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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
[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
Martin & Peterson Expires June 08, 2014 [Page 2]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
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.
For the same MX RR preference, when there is more than one IP address
(IPv4/IPv6) to be tried, the sender-SMTP SHOULD sort this list either
following the application configuration, recognition of an easily
reached or well performing address, or the operating system's address
selection [RFC6724] or when there is no clear reason to favor one,
then the sender-SMTP MUST randomize them to spread the load across
multiple mail exchangers for a specific domain name.
Most sender-SMTP limit the number of IP addresses that will be tried
for each MX record. For the same MX RR preference, the sender-SMTP
SHOULD ensure that at least two IP addresses of the other address
family are tried before that limit.
If the sender-SMTP orders the IP list for the same MX RR preference
based on some reacheability criteria, it will not prefer IP addresses
for instance 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 better connection. It is however not similar, because the
sender-SMTP MUST 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.
Network paths and servers are constantly breaking and being repaired.
Thus, implementations MUST occasionally age out their IPv6 or IPv4
Martin & Peterson Expires June 08, 2014 [Page 3]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
preferences. It is RECOMMENDED that this information be aged out
several times before the maximum retry time of a message while also
following the DNS TTL [RFC1035] of the MX RR.
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.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012.
Martin & Peterson Expires June 08, 2014 [Page 4]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
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
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:
Martin & Peterson Expires June 08, 2014 [Page 5]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
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:
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
A.1.4. Example 4: SMTP-Sender limiting the number of IP tried per MX
For example, a SMTP-server that prefers IPv6 over IPv4 as per
[RFC6724] default address preference is trying to send a message to
example.org with the following configuration:
Martin & Peterson Expires June 08, 2014 [Page 6]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
example.org. IN MX 10 mail1.example.org.
example.org. IN MX 20 mail2.example.org.
mail1.example.org. IN A 192.0.2.1
mail1.example.org. IN A 192.0.2.2
mail1.example.org. IN A 192.0.2.3
mail1.example.org. IN A 192.0.2.4
mail1.example.org. IN A 192.0.2.5
mail1.example.org. IN A 192.0.2.6
mail1.example.org. IN AAAA 2001:db8::1
mail1.example.org. IN AAAA 2001:db8::2
mail1.example.org. IN AAAA 2001:db8::3
mail1.example.org. IN AAAA 2001:db8::4
mail1.example.org. IN AAAA 2001:db8::5
mail1.example.org. IN AAAA 2001:db8::6
mail2.example.org. IN A 192.0.2.100
mail2.example.org. IN AAAA 2001:db8::100
The sender-SMTP would prefer mail1 (preference 10) and then mail2
(preference 20). An SMTP-server implementation that does not
implement this specification and has a 6 address limit per MX record
and [RFC6724] default address preference, would attempt to connect to
the 6 IPv6 addresses belonging to mail1. If the IPv6 path is broken,
these connection attempts would all fail (after a timeout), as would
the connection attempt to mail2's IPv6 address. Finally, after those
connection timeouts, the IPv4 address for mail2 would succeed. In
contrast, an SMTP-server which is following the normative procedure
above would succeed in connecting to mail1 if the IPv4 path is
working but the IPv6 path is broken. This SMTP-server implementation
would attempt to connect to the four IPv6 addresses belonging to
mail1 and then to the IPv4 addresses belonging to mail2 -- resulting
in mail delivery to mail1 (which is preferable for the receiving
domain) and faster mail delivery. Following the procedure above with
a 6 address limit per MX record and [RFC6724] default address
preference, the connections attempted would be:
2001:db8::1 ; mail1, connection attempt 1
2001:db8::2 ; mail1, connection attempt 2
2001:db8::3 ; mail1, connection attempt 3
2001:db8::4 ; mail1, connection attempt 4
192.0.2.1 ; mail1, connection attempt 5 (IPv4)
192.0.2.2 ; mail1, connection attempt 6 (IPv4)
; the other addresses for mail1 are not used by this particular
; sender-SMTP because of its 6 address limit per MX record.
2001:db8::100 ; mail2, connection attempt 1 (IPv6)
192.0.2.100 ; mail2, connection attempt 2 (IPv4)
Martin & Peterson Expires June 08, 2014 [Page 7]
Internet-Draft SMTP host selection with IPv4/v6 December 2013
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
Martin & Peterson Expires June 08, 2014 [Page 8]
