Network Working Group C. Hutzler
Internet-Draft AOL
Expires: September 6, 2005 D. Crocker
Brandenburg Internetworking
P. Resnick
QUALCOMM Incorporated
R. Sanders
Earthlink, Inc.
E. Allman
Sendmail, Inc.
March 26, 2004
Updated September 15, 2004
Updated March 7, 2005
Email Submission Between Independent Networks
draft-hutzler-spamops-03
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
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."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 6, 2005
Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
By submitting this Internet-Draft, any applicable patent or other IPR
claims of
which I am aware have been disclosed in accordance with RFC 3668
Full Copyright Statement
Copyright (C) The Internet Society (2005). All Rights Reserved. This
document is
subject to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights."
"This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
Abstract
Email has become a popular distribution service for a variety of
socially unacceptable, mass-effect purposes. The most obvious ones
include spam and worms. This note recommends conventions for the
operation of email submission and transport services between
independent operators, such as enterprises and Internet Service
Hutzler, et al. Expires September 6, 2005 [Page 1]
Internet-Draft Email Port Access March 2005
Providers. Its goal is to improve lines of accountability for
controlling abusive uses of the Internet mail service. Consequently
the document offers recommendations for constructive operational
policies between independent operators of email transmission
services.
The document will seek BCP status. Comments and discussion of this
document should be addressed to the ietf-smtp@imc.org mailing list
or the Ant-Spam Research Group (ASRG).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Submission, Relaying, Delivery . . . . . . . . . . . . . . . . 5
4. External Submission . . . . . . . . . . . . . . . . . . . . . 6
5. Message Submission Authentication Technologies . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 11
Informative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 14
Hutzler, et al. Expires September 6, 2005
[Page 2]
Internet-Draft Email Port Access March 2005
1. Introduction
The very characteristics that make email such a convenient
communications medium - its near ubiquity, rapid delivery and low
cost - have made it a fertile ground for the distribution of unwanted
or malicious content. Spam, fraud and worms have become a serious
problem, threatening the viability of email and costing end users and
providers millions of dollars in damages and lost productivity. In
recent years, independent operators including enterprises and ISPs
have turned to a number of different technologies and processes, in
an attempt to combat these problems, with varying effect and with
vastly different impacts on users and on the Internet mail
infrastructure.
Email will often travel between multiple independent providers of
email transmission services, en route to its final destination. They
will generally have no prior arrangement with one another and may
employ different rules on the transmission. It is therefore difficult
both to debug problems that occur in mail transmission and to assign
accountability if undesired or malicious mail is injected into the
Internet mail infrastructure.
This document suggests operational policies that independent
operators of email transmission services may adopt, to assist in
providing continued, smooth operation of Internet email, but with
controls in place to improve accountability. These policies are
appropriate for operators of all sizes and may be implemented by
operators independently, without regard for whether the other side of
an email exchange has implemented them.
It is important to note that the adoption of these policies alone
will not solve the problems of spam and other undesirable email.
However they provide a useful step in clarifying lines of
accountability and interoperability between operators. This will help
raise the bar for abusers, and will pave the way for additional tools
to preserve the utility of the Internet email infrastructure.
Hutzler, et al. Expires September 6, 2005 [Page 3]
Internet-Draft Email Port Access March 2005
2. Terminology
The Internet email architecture distinguishes four message- handling
components:
o Mail User Agents (MUAs)
o Mail Submission Agents (MSAs)
o Mail Transfer Agents (MTAs)
o Mail Delivery Agents (MDAs)
At the origination end, an MUA works on behalf of end-users to create
a message and performs initial "submission" into the transmission
infrastructure, via an MSA. An MSA accepts the message submission,
performs any necessary pre- processing on the message and relays the
message to an MTA for transmission. MTAs relay messages to other
MTAs, in a sequence reaching a destination MDA that, in turn,
delivers the email to the recipient's inbox. The inbox is part of the
recipient-side MUA that works on behalf of the end-user to process
received mail.
These architectural components are often compressed, such as having
the same software do MSA, MTA and MDA functions. However the
requirements for each of these components of the architecture are
becoming more extensive, so that their separation is increasingly
common.
Note: 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 [3].
Hutzler, et al. Expires September 6, 2005 [Page 4]
Internet-Draft Email Port Access March 2005
3. Submission, Relaying, Delivery
The MSA, MTA and MDA functions used to be considered as the same set
of functions. This has been reflected in the history of Internet
mail by having MSA, MTA and MDA transfers all be performed with SMTP
[2], over TCP Port 25. Internet mail permits email to be exchanged
with no prior arrangement. Hence Port 25 exchanges occur without
sender authentication. That is, the sender is not necessarily known
by the relaying MTAs or the MDA.
It is important to distinguish MUA-to-MSA email submission, versus
MTA relaying, versus the final MTA-to-MDA transmission, prior to
MDA-to-MUA delivery. Submission typically does entail a relationship
between client and server; equally, the MDA can determine that it
will be effecting final delivery and has an existing relationship
with the recipient. That is, MSAs and MDAs can take advantage of
having prior relationships with users, in order to constrain their
transfer activities.
Specifically, an MSA can choose to reject all postings from MUAs for
which it has no existing relationship. Similarly, an MDA can choose
to reject all mail to recipients for which that MDA has no
arrangement to perform delivery. Indeed, both of these policies are
already in common practice.
Best practices are:
o Operators of MSAs MUST perform authentication during mail
submission, based on an existing relationship with the submitting
entity. This requirement applies to all mail submission
mechanisms.
o For email being received from outside their local operational
environment, email service operators MUST distinguish between mail
that will be delivered inside that environment, from mail that is
to be relayed back out to the Internet. This prevents the problem
embodied by "open" relays.
o Mail coming from outside an email operator's local environment,
and having a RCPT-TO address that resolves to a destination
outside the local environment, MUST be treated as mail submission,
rather than mail relaying.
o MDAs SHALL NOT accept mail to recipients for which that MDA has no
arrangement to perform
delivery.
Hutzler, et al. Expires September 6, 2005 [Page 5]
Internet-Draft Email Port Access March 2005
4. External Submission
An MUA, such as one desiring enforced privacy, may need to submit
mail across the Internet, rather than to a local MSA. This
requirement creates a challenge for the provider operating the
network that hosts the MUA. It makes that provider an involuntary
recruit to the task of solving mass- effect email problems. When the
MUA participates in a problem that affects large numbers of Internet
users, the operator is expected to effect remedies and is often
expected to prevent such occurrences.
A proactive technique used by some providers is to block all outbound
Port 25 SMTP traffic or to force this traffic through a local SMTP
proxy, except for hosts that are explicitly authorized. This can be
problematic for some users, notably legitimate mobile users
attempting use their "home" MSA, even though those users might
already employ legitimate, Port 25-based authentication.
This document offers no recommendation concerning the blocking of
SMTP Port 25.
Rather, it pursues the constructive benefit of using the official
SUBMISSION Port 587 [1].
Best practices are:
o MSAs MUST support the SUBMISSION port, for MUAs accessing from
outside the MSA's local environment.
o MSAs MUST perform authentication during all mail transactions on
the SUBMISSION port, even for a message having a RCPT TO address
that would not cause the message to be relayed.
o Access Providers SHALL NOT block users from accessing the external
Internet using the SUBMISSION port.
o MUAs SHOULD use the SUBMISSION port for message submission.
Note that the requirement for authentication, on the part of the MSA,
thereby makes that MSA responsible for the ensuing traffic it
generates.
Hutzler, et al. Expires September 6, 2005 [Page 6]
Internet-Draft Email Port Access March 2005
Figure 1 depicts examples of a local user (MUA.l) submitting a
message to an MSA (MSA). It also shows a remote user (MUA.r), such as
might be in a coffee shop offering "hotspot" wireless access,
submitting a message to their "home" MSA via an Authenticated Port
587 transaction.
"Home" Network smtp /--------\ Destination
+-------+ +-----+ port 25 | | +----------+
| MUA.l | -> | MSA | ------> | | -> | MDA |
+-------+ 25 +-----+ | INTERNET | 25 +----------+
or ^ | |
587 \--------<---|---\ |
\---\----/
^ SUBMISSION
| Port 587
+--------+
| MUA.r |
+--------+
"HotSpot"
Figure 1: Example of Port 587 Usage
Hutzler, et al. Expires September 6, 2005 [Page 7]
Internet-Draft Email Port Access March 2005
5. Message Submission Authentication Technologies
There are many different technologies available to authenticate
message submission transactions. These range from simple mechanisms
like POP authorization before SMTP and IP Address access lists all
the way to SMTP AUTH [4] and client side certificates using Transport
Layer Security (TLS) [5]. Depending on the environment, each of these
mechanisms can be more or less effective and convenient.
Organizations are encouraged to choose the most secure approach that
is practical.
For example, SMTP AUTH [4] using a secure authentication method like
CRAM-MD5 or DIGEST-MD5 may be sufficient. However, in some
environments, it is impractical to use one of the secure methods,
meaning that SMTP AUTH [4] would be transmitting the username and the
password in clear text over insecure networks. This could allow
attackers to listen for this traffic and steal account data. In these
cases, using STARTTLS [5] to establish an encrypted channel for
transmission of the SMTP AUTH [4] username and password would be
preferred. Similarly, STARTTLS [5] with client side certificates
could be used with the SMTP AUTH [4] EXTERNAL mechanism to achieve
secure authentication.
Hutzler, et al. Expires September 6, 2005 [Page 8]
Internet-Draft Email Port Access March 2005
6. Security Considerations
Email transfer between independent administrations can be the source
of large volumes of unwanted email and email containing malicious
content designed to attack the recipient's system. This document
addresses the requirements to permit such exchanges while reducing
the likelihood that malicious mail will be transmitted.
Hutzler, et al. Expires September 6, 2005 [Page 9]
Internet-Draft Email Port Access March 2005
Appendix A. Acknowledgments
These recommendations were first formulated during informal
discussions among members of Anti-Spam Technical Alliance (ASTA) and
some participants from the Internet Research Task Force's Anti-Spam
Research Group (ASRG).
Hutzler, et al. Expires September 6, 2005 [Page 10]
Internet-Draft Email Port Access March 2005
Normative References
[1] Gellens, R. and J. Klensin, "Message Submission", RFC 2476,
December 1998.
[2] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
Hutzler, et al. Expires September 6, 2005 [Page 11]
Internet-Draft Email Port Access March 2005
Informative References
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Myers, J., "SMTP Service Extension for Authentication", RFC
2554, March 1999.
[5] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002.
Authors' Addresses
Carl Hutzler
America Online
12100 Sunrise Valley Drive
Reston, VA
20191
Phone: +1 703 265 5521
EMail: cdhutzler@aol.com
URI:
Dave Crocker
Brandenburg Internetworking
675 Spruce Drive
Sunnyvale, CA 94086
US
Phone: +1 408 246 8253
EMail: dcrocker@brandenburg.com
URI: http://www.brandenburg.com/
Peter W. Resnick
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121-1714
US
Phone: +1 858 651 4478
EMail: presnick@qualcomm.com
URI: http://www.qualcomm.com/~presnick/
Hutzler, et al. Expires September 6, 2005 [Page 12]
Internet-Draft Email Port Access March 2005
Robert Sanders
Earthlink, Inc.
1375 Peachtree Street
Atlanta, GA 30309
US
Phone: +1 404 748 7021
EMail: sandersr@corp.earthlink.net
URI: http://home.mindspring.com/~rsanders/
Eric Allman
Sendmail, Inc.
6425 Christie Avenue, Suite 400
Emeryville, CA 94608
US
Phone: +1 510 594 5501
EMail: eric@sendmail.com
Hutzler, et al. Expires September 6, 2005 [Page 13]
Internet-Draft Email Port Access March 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved. This
document is
subject to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights."
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Hutzler, et al. Expires September 6, 2005 [Page 14]
Internet-Draft Email Port Access March 2005
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hutzler, et al. Expires September 6, 2005 [Page 15]