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Versions: 00 01 02 03 04                                   Informational
SPFBIS Working Group                                        M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Informational                         February 21, 2012
Expires: August 24, 2012

                      The SPF/Sender-ID Experiment


   In 2006 the IETF published a suite of protocol documents comprising
   SPF and Sender-ID, two proposed email authentication protocols.
   Because of interoperability concerns created by simultaneous use of
   the two protocols by a receiver, and some concerns with Sender-ID and
   compatibility with existing standards, the IESG required them to have
   Experimental status and invited the community to observe their
   deployments for a period of time, hoping convergence would be
   possible later.

   After six years, sufficient experience and evidence have been
   collected that the experiment thus created can be considered
   concluded, and a common path forward can be selected.  This memo
   presents those findings.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 August 24, 2012.

Copyright Notice

   Copyright (c) 2012 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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  The Need For Consensus  . . . . . . . . . . . . . . . . . . . . 3
   3.  Evidence of Deployment  . . . . . . . . . . . . . . . . . . . . 4
   4.  Evidence of Differences . . . . . . . . . . . . . . . . . . . . 4
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Informative References  . . . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   In April, 2006, the IETF published the [SPF] and [SUBMITTER]/
   [SENDER-ID]/[PRA] email authentication protocols.  Both of these
   enabled one to publish via the Domain Name System a policy declaring
   which mail servers were authorized to send email on behalf of a
   specific domain name.  The two protocols made use of this policy
   statement and some specific (but different) logic to evaluate whether
   or not the email client sending or relaying a message was authorized
   to do so.

   Because Sender-ID could use the same policy statement as SPF, the
   IESG at the time was concerned that an implementation of Sender-ID
   might erroneously apply that statement to a message and, depending on
   selected recipient actions, could improperly interfere with message
   delivery.  As a result, the IESG required the publication of all of
   these documents as Experimental, and requested that the community
   observe deployment and operation of the protocols over a period of
   two years from publication in order to determine a reasonable path
   forward.  (For further details about the IESG's concern, see the IESG
   Note prepended to all of those documents.)

   Accordingly, this working group has convened to resolve this
   experiment and propose advancement of a single protocol going
   forward.  This memo presents evidence on both deployment and efficacy
   of the two protocols, and further discusses the increasing need for
   consensus.  At the end it presents conclusions and recommends a path
   forward, as the IESG requested.

2.  The Need For Consensus

   These two protocols fall into a family of protocols that provide
   domain-level email authentication services.  Another prominent one is
   [DKIM].  Various efforts exist that use these as building blocks to
   increased abuse filtering capabilties, and indeed this sort of work
   has spawned another working group in the Applications area, with
   still more of these incubating in associations and trade groups
   outside of the IETF.

   There is thus some palpable interest in having a path authorization
   scheme, as well as a domain-level signing scheme, on the Standards
   Track so that these newer technologies can develop with confidence.
   This is, in part, why the community has decided to expend the effort
   to bring this experiment to a conclusion and document the results,
   and then advance a single path authorization technology.

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3.  Evidence of Deployment

   Two different sources (see Appendix A) report that over 239,000
   domains have been observed to be publishing SPF records of some kind.
   This includes either TXT or SPF resource record types, with the
   prefix "v=spf1" found.  The same two reporting sources indicated far
   fewer that published "spf2.0" records that would indicate the
   publisher requested Sender-ID handling only; one source projected
   approximately 1,200 such domains while the other observed fewer than

   It is likely impossible to determine from a survey which MTAs have
   SPF or Sender-ID checking enabled at message ingress since it does
   not appear, for example, in the reply to the EHLO command from
   extended [SMTP].

   A survey of over 20 MTAs in current or recent use includes only two
   implementations of the SMTP SUBMITTER extension that could act as an
   enabler to [SENDER-ID].  An unknown number of clients implement it;
   although there is substantial activity showing its use in logs, it is
   unclear whether these are separate implementations by legitimate
   senders, or merely instances of distributed automated malware seeking
   to improve their odds of reaching the end user.

   The [OPENSPF] web site maintains a list of known implementations of
   SPF.  At the time of this memo's writing it listed six libraries, 22
   MTAs with built-in SPF implementations, and numerous patches for MTAs
   and mail clients.  No such summary about Sender-ID support in
   software could be found; two specific product implementations for
   inbound checking were discovered.

   [pending: passive DNS query report from John Levine]

   [pending: active DNS query report from TDP]

   [other data TBD]

4.  Evidence of Differences

   It is plain from inspection of the two protocols that they have much
   in common: For a single message, both require the same number of DNS
   queries, and both require the same code to parse the result.  The PRA
   algorithm applied by Sender-ID is, however, more expensive than
   simply extracting the domain name from the omnipresent
   RFC5321.MailFrom.  Thus, SPF is cheaper to apply to a message.

   One set of specific data collected by a working group contributor

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   shows that in more than 95.5% of cases, Sender-ID and SPF reach the
   same conclusion about a message, meaning either both protocols return
   a "pass" result or both return a "fail" result.  The data set
   yielding this response could not further characterize the cases in
   which the answers differed.

   Another data set from a different (much larger) source showed that in
   fewer than 5% of cases did the RFC5321.MailFrom domain and the
   RFC5322.From domain differ.  This is somewhat meaningful when
   determining the difference between the two protocols, but is more
   important when considering the fact that most Mail User Agents
   (MUAs), which actually present mail to the end user, typically only
   render the contents of the RFC5322.From field.

   [pending: updated field comparison report from Hotmail]

   [other data TBD]

5.  Conclusions

   It is standard procedure within the IETF to document as standard
   those protocols and practices that have come into sufficient common
   use as to become part of the basic infrastructure.

   Given the evidence above, the working group feels that the experiment
   allows the following conclusions:

   1.  [WG conclusions here]

6.  IANA Considerations

   This memo presents no actions for IANA.  [RFC Editor: Please remove
   this section prior to publication.]

7.  Security Considerations

   This memo contains information for the community only, akin to an
   implementation report, and does not introduce any new security
   concerns.  Its implications could, in fact, resolve some.

8.  Informative References

   [DKIM]     Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,

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              September 2011.

   [OPENSPF]  "Sender Policy Framework: Project Overview",

   [PRA]      Lyon, J., "Purported Responsible Address in E-Mail
              Messages", RFC 4407, April 2006.

              Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail",
              RFC 4406, April 2006.

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

   [SPF]      Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

              Allman, E. and H. Katz, "SMTP Service Extension for
              Indicating the Responsible Submitter of an E-Mail
              Message", RFC 4405, April 2006.

Appendix A.  Acknowledgments

   The following provided operational data that contributed to the
   findings presented above:

   Cisco:  contributed data about observed Sender-ID and SPF records in
      the DNS for a large number of domains

   Hotmail:  contributed data about the difference between
      RFC5321.MailFrom and RFC5322.From domains across large mail

   Santronics:  contributed data about the use of the SUBMITTER
      extension in aggregate SMTP client traffic

   The Trusted Domain Project:  contributed data about the difference
      between Sender-ID and SPF results, and counts of unique domains
      appearing to publish different kinds of SPF and Sender-ID records

   The author would also like to thank the following for their
   contributions to the development of this memo: Dave Crocker, Scott

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Author's Address

   Murray S. Kucherawy
   128 King St., 2nd Floor
   San Francisco, CA  94107

   Phone: +1 415 946 3800
   Email: msk@cloudmark.com

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