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Resolution of The SPF/Sender-ID Experiment

The information below is for an old version of the document.
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This is an older version of an Internet-Draft that was ultimately published as RFC 6686.
Author Murray Kucherawy
Last updated 2012-04-08 (Latest revision 2012-04-06)
Replaces draft-kucherawy-spfbis-experiment
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SPFBIS Working Group                                        M. Kucherawy
Internet-Draft                                                 Cloudmark
Intended status: Informational                             April 6, 2012
Expires: October 8, 2012

               Resolution of 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 single protocol can be advanced.  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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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 October 8, 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
   ( 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
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   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 . . . . . . . . . . . . . . . . . . . . 5
   5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     5.1.  From the Evidence . . . . . . . . . . . . . . . . . . . . . 5
     5.2.  Recommendations to the IESG . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   8.  Informative References  . . . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Experiences Developing SPF . . . . . . . . . . . . . . 8
   Appendix B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . 9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 9

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1.  Introduction

   In April, 2006, the IETF published the [SPF] and Sender-ID email
   authentication protocols, the latter consisting of three documents
   ([SUBMITTER], [SENDER-ID], and [PRA]).  Both of these enable 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 memo resolves the experiment by presenting evidence
   regarding both deployment and efficacy of the two protocols, and
   further discusses the increasing need for consensus.  At the end it
   presents conclusions based on the data collected.

2.  The Need For Consensus

   These two protocols fall into a family of protocols that provide
   domain-level email authentication services.  For reference, another
   prominent one is [DKIM].  Various efforts exist that use these as
   building blocks to increased abuse filtering capabilities, 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 large-scale DNS surveys were run that looked for the two
   supported kinds of resource records (RR) that can contain SPF policy

   One data source for this report requested SPF records from one
   million domains.  Approximately 287,267 domains included non-error
   and non-empty replies.  Of these, 287,250 included type 16 (DNS RR
   TXT) replies, 4,613 included type 99 (DNS RR SPF) replies, and 4,596
   included both types.

   Another source requested SPF records from 251,651 domains for which
   there was a history of previous observed SPF evaluations.  Of these,
   136,018 returned type 16 answers, 2,605 returned type 99 answers,
   2,439 returned both types, and 115,465 returned neither.  Of those
   answers retrieved, 6,479 included records that start with the string
   "spf2.0/pra" which are specific requests for Sender-ID processing by

   During this second survey, some domains were observed to provide
   immediate answers for type 16 queries, but would time out waiting for
   replies to type 99 queries.  For example, it was observed that 3,953
   distinct domains in the survey returned a result of some kind (a
   record or an error) for the TXT query in time N, while the SPF query
   ultimately failed but only after at least time 4N.

   It is likely impossible to determine from a survey which MTAs have
   SPF and/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].  We therefore rely on evidence found via web
   searches, and observed the following:

   o  A web site [SID-IMPL] dedicated to highlighting Sender-ID
      implementations last updated in late 2007 listed 13
      implementations, which we assume means they implement the PRA
      checks.  At least one of them is known no longer to be supported
      by its vendor.

   o  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.

   In a survey of numerous MTAs in current or recent use, only two
   (Santronics WinServer and McAfee MxLogic) were found to contain
   implementations of the SMTP SUBMITTER extension as part of the MTA
   service, which could act as an enabler to Sender-ID.  An unknown

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   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.

   A survey was done of queries to type 16 and type 99 records by
   observing nameserver logs.  Only a few queries were ever received for
   type 99 records, and those almost exclusively came from one large
   email service provider that queried for both types.  The vast
   majority of other querying agents only ever requested type 16.

   [pending: SPF query results from Hotmail]

   [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
   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.

   [pending: MAIL FROM/PRA 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.

5.1.  From the Evidence

   Given the six years that have passed since the publication of the
   experimental RFCs, and the evidence reported in the earlier sections

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   of this document, the following conclusions are supported:

   1.  There has not been substantial adoption of the type 99 (SPF) DNS
       resource record.  In both large-scale surveys performed for this
       work, less than 2% of responding domains published type 99
       records, and almost no clients requested them.

   2.  Of the records retrieved, fewer than 5% requested processing of
       messages using the PRA algorithm that was an integral part of

   3.  No data collected showed any substantial operational benefit
       (e.g., cheaper processing, improved accuracy) to using Sender-ID
       over SPF.

   4.  A survey of implementations shows significant support for both
       protocols, though there were more implementations in support of
       SPF than of Sender-ID.  Further, the SPF implementations showed
       better upkeep and current interest than the Sender-ID

   5.  A survey of implementations shows no significant use of the
       SUBMITTER extension by servers, but some by clients.

5.2.  Recommendations to the IESG

   In light of the above, the working group recommends to the IESG the

   1.  that the experiment comprising the series of RFCs defining the
       SUBMITTER SMTP extension, the Sender-ID mechanism, the Purported
       Responsible address algorithm, and SPF, be considered concluded;

   2.  that [SPF] be advanced to the standards track after appropriate
       technical review with respect to the deployed base;

   3.  that this revision to SPF deprecate the use of the type 99 DNS
       resource record by both clients and servers;

   4.  that [SUBMITTER], [SENDER-ID], and [PRA] have failed to gain any
       substantial adoption and are thus, de facto, obsolete (and thus
       could be marked "Obsoleted" and "Historic" by some future

   Appendix A is offered as a cautionary review of problems that
   affected the process of developing SPF and Sender-ID in terms of use
   of the DNS.

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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,
              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.

              "Sender ID Framework Industry Support and Solutions",
              October 2007, <

   [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.

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Appendix A.  Experiences Developing SPF

   SPF was originally developed by a community of interested developers
   outside the IETF.  It was brought to the IETF for standardization
   only after the specification was relatively mature and ready for the
   rigors of the IETF publication process.

   At the time, the prospect of getting a DNS resource record (RR) type
   allocated for SPF was not seriously considered, partly because it was
   perceived to have high barriers to entry.  As a result, by the time
   the working group was formed, there was already a substantial and
   growing installed base that had SPF running using TXT RRs.
   Eventually the application was made for the new RR type as a result
   of pressure from the DNS experts in the community, who encouraged
   doing so as the preferred path toward using the DNS for storing such
   things as policy data.

   Later, after type 99 was assigned (long after IESG approval of the
   document, in fact), a plan was put into place to effect a gradual
   transition to using type 99 instead of using type 16.  This plan
   failed to take effect for four primary reasons:

   1.  there was hesitation to make the transition because of concerns
       that nameservers (and, in fact, DNS-aware firewalls) would drop
       or reject requests for unknown RR types (see Section 3 for
       evidence of this), which means successful rollout of a new RR
       type is contingent upon widespread adoption of updated
       nameservers and resolver functions;

   2.  many DNS provisioning tools (e.g., web interfaces to controlling
       DNS zone data) were, and still are, typically lethargic about
       adding support for new RR types;

   3.  the substantial deployed base was already using type 16, and it
       was working just fine, leading to inertia;

   4.  [SPF] itself included a faulty transition plan: It said a server
       SHOULD publish both types and MUST publish at least one, while a
       client can query either or both, which means both can claim to be
       fully compliant while failing utterly to interoperate.

   It is likely that this will happen again if the bar to creating new
   RR types even for experimental development purposes is not lowered,
   and handling of unknown RR types becomes generally more graceful.

   There are DNS experts within the community that will undoubtedly
   point to DNS servers and firewalls that mistreat queries for unknown
   RR types, and claim they are broken, as a way of answering this

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   concern.  This is undoubtedly correct, but the reality is that they
   are among us and likely will be for some time, and this needs to be
   considered as new protocols and IETF procedures are developed.

Appendix B.  Acknowledgments

   The following provided operational data that contributed to the
   evidence 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
      volumes, and a survey of DNS queries observed in response to
      outgoing mail traffic

   John Levine:  conducted a survey of DNS server logs to evaluate SPF-
      related query traffic

   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 conducted one of the two
      detailed TXT/SPF record surveys including collecting timing data

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

Author's Address

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

   Phone: +1 415 946 3800

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