DKIM Working Group                                             E. Allman
Internet-Draft                                            Sendmail, Inc.
Intended status:  Standards Track                              M. Delany
Expires:  August 4, 2008                                     Yahoo! Inc.
                                                               J. Fenton
                                                     Cisco Systems, Inc.
                                                        February 1, 2008

                     DKIM Sender Signing Practices

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   DomainKeys Identified Mail (DKIM) defines a domain-level
   authentication framework for email using public-key cryptography and
   key server technology to permit verification of the source and
   contents of messages by either Mail Transport Agents (MTAs) or Mail
   User Agents (MUAs).  The primary DKIM protocol is described in

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   This document describes the records that authors' domains can use to
   advertise their practices regarding signing their outgoing mail, and
   how other hosts can access, parse and interpret those records.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

(Unresolved Issues/To Be Done)

   o  Need to consider handling of multiple responses to a DNS query for
      the SSP record.

   o  Security Considerations needs a detailed examination.

   o  IANA Considerations should be formalized (e.g., as in 4871).

   o  Check over the references.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Language and Terminology . . . . . . . . . . . . . . . . . . .  4
     2.1.  Terms Imported from DKIM Signatures Specification  . . . .  5
     2.2.  Evaluator  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  SSP Checker  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Valid Signature  . . . . . . . . . . . . . . . . . . . . .  5
     2.5.  Alleged Author . . . . . . . . . . . . . . . . . . . . . .  5
     2.6.  Author Address . . . . . . . . . . . . . . . . . . . . . .  6
     2.7.  Author Domain  . . . . . . . . . . . . . . . . . . . . . .  6
     2.8.  Author Signature . . . . . . . . . . . . . . . . . . . . .  6
     2.9.  Sender Signing Practices Record  . . . . . . . . . . . . .  6
   3.  Operational Description  . . . . . . . . . . . . . . . . . . .  6
     3.1.  Publication of SSP Records . . . . . . . . . . . . . . . .  6
     3.2.  Lookup of SSP Records  . . . . . . . . . . . . . . . . . .  8
     3.3.  SSP Record Syntax  . . . . . . . . . . . . . . . . . . . .  9
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     5.1.  DNS Attacks  . . . . . . . . . . . . . . . . . . . . . . . 11
     5.2.  DNS Wildcards  . . . . . . . . . . . . . . . . . . . . . . 11
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Appendix A.  Usage Examples (INFORMATIVE)  . . . . . . . . . . . . 13
     A.1.  Single Location Domains  . . . . . . . . . . . . . . . . . 13
     A.2.  Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 13
     A.3.  Bulk Mailing Domains with Discardable Mail . . . . . . . . 14
     A.4.  Third Party Senders  . . . . . . . . . . . . . . . . . . . 14
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 14
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 14
     C.1.  Changes since -ietf-dkim-ssp-01  . . . . . . . . . . . . . 15
     C.2.  Changes since -ietf-dkim-ssp-00  . . . . . . . . . . . . . 16
     C.3.  Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 16
     C.4.  Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 16
     C.5.  Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

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1.  Introduction

   DomainKeys Identified Mail (DKIM) defines a mechanism by which email
   messages can be cryptographically signed, permitting a signing domain
   to claim responsibility for the introduction of a message into the
   mail stream.  Message recipients can verify the signature by querying
   the signer's domain directly to retrieve the appropriate public key,
   and thereby confirm that the message was attested to by a party in
   possession of the private key for the signing domain.

   However, the legacy of the Internet is such that not all messages
   will be signed.  Therefore, the absence of a signature is not an a
   priori indication of forgery.  In fact, during early phases of DKIM
   deployment it must be expected that most messages will remain
   unsigned.  Nevertheless, some domains may find it highly desirable to
   advertise that they sign all of their outgoing mail making the
   absence of a valid signature a potential indication of forgery.
   Without a mechanism to do so, the benefits of DKIM are limited to
   cases in which a valid signature exists and cannot be extended to
   cases in which signatures are missing or are invalid.  Defining such
   a mechanism is the purpose of Sender Signing Practices (SSP).

   This specification focuses on information which is relevant in the
   absence of an acceptable signature.  Expressions of signing practice
   which require outside auditing are out of scope for this
   specification because they fall under the purview of reputation and
   accreditation.  Sender Signing Practices can be extended in the
   future to include additional information that a receiver might use as
   input to a processing decision.

   More specifically, this specification defines the SSP Checker, a
   module that retrieves the SSP information for a given domain, and the
   format of the data returned.  An module called the Evaluator combines
   information from DKIM signatures, SSP Checker results, and any other
   data sources it cares to use in order to make a decision regarding
   how the message should be processed.  The Evaluator is explicitly out
   of scope of this document, and is described herein in order to make
   the limits of this specification clear.

   The detailed requirements for Sender Signing Practices are given in
   [RFC5016], which the protocol described in this document attempts to
   satisfy.  This document refers extensively to [RFC4871], which should
   be read as a prerequisite to this document.

2.  Language and Terminology

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2.1.  Terms Imported from DKIM Signatures Specification

   Some terminology used herein is derived directly from [RFC4871].

   o  A "Signer" is the agent that signs a message.  In many cases it
      will correspond closely with the original author of the message or
      an agent working on the author's behalf.

   o  "Selectors" describe the keys published by a signing domain.
      Signing domains may have multiple Selectors.  Selectors subdivide
      the address space to allow a single sending domain to publish
      multiple keys.

   o  A "Verifier" is the agent that verifies a message by checking
      actual signature(s) in the message header against the message
      itself using the public key published in the Selector referenced
      by a given signature.

2.2.  Evaluator

   The "Evaluator" is the module that makes the ultimate decision on how
   an incoming message should be processed at a given site.  In some
   cases it may be colocated with the Verifier.  The Evaluator combines
   information from the DKIM signature(s) (if any), the output of the
   SSP Checker, and any other information it cares to consult in order
   to make a processing decision about the message.  The specification
   of the Evaluator is out of scope of this document.

2.3.  SSP Checker

   The "SSP Checker" module performs the SSP queries on behalf of the
   Evaluator.  It is the primary module defined by this document.  The
   input to the SSP Checker is an address extracted from the From header
   field of the message being evaluated; the output is either the Sender
   Signing Practices associated with that domain, or an error code.

2.4.  Valid Signature

   A "Valid Signature" is any signature on a message which correctly
   verifies using the procedure described in section 6.1 of [RFC4871].

2.5.  Alleged Author

   An "Alleged Author" is the Author Address of a message received by an
   Evaluator; it is "alleged" because it has not yet been verified.

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2.6.  Author Address

   The "Author Address" is an email address in the From header field of
   a message [RFC2822].  If the From header field contains multiple
   addresses, the message has multiple Author Addresses, which may
   potentially cause the Evaluator to perform multiple SSP Checks for a
   given message.

2.7.  Author Domain

   The "Author Domain" is everything to the right of the "@" in the
   Author Address (excluding the "@" itself).

2.8.  Author Signature

   An "Author Signature" is any Valid Signature where the identity of
   the user or agent on behalf of which the message is signed (listed in
   the "i=" tag or its default value from the "d=" tag) matches an
   Author Address in the message.

2.9.  Sender Signing Practices Record

   A "Sender Signing Practices Record" consists of a machine-readable
   record published by the domain of an Alleged Author which includes
   information on whether that domain signs all of their email, and
   related information.  That record is defined in detail in section
   Section 3.3.

3.  Operational Description

   The use of Sender Signing Practices consists of two parts:

      Publication of SSP records by author domains wishing to do so

      Lookup of SSP records by an SSP Checker under the direction of an

3.1.  Publication of SSP Records

3.1.1.  DNS Representation

   Sender Signing Practices Records are published using the DNS "TXT"
   resource record type.

      been considerable discussion on the DKIM WG mailing list regarding
      the relative advantages of TXT and a new resource record (RR)

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      type.  Many DNS server and resolver implementations are incapable
      of quickly and easily supporting new resource record types.  For
      this reason, support of TXT records is required whether a new RR
      type is defined or not.  However, without a "flag day" on which
      SSP TXT record support is to be withdrawn, such support is likely
      to continue indefinitely.  As a result, this specification defines
      no new RR type for SSP.

      Another alternative proposed by P. Hallam-Baker is the publication
      of both a TXT record and, when implementations permit, a new RR,
      referred to as XPTR, which gives the location from which SSP and
      other policy information relating to a give domain can be
      retrieved.  This has the advantage of supporting a variety of
      policies in a scalable manner, with better handling of wildcards
      and centralized publication of policy records, with caching
      advantages.  However, the above implementation issues also apply
      to XPTR, and an additional lookup is required to retrieve SSP via
      the XPTR method.  At the time of publication of this draft,
      consensus on this proposal was unclear.*]]*

   The RDATA for SSP resource records is textual in format, with
   specific syntax and semantics relating to their role in describing
   sender signing practices.  SSP records follow the tag-list syntax
   described in section 3.2 of [RFC4871], including the restriction on
   duplicate tags, the use of white space, and case sensitivity.
   Records not in overall compliance with that syntax MUST be ignored
   (considered equivalent to a "NODATA" result), although they MAY cause
   the logging of warning messages via an appropriate system logging
   mechanism.  All syntactically valid tags MUST be made available to
   the Evaluator.

3.1.2.  Location of SSP Records

   SSP records for a domain are published at a location in the domain's
   DNS hierarchy prefixed by "_ssp._domainkey"; e.g., the SSP record for
   "" would be a "TXT" record that is published at

   Sender Signing Practices are intended to apply to all mail sent from
   the domain of an Alleged Author.  In order to ensure that SSP applies
   to any hosts within that domain (e.g.,,, etc.) the SSP lookup algorithm looks up one level in
   the domain tree.  For example, mail signed by may
   optionally be covered by the SSP record for  This
   prevents administrators from having to include an SSP record for
   every name within a given domain.

   Normally, a domain expressing Sender Signing Practices will want to

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   do so for both itself and all of its "descendents" (child domains at
   all lower levels).  Domains wishing to do so MUST publish SSP records
   for the domain itself and any subdomains.

   Wildcards within a domain publishing SSP records pose a particular
   problem.  This is discussed in more detail in Section 5.2.

3.2.  Lookup of SSP Records

   NON-NORMATIVE NOTE:  While the operation of the Evaluator is outside
   the scope of this specification, it is generally not worthwhile for
   an Evaluator to request an SSP check when the results of that check
   will not affect the disposition of the message.  Since the
   information provided by SSP is only relevant in the absence of valid
   Author Signature(s), there is little to be gained by performing an
   SSP check on domains corresponding to valid Author Signatures.  SSP
   checks may also be unnecessary when the Evaluator has some other
   basis for deciding to process the message "normally", including, but
   not limited to, the presence of a DKIM signature that the Evaluator
   has some basis to trust sufficiently for this purpose.

3.2.1.  SSP Checker Results

   A Sender Signing Practices check produces one of four possible
   results for use by the Evaluator:

   1.  The domain does not exist in DNS.

   2.  The domain does exist, but no SSP Record is present.

   3.  The SSP Record exists, and that value is also returned.

   4.  The DNS information could not be determined due to a transient
       error such as "SERVFAIL".

3.2.2.  SSP Lookup Algorithm

   SSP Checkers doing an SSP lookup MUST produce a result that is
   semantically equivalent to applying the following steps in the order
   listed below.  In practice, several of these steps can be performed
   in parallel in order to improve performance.  However,
   implementations SHOULD avoid doing unnecessary DNS lookups.  For the
   purposes of this section a "valid SSP record" is one that is both
   syntactically and semantically correct; in particular, it must match
   the ABNF for a "tag-list" and must include a defined "dkim=" tag.

   1.  _Fetch Named SSP Record._ The SSP Checker MUST query DNS for a
       TXT record corresponding to the Author Domain prefixed by

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       ""_ssp._domainkey."" (note the trailing dot).  If the result of
       this query is a "NOERROR" response with one or more answers which
       are valid SSP records, return that record for interpretation by
       the Evaluator; otherwise, continue to the next step.

   2.  _Verify Domain Exists._ The SSP Checker MUST perform a DNS query
       for a record corresponding to the Author Domain (with no prefix).
       The type of the query can be of any type, since this step is only
       to determine if the domain itself exists in DNS.  This query MAY
       be done in parallel with the query made in step 2.  If the result
       of this query is an "NXDOMAIN" error, the SSP Checker MUST return
       an appropriate error to the Evaluator and terminate the

          NON-NORMATIVE DISCUSSION:  Any resource record type could be
          used for this query since the existence of a resource record
          of any type will prevent an "NXDOMAIN" error.  "MX" is a
          reasonable choice for this purpose is because this record type
          is thought to be the most common for likely domains, and will
          therefore result in a result which can be more readily cached
          than a negative result.

   3.  _Try Parent Domain._ The SSP Checker MUST query DNS for a TXT
       record for the immediate parent domain, prefixed with
       ""_ssp._domainkey.""  If the result of this query is anything
       other than a "NOERROR" response with a valid SSP record, the
       algorithm terminates returning a result indicating that no SSP
       record was present.  If the SSP "t" tag exists in the response
       and any of the flags is "s" (indicating it should not apply to a
       subdomain), the SSP Checker MUST also return a "No SSP Record"
       result.  Otherwise, return that record for interpretation by the

   If any of the queries involved in the Sender Signing Practices Check
   result in a "SERVFAIL" error response, the SSP Checker MUST return
   that information to the Evaluator; possible actions include queuing
   the message or returning an SMTP error indicating a temporary

3.3.  SSP Record Syntax

   SSP Records MUST match the "tag-list" syntax defined in [RFC4871].
   The specific tags used in SSP records are described below.
   Unrecognized tags MUST be ignored.

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   dkim=  Outbound signing practices for the domain (plain-text;
      REQUIRED).  Possible values are as follows:

      unknown  The domain may sign none, some, or all email.

      all  All mail from the domain is signed with an Author Signature.

      discardable  All mail from the domain is signed with an Author
         Signature.  Furthermore, if a message arrives without a valid
         Author Signature due to modification in transit, submission via
         a path without access to a signing key, or other reason, the
         domain encourages the recipient(s) to discard it.

         NON-NORMATIVE DISCUSSION:  Sender signing practices of
         "discardable" would be usually inappropriate for domains of end
         users, because of the potential for mailing lists and similar
         agents to modify messages in such a way as to render the
         signature invalid.  Domains sending mail that is expected to
         pass with no significant modification to the recipient, such as
         domains sending only transactional messages, are appropriate
         places to consider the publication of a "discardable" practice.
         See [RFC5016] section 5.3 and Appendix A for further


   ssp-dkim-tag   = "dkim" *WSP "=" *WSP ("unknown" /
                     "all" / "discardable")

   t= Flags, represented as a colon-separated list of names (plain-text;
      OPTIONAL, default is that no flags are set).  Flag values are:

      s  The signing practices apply only to the named domain, and not
         to subdomains.


   ssp-t-tag       = %x75 *WSP "=" *WSP ssp-t-tag-flag
                     0*( *WSP ":" *WSP ssp-t-tag-flag )
   ssp-t-tag-flag  = "s" / hyphenated-word
                           ; for future extension
   hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
                     (ALPHA / DIGIT) ]

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      Unrecognized flags MUST be included in the result that is provided
      to the Evaluator.

4.  IANA Considerations

   IANA is requested to create a "DKIM selector name" registry and to
   reserve the selector name ""_ssp"" to avoid confusion between DKIM
   key records and SSP records.

   *<<< Needs to be updated to be more complete; see 4871 for examples

5.  Security Considerations

   Security considerations in the Sender Signing Practices are mostly
   related to attempts on the part of malicious senders to represent
   themselves as other authors, often in an attempt to defraud either
   the recipient or an Alleged Author.

   Additional security considerations regarding Sender Signing Practices
   may be found in the DKIM threat analysis [RFC4686].


5.1.  DNS Attacks

   An attacker might attack the DNS infrastructure in an attempt to
   impersonate SSP records.  However, such an attacker is more likely to
   attack at a higher level, e.g., redirecting "A" or "MX" record
   lookups in order to capture traffic that was legitimately intended
   for the target domain.  Domains concerned about this should use
   DNSSEC [RFC4033].

   Because SSP operates within the framework of the legacy e-mail
   system, the default result in the absence of an SSP record is that
   the domain does not sign all of its messages.  It is therefore
   important that the SSP Checker distinguish a DNS failure such as
   SERVFAIL from other DNS errors so that appropriate actions can be

5.2.  DNS Wildcards

   Wildcards within a domain publishing SSP records, including but not
   limited to wildcard "MX" records, pose a particular problem.  While
   referencing the immediate parent domain allows the discovery of an
   SSP record corresponding to an unintended immediate-child subdomain,

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   wildcard records apply at multiple levels.  For example, if there is
   a wildcard "MX" record for "", the domain
   "" can receive mail through the named mail
   exchanger.  Conversely, the existence of the record makes it
   impossible to tell whether "" is a legitimate name
   since a query for that name will not return an "NXDOMAIN" error.  For
   that reason, SSP coverage for subdomains of domains containing a
   wildcard record is incomplete.

      NON-NORMATIVE NOTE:  Complete SSP coverage of domains containing
      (or where any parent contains) wildcards generally cannot be

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.

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

   [RFC4234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 4234, October 2005.

   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

6.2.  Informative References

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, September 2006.

   [RFC5016]  Thomas, M., "Requirements for a DomainKeys Identified Mail
              (DKIM) Signing Practices Protocol", RFC 5016,
              October 2007.

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Appendix A.  Usage Examples (INFORMATIVE)

   These examples are intended to illustrate typical uses of SSP.  They
   are not intended to be exhaustive, nor to apply to every domain or
   mail system's individual situation.

A.1.  Single Location Domains

   A common mail system configuration handles all of a domain's users'
   incoming and outgoing mail through a single MTA or cluster of MTAs.
   In that case, the MTA(s) can be configured to sign outgoing mail with
   an Author Signature.

   In this situation it might be appropriate to publish an SSP record
   for the domain containing "all", depending on whether the users also
   send mail through other MTAs that do not apply an Author Signature.
   Such MTAs could include MTAs at hotels or hotspot networks used by
   travelling users, or web sites that provide "mail an article"

   Domain managers are advised to consider the ways that mail processing
   can modify messages in ways that will invalidate an existing DKIM
   signature, such as mailing lists, courtesy forwarders, and other
   paths that could add or modify headers, or modify the message body.
   In that case, if the modifications invalidate the DKIM signature,
   recipient MTAs will consider the mail not to have an Author
   Signature, even though the signature was present when the mail was
   originally sent.

A.2.  Bulk Mailing Domains

   Another common configuration uses a domain solely for bulk or
   broadcast mail, with no individual human users, again typically
   sending all the mail through a single MTA or cluster of MTAs that can
   apply an Author Signature.  In this case, the domain's management can
   be confident that all of its outgoing mail will be sent through the
   signing MTA.  Lacking individual users, the domain is unlikely to
   participate in mailing lists, but could still send mail through other
   paths that might invalidate signatures.

   Domain owners often use specialist mailing providers to send their
   bulk mail.  In that case, the mailing provider needs access to a
   suitable signing key in order to apply an Author Signature.  One
   possible route would be for the domain owner to generate the key and
   give it to the mailing provider.  Another would be for the domain to
   delegate a subdomain to the mailing provider, for example,
   bigbank.example might delegate email.bigbank.example to such a
   provider.  In that case, the provider can generate the keys and DKIM

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   DNS records itself and use the subdomain in the Author Address in the

A.3.  Bulk Mailing Domains with Discardable Mail

   In some cases, a domain might sign all its outgoing mail with an
   Author Signature, but prefers that recipient systems discard mail
   without a valid Author Signature to avoid confusion from mail sent
   from sources that do not apply an Author Signature.  (This latter
   kind of mail is sometimes loosely called "forgeries".)  In that case,
   it may be appropriate to publish an SSP record containing
   "discardable".  Note that a domain SHOULD NOT publish a "discardable"
   record if it wishes to maximize the likelihood that mail from the
   domain is delivered, since it could cause some fraction of the mail
   the domain sends to be discarded.

   As a special case, if a domain sends no mail at all, it can safely
   publish a "discardable" SSP record, since any mail with an author
   address in the domain is a forgery.

A.4.  Third Party Senders

   Another common use case is for a third party to enter into an
   agreement whereby that third party will send bulk or other mail on
   behalf of a designated author domain, using that domain in the
   RFC2822 From:  or other headers.  Due to the many and varied
   complexities of such agreements, third party signing is not addressed
   in this specification.  The authors anticipate that as mail systems
   gain experience with DKIM, it will become possible to codify best
   practices of this and other usages of DKIM.

Appendix B.  Acknowledgements

   The authors wish to thank many members of the ietf-dkim mailing list
   for valuable suggestions and constructive criticism of earlier
   versions of this draft.

   This draft incorporates content from a parallel "DKIM Author Signing
   Policies" document edited by John Levine.  The authors appreciate
   this contribution.

Appendix C.  Change Log

   *NOTE TO RFC EDITOR:  This section may be removed upon publication of
   this document as an RFC.*

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C.1.  Changes since -ietf-dkim-ssp-01

   o  Reworded introduction for clarity.

   o  Various definition clarifications.

   o  Changed names of practices to unknown, all, and discardable.

   o  Removed normative language mandating use of SSP in particular
      situations (issue 1538).

   o  Clarified possible confusion over handling of syntax errors.

   o  Removed normative language from Introduction (issue 1538).

   o  Changed "Originator" to "Author" throughout (issue 1529).

   o  Removed all references to Third-Party Signatures (issues 1512,

   o  Removed all mention of "Suspicious" (issues 1528, 1530).

   o  Removed "t=y" (testing) flag (issue 1540).

   o  Removed "handling" tag (issue 1513).

   o  Broke up the "Sender Signing Practices Check Procedure" into two
      algorithms:  fetching the SSP record and interpretation thereof
      (issues 1531, 1535; partially addresses issue 1520).
      Interpretation is now the responsibility of the Evaluator.

   o  Document restructuring for better flow and remove redundancies
      (some may address issue 1523, but I'm not sure I understand that
      issue completely; also issues 1532, 1537).

   o  Removed all mention of how this interacts with users, even though
      it makes parts of the document harder to understand (issue 1526).

   o  Introduced the concepts of "SSP Checker" and "Evaluator".

   o  Multiple author case now handled my separate invocations of SSP
      checker by Evaluator (issue 1525).

   o  Removed check to avoid querying top-level domains.

   o  Changed ABNF use of whitespace from [FWS] to *WSP (partially
      addresses issue 1543).

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C.2.  Changes since -ietf-dkim-ssp-00

   o  Clarified Operation Overview and eliminated use of Legitimate as
      the counterpart of Suspicious since the words have different

   o  Improved discussion (courtesy of Arvel Hathcock) of the use of TXT
      records in DNS vs. a new RR type.

   o  Clarified publication rules for multilevel names.

   o  Better description of overall record syntax, in particular that
      records with unknown tags are considered syntactically correct.

   o  Clarified Sender Signing Practices Check Procedure, primarily by
      use of new term Author Domain.

   o  Eliminated section "Third-Party Signatures and Mailing Lists" that
      is better included in the DKIM overview document.

   o  Added "handling" tag to express alleged sending domain's
      preference about handling of Suspicious messages.

   o  Clarified handling of SERVFAIL error in SSP check.

   o  Replaced "entity" with "domain", since with the removal of user-
      granularity SSP, the only entities having sender signing policies
      are domains.

C.3.  Changes since -allman-ssp-02

   o  Removed user-granularity SSP and u= tag.

   o  Replaced DKIMP resource record with a TXT record.

   o  Changed name of the primary tag from "p" to "dkim".

   o  Replaced lookup algorithm with one which traverses upward at most
      one level.

   o  Added description of records which must be published, and effect
      of wildcard records within the domain, on SSP.

C.4.  Changes since -allman-ssp-01

   o  Changed term "Sender Signing Policy" to "Sender Signing

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   o  Changed query methodology to use a separate DNS resource record
      type, DKIMP.

   o  Changed tag values from SPF-like symbols to words.

   o  User level policies now default to that of the domain if not

   o  Removed the "Compliance" section since we're still not clear on
      what goes here.

   o  Changed the "parent domain" policy to only search up one level
      (assumes that subdomains will publish SSP records if appropriate).

   o  Added detailed description of SSP check procedure.

C.5.  Changes since -allman-ssp-00

   From a "diff" perspective, the changes are extensive.  Semantically,
   the changes are:

   o  Added section on "Third-Party Signatures and Mailing Lists"

   o  Added "Compliance" (transferred from -base document).  I'm not
      clear on what needs to be done here.

   o  Extensive restructuring.

Authors' Addresses

   Eric Allman
   Sendmail, Inc.
   6475 Christie Ave, Suite 350
   Emeryville, CA  94608

   Phone:  +1 510 594 5501

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   Mark Delany
   Yahoo! Inc.
   701 First Avenue
   Sunnyvale, CA  94089

   Phone:  +1 408 349 6831

   Jim Fenton
   Cisco Systems, Inc.
   MS SJ-9/2
   170 W. Tasman Drive
   San Jose, CA  95134-1706

   Phone:  +1 408 526 5914

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