DKIM Working Group                                             M. Thomas
Internet-Draft                                             Cisco Systems
Intended status: Informational                            September 2006
Expires: March 5, 2007

           Requirements for a DKIM Signing Practices Protocol

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Copyright Notice

   Copyright (C) The Internet Society (2006).

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   DomainKeys Identified Mail [DKIM] [I-D.ietf-dkim-base] provides a
   cryptographic mechanism for domains to assert responsibility for the
   messages they handle.  A related mechanism would allow an
   administrator to publish various statements about their DKIM signing
   practices.  This draft defines the requirement for this mechanism.

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Preface  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5

   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6

   3.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  7

   4.  SSP Problem Scenarios  . . . . . . . . . . . . . . . . . . . .  8
     4.1.  Problem Scenario 1: All Mail Signed with DKIM  . . . . . .  8
     4.2.  Problem Scenario 2: Illegitimate Domain Name Use . . . . .  9
     4.3.  Problem Scenario 3: Domain Sends No Mail . . . . . . . . . 10

   5.  SSP Deployment Scenarios . . . . . . . . . . . . . . . . . . . 11
     5.1.  Deployment Scenario 1: Outsourced Signing  . . . . . . . . 11
     5.2.  Deployment Scenario 2: Determinism in Lookup Mechanism . . 11
     5.3.  Deployment Scenario 3: Resent Original Mail  . . . . . . . 11
     5.4.  Deployment Scenario 4: Incremental Deployment of
           Signing  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.5.  Deployment Scenario 5: Transport Scenarios . . . . . . . . 12
     5.6.  Deployment Scenario 6: Human Legibility of Practices . . . 13
     5.7.  Deployment Scenario 7: Cryptographic Downgrade Attacks . . 13
     5.8.  Deployment Scenario 8: Extensibility . . . . . . . . . . . 13
     5.9.  Deployment Scenario 9: Security  . . . . . . . . . . . . . 13

   6.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  Discovery Requirements . . . . . . . . . . . . . . . . . . 15
     6.2.  Transport requirements . . . . . . . . . . . . . . . . . . 16
     6.3.  Practice and Expectation Requirements  . . . . . . . . . . 16
     6.4.  Extensibility and Forward Compatibility Requirements . . . 19

   7.  Security Requirements  . . . . . . . . . . . . . . . . . . . . 20

   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21

   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22

   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23

   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 24

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     11.2. Informative References . . . . . . . . . . . . . . . . . . 24

   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
   Intellectual Property and Copyright Statements . . . . . . . . . . 26

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1.  Preface

   The purpose of this draft is get out into the open a range of issues
   related to the perceived need for a signing practices information
   service primarily focused on DKIM.  This document is intended to
   document well-agreed upon problems and requirements, in addition to
   less well-agreed upon requirements in an attempt to capture the issue
   as well as generalize the requirement as much as possible.  These
   latter requirements will be noted as "[PROVISIONAL]" to indicate that
   there is not yet solid consensus, or that the problem is not well
   understood.  A winnowing process is envisioned where the more
   difficult and/or speculative problems/requirement will be eliminated
   unless concrete problems with proven constituencies can be
   demonstrated, along with reasonable plausibility that they do not
   contradict more well agreed upon requirements.

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2.  Definitions

   o  Domain Holder: the entity that ultimately controls the contents of
      the DNS subtree starting at the domain, either directly or by
      delegation via NS records it controls.

   o  First Party Address: For DKIM, a first party address is defined to
      be the [RFC2822].From address in the message header; a first party
      address is also known as an Author address

   o  First Party Signature: a first party signature is a valid
      signature where the domain tag (d= or the more specific identity
      i= tag) matches the first party address.  "Matches" in this
      context is defined in [I-D.ietf-dkim-base]

   o  Third Party Signature: a third party signature is a valid
      signature that does not qualify as a First Party Signature.  Note
      that a DKIM third party signature does is not required to
      correspond to a third party address such as Sender or Listid, etc.

   o  DKIM Signer Complete: the state where the domain holder believes
      that all legitimate mail purportedly from the domain was sent with
      a valid DKIM signature.

   o  The Protocol: in this document, The Protocol is used as
      placeholder for a protocol that will meet the requirements set in
      this draft.

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3.  Introduction

   The DomainKeys Identified Mail working group is chartered to create a
   base signing mechanism for email.  This work is contained in
   [I-D.ietf-dkim-base].  In addition there are two other documents
   [I-D.ietf-dkim-overview] and [I-D.ietf-dkim-threats] which give an
   overview and a threat analysis of the chartered work.  This draft
   reflects the requirements for the last part of the chartered work to
   define a protocol to publish DKIM signing practices.

   While the base signing document defines a mechanism for signing and
   verifying DKIM signatures, there has been a great deal of interest in
   a signing practices protocol.  The most pressing case seems to be the
   bid down attack inherent with almost all systems that allow optional
   authentication: how does a receiver know whether or not it should
   expect a message to contain authentication information?  For email
   this is an especially difficult problem since there is generally no a
   priori knowledge of a sending domain's practices.  If a protocol
   could be developed for a domain to publish its DKIM signing
   practices, a message verifier could take that into account when it
   receives a unsigned piece of email.

   This draft is organized into two main sections: a Problem and
   Deployment Scenario section which describes the problems that The
   Protocol is intended to address as well as the deployment issues
   surrounding the base problems.  The second section is the
   Requirements that arise because of those scenarios.

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4.  SSP Problem Scenarios

   The email world is a diverse place with many deployment scenarios.
   This section tries to outline some usage scenarios that it is
   expected that DKIM signing/verifying will take place in, and how a
   new protocol might be helpful to clarify the relevance of DKIM signed

4.1.  Problem Scenario 1: All Mail Signed with DKIM

   After auditing their outgoing mail and deploying DKIM signing for all
   of their legitimate outgoing mail, a domain could be said to be DKIM
   signing complete.  That is, the domain has to the best of its ability
   insured that all mail legitimately purporting to have come from that
   domain contains a valid DKIM signature.

   A receiver in the general case doesn't know what the practices are
   for a given domain, or what their expectations are for unsigned mail.
   Thus the receiver is at a disadvantage in that it does not know if it
   should expect mail to be signed from a given domain or not.  This
   knowledge gap leads to a trivially exploitable bid-down attack where
   the attacker merely sends unsigned mail; since the receiver doesn't
   know the practices of the signing domain, it cannot treat the message
   any more harshly for lack of a valid signature.

   An information service which allowed a receiver to query for the
   practices and expectations of the sending domain when no valid
   signature is found could be useful in closing this gap.  A receiver
   could use this information to treat such questionable mail with
   varying degrees of prejudice.

   Note that for the foreseeable future, DKIM signature breakage for
   unrestricted use patterns (eg where users are members of mailing
   lists, etc) will likely suffer occasional non-malicious signature
   failure in transit.  While probably not a large percentage of total
   traffic, the kind (quality) of breakage may be a significant concern
   for those usage patterns.  This scenario defines where the sender
   cannot set any expectation as to whether an individual message will
   arrive intact.

   Even without that expectation, a receiver may be able to take
   advantage of the knowledge that the domain's practice is to sign all
   mail and bias filters against unsigned or damaged in transit mail.
   This information should not be expected to be used in a binary yes/no
   fashion, but instead as a data point among others in a filtering

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   1.  Mail with a [RFC2822].From A purportedly sends to B with a
       missing or broken DKIM signature from A

   2.  B would like to know whether that is an expected state of

   3.  A provides information that it signs all outgoing mail, but
       places no expectation on whether it will arrive with an intact

   4.  B could use this information to bias its filters such that it
       looks somewhat more suspicious.

4.2.  Problem Scenario 2: Illegitimate Domain Name Use

   There seems to be a class of mail -- mostly transactional mail from
   high value domains -- that are the target of phishing attacks.  In
   particular, many phishing scams forge the [RFC2822].From address in
   addition to spoofing much of the content to trick unsuspecting users
   into revealing sensitive information.  Domain holders sending this
   kind of mail would like the ability to give an enhanced guarantee
   that mail sent in their name should always arrive with the provable
   consent of the domain holder.

   From a receiver's standpoint, knowing that a domain not only signs
   all of its mail, but also expects that legitimate mail from the
   domain will be received with a valid signature is quite interesting.
   This assertion from the signing domain is quite a bit stronger than
   the assertion in Problem Scenario 1; even though a signer can never
   know the true path mail will take before delivery, the implication is
   that if the message is lacking a valid signature the message is
   either malicious or is the responsibility of the signing domain to
   avoid whatever broke the signature.

      [Informative Note: in terms of a receiving filter, one may choose
      to treat scenario 2 much more harshly than scenario 1; where
      scenario 1 looks odd, scenario 2 looks like something is very

   1.  Mail with a [RFC2822].From A purportedly sends to B with a
       missing or broken DKIM signature from A

   2.  B would like to know whether that is an expected state of

   3.  A provides information that it signs all outgoing mail, but
       places an expectation that it should arrive with an intact
       signature, and that the receiver should be suspicious if it does

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   4.  B could use this information to bias its filters such that it
       looks much more suspicious.

4.3.  Problem Scenario 3: Domain Sends No Mail

   A domain may not intend to send mail at all.  In such a case, it
   could be advantageous for a receiver to know the domain's intent and
   would be likely to treat such mail very suspiciously.  It has been
   noted that a solution to Problem Scenario 2 could potentially be used
   to emulate this practice.  In reality, they are close but not
   precisely the same semantics.  That is, a piece of email purporting
   to come from a domain which claims to send none is illegitimate on
   its face, whereas there may be some lingering doubt with Problem
   Scenario 2 given the possibility in deployments between whether one
   should publish scenario 1 and 2, etc.

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5.  SSP Deployment Scenarios

   Given the problems enumerated above for which we'd like The Protocol
   to provide information to recipients, there are a number of scenarios
   that are not related to the problems that are to be solved, per se,
   but the actual mechanics of implementing/deploying the information
   service that The Protocol would provide.

5.1.  Deployment Scenario 1: Outsourced Signing

   Many domains do not run their own mail infrastructure, or may
   outsource parts of it to third parties.  It is desirable for a domain
   holder to have the ability delegate to other entities the ability to
   sign for the domain holder.  One obvious use scenario is a domain
   holder from a small domain that needs to have the ability for their
   outgoing ISP to sign all of their mail on behalf of the domain
   holder.  Other use scenarios include outsourced bulk mail for
   marketing campaigns, as well as outsourcing various business
   functions such as insurance benefits, etc.

5.2.  Deployment Scenario 2: Determinism in Lookup Mechanism

   The Protocol's client will generally be deployed on incoming mail
   streams to provide the information as proposed in the problem
   scenarios.  As such, it is envisioned that the RFC2822.From address
   would be used as a basis for the lookup.  In particular, the domain
   part of the address would be consulted in some manner to fetch the
   published information.  There is a fairly trivial attack against a
   naive use of this algorithm which is called the subdomain attack:
   that is, a domain needs to not only publish a policy for a given DNS
   label it controls, but it also may need to protect all subdomains of
   that label as well.  If this characteristic is not met, an attacker
   would need only create a possibly fictitious subdomain which was not
   covered by The Protocol's information service

   In widening the scope to have the possibility of all subdomains
   inherit the parent practice, a number of algorithms could be employed
   -- all seemingly with their own set of engineering tradeoffs.  A
   common theme in the production of this draft was that the number of
   lookups, on average should be small, and that the discovery process
   should always be bound to some small finite number of queries.

5.3.  Deployment Scenario 3: Resent Original Mail

   Resent mail is a common occurrence in many scenarios in the email
   world of today.  For example, Alice sends a DKIM signed message with
   a published practice of signing all messages to Bob's mailing list.
   Bob, being a good net citizen, wants to be able to take his part of

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   the responsibility of the message in question, so he DKIM signs the
   message, perhaps corresponding to the Sender address.

   Note that this scenario is completely orthogonal to whether Alice's
   signature survived Bob's mailing list: Bob merely wants to assert his
   part in the chain of accountability for the benefit of the ultimate
   receivers.  It would be useful for this practice to be encouraged as
   it gives a more accurate view of who handled the message.  It also
   has the side benefit that remailers that are not friendly to DKIM
   first party signatures (ie, break them) can be potentially assessed
   by the receiver based on the receiver's opinion of the signing
   domains that actually survived.

5.4.  Deployment Scenario 4: Incremental Deployment of Signing

   As a practical matter, it may be difficult for a domain to roll out
   DKIM signing such that they can publish the DKIM Signing Complete
   practice given the complexities of the user population, outsourced
   vendors sending on its behalf, etc.  This leaves open an exploit that
   high-value mail such as in Problem Scenario 2 must be classified to
   the least common denominator of the published practices.  It would be
   desirable to allow a domain holder to publish a list of exceptions
   which would have a stronger practices statement.

   For example, might be ready to say that is always signed, but the rest of the
   domain, say, is not.  Another situation is that the practices of some
   address local parts in a given domain are not the same as practices
   of other local parts.  Using the same example of being a transactional kind of email
   which would like to publish very strong practices, mixed in with the
   rest of the user population local parts which may go through mailing
   lists, etc, for which a less strong statement is appropriate.

   It should be said that DKIM, through the use of subdomains, can
   already support this kind of differentiation.  That is, in order to
   publish a strong practice, one only has to segregate those cases into
   different subdomains.  For example: *
   would publish a strong practice while * could
   publish a more permissive practice.

5.5.  Deployment Scenario 5: Transport Scenarios

   Email service provides an any-any mesh of potential connections: all
   that is required is the publication of an MX record and a SMTP
   listener to receive mail.  Thus the use of The Protocol is likely to
   fall into two main scenarios, the first of which are large, well
   known domains who are in constant contact with one another.  In this

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   case caching of records is essential for performance, including the
   caching of the non-existence of records (ie, negative caching).

   The second main scenario is when a domain exchanges mail with a much
   smaller volume domain.  This scenario can be both perfectly normal as
   with the case of vanity domains, and sadly a vector for those sending
   mail for anti-social reasons.  In this case we'd like the burden to
   The Protocol querier to be low, since many of the lookups will not
   provide a useful answer.  Likewise, it would be advantageous to have
   upstream caching here as well so that, say, a mailing list exploder
   on a small domain does not result in an explosion of queries back at
   the root server for the small domain.

5.6.  Deployment Scenario 6: Human Legibility of Practices

   While The Protocol records are likely to be primarily consumed by an
   automaton, for the foreseeable future they are also likely to be
   inspected by hand.  It would be nice to have the practices stated in
   a fashion which is also intuitive to the human inspectors.

      [Author's $.02: it's been amply demonstrated that simple human
      readable labels are often misconstrued.  Opaque symbols do have
      the advantage that you need to consult the definition to determine
      its meaning rather than just intuiting what it "ought" to mean.

5.7.  Deployment Scenario 7: Cryptographic Downgrade Attacks

   There is a downgrade attack possible where a DKIM signature is hashed
   with a previously acceptable but now insecure hash algorithm.  This
   could allow attackers to send their chosen text which is apparently
   signed by the targeted domain.  It would be advantageous for a domain
   to publish what the allowable signing/hashing algorithms are to
   prevent this downgrade attack.

5.8.  Deployment Scenario 8: Extensibility

   While this document pertains only to requirements surrounding DKIM
   signing practices, it would be beneficial for the protocol to be able
   to extend to other protocols.

5.9.  Deployment Scenario 9: Security

   The protocol must be able to withstand life in a hostile open
   internet environment.  These include DoS attacks, and especially DoS
   attacks that leverage themselves through amplification inherent in
   the protocol.  In addition, while a useful protocol may be built
   without strong source authentication provided by the information

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   service, a path to strong source authentication should be provided by
   the protocol, or underlying protocols.

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6.  Requirements

   This section defines the requirements for The Protocol.  As with most
   requirements drafts, these requirements define the MINIMUM
   requirements that a candidate protocol must provide.  It should also
   be noted that The Protocol must fulfill all of the requirements.

      [Informative Note: it's not clear to the author that all of the
      provisional requirements can fulfill the harder requirements.  If
      this is determined to be true, the provisional requirement should
      either be dropped or the harder requirements revised]

6.1.  Discovery Requirements

   Receivers need a means of obtaining information about a sender's DKIM
   practices.  This requires a means of discovering where the
   information is and what it contains.

   1.  The author is the first-party sender of a message, as specified
       in the [rfc2822].From field.  The Protocol's information is
       associated with the author's domain name and is published
       subordinate to that domain name.

   2.  In order to limit the cost of its use, any query service
       supplying The Protocol's information must provide a definitive
       responsive within a small, deterministic number of query

          [Informative Note: this, for all intents and purposes is a
          prohibition on anything that might produce loops or result in
          extended delays and overhead; also though "deterministic"
          doesn't specify how many exchanges, the expectation is "few".]

          [Refs: Deployment Scenario 2]

   3.  The Protocol's publishing mechanism MUST be defined to produce
       unambiguous semantics, particularly with respect to other
       information that might share the publication mechanism.

          [Informative note: An example of ambiguity is sharing resource
          record types within a common DNS leaf.  Hence, uniquely
          defined leaf names or uniquely defined resource record types
          will ensure unambiguous reference.]

          [Refs: Deployment Scenario 2]

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6.2.  Transport requirements

   1.  Widespread deployment of the transport layer would be highly
       desirable, especially if riding on top of a true transport layer
       (eg, TCP, UDP).

          [Refs: Deployment Scenario 5, 8]

   2.  A low-cost query/response in terms of latency time and the number
       of packets involved is highly desirable.

          [Refs: Deployment Scenario 5]

   3.  If the infrastructure doesn't provide caching (ala DNS), the
       records retrieved will need time-to-live values to allow querying
       verifiers to maintain their own caches.  Existing caching
       infrastructure is, however, highly desirable.

          [Refs: Deployment Scenario 5]

   4.  Multiple geographically and topologically diverse servers must be
       supported for high availability

          [Refs: Deployment Scenario 5, 8]

6.3.  Practice and Expectation Requirements

   In this section, a Practice is defined as a true statement according
   to the [RFC2822].From domain holder of its intended externally
   visible behavior.  An Expectation combines with a Practice to convey
   what the domain holder considers the likely outcome of the
   survivability of the Practice for a receiver.  For example, a
   Practice that X is true when it leaves the domain, and an Expectation
   that it will|will-not|may|may-not remain true for some/all receivers.

   1.   The Protocol MUST be able to make Practices and Expectation
        assertions about the [RFC2822].From address in the context of
        DKIM.  The Protocol will not make assertions about other
        addresses for DKIM at this time.

           [Refs: Problem Scenario 1,2]

   2.   [PROVISIONAL] The Protocol MUST be able to publish a Practice
        which is indicative that domain doesn't send mail.

           [Refs: Problem Scenario 3]

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   3.   If the Discovery process would be shortened by publication of a
        "null" practice, the protocol SHOULD provide a mechanism to
        publish such a practice.

           [INFORMATIVE NOTE: there seems to be widespread consensus
           that a "neutral" or "I sign some mail" practice is useless to
           receivers.  However, a null practice may help to cut short
           the policy lookup mechanism if it's published, and if that's
           the case it seems worthwhile.  Also, a null policy may have
           some forensic utility, such as gaging the number of domains
           considering/using DKIM for example.]

           [Refs: Deployment Scenario 2]

   4.   The Protocol MUST be able to publish a Practice that the
        domain's signing behavior is "DKIM Signing Complete"

           [Refs: Problem Scenario 1]

   5.   The Protocol MUST be able to publish an Expectation that a
        verifiable First Party DKIM Signature should be expected on
        receipt of a message.

           [Refs: Problem Scenario 2]

   6.   Practices and Expectations MUST be presented in the Protocol
        syntax using as intuitive a descriptor as possible.  For
        example, p=? would be better represented as p=unknown.

           [Refs: Deployment Scenario 6]

   7.   The Protocol MUST NOT invent a different means of allowing
        affiliated parties to sign on a domain's behalf.  Because DKIM
        uses DNS to store selectors, there is always the ability for a
        domain holder to delegate all or parts of the _domainkey
        subdomain to an affiliated party of the domain holder's
        choosing.  That is, the domain holder may be able to set a NS
        record for to, say, an email provider who
        manages that namespace.  There is also the ability for the
        domain holder to partition its namespace into subdomains to
        further constrain how third parties.  For example, a domain
        holder could delegate only to a
        third party to constrain the third party to only be able to
        produce valid signatures in the subdomain.
        Last, a domain holder can even use CNAME's to delegate
        individual leaf nodes.

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   8.   [PROVISIONAL] The protocol MUST have the ability to provide
        practices and expectations keyed off of the local part of the
        [RFC2822].From address.  As with all provisional requirements,
        this requirement must not be in conflict with other
        requirements, including DNS considerations, etc.

           [INFORMATIVE NOTE: this requirement seems to have rather weak
           support.  It's mainly been added so that it can be issue-
           tracked. /mat]

           [Refs: Deployment Scenario 4]

   9.   Practices and Expectations MUST be presented as an information
        service from the signing domain to be consumed as an added
        factor to the receiver's local policy.  In particular, a
        Practice or Expectation MUST NOT mandate any disposition stance
        on the receiver.

           [Refs: Problem Scenario 1, 2, 3]

   10.  There is no requirement that The Protocol publish a Practices of
        any/all third parties that MUST NOT sign on the domain holder's
        behalf.  This should be considered out of scope.

           [INFORMATIVE NOTE: this is essentially saying that the
           protocol doesn't have to concern itself with being a
           blacklist repository.]

           [Refs: Problem Scenario 1-2]

   11.  The Protocol MUST NOT be required to be invoked if a valid first
        party signature is found.

           [Refs: Deployment Scenario 2]

   12.  [PROVISIONAL] A domain holder MUST be able to publish a Practice
        which enumerates the acceptable cryptographic algorithms for
        signatures purportedly from that domain.

           [Refs: Deployment Scenario 7]

   13.  The protocol MUST NOT provide a mechanism which impugns the
        existence of non-first party signatures in a message.  A
        corollary of this requirement is that the protocol MUST NOT link
        practices of first party signers with the practices of third
        party signers.

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           [INFORMATIVE NOTE: the main thrust of this requirement is
           that practices should only be published for that which the
           publisher has control, and should not meddle in what is
           ultimately the local policy of the receiver.]

           [Refs: Deployment Scenario 3]

6.4.  Extensibility and Forward Compatibility Requirements

   1.  The Protocol MUST NOT extend to any other protocol than DKIM for
       email at this time.  The Protocol SHOULD be able to extend for
       protocols other than DKIM.

          [Refs: Deployment Scenario 8]

   2.  The Protocol MUST be able to add new Practices and Expectations
       within the existing discovery/transport/practices in a backward
       compatible fashion.

          [Refs: Deployment Scenario 8]

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7.  Security Requirements

   1.  Minimize DoS potential: The Protocol for a high-value domain is
       potentially a high-value DoS target, especially since the
       unavailability of The Protocol's record could make unsigned
       messages less suspicious.

   2.  Amplification Attacks: The Protocol MUST NOT make highly
       leveraged amplification or make-work attacks possible.  In
       particular any amplification must be O(1).

          [Author's question: is it really O(1)? or O(n)?]

          [Refs: Deployment Scenario 9]

   3.  Authenticity: The Protocol MUST have the ability for a domain
       holder to provide The Protocol's data such that a receiver can
       determine that it is authentically from the domain holder with a
       large degree of certainty.  The Protocol may provide means which
       provide less certainty in trade off for ease of deployment.

          [Refs: Deployment Scenario 9]

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8.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

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9.  Security Considerations

   This draft defines requirements for a new protocol and the security
   related requirements are defined above.  There is an expectation that
   The Protocol will not always be required to have source
   authentication of the practices information which is noteworthy.

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10.  Acknowledgments

   Due to flipping in the market and raising interest rates, this home
   is no longer free.  Dave Crocker and Jim Fenton helped me raise the
   walls on this draft and are accorded a room at the inn.  The inn is
   not yet full, however.

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11.  References

11.1.  Normative References

              Allman, E., "DomainKeys Identified Mail (DKIM)
              Signatures", draft-ietf-dkim-base-04 (work in progress),
              July 2006.

   [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.

11.2.  Informative References

              Hansen, T., "DomainKeys Identified Mail (DKIM) Service
              Overview", draft-ietf-dkim-overview-01 (work in progress),
              June 2006.

              Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", draft-ietf-dkim-threats-03 (work
              in progress), May 2006.

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Author's Address

   Michael Thomas
   Cisco Systems
   606 Sanchez St
   San Francisco, California  94114

   Phone: +1-408-525-5386
   Fax:   +1-408-525-5386

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Full Copyright Statement

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