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INFORMATIONAL

Network Working Group                                          M. Thomas
Request for Comments: 5016                                 Cisco Systems
Category: Informational                                     October 2007


                          Requirements for a
      DomainKeys Identified Mail (DKIM) Signing Practices Protocol

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   DomainKeys Identified Mail (DKIM) provides a cryptographic mechanism
   for domains to assert responsibility for the messages they handle.  A
   related mechanism will allow an administrator to publish various
   statements about their DKIM signing practices.  This document defines
   requirements for this mechanism, distinguishing between those that
   must be satisfied (MUST), and those that are highly desirable
   (SHOULD).




























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Table of Contents

   1. Introduction ....................................................2
   2. Definitions and Requirements Language ...........................3
   3. SSP Problem Scenarios ...........................................4
      3.1. Problem Scenario 1: Is All Mail Signed with DKIM? ..........4
      3.2. Problem Scenario 2: Illegitimate Domain Name Use ...........5
   4. SSP Deployment Considerations ...................................6
      4.1. Deployment Consideration 1: Outsourced Signing .............6
      4.2. Deployment Consideration 2: Subdomain Coverage .............6
      4.3. Deployment Consideration 3: Resent Original Mail ...........7
      4.4. Deployment Consideration 4: Incremental Deployment
           of Signing .................................................7
      4.5. Deployment Consideration 5: Performance and Caching ........8
      4.6. Deployment Consideration 6: Human Legibility of Practices ..8
      4.7. Deployment Consideration 7: Extensibility ..................8
      4.8. Deployment Consideration 8: Security .......................8
   5. Requirements ....................................................9
      5.1. Discovery Requirements .....................................9
      5.2. SSP Transport Requirements ................................10
      5.3. Practice and Expectation Requirements .....................10
      5.4. Extensibility and Forward Compatibility Requirements ......13
   6. Requirements for SSP Security ..................................13
   7. Security Considerations ........................................13
   8. Acknowledgments ................................................13
   9. References .....................................................14
      9.1. Normative References ......................................14

1.  Introduction

   DomainKeys Identified Mail [RFC4871] defines a message level signing
   and verification mechanism for email.  While a DKIM signed message
   speaks for itself, there is ambiguity if a message doesn't have a
   valid first party signature (i.e., on behalf of the [RFC2822].From
   address): is this to be expected or not?  For email, this is an
   especially difficult problem since there is no expectation of a
   priori knowledge of a sending domain's practices.  This ambiguity can
   be used to mount a bid down attack that is inherent with systems like
   email that allow optional authentication: if a receiver doesn't know
   otherwise, it should not assume that the lack of a valid signature is
   exceptional without other information.  Thus, an attacker can take
   advantage of the ambiguity and simply not sign messages.  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 an unsigned piece of email.






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   This document defines the requirements for a mechanism that permits
   the publication of Sender Signing Practices (SSP).  The document is
   organized into two main sections: first, a Problem and Deployment
   Scenario section that describes the problems that SSP is intended to
   address as well as the deployment issues surrounding the base
   problems, and the second section is the Requirements that arise
   because of those scenarios.

2.  Definitions and Requirements Language

   o  Domain Holder: the entity that 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 signing identity (the d= tag or the more
      specific identity i= tag) matches the first party address.
      "Matches" in this context is defined in [RFC4871].

   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 is not required to correspond to
      a header field address such as the contents of Sender or List-Id,
      etc.

   o  Practice: a statement according to the [RFC2822].From domain
      holder of externally verifiable behavior in the email messages it
      sends.

   o  Expectation: an expectation combines with a practice to convey
      what the domain holder considers the likely survivability of the
      practice for a receiver, in particular receivers that may be more
      than one SMTP hop away.

   o  DKIM Signing Complete: a practice where the domain holder asserts
      that all legitimate mail will be sent with a valid first party
      signature.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].






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3.  SSP Problem Scenarios

   The email world is a diverse place with many deployment
   considerations.  This section outlines expected usage scenarios where
   DKIM signing/verifying will take place, and how a new protocol might
   help to clarify the relevance of DKIM-signed mail.

3.1.  Problem Scenario 1: Is 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, ensured that all legitimate mail 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.  Thus, the receiver is at a disadvantage in not
   knowing whether or not it should expect all mail to be signed from a
   given domain.  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 that allows a receiver to query for the
   practices and expectations of the first party domain when no valid
   first party 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, unrestricted use patterns of
   mail (e.g., where users may be members of mailing lists, etc.) will
   likely suffer occasional, non-malicious signature failure in transit.
   While probably not a large percentage of total traffic, this kind 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 its 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 system.







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   The following exchange illustrates problem scenario 1.

   1.  Mail with a [RFC2822].From domain Alice is sent to domain Bob
       with a missing or broken DKIM first party signature from Alice.

   2.  Domain Bob would like to know whether that is an expected state
       of affairs.

   3.  Domain Alice provides information that it signs all outgoing
       mail, but places no expectation on whether it will arrive with an
       intact first party signature.

   4.  Domain Bob could use this information to bias its filters to
       examine the message with some suspicion.

3.2.  Problem Scenario 2: Illegitimate Domain Name Use

   A class of mail typified by transactional mail from high-value
   domains is currently 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 mail
   would like the ability to give an enhanced guarantee that mail sent
   with their domain name should always arrive with the proof that the
   domain holder consented to its transmission.  That is, the message
   should contain a valid first party signature as defined above.

   From a receiver's standpoint, knowing that a domain not only signs
   all of its mail, but places a very high value on the receipt of a
   valid first party signature from that domain is helpful.  Hence, a
   receiver knows that the sending domain signs all its mail, and that
   the sending domain considers mail from this domain particularly
   sensitive in some sense, and is asking the receiver to be more
   suspicious than it otherwise might be of a broken or missing first-
   party signature.  A receiver with the knowledge of the sender's
   expectations in hand might choose to process messages not conforming
   to the published practices in a special manner.  Note that the
   ability to state an enhanced guarantee of a valid signature means
   that senders should expect mail that traverses modifying
   intermediaries (e.g., mailing lists, etc.) will likely be quarantined
   or deleted; thus, this scenario is more narrow than problem scenario
   1.

      Informative Note: a receiving filter may choose to treat scenario
      2 much more harshly than scenario 1; where scenario 1 looks odd,
      scenario 2 looks like something is very wrong.





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   1.  Mail with a [RFC2822].From domain Alice is sent to domain Bob
       with a missing or broken first party DKIM signature from domain
       Alice.

   2.  Domain Bob would like to know whether that is an expected state
       of affairs.

   3.  Domain Alice provides information that it signs all outgoing
       mail, and furthermore places an expectation that it should arrive
       with an intact first party signature, and that the receiver
       should be much more wary if it does not.

   4.  Domain Bob could use this information to bias its filters such
       that it examines the message with great suspicion.

4.  SSP Deployment Considerations

   Given the problems enumerated above for which we'd like SSP 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 SSP would provide.

4.1.  Deployment Consideration 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 to 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.

4.2.  Deployment Consideration 2: Subdomain Coverage

   An SSP client will perform lookups on incoming mail streams to
   provide the information as proposed in the problem scenarios.  The
   domain part of the first address of the [RFC2822].From will form the
   basis to fetch the published information.  A trivial attack to
   circumvent finding the published information can be mounted by simply
   using a subdomain of the parent domain that doesn't have published
   information.  This attack is called the subdomain attack: that is, a
   domain wants to not only publish a policy for a given DNS label it
   controls, but it would also like 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 that was not covered



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   by the SSP's information service.  Thus, it would be advantageous for
   SSP to not only cover a given domain, but all subdomains of that
   domain as well.

4.3.  Deployment Consideration 3: Resent Original Mail

   Resent mail is a common occurrence in many scenarios in the email
   world of today.  For example, domain Alice sends a DKIM-signed
   message with a published practice of signing all messages to domain
   Bob's mailing list.  Bob, being a good net citizen, wants to be able
   to take his part of 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 break DKIM first party
   signatures can be potentially assessed by the receiver based on the
   receiver's opinion of the signing domains that actually survived.

4.4.  Deployment Consideration 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, the
   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 that would have a more restrictive practices
   statement.  NB: this consideration has been deemed met by the
   mechanisms provided by the base DKIM signing mechanism; it is merely
   documented here as having been an issue.

   For example, bigbank.example.com might be ready to say that
   statements@bigbank.example.com 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
   statements@bigbank.example.com being a transactional kind of email
   that 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.





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   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: accounts.bigbank.example.com
   would publish constrained practices, while
   corporateusers.bigbank.example.com might publish more permissive
   practices.

4.5.  Deployment Consideration 5: Performance and Caching

   Email service provides an any-any mesh of potential connections: all
   that is required is the publication of an MX record and an SMTP
   listener to receive mail.  Thus, the use of SSP is likely to fall
   into two main scenarios, the first of which are large, well-known
   domains that are in constant contact with one another.  In this case,
   caching of records is essential for performance, including the
   caching of the non-existence of records (i.e., 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 unfortunately, a vector for
   those sending mail for anti-social reasons.  In this case, we'd like
   the message exchange burden to SSP 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 and authoritative server for
   the small domain.

4.6.  Deployment Consideration 6: Human Legibility of Practices

   While SSP 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 that is also intuitive to the human inspectors.

4.7.  Deployment Consideration 7: 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.

4.8.  Deployment Consideration 8: Security

   SSP 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



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

5.  Requirements

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

5.1.  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.  SSP'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 SSP's information MUST provide a definitive response
       within a small, deterministic number of message exchanges under
       normal operational conditions.

         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 Considerations, Sections 4.2 and 4.5.

   3.  SSP's publishing mechanism MUST be defined such that it does not
       lead to multiple resource records of the same type for different
       protocols residing at the same location.

         Informative note: an example is multiple resource record of the
         same type within a common DNS leaf.  Hence, uniquely defined
         leaf names or uniquely defined resource record types will
         ensure unambiguous referencing.

         Refs: Deployment Consideration, Section 4.2.







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   4.  SSP retrieval SHOULD provide coverage for not only a given domain
       but all of its subdomains as well.  It is recognized that there
       is some reasonable doubt about the feasibility of a widely
       accepted solution to this requirement.  If the working group does
       not achieve rough consensus on a solution, it MUST document the
       relevant security considerations in the protocol specification.

         Refs: Deployment Considerations, Sections 4.2 and 4.5.

5.2.  SSP Transport Requirements

   The publication and query mechanism will operate as an internet-based
   message exchange.  There are multiple requirements for this lower-
   layer service:

   1.  The exchange SHOULD have existing widespread deployment of the
       transport layer, especially if riding on top of a true transport
       layer (e.g., TCP, UDP).

         Refs: Deployment Considerations, Sections 4.5 and 4.7.

   2.  The query/response in terms of latency time and the number of
       messages involved MUST be low (less than three message exchanges
       not counting retransmissions or other exceptional conditions).

         Refs: Deployment Consideration, Section 4.5.

   3.  If the infrastructure doesn't provide caching (a la DNS), the
       records retrieved MUST provide initiators the ability to maintain
       their own cache.  The existing caching infrastructure is,
       however, highly desirable.

         Refs: Deployment Consideration, Section 4.5.

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

         Refs: Deployment Considerations, Sections 4.5 and 4.7.

5.3.  Practice and Expectation Requirements

   As stated in the definitions section, a practice is a statement
   according to the [RFC2822].From domain holder of externally
   verifiable behavior in the email messages it sends.  As an example, a
   practice might be defined such that all email messages will contain a
   DKIM signature corresponding to the [RFC2822].From address.  Since
   there is a possibility of alteration between what a sender sends and
   a receiver examines, an expectation combines with a practice to



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   convey what the [RFC2822].From domain considers the likely outcome of
   the survivability of the practice at a receiver.  For example, a
   practice that a valid DKIM for the [RFC2822].From address is present
   when it is sent from the domain, and an expectation that it will
   remain present and valid for all receivers whether topologically
   adjacent or not.

   1.  SSP MUST be able to make practices and expectation assertions
       about the domain part of a [RFC2822].From address in the context
       of DKIM.  SSP will not make assertions about other addresses for
       DKIM at this time.

         Refs: Problem Scenarios 1 and 2, Sections 3.1 and 3.2.

   2.  SSP MUST provide a concise linkage between the [RFC2822].From and
       the identity in the DKIM i= tag, or its default if it is missing
       in the signature.  That is, SSP MUST precisely define the
       semantics of what qualifies as a first party signature.

         Refs: Problem Scenarios 1 and 2, Sections 3.1 and 3.2.

   3.  SSP MUST be able to publish a practice that the domain's signing
       behavior is "DKIM Signing Complete".  That is, all messages were
       transmitted with a valid first party signature.

         Refs: Problem Scenario 1, Section 3.1.

   4.  SSP 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, Section 3.2.

   5.  Practices and expectations MUST be presented in SSP syntax using
       as intuitive a descriptor as possible.  For example, p=? would be
       better represented as p=unknown.

         Refs: Deployment Consideration, Section 4.6.

   6.  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 set an NS
       record for _domainkey.example.com to delegate to an email
       provider who manages the entire namespace.  There is also the
       ability for the domain holder to partition its namespace into
       subdomains to further constrain third parties.  For example, a
       domain holder could delegate only _domainkey.benefits.example.com



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       to a third party to constrain the third party to only be able to
       produce valid signatures in the benefits.example.com subdomain.
       Last, a domain holder can even use CNAME's to delegate individual
       leaf nodes.  Given the above considerations, SSP need not invent
       a different means of allowing affiliated parties to sign on a
       domain's behalf at this time.

         Refs: Deployment Consideration, Section 4.4.

   7.  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 Scenarios 1 and 2, Sections 3.1 and 3.2.

   8.  There is no requirement that SSP publish 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 Scenarios 1 and 2, Sections 3.1 and 3.2.

   9.  SSP MUST NOT be required to be invoked if a valid first party
       signature is found.

         Refs: Deployment Consideration, Section 4.2.

   10. SSP MUST NOT provide a mechanism that 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.

         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 Consideration, Section 4.3.








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5.4.  Extensibility and Forward Compatibility Requirements

   1.  SSP MUST NOT extend to any protocol other than DKIM for email at
       this time.  SSP SHOULD be extensible for protocols other than
       DKIM.

         Refs: Deployment Consideration, Section 4.7.

   2.  SSP MUST be able to add new practices and expectations within the
       existing discovery/transport/practices in a backward compatible
       fashion.

         Refs: Deployment Consideration, Section 4.7.

6.  Requirements for SSP Security

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

   2.  SSP MUST NOT make highly leveraged amplification or make-work
       attacks possible.  In particular, the work and message exchanges
       involved MUST be order of a constant.

         Refs: Deployment Consideration, Section 4.8.

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

         Refs: Deployment Consideration, Section 4.8.

7.  Security Considerations

   This document defines requirements for a new protocol and the
   security related requirements are defined above.  Since it is
   expected that the new protocol will use the DNS as a basis for the
   published SSP information, most if not all of the threats described
   in [RFC4686] will be applicable.

8.  Acknowledgments

   Dave Crocker and Jim Fenton provided substantial review of this
   document.  Thanks also to Vijay Gurbani and David Harrington for
   their helpful last call reviews.




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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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

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

Author's Address

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

   Phone: +1-408-525-5386
   Fax:   +1-408-525-5386
   EMail: mat@cisco.com























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   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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Thomas                       Informational                     [Page 15]


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