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A Session Initiation Protocol (SIP) Response Code for Rejected Calls

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Eric Burger
Last updated 2018-07-16
Replaced by draft-ietf-sipcore-rejected, RFC 8688
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SIPCORE                                                        E. Burger
Internet-Draft                                     Georgetown University
Intended status: Standards Track                           July 16, 2018
Expires: January 17, 2019

  A Session Initiation Protocol (SIP) Response Code for Rejected Calls


   This document defines the 608 (Rejected) SIP response code.  This
   response code enables calling parties to learn their call was
   rejected by an intermediary and will not be answered.  As a 6xx code,
   the caller will be aware that future attempts to contact the same UAS
   will be likely to fail.  The present use case driving the need for
   the 608 response code is when the intermediary is an analytics
   engine.  In other words, the rejection is by a machine or other
   process, as opposed to a human at the target UAS indicating the call
   was not wanted.  This document also defines the use of the Call-Info
   header in 608 responses to enable rejected callers to contact
   entities that blocked their calls in error.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 17, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   The IETF has been addressing numerous issues surrounding how to
   handle unwanted and, depending on the jurisdiction, illegal calls
   [RFC5039].  Technologies such as STIR [RFC7340] and SHAKEN [SHAKEN]
   address cryptographic signing and attestation, respectively, of
   signaling to ensure the integrity and authenticity of the asserted

   This document describes a new SIP response code, 608, which allows
   calling parties to learn their call was rejected by an intermediary.
   As will be described below, we need a distinct indicator to
   differentiate between a user rejection and an intermediary's
   rejection of a call.  In many jurisdictions, some calls, even if
   unwanted by the user, may not be blocked unless there is an explicit
   user request.  Moreover, users may misidentify the nature of a
   caller.  For example, a legitimate caller may call a user who finds
   the call to be unwanted.  However, instead of marking the call as
   unwanted, the user may mark the call as illegal.  With that
   information, an analytics engine may determine that all calls from
   that source should be blocked.  However, in many jurisdictions
   blocking calls from that source from other users may not be legal.
   Likewise, one can envision jurisdictions that allow an operator to
   block such calls, but only if there is a remediation mechanism in
   place to address false positives.

   Today, some call blocking services may return responses such as 604
   (Does Not Exist Anywhere).  This might be a strategy to attempt to
   get a destination's address removed from a calling database.
   However, other network elements might interpret this to mean the user
   truly does not exist and result in the user not being able to receive
   calls.  As well, in many jurisdictions, providing false signaling is

   The 608 response code addresses this need of addressing falsely
   blocked calls.  Specifically, this code informs the UAC the call was
   blocked, the call was blocked by an intermediary, and, to satisfy
   some jurisdiction's requirements for providing a redress mechanism,
   how to contact the operator of the intermediary.

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   In the call handling ecosystem, users can explicitly reject a call or
   later mark a call is being unwanted by issuing a 607 SIP response
   code (Unwanted) [RFC8197].  Figure 1 shows the operation of the 607
   SIP response code.  The UAS indicates the call was unwanted.  As
   RFC8197 explains, not only does the called party desire to reject
   that call, they wish to let their proxy know they do not ever want to
   get calls from that source.  The proxy may send call information to a
   call analytics engine.  For various reasons described in RFC8197, if
   a network operator receives multiple reports of unwanted calls, that
   may indicate the entity placing the calls is likely to be a source of
   unwanted calls for many people.  As such, other users of the service
   provider's service may wish the service provider to automatically
   reject calls on their behalf based on that and other analytics.

   Another value of the 607 rejection is presuming the proxy forwards
   the response code to the UAC, the calling UAC or intervening proxies
   know the user is not interested in receiving calls from that sender.

                      |   Call    |
                      | Analytics |
                      |  Engine   |
                            ^ (likely not SIP)
   +-----+    607     |  Called   |    607    +-----+
   | UAS | <--------> |  Party    | <-------> | UAC |
   +-----+            |  Proxy    |           +-----+

                    Figure 1: Unwanted (607) Call Flow

   For calls rejected with a 607 from a legitimate caller, receiving a
   607 response code can inform the caller to stop attempting to call
   the user.  Moreover, if the legitimate caller believes the user is
   rejecting their calls in error, they can use other channels to
   contact the user.  For example, if a pharmacy calls a user to let
   them know their prescription is available for pickup and the user
   mistakenly thinks the call is unwanted and issues a 607 response
   code, the pharmacy, having an existing relationship with the
   customer, can send the user an email, also noting they might consider
   not rejecting their calls in the future.

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                    +--------+          +-----------+
                    | Called |          |   Call    |
   +-----+          | Party  |          | Analytics |  +-----+
   | UAC |          | Proxy  |          |  Engine   |  | UAS |
   +--+--+          +---+----+          +----+------+  +--+--+
      |  INVITE         |                    |            |
      | --------------> |  INVITE            |            |
      |                 | ------------------------------> |
      |                 |                    |            |
      |                 |                    |       607  |
      |                 | <------------------------------ |
      |                 |                    |            |
      |                 |  Unwanted call     |            |
      |            607  | -----------------> |            |
      | <-------------- |  indicator         |            |
      |                 |                    |            |

                  Figure 2: Unwanted (607) Ladder Diagram

   However, things get more complicated if an intermediary, such as a
   third-party provider of call management services that classify calls
   based on the relative likelihood the call is unwanted, misidentifies
   the call as unwanted.  Figure 3 shows this case.  In this situation,
   it would be beneficial for the caller to be able to learn who
   rejected the call, so they might be able to correct the

   In this situation, one might be tempted to have the intermediary use
   the 607 response code.  607 indicates to the caller the subscriber
   did not get the call and they do not want the call.  However, RFC8197
   specifies that one of the uses of 607 is to inform analytics engines
   that a user (human) has rejected a call.  The problem here is network
   elements downstream from the intermediary might interpret the 607 as
   a user (human) marking the call as unwanted, as opposed to a
   statistical, machine learning, vulnerable to the base rate fallacy
   [BaseRate] algorithm rejecting the call.  In other words, those
   downstream entities should not be relying on another entity
   'deciding' the call is unwanted.  By distinguishing between a (human)
   user rejection and an intermediary's statistical rejection, a
   downstream network element that sees a 607 response code can weight
   it as a human rejection in its call analytics.

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                      |   Call    |
                      | Analytics |
                      |  Engine   |
                         ^     | (likely not SIP)
                         |     v
   +-----+            |  Called   |    608    +-----+
   | UAS |            |  Party    | <-------> | UAC |
   +-----+            |  Proxy    |           +-----+

                    Figure 3: Rejected (608) Call Flow

   It is useful for blocked callers to have a redress mechanism.  One
   can imagine that some jurisdictions will require it.  However, we
   must be mindful that most of the calls that will be blocked will, in
   fact, be illegal and eligible for blocking.  Thus, providing
   alternate contact information for a user would be counterproductive
   to protecting that user from illegal communications.  This is another
   reason we do not propose to simply allow alternate contact
   information in a 607 response message.

                    +--------+          +-----------+
                    | Called |          |   Call    |
   +-----+          | Party  |          | Analytics |  +-----+
   | UAC |          | Proxy  |          |  Engine   |  | UAS |
   +--+--+          +---+----+          +----+------+  +--+--+
      |  INVITE         |                    |            |
      | --------------> |  Information from  |            |
      |                 | -----------------> |            |
      |                 |  INVITE            |            |
      |                 |            Reject  |            |
      |            608  | <----------------- |            |
      | <-------------- |            call    |            |
      |                 |                    |            |

                  Figure 4: Rejected (608) Ladder Diagram

   As such, we need a mechanism for indicating an intermediary rejected
   a call while providing contact information for the operator of the
   intermediary that provides call rejection services to the called
   party, without exposing the target user's contact information.

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2.  Terminology

   This document uses the terms "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "OPTIONAL" as described in BCP14 [RFC2119][RFC8174] when, and only
   when, they appear in all capitals, as shown here.

3.  Protocol Operation

   For clarity, this section uses the term 'intermediary' as the entity
   that acts as a SIP User Agent Server (UAS) on behalf of the user in
   the network, as opposed to the user's UAS (colloquially, but not
   necessarily, their phone).  The intermediary could be a back-to-back
   user agent (B2BUA) or a SIP Proxy.

3.1.  Intermediary Operation

   An intermediary MAY issue the 608 code in a failure response for an
   INVITE, MESSAGE, SUBSCRIBE, or other out-of-dialog SIP [RFC3261]
   request to indicate that an intermediary rejected the offered
   communication as unwanted by the user.  An intermediary MAY issue the
   608 as the value of the "cause" parameter of a SIP reason-value in a
   Reason header field [RFC3326].

   Unless there are indicators the calling party will use the contents
   of the Call-Info header for malicious purposes (see Section 6), if an
   intermediary issues a 608 code, the intermediary MUST include a Call-
   Info header in the response.

   If there is a Call-Info header, it MUST have the 'purpose' parameter
   of 'card'.  The value of the Call-Info header MUST refer to a valid
   vCard [RFC6350] object.

   The vCard referenced in the Call-Info header MUST include at least
   one of the URL, EMAIL, TEL, or ADR properties.  UACs supporting this
   specification MUST be prepared to receive a full vCard.  Call
   originators (at the UAC) can use the information returned by the
   vCard to contact the intermediary that rejected the call to appeal
   the intermediary's future blocking of the call attempt.  What the
   intermediary does if the blocked caller contacts the intermediary is
   outside the scope of this document.

   Proxies need to be mindful that a downstream intermediary may reject
   the attempt with a 608 while other paths may still be in progress.
   In this situation, the requirements stated in Section 16.7 of RFC3261
   [RFC3261] apply.  Specifically, the proxy should cancel pending
   transactions and must not create any new branches.  Note this is not

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   a new requirement but simply pointing out the existing 6xx protocol

3.2.  UAC Operation

   A UAC conforming to this specification MUST include the sip.608
   feature capability tag in the INVITE request.

   In any event, upon receiving a 608 response, UACs perform normal SIP
   processing for 6xx responses.

3.3.  Legacy Interoperation

   If the UAC indicates support for 608 and the intermediary issues a
   608, life is good as the UAC will receive all the information it
   needs to remediate an erroneous block by an intermediary.  However,
   what if the UAC does not understand 608?  Besides a UAC predating
   this specification, the could occur for callers from the legacy, non-
   SIP public switched network connecting to the SIP network via a media

   We address this situation by having the closest conforming network
   element (proxy or B2BUA) play an announcement in the media.  See
   Section 3.4 for requirements on the announcement.  The simple rule is
   a network element that inserts the sip.608 feature capability MUST be
   able to convey at a minimum whom to contact, ideally how to contact,
   the operator of the intermediary that rejected the call attempt.

   The degenerate case is the intermediary is the only element that
   understands the semantics of the 608 result code.  Obviously, any SIP
   device will understand that a 608 result code is a 6xx error.
   However, there are no other elements in the call path that understand
   the meaning of the value of the Call-Info header.  The intermediary
   knows this is the case as the INVITE request will not have the
   sip.608 feature capability.  In this case, one can consider the
   intermediary to be the element 'inserting' a virtual sip.608 feature
   capability.  As such, the intermediary MUST play the announcement,
   with the caveats described in Section 3.4 and Section 6.

   Now we take the case where a network element that understands the 608
   result code receives an INVITE for further processing.  A network
   element conforming with this specification MUST insert the sip.608
   feature capability, per the behaviors described in Section 4.2 of
   [RFC6809].  This information will be in the vCard referenced by the
   Call-Info header in the 608 response message.  Note this
   specification does not specify the mechanism for such notification to
   the UAC (see Section 3.4).

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   Do note that even if a network element plays an announcement
   describing the contents of the 608 result message, the network
   element MUST also ultimately send the 608 result code message as the
   final response to the INVITE.

   One aspect of using a feature capability is only the network elements
   that will consume (UAC) or play an announcement (media gateway, SBC,
   or proxy) need understand the sip.608 feature capability.  All other
   (existing) infrastructure can remain without modification, assuming
   they are conformant to Section 16.6 of [RFC3261], specifically they
   will pass headers such as "Feature-Capability: sip.608" unmodified.

3.4.  Announcement Requirements

   There are a few requirements on the element that will be doing the
   announcement for legacy interoperation.

   As noted above, the element that inserts the sip.608 feature
   capability is responsible for conveying the information referenced by
   the Call-Info header in the 608 response message.  However, this
   specification does not mandate the modality for conveying that

   Let us take the case where a telecommunications service provider
   controls the element inserting the sip.608 feature capability.  It
   would be reasonable to expect the service provider would play an
   actual announcement in the media path towards the UAC (caller).  It
   is important to note that the network element should be mindful of
   the media type requested by the UAC as it formulates the
   announcement.  For example, it would make sense for an INVITE that
   only indicated audio codecs in the SDP [RFC4566] to result in an
   audio announcement.  However, if the INVITE only indicated a real-
   time text codec, for example, the network element SHOULD send the
   information in a text format, not an audio format.

   It is also possible for the network element inserting the sip.608
   feature capability to be under the control of the same entity that
   controls the UAC.  For example, a large call center might have legacy
   UACs, but have a modern outbound calling proxy that understands the
   full semantics of the 608 result code.  In this case, it is enough
   for the outbound calling proxy to digest the Call-Info information
   and handle the information digitally, rather than 'transcoding' the
   Call-Info information for presentation to the caller.

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4.  Example

   Given an INVITE (shamelessly taken from [SHAKEN]:

   Max-Forwards: 69
   Contact: <sip:+12155551212@;rinstance=9da3088f36cc>
   To: <>
   From: "Alice" <>;tag=614bdb40
   P-Asserted-Identity: "Alice"<>,
   CSeq: 2 INVITE
   Content-Type: application/sdp
   Date: Tue, 16 Aug 2016 19:23:38 GMT
   Content-Length: 153

   o=- 13103070023943130 1 IN IP4
   c=IN IP4
   t=0 0
   m=audio 54242 RTP/AVP 0

   An intermediary could reply:

   SIP/2.0 608 Rejected
   Via: SIP/2.0/UDP;branch=z9hG4bK-524287-1
   From: "Alice" <>;tag=614bdb40
   To: <>
   CSeq: 2 INVITE
   Call-Info: <>;purpose=card

   A minimal vCard, in this example at
   complaints.vcf, could contain:

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   FN:Robocall Adjudication

   For an intermediary that provides a Web site for adjudication, the
   vCard could contain:

   FN:Robocall Adjudication

   For an intermediary that provides a telephone number and a postal
   address, the vCard could contain:

   FN:Robocall Adjudication
   ADR;TYPE=work;Argument Clinic;12 Main St;Anytown;AP;000000;Somewhere

   Note that it is up to the receiver to decide which modality, if any,
   it will use.

   Figure 5 depicts a call flow illustrating legacy interoperability.
   In this non-normative example, we see a UAC that does not support the
   full semantics for 608.  However, there is an SBC that does support
   608.  Per RFC6809 [RFC6809], the SBC can insert "sip.608" into the
   Feature-Caps header for the INVITE.  When the intermediary, labeled
   "Called Party Proxy" in the figure, rejects the call, it knows it can
   simply perform the processing described in this document.  Since the
   intermediary saw the sip.608 feature capability, it knows it does not
   need to send any media describing whom to contact in the event of an
   erroneous rejection.

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   |  Call   |
   | Engine  |
      ^   |
      |   v
   | Called  |     +-----+     +-----+     +---+     +-----+     +---+
   | Party   | <---+Proxy| <---+Proxy| <---+SBC| <---+Proxy| <---+UAC|
   | Proxy   |     +-----+     +-----+     +---+     +-----+     +---+
   +---------+                               |                     |
        |                                    |              INVITE |
        |                             INVITE |<--------------------|
        |<-----------------------------------|                     |
        |              Feature-Caps: sip.608 |                     |
        |                                    |                     |
        | 608 Rejected                       |                     |
        |----------------------------------->| 183                 |
        | Call-Info: <...>                   |-------------------->|
        |     [path for Call-Info elided     | SDP for media       |
        |      for illustration purposes]    |                     |
        |                                    |=== Announcement ===>|
        |                                    |                     |
        |                                    | 608                 |
        |                                    |-------------------->|
        |                                    | Call-Info: <...>    |

                        Figure 5: Legacy Operation

   When the SBC receives the 608 result code, it correlates that with
   the original INVITE from the UAC.  The SBC remembers that it inserted
   the sip.608 feature capability, which means it is responsible for
   somehow alerting the UAC the call failed and whom to contact.  At
   this point the SBC can play a prompt, either natively or through a
   mechanism such as NETANN [RFC4240], that sends the relevant
   information in the appropriate media to the UAC.  Note the SBC also
   still sends the full 608 response code, including the Call-Info
   header, towards the UAC.

5.  IANA Considerations

5.1.  SIP Response Code

   This document registers a new SIP response code, 608.  Please
   register the response code in the "Response Codes" subregistry of the

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   "Session Initiation Protocol (SIP) Parameters" registry at

   Response code: 608

   Description: Rejected

   Reference: [RFCXXXX]

5.2.  SIP Global Feature-Capability Indicator

   This document defines the feature capability sip.608 in the "SIP
   Feature-Capability Indicator Registration Tree" registry defined in

   Name: sip.608

   Description: This feature capability indicator, when included in a
   Feature-Caps header field of an INVITE request, indicates that the
   entity that inserted the sip.608 Feature-Caps value will be
   responsible for indicating to the caller any information contained in
   the 608 SIP response code, specifically the value referenced by the
   Call-Info header.

   Reference: [RFCXXXX]

6.  Security Considerations

   Intermediary operators need to be mindful of whom they are sending
   the 608 response to.  There is a risk that a truly malicious caller
   is being rejected.  This raises two issues.  The first is the caller,
   being alerted their call is being automatically rejected, may change
   their call behavior to defeat call blocking systems.  The second, and
   more significant risk, is that by providing a contact modality in the
   Call-Info field, the intermediary may be giving the malicious caller
   a vector for attack.  In other words, the intermediary will be
   publishing an address that a malicious actor may use to launch an
   attack on the intermediary.  Because of this, intermediary operators
   may wish to configure their response to only include a Call-Info
   field for INVITE or other initiating methods that are signed and pass
   validation by STIR [RFC8224].

   Another risk is for an attacker to purposely not include the sip.608
   feature capability in a flood of INVITE requests, direct those
   requests to stateless proxies, and direct the Contact header to a
   victim device.  Because the mechanism described here can result in an
   audio file being sent to the target of the Contact header, an
   attacker could use the mechanism described by this document as an

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   amplification attack, given a SIP INVITE can be under 1 kilobyte, and
   an audio file can be hundreds of kilobytes.  One remediation for this
   is for devices that insert a sip.608 feature capability only transmit
   media to what is highly likely to be the actual source of the call
   attempt.  A method for this is to only play media in response to an
   INVITE that is signed and passed validation by STIR [RFC8224].

7.  Acknowledgements

   This document liberally lifts from [RFC8197] in its text and
   structure.  However, the mechanism and purpose is quite different.
   Any errors are the current editor's and not the editor of RFC8197.
   Thanks also go to Ken Carlberg of the FCC, Russ Housley, Paul
   Kyzivat, and Tolga Asveren for their suggestions on improving the
   draft.  Tolga's suggestion to provide a mechanism for legacy
   interoperability served to expand the draft by 50%.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
              Header Field for the Session Initiation Protocol (SIP)",
              RFC 3326, DOI 10.17487/RFC3326, December 2002,

   [RFC6350]  Perreault, S., "vCard Format Specification", RFC 6350,
              DOI 10.17487/RFC6350, August 2011,

   [RFC6809]  Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism to
              Indicate Support of Features and Capabilities in the
              Session Initiation Protocol (SIP)", RFC 6809,
              DOI 10.17487/RFC6809, November 2012,

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

8.2.  Informative References

              Bar-Hillel, M., "The Base-Rate Fallacy in Probability
              Judgements", 4 1977,

   [RFC4240]  Burger, E., Ed., Van Dyke, J., and A. Spitzer, "Basic
              Network Media Services with SIP", RFC 4240,
              DOI 10.17487/RFC4240, December 2005,

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [RFC5039]  Rosenberg, J. and C. Jennings, "The Session Initiation
              Protocol (SIP) and Spam", RFC 5039, DOI 10.17487/RFC5039,
              January 2008, <>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

   [RFC8197]  Schulzrinne, H., "A SIP Response Code for Unwanted Calls",
              RFC 8197, DOI 10.17487/RFC8197, July 2017,

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,

   [SHAKEN]   Alliance for Telecommunications Industry Solutions (ATIS)
              and the SIP Forum, "Signature-based Handling of Asserted
              information using toKENs (SHAKEN)", ATIS 1000074, 1 2017,

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Author's Address

   Eric W. Burger
   Georgetown University
   37th & O St, NW
   Washington, DC  20057


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