SIPCORE                                                        E. Burger
Internet-Draft                                     Georgetown University
Intended status: Standards Track                                B. Nagda
Expires: September 28, 2019        Massachusetts Institute of Technology
                                                          March 27, 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 that an intermediary
   rejected their call attempt.  The call will not be answered.  As a
   6xx code, the caller will be aware that future attempts to contact
   the same UAS will likely fail.  The present use case driving the need
   for the 608 response code is when the intermediary is an analytics
   engine.  In this case, the rejection is by a machine or other
   process.  This contrasts with the 607 (Unwanted) SIP response code,
   which a human at the target UAS indicated the call was not wanted.
   In some jurisdictions this distinction is important.  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.  This provides a remediation mechanism for legal callers
   that find their calls blocked.

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
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   This Internet-Draft will expire on September 28, 2019.

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Copyright Notice

   Copyright (c) 2019 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Protocol Operation  . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Intermediary Operation  . . . . . . . . . . . . . . . . .   8
     3.2.  jCard Construction  . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  JOSE Header . . . . . . . . . . . . . . . . . . . . .   8
       3.2.2.  JWT Payload . . . . . . . . . . . . . . . . . . . . .   8
       3.2.3.  JWS Signature . . . . . . . . . . . . . . . . . . . .   9
     3.3.  UAC Operation . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Legacy Interoperation . . . . . . . . . . . . . . . . . .   9
     3.5.  Announcement Requirements . . . . . . . . . . . . . . . .  10
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Full Exchange . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Web Site jCard  . . . . . . . . . . . . . . . . . . . . .  14
     4.3.  Multi-modal jCard . . . . . . . . . . . . . . . . . . . .  15
     4.4.  Legacy Interoperability . . . . . . . . . . . . . . . . .  15
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  SIP Response Code . . . . . . . . . . . . . . . . . . . .  17
     5.2.  SIP Feature-Capability Indicator  . . . . . . . . . . . .  17
     5.3.  JSON Web Token Claim  . . . . . . . . . . . . . . . . . .  17
     5.4.  Call-Info Purpose . . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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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 the cryptographic signing and attestation, respectively, of
   signaling to ensure the integrity and authenticity of the asserted
   caller identity.

   This document describes a new SIP response code, 608, which allows
   calling parties to learn that an intermediary rejected their call.
   As described below, we need a distinct indicator to differentiate
   between a user rejection and an intermediary's rejection of a call.
   In some jurisdictions, 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 some jurisdictions blocking calls from that
   source for 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.

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

   The 608 response code addresses this need of remediating falsely
   blocked calls.  Specifically, this code informs the UAC that an
   intermediary blocked the call and provides a redress mechanism that
   allows callers to contact the operator of the intermediary.

   In the current call handling ecosystem, users can explicitly reject a
   call or later mark a call as being unwanted by issuing a 607 SIP
   response code (Unwanted) [RFC8197].  Figure 1 and Figure 2 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 can let their proxy know that they
   consider future calls from that source unwanted.  Upon receipt of the
   607 response from the UAS, the proxy may send call information to a
   call analytics engine.  For various reasons described in RFC8197, if

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   a network operator receives multiple reports of unwanted calls, that
   may indicate that the entity placing the calls is likely to be a
   source of unwanted calls for many people.  As such, other customers
   of the service provider may want the service provider to
   automatically reject calls on their behalf.

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

                         |   Call    |
                         | Analytics |
                         |  Engine   |
                            ^     | (likely not SIP)
                            |     v
      +-----+    607     |  Called   |    607    +-----+
      | UAC | <--------- |  Party    | <-------- | UAS |
      +-----+            |  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 a 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 or push a note to the pharmacist
   to ask the customer to consider not rejecting their calls in the

   Many systems that allow the user to mark the call unwanted (e.g.,
   with the 607 response code) also allow the user to change their mind
   and unmark such calls.  This mechanism is relatively easy to
   implement as the user usually has a direct relationship with the
   service provider that is blocking calls.

   However, things become more complicated if an intermediary, such as a
   third-party provider of call management services that classifies
   calls based on the relative likelihood that the call is unwanted,

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   misidentifies the call as unwanted.  Figure 3 shows this case.  Note
   that the UAS typically does not receive an INVITE since the called
   party proxy rejects the call on behalf of the user.  In this
   situation, it would be beneficial for the caller to learn who
   rejected the call, so they can correct the misidentification.

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

                  Figure 2: Unwanted (607) Ladder Diagram

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

                    Figure 3: Rejected (608) Call Flow

   In this situation, one might be tempted to have the intermediary use
   the 607 response code. 607 indicates to the caller the subscriber
   does 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

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   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
   engine's statistical rejection, a downstream network element that
   sees a 607 response code can weight it as a human rejection in its
   call analytics.

   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.

   One might ask why we cannot use the same mechanism an analytics
   service provider offers their customers that lets them correct a call
   blocked in error?  The reason is while there is an existing
   relationship between the customer (called party) and the analytics
   service provider, it is unlikely there is a relationship between the
   caller and the analytics service provider.  Moreover, there are
   numerous call blocking providers in the ecosystem.  As such, we need
   a mechanism for indicating an intermediary rejected a call that also
   provides contact information for the operator of that intermediary,
   without exposing the target user's contact information.

   The protocol described in this document uses existing IETF protocol
   mechanisms for specifying the redress mechanism.  In the SIP header
   passed back to the UAC, we send additional information specifying a
   redress address.  We choose to encode the redress address using jCard
   [RFC7095].  Conveniently, we use jCard rather than vCard [RFC6350] as
   we have a standard marshaling mechanism for creating a canonical
   representation of a JSON [RFC8259] object, such as a jCard, and a
   standard presentation format for such an object, namely JWS
   [RFC7515].  The SIP community is familiar with this concept as it is
   the mechanism used by STIR [RFC8224].

   The jCard encoding might seem unnecessary at first, but it is
   essential to preventing potential network attacks.  Suppose, for
   example, that the redress address was simply passed as a header
   value.  One can imagine an adverse agent that maliciously spoofs a
   608 response with the same redress address to a large number of
   active callers, who may then all send redress requests to the
   specified address (the basis for a denial-of-service attack).  The

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   process would occur as follows: (1) a malicious agent senses INVITE
   requests from a variety UACs and (2) spoofs 608 responses with an
   unsigned redress address before the intended receivers can respond,
   causing (3) the UACs to all contact the redress address at once.  The
   jCard encoding allows the UAC to verify the blocking intermediary's
   identity before contacting the redress address.  This guards against
   a malicious agent spoofing 608 responses, preventing the denial-of-
   service attack.  Thus if the jCard address is unreachable or
   undecipherable, either (1) a malicious agent is lying about the jCard
   or (2) the redress mechanism is misconfigured.

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.

   Figure 4 shows an overview of the call flow for a rejected call.

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

                  Figure 4: Rejected (608) Ladder Diagram

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3.1.  Intermediary Operation

   An intermediary MAY issue the 608 response 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].

   If an intermediary issues a 608 code and there are not indicators the
   calling party will use the contents of the Call-Info header for
   malicious purposes (see Section 6), 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 'jwscard'.  The value of the Call-Info header MUST refer to a
   valid JWS [RFC7515] encoding of a jCard [RFC7095] object.

   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&nbsp;16.7 of RFC3261
   [RFC3261] apply.  Specifically, the proxy should cancel pending
   transactions and must not create any new branches.  Note this is not
   a new requirement but simply pointing out the existing 6xx protocol
   mechanism in SIP.

3.2.  jCard Construction

   The intermediary constructs the JWS as follows.

3.2.1.  JOSE Header

   The JOSE header MUST include the typ, alg, and x5u parameters from
   JWS [RFC7515].  The typ parameter MUST have the value "vcard+json".
   Implementations MUST support ES256 as JWA [RFC7518] defines it, and
   MAY support other registered signature algorithms.  Finally, the x5u
   parameter MUST be a URI that resolves to the public key certificate
   corresponding to the key used to digitally sign the JWS.

3.2.2.  JWT Payload

   The payload contains two JSON values.  The first JWT claim that MUST
   be present is the iat (issued at) claim [RFC7519].  The "iat" MUST be
   set to the date and time of the issuance of the 608 response.  This
   mandatory component protects the response from replay attacks.

   The second JWT claim that MUST be present is the jcard claim.
   Section 5.3 describes the registration.  In the construction of the

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

3.2.3.  JWS Signature

   JWS [RFC7515] specifies the procedure for calculating the signature
   over the jCard JWT.  Section 4 of this document has a detailed
   example on constructing the JWS, including the signature.

3.3.  UAC Operation

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

   Upon receiving a 608 response, UACs perform normal SIP processing for
   6xx responses.

3.4.  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?  For example, how can we
   support callers from a legacy, non-SIP public switched network
   connecting to the SIP network via a media gateway?

   We address this situation by having the first network element that
   conforms with this specification play an announcement in the media.
   See Section 3.5 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 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 response code.  Obviously, any
   SIP device will understand that a 608 response 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.  If the caveats described in Section 3.5 and Section 6 do
   not hold, the intermediary MUST play the announcement.

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   Now we take the case where a network element that understands the 608
   response 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&nbsp;4.2 of

   Do note that even if a network element plays an announcement
   describing the contents of the 608 response message, the network
   element MUST forward the 608 response 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&nbsp;16.6 of [RFC3261], specifically they
   will pass headers such as "Feature-Capability: sip.608" unmodified.

3.5.  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
   announcement in the media path towards the UAC (caller).  It is
   important to note 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 and the
   network element is able to render the information in the requested
   media format, the network element MUST 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 response code.  In this case, it is enough
   for the outbound calling proxy to digest the Call-Info information

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   and handle the information digitally, rather than 'transcoding' the
   Call-Info information for presentation to the caller.

4.  Examples

   These examples are not normative, do not include all protocol
   elements, and may have errors.  Review the protocol documents for
   actual syntax and semantics of the protocol elements.

4.1.  Full Exchange

   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
   Feature-Caps: sip.608
   Content-Length: 153

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

   An intermediary could reply:

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   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=jwscard

   The location resolves to a
   JWS.  The JWS would be constructed as follows.

   The JWS header of this example jCard could be:

   { {"alg":"ES256"},
     {"x5u":""} }

   Now, let us construct a minimal jCard.  For this example, the jCard
   refers the caller to an email address,

       ["version", {}, "text", "4.0"],
       ["fn", {}, "text", "Robocall Adjudication"],
       ["email", {"type":"work"},
                 "text", ""]

   With this jCard, we can now construct the JWT:

         ["version", {}, "text", "4.0"],
         ["fn", {}, "text", "Robocall Adjudication"],
         ["email", {"type":"work"},
                   "text", ""]

   In order to calculate the signature, we need to encode the JOSE
   header and JWT into base64.  As an implementation note, one can trim

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   whitespace in the JSON objects to save a few bytes.  UACs MUST be
   prepared to receive pretty printed, compact, or bizarrely formatted
   JSON.  For the purposes of this example, we leave the objects with
   pretty whitespace.  Speaking of pretty vs. machine formatting, these
   examples have line breaks in the base64 encodings for ease of
   publication in the RFC format.  The specification of base64 allows
   for these line breaks and the decoded text works just fine.  However,
   those extra line break octets would affect the calculation of the
   signature.  As such, implementations MUST NOT insert line breaks into
   the base64 encodings of the JOSE header or JWT.  This also means UACs
   MUST be prepared to receive arbitrarily long octet streams from the
   URI referenced by the Call-Info SIP header.

   base64 of JOSE header:

   base64 of JWT:

   In this case, the object to be signed (remembering this is just a
   single, long line; the line breaks are for ease of review but do not
   appear in the actual text being signed is as follows:


   We use the following X.509 PKCS #8-encoded ECDSA private key, also
   shamelessly taken from [SHAKEN]), as an example key for signing the
   hash of the above text.  Do NOT use this key in real life!  It is for
   exemplary purposes only.  At the very least, we would strongly
   recommend the key being encrypted at rest.

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   -----BEGIN PRIVATE KEY-----
   -----END PRIVATE KEY-----

   The resulting JWS, using the above key on the above object, renders
   the following ECDSA P-256 SHA-256 digital signature.


   Thus, the JWS stored at,
   would contain:


4.2.  Web Site jCard

   For an intermediary that provides a Web site for adjudication, the
   jCard could contain the following.  Note the calculation of the JWS
   is not shown; the URI reference in the Call-Info header would be to
   the JWS of the signed jCard.

       ["version", {}, "text", "4.0"],
       ["fn", {}, "text", "Robocall Adjudication"],
       ["url", {"type":"work"},
               "text", ""]

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4.3.  Multi-modal jCard

   For an intermediary that provides a telephone number and a postal
   address, the jCard could contain the following.  Note the calculation
   of the JWS is not shown; the URI reference in the Call-Info header
   would be to the JWS of the signed jCard.

       ["version", {}, "text", "4.0"],
       ["fn", {}, "text", "Robocall Adjudication"],
       ["adr", {"type":"work"}, "text",
         ["Argument Clinic",
          "12 Main St","Anytown","AP","000000","Somecountry"]
       ["tel", {"type":"work"}, "uri", "tel:+1-555-555-1212"]

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

4.4.  Legacy Interoperability

   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 response 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.

   As an example, the SBC could extract the FN and TEL jCard fields and
   play something like a special information tone (see Telcordia SR-2275
   [SR-2275] section or ITU-T E.180 [ITU.E.180.1998] section
   7), followed by "Your call has been rejected by ...", followed by a
   text-to-speech translation of the FN text, followed by "You can reach
   them on", followed by a text-to-speech translation of the telephone
   number in the TEL field.

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   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 defines a new SIP response code, 608 in the "Response
   Codes" subregistry of the "Session Initiation Protocol (SIP)
   Parameters" registry defined in [RFC3261].

   Response code:  608

   Description:  Rejected

   Reference:  [RFCXXXX]

5.2.  SIP 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]

5.3.  JSON Web Token Claim

   This document defines the new JSON Web Token claim in the "JSON Web
   Token Claims" sub-registry created by [RFC7519].  Section 3.2.2
   defines the syntax.  The required information is:

   Claim Name:  jcard

   Claim Description:  jCard data

   Change Controller:  IESG

   Reference:  [RFCXXXX], [RFC7095]

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5.4.  Call-Info Purpose

   This document defines the new predefined value "jwscard" for the
   "purpose" header field parameter of the Call-Info header field.  This
   modifies the registry header field parameters and parameter values by
   adding this RFC as a reference to the line for the header field
   "Call-Info" and parameter name "purpose":

   Header Field:  Call-Info

   Parameter Name:  purpose

   Predefined Values:  Yes

   Reference:  [RFCXXXX]

6.  Security Considerations

   Intermediary operators need to be mindful of whom they are sending
   the 608 response.  There is a risk that a truly malicious caller is
   being rejected.  This raises two issues.  The first is the caller,
   now alerted that the 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 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 proxies known to insert the sip.608 feature, and direct
   the SDP 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 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].

   Yet another risk is a malicious entity or the intermediary itself can
   generate a malicious 608 response with a jCard referring to a

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   malicious agent.  For example, the recipient of a 608 may receive a
   TEL URI in the vCard.  When the recipient calls that address, the
   malicious agent could ask for personally identifying information.
   However, instead of using that information to verify the recipient's
   identity, they are pharming the information for nefarious ends.  As
   such, we strongly recommend the recipient validates to whom they are
   communicating with if asking to adjudicate an erroneously rejected
   call attempt.  Since we may also be concerned about intermediate
   nodes modifying contact information, we can address both of these
   issues with a single solution.  The remediation is to require the
   intermediary to sign the jCard.  Signing the jCard provides integrity
   protection.  In addition, one can imagine mechanisms such as used by
   SHAKEN [SHAKEN] to use signing certificate issuance as a mechanism
   for traceback to the entity issuing the jCard, for example tying the
   identity of the subject of the certificate to the To field of the
   initial SIP request, as if the intermediary was vouching for the From
   field of a SIP request with that identity.

   Since the decision of whether to include Call-Info in the 608
   response is a matter of policy, one thing to consider is whether a
   legitimate caller can ascertain whom to contact without such
   information being included in the 608.  For example, in some
   jurisdictions, if the terminating service provider is the
   intermediary, the caller can lookup who the terminating service
   provider is based on the routing information for the dialled number.
   As such, the Call-Info jCard could be redundant information.
   However, the factors going into a particular service provider's or
   jourisdiction's choice of whether or not to include Call-Info is
   outside the scope of this document.

7.  Acknowledgements

   This document liberally lifts from [RFC8197] in its text and
   structure.  However, the mechanism and purpose of 608 is quite
   different than 607.  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%. In
   addition, Tolga came up with the jCard attack.

   Finally, Bhavik Nagda provided clarifying edits as well and more
   especially wrote and tested an implementation of the 608 response
   code in Kamailio.  Code is available at <
   nagdab/608_Implementation> .

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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,

   [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,

   [RFC7095]  Kewisch, P., "jCard: The JSON Format for vCard", RFC 7095,
              DOI 10.17487/RFC7095, January 2014,

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <>.

   [RFC7518]  Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
              DOI 10.17487/RFC7518, May 2015,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

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

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8.2.  Informative References

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

              International Telecommunications Union, "Technical
              characteristics of tones for the telephone service",
              ITU Recommendation E.180/Q.35, March 1998.

   [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, <>.

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

   [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,

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

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

   [SR-2275]  Telcordia, "Bellcore Notes on the Networks", Telcordia SR-
              2275, October 2000.

Authors' Addresses

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


   Bhavik Nagda
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA  02139


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