1. RFC 8688
Internet Engineering Task Force (IETF)                       E.W. Burger
Request for Comments: 8688                         Georgetown University
Category: Standards Track                                       B. Nagda
ISSN: 2070-1721                    Massachusetts Institute of Technology
                                                           December 2019

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


   This document defines the 608 (Rejected) Session Initiation Protocol
   (SIP) response code.  This response code enables calling parties to
   learn that an intermediary rejected their call attempt.  No one will
   deliver, and thus answer, the call.  As a 6xx code, the caller will
   be aware that future attempts to contact the same User Agent Server
   will likely fail.  The initial 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 in which a human
   at the target User Agent Server indicates the user did not want the
   call.  In some jurisdictions, this distinction is important.  This
   document also defines the use of the Call-Info header field 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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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
   (https://trustee.ietf.org/license-info) in effect on the date of
   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.

Table of Contents

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

1.  Introduction

   The IETF has been addressing numerous issues surrounding how to
   handle unwanted and, depending on the jurisdiction, illegal calls
   [RFC5039].  Secure Telephone Identity Revisited (STIR) [RFC7340] and
   Signature-based Handling of Asserted information using toKENs
   (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 Session Initiation Protocol (SIP)
   [RFC3261] 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,
   service providers may not be permitted to block calls, even if
   unwanted by the user, 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 to block all calls from that source.
   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, which might result in the user not being able
   to receive calls from anyone, even if they wanted to receive the
   calls.  In many jurisdictions, providing such false signaling is also

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

   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].  Figures 1 and 2 show the
   operation of the 607 SIP response code.  The User Agent Server (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
   unwanted call indicators, such as the value of the From header field
   and other information elements, to a call analytics engine.  For
   various reasons described in [RFC8197], if 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.

   There is another value of the 607 rejection code.  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,
   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         |                    |            |
           | --------------> |  Is call OK?       |            |
           |                 |------------------->|            |
           |                 |                    |            |
           |                 |               Yes  |            |
           |                 |<-------------------|            |
           |                 |                    |            |
           |                 | INVITE             |            |
           |                 | ------------------------------> |
           |                 |                    |            |
           |                 |                    |       607  |
           |                 | <------------------------------ |
           |                 |                    |            |
           |                 |  Unwanted call     |            |
           |            607  | -----------------> |            |
           | <-------------- |  indicators        |            |
           |                 |                    |            |

                  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 consider having the intermediary use the
   607 response code. 607 indicates to the caller that 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 that network elements
   downstream from the intermediary might interpret the 607 as coming
   from a user (human) who has marked the call as unwanted, as opposed
   to coming from an algorithm using statistics or machine learning to
   reject the call.  An algorithm can be vulnerable to the base-rate
   fallacy [BaseRate] rejecting the call.  In other words, those
   downstream entities should not rely 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 weigh it as a human
   rejection in its call analytics, versus deciding whether to consider
   a 608 at all, and if so, weighing it appropriately.

   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 intermediaries block
   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.

   Why do we not use the same mechanism an analytics service provider
   offers their customers?  Specifically, why not have the analytics
   service provider allow the called party to correct a call blocked in
   error?  The reason is that 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.  Therefore, 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 SIP protocol
   mechanisms for specifying the redress mechanism.  In the Call-Info
   header field passed back to the UAC, we send additional information
   specifying a redress address.  We choose to encode the redress
   address using jCard [RFC7095].  As we will see later in this
   document, this information needs to have its own application-layer
   integrity protection.  Thus, we use jCard rather than vCard
   [RFC6350], as we have a marshaling mechanism for creating a
   JavaScript Object Notation (JSON) [RFC8259] object, such as a jCard,
   and a standard integrity format for such an object, namely, JSON Web
   Signature (JWS) [RFC7515].  The SIP community is familiar with this
   concept as it is the mechanism used by STIR [RFC8224].

   Integrity protecting the jCard with a cryptographic signature might
   seem unnecessary at first, but it is essential to preventing
   potential network attacks.  Section 6 describes the attack and why we
   sign the jCard in more detail.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Protocol Operation

   This section uses the term "intermediary" to mean the entity that
   acts as a SIP UAS on behalf of the user in the network as opposed to
   the user's UAS (usually, but not necessarily, their phone).  The
   intermediary could be a back-to-back user agent (B2BUA) or a SIP

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

                         +--------+         +-----------+
                         | Called |         |   Call    |
        +-----+          | Party  |         | Analytics |   +-----+
        | UAC |          | Proxy  |         |  Engine   |   | UAS |
        +-----+          +--------+         +-----------+   +-----+
           |  INVITE         |                    |            |
           | --------------> |  Is call OK?       |            |
           |                 |------------------->|            |
           |                 |                    |            |
           |                 |               Yes  |            |
           |                 |<-------------------|            |
           |                 |                    |            |
           |                 | INVITE             |            |
           |                 | ------------------------------> |
           |                 |                    |            |
           |                 |                    |       607  |
           |                 | <------------------------------ |
           |                 |                    |            |
           |                 |  Unwanted call     |            |
           |            607  | -----------------> |            |
           | <-------------- |  indicators        |            |
           |                 |                    |            |

                  Figure 4: Rejected (608) Ladder Diagram

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 no indicators the
   calling party will use the contents of the Call-Info header field for
   malicious purposes (see Section 6), the intermediary MUST include a
   Call-Info header field in the response.

   If there is a Call-Info header field, it MUST have the "purpose"
   parameter of "jwscard".  The value of the Call-Info header field MUST
   refer to a valid JSON Web Signature (JWS) [RFC7515] encoding of a
   jCard [RFC7095] object.  The following section describes the
   construction of the JWS.

   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] 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.  JWS Construction

   The intermediary constructs the JWS of the jCard as follows.

3.2.1.  JOSE Header

   The Javascript Object Signing and Encryption (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 JSON Web Algorithms (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 JSON Web Token (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

   The second JWT claim that MUST be present is the "jcard" claim.  The
   value of the jcard [RFC7095] claim is a JSON array conforming to the
   JSON jCard data format defined in [RFC7095].  Section 5.3 describes
   the registration.  In the construction of the 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 indicator in the Feature-Caps header field of the
   INVITE request.

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

   As for the disposition of the jCard itself, the UAC MUST check the
   "iat" claim in the JWT.  As noted in Section 3.2.2, we are concerned
   about replay attacks.  Therefore, the UAC MUST reject jCards that
   come with an expired "iat".  The definition of "expired" is a matter
   of local policy.  A reasonable value would be on the order of a
   minute due to clock drift and the possibility of the playing of an
   audio announcement before the delivery of the 608 response.

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.  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 field.  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 Sections 3.5 and 6
   do not hold, the intermediary MUST play the announcement.

   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 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 that only the network
   elements that will either consume (UAC) or play an announcement
   (media gateway, session border controller (SBC) [RFC7092], or proxy)
   need to understand the sip.608 feature capability.  If the other
   network elements conform to Section 16.6 of [RFC3261], they will pass
   header fields such as "Feature-Caps: *;+sip.608" unmodified and
   without need for upgrade.

   Because the ultimate disposition of the call attempt will be a
   600-class response, the network element conveying the announcement in
   the legacy direction MUST use the 183 Session Progress response to
   establish the media session.  Because of the small chance the UAC is
   an extremely old legacy device and is using UDP, the UAC MUST include
   support for 100rel [RFC3262] in its INVITE, the network element
   conveying the announcement MUST Require 100rel in the 183, and the
   UAC MUST issue a Provisional Response ACKnowledgement (PRACK) to
   which the network element MUST respond 200 OK PRACK.

3.5.  Announcement Requirements

   There are a few requirements on the element that handles 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 field in the 608 response message.  However,
   this specification does not mandate how to convey that information.

   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 Session Description Protocol (SDP) [RFC4566] to result
   in an audio announcement.  Likewise, if the INVITE only indicated
   real-time text [RFC4103] and the network element can render the
   information in the requested media format, the network element should
   send the information in a text 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
   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], with the line
   breaks in the Identity header field for display purposes only:

   INVITE sip:+12155550113@tel.one.example.net SIP/2.0
   Max-Forwards: 69
   Contact: <sip:+12155550112@[2001:db8::12]:50207;rinstance=9da3088f3>
   To: <sip:+12155550113@tel.one.example.net>
   From: "Alice" <sip:+12155550112@tel.two.example.net>;tag=614bdb40
   P-Asserted-Identity: "Alice"<sip:+12155550112@tel.two.example.net>,
   CSeq: 2 INVITE
   Content-Type: application/sdp
   Date: Tue, 16 Aug 2016 19:23:38 GMT
   Feature-Caps: *;+sip.608
   Identity: eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwicHB0Ijoic2hha2V
   Content-Length: 153

   o=- 13103070023943130 1 IN IP6 2001:db8::177
   c=IN IP6 2001:db8::177
   t=0 0
   m=audio 54242 RTP/AVP 0

   An intermediary could reply:

   SIP/2.0 608 Rejected
   Via: SIP/2.0/UDP [2001:db8::177]:60012;branch=z9hG4bK-524287-1
   From: "Alice" <sip:+12155550112@tel.two.example.net>;tag=614bdb40
   To: <sip:+12155550113@tel.one.example.net>
   CSeq: 2 INVITE
   Call-Info: <https://block.example.net/complaint-jws>;purpose=jwscard

   The location https://block.example.net/complaint-jws resolves to a
   JWS.  One would construct the JWS as follows.

   The JWS header of this example jCard could be:

   { "alg":"ES256",

   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", "remediation@blocker.example.net"]

   To calculate the signature, we need to encode the JSON Object Signing
   and Encryption (JOSE) header and JWT into base64url.  As an
   implementation note, one can trim 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
   base64url encodings for ease of publication in the RFC format.  The
   specification of base64url 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.  Implementations MUST
   NOT insert line breaks into the base64url 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 header field.

   base64url of JOSE header:


   base64url of JWT:


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


   We use the following X.509 PKCS #8-encoded Elliptic Curve Digital
   Signature Algorithm (ECDSA) 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 example purposes only.
   At the very least, we would strongly recommend encrypting the key at

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

   -----BEGIN PUBLIC KEY-----
   -----END PUBLIC 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 https://blocker.example.net/complaints-jws
   would contain:


4.2.  Web Site jCard

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

       ["version", {}, "text", "4.0"],
       ["fn", {}, "text", "Robocall Adjudication"],
       ["url", {"type":"work"},
        "text", "https://blocker.example.net/adjudication-form"]

4.3.  Multi-modal jCard

   For an intermediary that provides a telephone number and a postal
   address, the jCard could contain the following.  Note that we do not
   show the calculation of the JWS; the URI reference in the Call-Info
   header field 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:+1-555-555-0112"]

   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], the SBC can insert "*;+sip.608" into the
   Feature-Caps header field 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.  For illustrative
   purposes, the figure shows generic SIP Proxies in the flow.  Their
   presence or absence or the number of proxies is not relevant to the
   operation of the protocol.  They are in the figure to show that
   proxies that do not understand the sip.608 feature capability can
   still participate in a network offering 608 services.

                                                           |  Call   |
                                                           | Engine  |
                                                              ^  |
                                                              |  |
                                                              |  v
         +---+    +-----+    +---+    +-----+    +-----+    |Called|
         |UAC+----+Proxy+----+SBC+----+Proxy+----+Proxy+----+Party |
         +---+    +-----+    +---+    +-----+    +-----+    |Proxy |
           |                   |                            +------+
           | INVITE            |                               |
           |------------------>|                               |
           |                   | INVITE                        |
           |                   |------------------------------>|
           |                   | Feature-Caps: *;+sip.608      |
           |                   |                               |
           |                   |                  608 Rejected |
           |                   |<------------------------------|
           |               183 |              Call-Info: <...> |
           |<------------------|    [path for Call-Info elided |
           |     SDP for media |     for illustration purposes]|
           |                   |                               |
           | PRACK             |                               |
           |------------------>|                               |
           |                   |                               |
           |      200 OK PRACK |                               |
           |<------------------|                               |
           |                   |                               |
           |<== Announcement ==|                               |
           |                   |                               |
           |      608 Rejected |                               |
           |<------------------|                               |
           |  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 disclosing 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.  Since this
   is a potentially long transaction and there is a chance the UAC is
   using an unreliable transport protocol, the UAC will have indicated
   support for provisional responses, the SBC will indicate it requires
   a PRACK from the UAC in the 183 response, the UAC will provide the
   PRACK, and the SBC will acknowledge receipt of the PRACK before
   playing the announcement.

   As an example, the SBC could extract the FN and TEL jCard fields and
   play something like a special information tone (see Section
   of Telcordia [SR-2275] or Section 7 of ITU-T E.180 [ITU.E.180.1998]),
   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.

   Note that the SBC also still sends the full 608 response code,
   including the Call-Info header field, 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:        RFC 8688

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 associated with the indicator
                 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 field.
   Reference:    RFC 8688

5.3.  JSON Web Token Claim

   This document defines the new JSON Web Token claim in the "JSON Web
   Token Claims" subregistry 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:          RFC 8688, [RFC7095]

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 "Header Field Parameters and Parameter Values"
   subregistry of the "Session Initiation Protocol (SIP) Parameters"
   registry 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:          RFC 8688

6.  Security Considerations

   Intermediary operators need to be mindful to whom they are sending
   the 608 response.  The intermediary could be rejecting a truly
   malicious caller.  This raises two issues.  The first is the caller,
   now alerted that an intermediary is automatically rejecting their
   call attempts, 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 header 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 header field for INVITE, or other signed
   initiating methods, that pass validation by STIR [RFC8224].

   Another risk is as follows.  Consider an attacker that floods a proxy
   that supports the sip.608 feature.  However, the SDP in the INVITE
   request refers to a victim device.  Moreover, the attacker somehow
   knows there is a 608-aware gateway connecting to the victim who is on
   a segment that lacks the sip.608 feature capability.  Because the
   mechanism described here can result in sending an audio file to the
   target of the SDP, 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 to 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 a STIR-signed INVITE that passes validation.
   Beyond requiring a valid STIR signature on the INVITE, the
   intermediary can also use remediation procedures such as doing the
   connectivity checks specified by Interactive Connectivity
   Establishment [RFC8445].  If the target did not request the media,
   the check will fail.

   Yet another risk is a malicious intermediary that generates a
   malicious 608 response with a jCard referring to a 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 phishing the information for nefarious ends.  A similar scenario
   can unfold if the malicious agent inserts a URI that points to a
   phishing or other site.  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 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].

   Similarly, one can imagine an adverse agent that maliciously spoofs a
   608 response with a victim's contact address to many active callers
   who may then all send redress requests to the specified address (the
   basis for a denial-of-service attack).  The process would occur as
   follows: (1) a malicious agent senses INVITE requests from a variety
   of 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.  Specifically, because the sender
   signs the jCard, we can cryptographically trace the sender of the
   jCard.  Given the protocol machinery of having a signature, one can
   apply local policy to decide whether to believe that the sender of
   the jCard represents the owner of the contact information found in
   the jCard.  This guards against a malicious agent spoofing 608

   Specifically, one could use policies around signing certificate
   issuance as a mechanism for traceback to the entity issuing the
   jCard.  One check could be verifying that the identity of the subject
   of the certificate relates to the To header field of the initial SIP
   request, similar to validating that the intermediary was vouching for
   the From header field of a SIP request with that identity.  Note that
   we are only protecting against a malicious intermediary and not a
   hidden intermediary attack (formerly known as a "man-in-the-middle
   attack").  Thus, we only need to ensure the signature is fresh, which
   is why we include "iat".  For most implementations, we assume that
   the intermediary has a single set of contact points and will generate
   the jCard on demand.  As such, there is no need to directly correlate
   HTTPS fetches to specific calls.  However, since the intermediary is
   in control of the jCard and Call-Info response, an intermediary may
   choose to encode per-call information in the URI returned in a given
   608 response.  However, if the intermediary does go that route, the
   intermediary MUST use a non-deterministic URI reference mechanism and
   be prepared to return dummy responses to URI requests referencing
   calls that do not exist so that attackers attempting to glean call
   metadata by guessing URIs (and thus calls) will not get any
   actionable information from the HTTPS GET.

   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 including
   such information in the 608.  For example, in some jurisdictions, if
   only the terminating service provider can be the intermediary, the
   caller can look up who the terminating service provider is based on
   the routing information for the dialed number.  Thus, the Call-Info
   jCard could be redundant information.  However, the factors going
   into a particular service provider's or jurisdiction's choice of
   whether to include Call-Info is outside the scope of this document.

7.  References

7.1.  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,

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, DOI 10.17487/RFC3262, 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, <https://www.rfc-editor.org/info/rfc7515>.

   [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, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [BaseRate] Bar-Hillel, M., "The Base-Rate Fallacy in Probability
              Judgements", April 1977,

              ITU-T, "Technical characteristics of tones for the
              telephone service", ITU-T Recommendation E.180/Q.35, March

   [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
              Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,

   [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, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC5039]  Rosenberg, J. and C. Jennings, "The Session Initiation
              Protocol (SIP) and Spam", RFC 5039, DOI 10.17487/RFC5039,
              January 2008, <https://www.rfc-editor.org/info/rfc5039>.

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

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents",
              RFC 7092, DOI 10.17487/RFC7092, December 2013,

   [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,

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,

   [SHAKEN]   ATIS/SIP Forum IP-INNI Task Group, "Signature-based
              Handling of Asserted information using toKENs (SHAKEN)",
              ATIS 1000074, January 2017,

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


   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 RFC 8197.  Thanks also go to Ken Carlberg of the FCC, Russ
   Housley, Paul Kyzivat, and Tolga Asveren for their suggestions on
   improving the document.  Tolga's suggestion to provide a mechanism
   for legacy interoperability served to expand the document by 50%. In
   addition, Tolga came up with the jCard attack.  Finally, Christer
   Holmberg, as always, provided a close reading and fixed a SIP
   feature-capability bug found by Yehoshua Gev.

   Of course, we appreciated the close read and five pages of comments
   from our estimable Area Director, Adam Roach.  In addition, we
   received valuable comments during IETF Last Call and JWT review from
   Ines Robles, Mike Jones, and Brian Campbell, and IESG review from
   Alissa Cooper, Eric Vyncke, Alexey Melnikov, Benjamin Kaduk, Barry
   Leiba, and with most glee, Warren Kumari.

   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 https://github.com/
   nagdab/608_Implementation.  Grace Chuan from MIT regenerated and
   verified the JWT while working at the FCC.

Authors' Addresses

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

   Email: eburger@standardstrack.com

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

   Email: nagdab@gmail.com
  1. RFC 8688