A Session Initiation Protocol (SIP) Response Code for Rejected Calls
RFC 8688

Document Type RFC - Proposed Standard (December 2019; No errata)
Authors Eric Burger  , Bhavik Nagda 
Last updated 2019-12-04
Replaces draft-burger-sipcore-rejected
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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
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