1. RFC 8197
Internet Engineering Task Force (IETF)                    H. Schulzrinne
Request for Comments: 8197                                           FCC
Category: Standards Track                                      July 2017
ISSN: 2070-1721

                 A SIP Response Code for Unwanted Calls


   This document defines the 607 (Unwanted) SIP response code, allowing
   called parties to indicate that the call or message was unwanted.
   SIP entities may use this information to adjust how future calls from
   this calling party are handled for the called party or more broadly.

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) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Normative Language  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Behavior of SIP Entities  . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  SIP Response Code . . . . . . . . . . . . . . . . . . . .   5
     5.2.  SIP Global Feature-Capability Indicator . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In many countries, an increasing number of calls are unwanted
   [RFC5039]: they might be fraudulent or illegal telemarketing or maybe
   the receiving party does not want to be disturbed by, say, surveys or
   solicitation by charities.  Carriers and other service providers may
   want to help their subscribers avoid receiving such calls, using a
   variety of global or user-specific filtering algorithms.  One input
   into such algorithms is user feedback.  User feedback may be offered
   through smartphone apps, APIs or within the context of a SIP-
   initiated call.  This document addresses feedback within the SIP
   call.  Here, the called party either rejects the SIP [RFC3261]
   request as unwanted or terminates the session with a BYE request
   after answering the call.  INVITE and MESSAGE requests are most
   likely to trigger such a response.

   To allow the called party to express that the call was unwanted, this
   document defines the 607 (Unwanted) response code.  The user agent
   (UA) of the called party, based on input from the called party or
   some UA-internal logic, uses this to indicate that this call is
   unwanted and that future attempts are likely to be similarly
   rejected.  While factors such as identity spoofing and call
   forwarding may make authoritative identification of the calling party
   difficult or impossible, the network can use such a rejection --
   possibly combined with a pattern of rejections by other callees and/
   or other information -- as input to a heuristic algorithm for
   determining future call treatment.  The heuristic processing and
   possible treatment of persistently unwanted calls are outside the
   scope of this document.

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   When this document refers to "caller identity", it uses "identity" in
   the same sense as [SIP-IDENTITY], i.e., to mean either a canonical
   address-of-record (AOR) SIP URI employed to reach a user (such as
   'sip:alice@atlanta.example.com'), or a telephone number, which
   commonly appears in either a tel URI [RFC3966] or as the user portion
   of a SIP URI.

2.  Normative Language

   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.  Motivation

   None of the existing 4xx, 5xx, or 6xx response codes signify that
   this SIP request is unwanted by the called party.  For example, 603
   (Decline) might be used if the called party is currently at dinner or
   in a meeting, but does not want to indicate any specific reason.  As
   described in Section 21.6.2 [RFC3261], a 603 response may include a
   Retry-After header field to indicate a better time to attempt the
   call.  Thus, the call is rejected due to the called party's
   (temporary) status.  As described in Section 4, the called party
   invokes the "unwanted call" user interface and 607 (Unwanted)
   response indicating that it is instead the caller's identity that is
   causing the call to be rejected.

4.  Behavior of SIP Entities

   The response code 607 MAY be used in a failure response for an
   INVITE, MESSAGE, SUBSCRIBE, or other out-of-dialog SIP request to
   indicate that the offered communication is unwanted.  The response
   code MAY also be used as the value of the "cause" parameter of a SIP
   reason-value in a Reason header field [RFC3326], typically when the
   called party user agent issues a BYE request terminating an incoming
   call or a forking proxy issues a CANCEL request after receiving a 607
   response from one of the branches.  (Including a Reason header field
   with the 607 status code allows the called party user agent that
   receives a CANCEL request to make an informed choice whether and how
   to include such calls in their missed-call list or whether to show an
   appropriate indication to the user.)

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   The SIP entities receiving this response code are not obligated to
   take any particular action beyond those appropriate for 6xx
   responses.  Following the default handling for 6xx responses in
   [RFC5057], the 607 response destroys the transaction.  The service
   provider delivering calls or messages to the user issuing the
   response MAY take a range of actions, for example, add the calling
   party to a personal blacklist specific to the called party, use the
   information as input when computing the likelihood that the calling
   party is placing unwanted calls ("crowd sourcing"), initiate a
   traceback request, or report the calling party's identity to consumer
   complaint databases.  As discussed in Section 6, reversing the
   'unwanted' labeling is beyond the scope of this mechanism, as it will
   likely require a mechanism other than call signaling.

   The user experience is envisioned to be somewhat similar to email
   spam buttons where the detailed actions of the email provider remain
   opaque to the user.

   The mechanism described here is only one of many inputs likely to be
   used by call-filtering algorithms operated by service providers,
   using data on calls from a particular identifier such as a telephone
   number to establish handling for future calls from the same
   identifier.  Call handling for unwanted calls is likely to involve a
   combination of heuristics, analytics, and machine learning.  These
   may use call characteristics such as call duration and call volumes
   for a particular caller, including changes in those metrics over
   time, as well as user feedback via non-SIP approaches and the
   mechanism described here.  Implementations will have to make
   appropriate trade-offs between falsely labeling a caller as unwanted
   and delivering unwanted calls.

   Systems receiving 607 responses could decide to treat pre-call and
   mid-call responses differently, given that the called party has had
   access to call content for mid-call rejections.

   Depending on the implementation, the response code does not
   necessarily automatically block all calls from that caller identity.
   The same user interface action might also trigger addition of the
   caller identity to a local, on-device blacklist or graylist, e.g.,
   causing such calls to be flagged or alerted with a different ring

   The actions described here do not depend on the nature of the SIP
   URI, e.g., whether or not it describes a telephone number; however,
   the same anonymous SIP URI [RFC3323] may be used by multiple callers;
   thus, such URIs are unlikely to be appropriate for URI-specific call
   treatment.  SIP entities tallying responses for particular callers
   may need to consider canonicalzing SIP URIs, including telephone

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   numbers, as described in [SIP-IDENTITY].  The calling party may be
   identified in different locations in the SIP header, e.g., the From
   header field, P-Asserted-Identity or History-Info, and may also be
   affected by diverting services.

   This document defines a SIP feature-capability [RFC6809], sip.607,
   that allows the registrar to indicate that the corresponding proxy
   supports this particular response code.  This allows the UA, for
   example, to provide a suitable user-interface element, such as a
   "spam" button, only if its service provider actually supports the
   feature.  The presence of the feature capability does not imply that
   the provider will take any particular action, such as blocking future
   calls.  A UA may still decide to render a "spam" button even without
   such a capability if, for example, it maintains a device-local
   blacklist or reports unwanted calls to a third party.

5.  IANA Considerations

5.1.  SIP Response Code

   This document registers a new SIP response code.  This response code
   is defined by the following information, which has been added to the
   "Response Codes" subregistry under the "Session Initiation Protocol
   (SIP) Parameters" registry <http://www.iana.org/assignments/sip-

   Response Code:  607

   Description:  Unwanted

   Reference:  [RFC8197]

5.2.  SIP Global Feature-Capability Indicator

   This document defines the feature capability sip.607 in the "SIP
   Feature-Capability Indicator Registration Tree" registry defined in

   Name:  sip.607

   Description:  This feature-capability indicator, when included in a
      Feature-Caps header field of a REGISTER response, indicates that
      the server supports, and will process, the 607 (Unwanted) response

   Reference:  [RFC8197]

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6.  Security Considerations

   If the calling party address is spoofed, users may report the caller
   identity as placing unwanted calls, possibly leading to the blocking
   of calls from the legitimate user of the caller identity in addition
   to the unwanted caller, i.e., creating a form of denial-of-service
   attack.  Thus, the response code SHOULD NOT be used for creating
   global call filters unless the calling party identity has been
   authenticated using [SIP-IDENTITY] as being assigned to the caller
   placing the unwanted call.  (The creation of call filters local to a
   user agent is beyond the scope of this document.)

   Even if the identity is not spoofed, a call or message recipient
   might flag legitimate caller identities, e.g., to exact vengeance on
   a person or business, or simply by mistake.  To correct errors, any
   additions to a personal list of blocked caller identities should be
   observable and reversible by the party being protected by the
   blacklist.  For example, the list may be shown on a web page or the
   subscriber may be notified by periodic email reminders.  Any
   additions to a global or carrier-wide list of unwanted callers needs
   to consider that any user-initiated mechanism will suffer from an
   unavoidable rate of false positives and tailor their algorithms
   accordingly, e.g., by comparing the fraction of delivered calls for a
   particular caller that are flagged as unwanted rather than just the
   absolute number and considering time-weighted filters that give more
   credence to recent feedback.

   If an attacker on an unsecured network can spoof SIP responses for a
   significant number of call recipients, it may be able to convince the
   call-filtering algorithm to block legitimate calls.  Use of TLS to
   protect signaling mitigates against this risk.

   Since caller identities are routinely reassigned to new subscribers,
   algorithms are advised to consider whether the caller identity has
   been reassigned to a new subscriber and possibly reset any related
   rating.  (In some countries, there are services that track which
   telephone numbers have been disconnected before they are reassigned
   to a new subscriber.)

   Some call services, such as 3PCC [RFC3725] and call transfer
   [RFC5359], increase the complexity of identifying who (if anyone)
   should be impacted by the receipt of 607 within BYE.  Such services
   might cause the wrong party to be flagged or prevent flagging the
   desired party.

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   For both individually authenticated and unauthenticated calls,
   recipients of response code 607 may want to distinguish responses
   sent before and after the call has been answered, ascertaining
   whether either response timing suffers from a lower false-positive

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,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [RFC3323]  Peterson, J., "A Privacy Mechanism for the Session
              Initiation Protocol (SIP)", RFC 3323,
              DOI 10.17487/RFC3323, November 2002,

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,

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   [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
              RFC 3966, DOI 10.17487/RFC3966, December 2004,

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

   [RFC5057]  Sparks, R., "Multiple Dialog Usages in the Session
              Initiation Protocol", RFC 5057, DOI 10.17487/RFC5057,
              November 2007, <http://www.rfc-editor.org/info/rfc5057>.

   [RFC5359]  Johnston, A., Ed., Sparks, R., Cunningham, C., Donovan,
              S., and K. Summers, "Session Initiation Protocol Service
              Examples", BCP 144, RFC 5359, DOI 10.17487/RFC5359,
              October 2008, <http://www.rfc-editor.org/info/rfc5359>.

              Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", Work in Progress,
              draft-ietf-stir-rfc4474bis-16, February 2017.


   Tolga Asveren, Ben Campbell, Peter Dawes, Spencer Dawkins, Martin
   Dolly, Keith Drage, Vijay Gurbani, Christer Holmberg, Olle Johansson,
   Paul Kyzivat, Jean Mahoney, Marianne Mohali, Adam Montville, Al
   Morton, Denis Ovsienko, Brian Rosen, Brett Tate, Chris Wendt, Dale
   Worley, and Peter Yee (Gen-ART reviewer) provided helpful comments.

Author's Address

   Henning Schulzrinne
   445 12th Street SW
   Washington, DC  20554
   United States of America

   Email: henning.schulzrinne@fcc.gov

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  1. RFC 8197