SIPCORE                                                   H. Schulzrinne
Internet-Draft                                                       FCC
Intended status: Standards Track                       December 12, 2016                         January 4, 2017
Expires: June 15, July 8, 2017

                 A SIP Response Code for Unwanted Calls


   This document defines the 666 (Unwanted) SIP response code, allowing
   called parties to indicate that the call was unwanted.  The
   terminating  SIP entity entities
   may use this information to adjust how future call
   handling behavior for calls from this calling
   party are handled for the called party or more broadly.

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Table of Contents

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

1.  Introduction

   In many countries, an increasing number of calls are unwanted
   [RFC5039], as
   [RFC5039]: they might be fraudulent, illegal telemarketing or 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 only the last mode, where
   the called party either rejects the SIP [RFC3261] request, typically
   requests using the INVITE or
   MESSAGE, MESSAGE methods, as unwanted or
   terminates the call with a BYE request after answering the call.  To
   allow the called party to express that the call was unwanted, this
   document defines the 666 (Unwanted) response code.  The called user
   agent (UAS), based on input from the called party or a automata acting on her behalf some UA-internal
   logic, uses this to indicate that future calls from the same caller
   are also unwanted.

2.  Normative Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

3.  Motivation

   None of the existing 4xx, 5xx or 6xx response codes allow the called
   party to convey signify that they not only reject
   calls from this call, e.g., using 480
   (Temporarily Unavailable), 486 (Busy Here), 600 (Busy Everywhere),
   603 (Decline) or 606 (Not Acceptable), but that the caller is
   unwanted. are unwanted by the called party.  The
   particular response code number was chosen to reflect the distaste
   felt by many upon receiving such calls.

4.  Behavior of SIP Entities

   The response code 666 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 fields field [RFC3326], typically when the
   UAS issues a BYE or CANCEL request terminating an incoming call.

   The SIP entities receiving this response code are not obligated to
   take any particular action.  The service provider delivering calls to
   the user issuing the response may, response, for example, MAY add the calling party
   to a personal blacklist specific to the called party, or may MAY use the
   information as input when computing the likelihood that the calling
   party is placing unwanted calls ("crowd sourcing"), might MAY initiate a
   traceback request, or could and MAY report the calling number to government

   Receiving systems 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.  In other words, depending on
   the implementation, the response code does not necessarily
   automatically block all calls from that number.  The same user
   interface action might also trigger addition of the number to a
   local, on-device blacklist or graylist, e.g., causing such calls to
   be flagged or alert alerted with a different ring tone.

   The actions described here do not depend on the nature of the SIP
   URI, e.g., whether it describes a telephone number or not; however,
   the same anonymous SIP URI [RFC3323] may be used by multiple callers
   and 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 numbers, as described in [I-D.ietf-stir-rfc4474bis].

   We define a SIP feature-capability [RFC6809], sip.666, that allows
   the registrar to indicate that it 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 as 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 register registers a new SIP response code.  This response code
   is defined by the following information, which is to be added to the
   method and response-code
   "Response Codes" sub-registry under

   Response Code Number  666

   Default Reason Phrase  Unwanted

   Reference  [this RFC]

5.2.  SIP Global Feature-Capability Indicator

   This document defines the feature capability sip.666 in the "SIP
   Feature-Capability Indicator Registration Tree" registry defined in

   Name  sip.666

   Description  This feature-capability indicator when used in a
      REGISTER response indicates that the server will process the 666
      response code.  This does not imply any specific action.

   Reference  [this RFC]

6.  Security Considerations

   If the calling party number is spoofed, users may report the number
   as placing unwanted calls, possibly leading to the blocking of calls
   from the legitimate user of the number 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 number has been authenticated using
   [I-D.ietf-stir-rfc4474bis] 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 number is not spoofed, a call recipient might flag
   legitimate numbers, e.g., to extract vengeance on a person or
   business, or simply by mistake.  Thus,  To correct errors, any additions to
   a personal list of blocked numbers should be observable and
   reversible by the subscriber, e.g., party being protected by the blacklist.  For
   example, the list may be shown on a web page or the subscriber may be
   notified by regular periodic email notification, and reservible. 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.

   Since telephone numbers are routinely re-assigned to new subscribers,
   algorithms are advised to consider whether the number has been re-
   assigned to a new subscriber and possibly reset any related rating.

   For both individually-authenticated and unauthenticated calls,
   recipients 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 rate.

7.  Acknowledgements

   Tolga Asveren, Peter Dawes, Martin Dolly, Keith Drage, Vijay Gurbani,
   Paul Kyzivat, Jean Mahoney, Brian Rosen and Rosen, Chris Wendt and Dale Worley
   provided helpful comments.

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,

8.2.  Informative References

              Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-15
              (work in progress), October 2016.

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

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

Author's Address

   Henning Schulzrinne
   445 12th Street SW
   Washington, DC  20554