SIPCORE                                                   H. Schulzrinne
Internet-Draft                                                       FCC
Intended status: Standards Track                        January 12, 2017
Expires: July 16, 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 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.

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Table of Contents

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

1.  Introduction

   In many countries, an increasing number of calls are unwanted
   [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 methods, as unwanted or
   terminates the session 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 some UA-internal
   logic, uses this to indicate that future calls from the same caller
   are also unwanted.

   As in [I-D.ietf-stir-rfc4474bis], we use the term "caller identity"
   or "calling party identity" in this document to mean either a
   canonical address-of-record (AoR) SIP URI employed to reach a user
   (such as ''), 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",

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   document are to be interpreted as described in BCP 14, RFC 2119

3.  Motivation

   None of the existing 4xx, 5xx or 6xx response codes signify that this
   SIP request is 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 field [RFC3326], typically when the
   UAS issues a BYE request terminating an incoming call or the UAC
   issues a CANCEL request when forking a call.  (Including a Reason
   header field with the 666 status code allows the UAS that receive a
   CANCEL request to make an informed choice whether and how to include
   such calls in their missed-call list.)

   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 666 response destroys the transaction.  The service
   provider delivering calls or messages to the user issuing the
   response, for example, MAY add the calling party to a personal
   blacklist specific to the called party, MAY use the information as
   input when computing the likelihood that the calling party is placing
   unwanted calls ("crowd sourcing"), MAY initiate a traceback request,
   and MAY report the calling party identity to government authorities.

   Systems receiving 666 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.  In other words,
   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 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

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   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].  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.666,
   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 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 registers a new SIP response code.  This response code
   is defined by the following information, which is to be added to the
   "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]

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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 [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 identity is not spoofed, a call or message recipient
   might flag legitimate caller identities, e.g., to extract 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.

   Since caller identities are routinely re-assigned to new subscribers,
   algorithms are advised to consider whether the caller identity has
   been re-assigned to a new subscriber and possibly reset any related

   For both individually-authenticated and unauthenticated calls,
   recipients of response code 666 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.  Acknowledgements

   Tolga Asveren, Peter Dawes, Martin Dolly, Keith Drage, Vijay Gurbani,
   Olle Johansson, Paul Kyzivat, Jean Mahoney, Marianne Mohali, Brian
   Rosen, Brett Tate, Chris Wendt and Dale Worley provided helpful

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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,

   [RFC5057]  Sparks, R., "Multiple Dialog Usages in the Session
              Initiation Protocol", RFC 5057, DOI 10.17487/RFC5057,
              November 2007, <>.

   [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,

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

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   [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


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