SIPCORE                                                   H. Schulzrinne
Internet-Draft                                       Columbia University
Intended status: Standards Track                         August 29, 2019
Expires: March 1, 2020

              SIP Call-Info Parameters for Labeling Calls


   Called parties often wish to decide whether to accept, reject or
   redirect calls based on the likely nature of the call.  For example,
   they may want to reject unwanted telemarketing or fraudulent calls,
   but accept emergency alerts from numbers not in their address book.
   This document describes SIP Call-Info parameters and a feature tag
   that allow originating, intermediate and terminating SIP entities to
   label calls as to their type, confidence and references to additional

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 1, 2020.

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
   ( 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

Schulzrinne               Expires March 1, 2020                 [Page 1]

Internet-Draft               Call-Info Spam                  August 2019

   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
   2.  Normative Language  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   4
   4.  Parameters  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Call Types  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  REGISTER Response . . . . . . . . . . . . . . . . . . . .   7
     6.2.  INVITE Request  . . . . . . . . . . . . . . . . . . . . .   7
   7.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  SIP Call-Info Header Field Parameters . . . . . . . . . .   8
     8.2.  SIP Global Feature-Capability Indicator . . . . . . . . .   8
     8.3.  SIP Call-Info Type Parameter  . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   In many countries, an increasing number of calls are unwanted
   [RFC5039], as they might be fraudulent, telemarketing or the
   receiving party does not want to be disturbed by, say, surveys or
   solicitation by charities.  Currently, called parties have to rely
   exclusively on the caller's number or, if provided, caller name, but
   unwanted callers may not provide their true name or may use a name
   that misleads, e.g., "Cardholder Services".  On the other hand, many
   calls from unknown numbers may be important to the called party,
   whether this is an emergency alert from their emergency management
   office or a reminder about a doctor's appointment.  Since many
   subscribers now reject all calls from unknown numbers, such calls may
   also inadvertently be left unanswered.  Users may also install
   smartphone apps that can benefit from additional information in
   making decisions as to whether to ring, reject or redirect a call to

   To allow called parties to make more informed decisions on how to
   handle incoming calls from unknown callers, we describe a new set of
   parameters for the SIP [RFC3261] Call-Info header field for labeling
   the nature of the call.

Schulzrinne               Expires March 1, 2020                 [Page 2]

Internet-Draft               Call-Info Spam                  August 2019

   This specification assumes that the user agent can trust its SIP
   provider to correctly label the nature of calls.  This may not always
   be the case and not all SIP service providers will label calls, so
   users may need to draw on other, third-party, sources of call
   information beyond the scope of this specification or may decide to
   disregard the call labeling offered by their service provider.
   (Service providers may, for example, be reluctant to label calls as
   spam.)  However, the SIP registrar already occupies a position of
   trust by necessity; also, the user agent is typically a customer of
   the operator of the registrar or within the same organization, e.g.,
   if the registrar is part of a PBX.  Thus, the entity inserting the
   Call-Info header field and the UAS relying on it SHOULD be part of
   the same trust domain [RFC3324].  Conversely, the entity signing the
   caller information [RFC8224] is likely either to be the caller itself
   or the originating service provider, neither of which is likely to
   label the caller as a category unlikely to be answered by the called

   The service provider inserting the Call-Info header field may draw on
   a wide variety of sources.  For example, service providers offering
   alerting or notification services (e.g., for packages or health
   alerts) may register their phone numbers, after suitable vetting, in
   shared databases.  Government agencies could publish electronic
   directories of official telephone numbers, drawing on the historical
   precedent of the "blue pages" found in printed phone directories.
   Government regulators for financial services, health care providers
   and charitable organizations could provide sources of telephone
   numbers and service types belonging to such organizations.  Finally,
   crowd-sourcing might also be used to populate databases of call
   types.  In the United States, industry organizations have proposed
   variations of such caller databases to prevent accidental blocking of
   calls based on their statistics such as frequency or duration alone.

   Providers may also find the SIP Priority header ([RFC3261],
   Section 20.26) field useful in helping called parties decide how to
   respond to an incoming call.

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.

Schulzrinne               Expires March 1, 2020                 [Page 3]

Internet-Draft               Call-Info Spam                  August 2019

3.  Overview of Operation

   This document describes a new set of optional parameters and usage
   for the SIP [RFC3261] Call-Info header field, with a purpose "info",
   for labeling the nature of the call.  The header field may be
   inserted by the call originator, an intermediate proxy or B2BUA or
   the terminating carrier, based on assertions by the caller, number-
   indexed databases, call analytics or other sources of information.
   The SIP provider serving the called party MUST remove any parameters
   enumerated in this specification that it does not trust.

   To ensure that an untrusted originating caller does not mislead the
   called party, a new feature capability indicator [RFC6809],
   info.spam, in the REGISTER response signals whether the terminating
   carrier supports the feature described in this document and thus will
   remove any untrusted 'confidence', 'origin', 'source' and 'type'
   Call-Info header field information parameters.  It is possible for
   the terminating carrier to support this feature by simply removing
   all parameters defined in the document, without inserting any of its
   own information, although this is likely to be unusual.  A user agent
   MUST ignore any of the parameters defined in this document unless the
   feature capability indicator is present in the response to the
   REGISTER request.  An example of the REGISTER response is shown in
   Section 6.1.

   SIP proxies or B2BUAs MUST add a new Call-Info "info" header field
   value, rather than add parameters to an existing value.  Thus, one
   SIP request MAY contain several Call-Info header instances of purpose
   "info", either as a single header with a comma-separated list of
   header values or separate headers, or some combination.

   As defined in [RFC3261], the Call-Info header field contains a URI
   that can provide additional information about the caller or call.
   For example, many call filtering services provide a web page with
   crowd-sourced information about the calling number.  If the entity
   inserting the header field does not have information it wants to link
   to, it MUST use an empty data URL [RFC2397] as a placeholder, as in
   "data:,".  (The Call-Info header field syntax makes the URI itself
   mandatory.)  An example is shown in Section 6.2.

4.  Parameters

   All of the parameters listed below are optional and may appear in any
   combination and order.  Their ABNF is defined in Section 7.  All
   except the 'type' parameter are optional.

   confidence  The 'confidence' parameter carries an estimated
      probability that the call is of the nature indicated in the 'type'

Schulzrinne               Expires March 1, 2020                 [Page 4]

Internet-Draft               Call-Info Spam                  August 2019

      parameter, expressed as a whole-number percentage between 0 and
      100, inclusive, with larger numbers indicating higher probability.
      The computation of the estimate is beyond the scope of this
      specification.  If a 'type' is not specified, this parameter
      estimates the likelihood that the call is unwanted spam by the
      called party.  If the confidence level is not specified, the
      sender considers the information reliable enough to act on,
      according to its local decision thresholds.

   origin  The origin parameter provides free-text information, as a
      quoted-text (UTF8-encoded) string, about the source of the 'type'
      or 'confidence' parameter and is meant to be used for debugging,
      rather than for display to the end user.  For example, it may
      indicate the name of an external information source, such as a
      list of known emergency alerters or a government agency.

   source  The source parameter identifies the entity, by host name,
      domain or IP address, that inserted the 'confidence', 'origin' and
      'type' parameters.  It uses the "host" ABNF syntax.

   type  The type parameter indicates the type of the call or caller.
      It is drawn from an extensible set of values, with the initial set
      listed below.  Gateways to analog phone systems MAY include the
      label in caller name (CNAM) information delivered to user
      equipement.  Automated call classification systems MAY use this
      information as one factor in deciding how to handle the call.
      Calls SHOULD be labeled with types that may make it more likely
      that the caller will answer (e.g., for alert and health-related
      calls) if the entity inserting the information is confident that
      the calling party number is valid, e.g., because the request has
      been signed [RFC8224].

5.  Call Types

   The following initial set of types are defined.  The call types are
   generally based on the caller's telephone number or possibly an
   assertion by a trusted caller, as the content cannot be not known.
   Each call is tagged with at most one type label, i.e., the labels are
   meant to be mutually exclusive.  The definitions are meant to be
   informal and reflect the common understanding of subscribers who are
   not lawyers.  By their very nature, this classification may sometimes
   be erroneous, e.g., if a number has been re-assigned to another
   entity or if crowd-sourced information is wrong, and thus should be
   treated as a hint or estimate.  Each entity inserting type
   information will need to define its own policy as to the level of
   certainty it requires before it inserts type information.

Schulzrinne               Expires March 1, 2020                 [Page 5]

Internet-Draft               Call-Info Spam                  August 2019

   Other strings may be used; there does not appear to be a need for
   defining vendor-defined strings as the likelihood of confusion
   between a service-provider-specific usage and a later extension to
   the list appears low.  Additional labels are registered with IANA.

   business  Calls placed by businesses, i.e., an entity or enterprise
      entered into for profit.  This type is used if no other, more
      precise, category fits.

   debt-collection  Calls related to collecting of debt owed or alleged
      to be owed by the called party.

   emergency-alert  Calls that provide the recipient warnings and alerts
      regarding a pending or on-going emergency.  (This call type is
      unrelated to emergency calls placed by individuals using emergency
      numbers such as 9-1-1 or 1-1-2.)

   fraud  The call is considered to be fraudulent.

   government  A call placed by a government entity, if no more specific
      label such as "health" or "debt-collection" is known or applies.

   health  Informational calls by health plans, health care
      clearinghouses or health care provider, where health care means
      care, services, or supplies related to the health of an

   informational  Calls intended to convey information to the called
      party about a transaction such as package delivery, appointment
      reminder, or order confirmation.

   not-for-profit  A call placed by a not-for-profit organization,
      including for soliciting donations or providing information.

   personal  A non-business, person-to-person, call, e.g., from a
      residential line or personal mobile number.

   political  Calls related to elections or other political purposes.

   public-service  Calls that provide the recipient information
      regarding public services, e.g., school closings.

   prison  Calls from jails, prisons and other correctional facilities.

   spam  A call that is likely unwanted, if not otherwise classified.

   spoofed  The calling number for this call has been spoofed.  (For
      example, the call has failed STIR validation [RFC8224] within the

Schulzrinne               Expires March 1, 2020                 [Page 6]

Internet-Draft               Call-Info Spam                  August 2019

      SIP service provider network or the telephone number is not a
      valid number or is known not to have been assigned.)

   survey  A call that solicits the opinions or data of the called

   telemarketing  Calls placed in order to induce the purchase of a
      product or service to the called party.

   trusted  The call is being placed by a trusted entity and falls
      outside the other categories listed.  This may include call backs,
      e.g., from a conferencing service, or messages from
      telecommunication carriers and utilities.

6.  Examples

6.1.  REGISTER Response

   The example below shows a partial REGISTER response showing that the
   registrar and proxy will remove any untrusted Call-Info header

   SIP/2.0 200 OK
   From: Bob <>;tag=a73kszlfl
   To: Bob <>;tag=34095828jh
   Feature-Caps: *;

6.2.  INVITE Request

       INVITE SIP/2.0
       Call-Info: <>
         ;purpose=info ;confidence=85 ;type=fraud
         ;origin="FTC fraud list"

7.  ABNF

Schulzrinne               Expires March 1, 2020                 [Page 7]

Internet-Draft               Call-Info Spam                  August 2019

                label-info-params = [ci-confidence] / [ci-source] / [ci-origin] / ci-type
                ci-confidence = "confidence" EQUAL 1*3DIGIT
                ci-origin = "origin" EQUAL quoted-string
                ci-source = "source" EQUAL host
                ci-type = "type" EQUAL ("business" / "debt-collection" / "emergency-alert" / "fraud" /
                            "government" / "health" / "informational" / "not-for-profit" /
                            "personal" / "political" / "public-service" / "prison" / "spam" /
                            "spoofed" / "survey" / "telemarketing" / "trusted" /

8.  IANA Considerations

8.1.  SIP Call-Info Header Field Parameters

   This document defines the 'confidence', 'origin', 'source' and 'type'
   parameters in the Call-Info header in the "Header Field Parameters
   and Parameter Values" registry defined by [RFC3968].

    | Header Field | Parameter Name | Predefined Values | Reference  |
    | [this RFC]   | Call-Info      | confidence        | No         |
    | Call-Info    | origin         | No                | [this RFC] |
    | Call-Info    | source         | No                | [this RFC] |
    | Call-Info    | type           | Yes               | [this RFC] |

8.2.  SIP Global Feature-Capability Indicator

   This document defines the feature capability in
   the "SIP Feature-Capability Indicator Registration Tree" registry
   defined in [RFC6809].


   Description  This feature-capability indicator when used in a
      REGISTER response indicates that the server will add, inspect,
      alter and possibly remove the Call-Info header field parameters
      defined in the reference.

   Reference  [this RFC]

8.3.  SIP Call-Info Type Parameter

   This specification establishes the "Call-Info Type" sub-registry
   under  Call-Info
   "type" parameters are used in the "type" parameter in the SIP Call-
   Info header field.  The initial values are listed in Section 5.

Schulzrinne               Expires March 1, 2020                 [Page 8]

Internet-Draft               Call-Info Spam                  August 2019

   Additional values are allocated by expert review [RFC5226]; only the
   token value, using the ABNF iana-token, and a brief description,
   typically no more than a few sentences, is required.  The ABNF for
   iana-token is defined in [RFC3261].  A specification is not required.

9.  Security Considerations

   The security considerations in [RFC3261] (Section 20.9) apply.  A
   user agent MUST ignore the parameters defined in this document unless
   the SIP REGISTER response contained the feature
   capability.  B2BUAs or proxies that maintain user registrations MUST
   remove any parameters defined in this document that were provided by
   untrusted third parties.

   The UAS SHOULD only consider Call-Info header field information that
   originates from a registrar that is part of the same trust domain

   The protection offered against rogue SIP entities by the feature
   capability relies on protecting the REGISTER response against man-in-
   the-middle attacks that maliciously add the capability indicator.
   Thus, a UAS SHOULD NOT trust the information in the "Call-Info"
   header field unless the SIP session between the entity inserting the
   header field and the UAS is protected by TLS [RFC8446].

   Labeling calls is likely only useful if the caller identity can be
   trusted, e.g., by having the call signaling requests signed
   [RFC8224], as otherwise spoofed calls would likely be mislabeled and
   thus increase the likelihood that the called party is mislead,
   answers unwanted calls or is defrauded.  Thus, this information MUST
   only be added calls with an attestation level of "Full Attestation"
   [RFC8588] or for calls where the SIP entity inserting the header
   knows to have correct calling number information, e.g., because the
   call originated within the same PBX or the same carrier and the
   operating entity ensures that caller ID spoofing is highly unlikely
   within their realm of responsibility.

10.  Acknowledgements

   Jim Calme and other members of the Robocall Strikeforce helped draft
   the initial list of call types.  Tolga Asveren, Ben Campbell, Keith
   Drage, Christer Holmberg, Paul Kyzivat and Dale Worley provided
   helpful comments on the document.

Schulzrinne               Expires March 1, 2020                 [Page 9]

Internet-Draft               Call-Info Spam                  August 2019

11.  References

11.1.  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,

   [RFC2397]  Masinter, L., "The "data" URL scheme", RFC 2397,
              DOI 10.17487/RFC2397, August 1998,

   [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,

   [RFC3324]  Watson, M., "Short Term Requirements for Network Asserted
              Identity", RFC 3324, DOI 10.17487/RFC3324, November 2002,

   [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
              (IANA) Header Field Parameter Registry for the Session
              Initiation Protocol (SIP)", BCP 98, RFC 3968,
              DOI 10.17487/RFC3968, December 2004,

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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, <>.

Schulzrinne               Expires March 1, 2020                [Page 10]

Internet-Draft               Call-Info Spam                  August 2019

   [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,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

11.2.  Informative References

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

   [RFC8588]  Wendt, C. and M. Barnes, "Personal Assertion Token
              (PaSSporT) Extension for Signature-based Handling of
              Asserted information using toKENs (SHAKEN)", RFC 8588,
              DOI 10.17487/RFC8588, May 2019,

Author's Address

   Henning Schulzrinne
   Columbia University
   450 Computer Science Bldg.
   New York, NY  10027


Schulzrinne               Expires March 1, 2020                [Page 11]