SIP Call-Info Parameters for Rich Call Data

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Table of Contents

1. Introduction

Signaling protocols in telephone networks have long supported the delivery of a 'calling name' from the originating side to the terminating side, though in practice, the terminating side is often left to derive a name from the calling-party number by consulting a local address book or an external database. SIP [RFC3261] similarly can carry a 'display-name' in the From header field value from the originating to terminating side, though it is an unsecured field that is not commonly trusted and is often replaced or ignored. The same can be considered true of information in the Call-Info header field in SIP.¶

To allow calling parties to initiate, and called parties to receive, a more comprehensive, deterministic, and extensible Rich Call Data (RCD) for incoming calls, this document defines a new parameter ('call-reason') for the SIP Call-Info header field [RFC3261] and also a new token ("rcd-jcard") for the 'purpose' parameter of the Call-Info header field. For this document and depending on the policies of the communications system, a calling party could be either the end user device (e.g., a SIP user agent (UA)) or a network service as part of a telephone service provider. Similarly, a called party could be an end user device or the network telephone service provider acting on behalf of the recipient of the call.¶

Used on its own, this specification assumes that the called party UA can trust the SIP network or the SIP provider to assign, deliver, and protect the correct RCD information as an end-to-end security policy. However, as is true in many interconnected communications services, this end-to-end trust cannot be guaranteed. Therefore, the recommended approach is that the entity inserting the Call-Info header field should also sign the caller information via STIR-defined protocol tools [RFC7340] for SIP [RFC8224] and specifically through the use of RCD or the "rcd" PASSporT defined in [I-D.ietf-stir-passport-rcd].¶

Alternatively, this specification can be utilized in conjunction with the protocols defined in [I-D.ietf-stir-passport-rcd] as part of the communications signaling path, specifically in the trusted UNI device interface at the terminating side as part of an authenticated, network-to-device, trusted signaling where a device may not have the ability to verify the "rcd" PASSporT, but it can receive the RCD information from the Call-Info header field as defined in this specification.¶

[RFC7852] provides a means of carrying additional data about callers for the purposes of emergency services (especially Section 4.4 (Owner/Subscriber Information) of [RFC7852]). This specification provides an overlapping functionality for non-emergency cases. Rather than overloading its "EmergencyCallData" Call-Info 'purpose' parameter value, this document defines a separate 'purpose' parameter for the more generic delivery of information via jCard [RFC7095]. This document borrows from [RFC7852] the capability to carry a data structure as a body, through the use of the "cid" URI scheme [RFC2392].¶

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "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. Overview

In this document, we provide a framework for the use of Call-Info header field to carry RCD in SIP [RFC3261]. The Call-Info header field (defined in [RFC3261], Section 20.9) defines a 'purpose' parameter. In addition to providing guidance on calling name practices and the use of the existing 'purpose' parameter token, "icon", this document expands on other types of RCD by defining a new 'purpose' token, "rcd-jcard", and a new generic parameter, 'call-reason', for the Call-Info header field to align with RCD as defined in the STIR framework [RFC8224] and with "rcd" PASSporTs defined in [I-D.ietf-stir-passport-rcd].¶

The 'purpose' parameter token "rcd-jcard" is used to associate RCD related to the identity of the calling party in the form of a jCard [RFC7095]. While there is a "card" token defined in [RFC3261] which could be considered to have an overlapping purpose, the "rcd-jcard" token is intended to denote the jCard profile defined in this document for use in the Call-Info header field for RCD. The choice of jCard in this specification is guided by two things. First, JSON has become the default and is generally the widely accepted, optimally supported format for transmission, parsing, and manipulation of data on IP networks, and jCard represents an extensible method of providing information about a person or business associated with a call. Second, jCard has been defined in [I-D.ietf-stir-passport-rcd] and has been adopted by PASSporT [RFC8225] because of the usage of JSON Web Tokens (JWT) [RFC7519].¶

The new Call-Info header field parameter 'call-reason' provides a string or other object that conveys the caller's intent or reason for calling to help the called party understand the context and intent of the call and why they may want to answer the call.¶

4. A Call-Info Framework for Carrying Rich Call Data

This specification extends the Call-Info header field to be compatible and complimentary to the RCD framework defined in [I-D.ietf-stir-passport-rcd]. Typically, a SIP-based call involves multiple hops through different trusted and untrusted networks. The STIR framework [RFC7340] addresses the protection of the carriage of call information and identities over untrusted networks, which wasn't addressed in the core SIP specifications. [RFC3261], Section 20.9 defines the Call-Info header field as the mechanism for carrying call- and caller-related information and also provides procedures for defining new 'purpose' parameter tokens. This document discusses the use of existing tokens and defines a new 'purpose' token to correspond to the RCD framework.¶

There are a number of RCD information types that can be transmitted in the Call-Info header field of a SIP request. The STIR RCD specification [I-D.ietf-stir-passport-rcd] defines calling name, a logo or icon associated with the caller, and a call reason string. It also discusses an extensible way of carrying caller information using jCard [RFC7095]. It may be that future specifications extend information types and, similar to how this document extends the Call-Info header field to provide corresponding functionality to STIR RCD, it is RECOMMENDED that future specifications also provide corresponding Call-Info extensions.¶

The RCD framework defined both in this document as well as in [I-D.ietf-stir-passport-rcd] carries call-specific information. The insertion of RCD is intended to be singular in that the receiving party should not be required to make any call-specific decisions based on redundant, duplicate, or conflicting RCD. With the use of the Call-Info header field for the transmission of RCD, any RCD-related information defined in this specification or future specifications that extend this mechanism MUST be contained in a single Call-Info header field containing all URI and 'purpose' tokens and parameters related to RCD. The RCD information is either intended to be added by a party that is authoritative over that information or to have been translated from a verified STIR RCD PASSporT and unmodified once in a trusted domain. Any additional parties involved in the call path SHOULD NOT modify the Call-Info header field or add additional Call-Info header fields related to RCD. The insertion of the RCD Call-Info header field should be considered a trusted action based on trusted information, and the information SHOULD NOT be considered modifiable as a best practice.¶

As discussed in [I-D.ietf-stir-passport-rcd], the calling name uses the display-name value of the From header field [RFC3261] of the request. Alternatively, for some calls, the calling name may come from the P-Asserted-ID header field [RFC3325]. This is out of scope for Call-Info header field, so will not be covered in this document further.¶

For logos or icons that can represent the calling party, the 'purpose' token "icon" [RFC3261] is used to indicate a URI for an image resource that can be displayed to the user receiving the SIP request. For the purpose of this document and the transmission of RCD, the "icon" 'purpose' token should be used as defined. Section 8.2 provides high-level guidance on image formatting and related information.¶

This document defines 'call-reason' as a new parameter for the Call-Info header field. This parameter carries a string indicating the reason for the call.¶

jCard is a comprehensive and extensible mechanism defined in the STIR RCD framework. While [RFC3261] specifies a "card" 'purpose' token, the intent of defining a new "rcd-jcard" 'purpose' token is to use the JSON jCard format [RFC7095] and to provide guidance for the use and non-use of jCard attributes to describe the calling party in a communications session as well to provide some security considerations around that information. These topics are covered in the next sections.¶

5. "rcd-jcard" Call-Info 'purpose' Token

The Call-Info 'purpose' token "rcd-jcard" indicates support of RCD associated with the identity of a calling party in a SIP call [RFC3261], Section 20.9. The format of a Call-Info header field when using the "rcd-jcard" token is as follows.¶

The Call-Info header field is defined to include a URI that points to a resource that is a jCard JSON object [RFC7095]. The media type for the JSON text MUST be set as application/json with a default encoding of UTF-8 [RFC8259]. This MAY be carried directly in the Call-Info header field URI using the "data" URI scheme. A jCard also MAY be carried in the body of the SIP request bearing this Call-Info header field via the "cid" URI scheme [RFC2392]. Alternatively, the URI MUST define the use HTTPS or a transport that can validate the integrity of the source of the resource as well as the transport channel through which the resource is retrieved. If, in the specific deployment environment of SIP, the source or integrity of the RCD information cannot be trusted, then the use of the STIR RCD framework defined in [I-D.ietf-stir-passport-rcd] should be considered.¶

The jCard is intended to contain multiple information elements about the calling party. A call and its corresponding single RCD-related Call-Info header field MUST only contain a single "rcd-jcard" token.¶

The fields like "fn", "photo", or "logo" if used with the use of "icon" calling name in From or P-Asserted-ID header field or purpose token, as described in the previous section, MUST either match or be avoided to allow the called party to clearly determine the intended calling name or icon.¶

An example of a Call-Info header field is:¶

Call-Info: <https://example.com/qbranch.json>;purpose=rcd-jcard

An example of the contents of a URL-linked jCard JSON file is shown as follows:¶

["vcard",
  [
    ["version",{},"text","4.0"],
    ["fn",{},"text","Q Branch"],
    ["org",{},"text","MI6;Q Branch Spy Gadgets"],
    ["photo",{},"uri","https://example.com/photos/q-256x256.png"],
    ["logo",{},"uri","https://example.com/logos/mi6-256x256.jpg"],
    ["logo",{},"uri","https://example.com/logos/mi6-64x64.jpg"]
  ]
]

An example SIP INVITE using the "data" URI scheme is as follows:¶

INVITE sip:alice@example.com SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Alice <sip:alice@example.com>
From: Bob <sip:12155551000@example.com;user=phone>;tag=1928301774>
Call-ID: a84b4c76e66710
Call-Info: <data:application/json,["vcard",[["version",{},"text",
"4.0"],["fn",{},"text","Q Branch"],["org",{},"text","MI6;Q Branch
Spy Gadgets"],["photo",{},"uri","https://example.com/photos/quart
ermaster-256x256.png"],["logo",{},"uri","https://example.com/log
os/mi6-256x256.jpg"],["logo",{},"uri","https://example.com/logos/
mi6-64x64.jpg"]]]\>;purpose=rcd-jcard;call-reason="Rendezvous for
Little Nellie"
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Fri, 25 Sep 2015 19:12:25 GMT
Contact: <sip:12155551000@gateway.example.com>
Content-Type: application/sdp

v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000

An example SIP INVITE using the "cid" URI scheme is as follows:¶

INVITE sip:alice@example.com SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Alice <sip:alice@example.com>
From: Bob <sip:12155551000@example.com;user=phone>;tag=1928301774>
Call-ID: a84b4c76e66710
Call-Info: <cid:12155551000@example.com>;purpose=rcd-jcard;
  call-reason="Rendezvous for Little Nellie"
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Fri, 25 Sep 2015 19:12:25 GMT
Contact: <sip:12155551000@gateway.example.com>
Content-Type: multipart/mixed; boundary=boundary1
Content-Length: ...

--boundary1

Content-Type: application/sdp

v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000

--boundary1

Content-Type: application/json
Content-ID: <12155551000@example.com>

["vcard",[["version",{},"text","4.0"],["fn",{},"text","Q Branch"],
["org",{},"text","MI6;Q Branch Spy Gadgets"],["photo",{},"uri","ht
tps://example.com/photos/quartermaster-256x256.png"],["logo",{},"u
ri","https://example.com/logos/mi6-256x256.jpg"],["logo",{},"uri",
"https://example.com/logos/mi6-64x64.jpg"]]]

6. 'call-reason' Call-Info Parameter

This specification defines a new parameter that extends the overall content of the RCD-related Call-Info header field. As other parameters may be defined in the future, this parameter is intended to be separate and distinct from the other URI and 'purpose' tokens that may proceed these parameters.¶

This new parameter of the Call-Info header field is called 'call-reason'. The 'call-reason' parameter is intended to convey a short textual message suitable for display to an end user during call alerting. As a general guideline, this message SHOULD be no longer than 64 characters; displays that support this specification may be forced to truncate messages that cannot fit onto a screen. This message conveys the caller's intention in contacting the callee. It is an optional parameter, and the sender of a SIP request cannot guarantee that its display will be supported by the terminating endpoint. The manner in which this reason is set by the caller is outside the scope of this specification.¶

An alternative approach would have been to use the value of Subject header field [RFC3261] to convey the reason for the call. However, because the Subject header field has seen little historical use in SIP implementations and its specification describes its potential use in filtering, it seemed prudent to define a new means of carrying a call reason indication.¶

An example of a Call-Info header field value with the "call-reason" parameter follows:¶

Call-Info: <https://example.com/jbond.json>;purpose=rcd-jcard;
  call-reason="For your ears only"

In the case that there is only a 'call-reason' parameter or any future parameters that may be defined and no need for a purpose parameter with no associated URI, it is RECOMMENDED to include a null data URI, "data:" as the URI. That purpose parameter MUST be "rcd-jcard" defined in this document to avoid any conflicts with existing implementations and previously defined purpose parameters. As an example:¶

Call-Info: <data:>;purpose=rcd-jcard;
  call-reason="For your ears only"

7. Usage and an Example of Call-Info for RCD

The procedures for the usage of URIs and 'purpose' parameter tokens should generally follow the procedures defined in [RFC3261]. The following example of a Call-Info header field shows the use of multiple 'purpose' parameters to indicate a jCard and an icon and also a 'call-reason' parameter:¶

Call-Info: <https://example.com/jbond.json>;purpose=rcd-jcard,
  <https://example.com/jbond.png>;purpose=icon;
  call-reason="For your ears only"

8. Usage of jCard and Property-Specific Usage

Beyond the definition of the specific properties or JSON arrays associated with each property, this specification defines a few rules above and beyond [RFC7095] that are specific to the use of jCard for Call-Info and RCD to ensure there is a minimum level of supported properties to which every implementation of this specification should adhere. This includes support for interpreting the value of these properties and the ability to render in some appropriate form the display capabilities of common telephone devices as well as applications, and also includes requirements specific to textual and graphics-capable displays.¶

  +-------------+--------------------------------------------------+
  | Cardinality | Meaning                                          |
  +-------------+--------------------------------------------------+
  |      1      | Exactly one instance per jCard MUST be present.  |
  |      *1     | Exactly one instance per jCard MAY be present.   |
  |      1*     | One or more instances per jCard MUST be present. |
  |      *      | One or more instances per jCard MAY be present.  |
  +-------------+--------------------------------------------------+

Example:
["fn", {}, "text", "Mr. John Q. Public\, Esq."]

Example:
["n", {}, "text", "Public;John;Quinlan;Mr.;Esq."]
["n", {}, "text", "Stevenson;John;Philip,Paul;Dr.;Jr.,M.D.,A.C.P."]

Example:
["nickname", {}, "text", "Robbie"]
["nickname", {}, "text", "Jim,Jimmie"]
["nickname", {}, "text", "TYPE=work:Boss"]

Example:
["photo", {}, "uri", "http://www.example.com/jqpublic-256x256.png"]

Example:
["adr", {"type":"work"}, "text",
  ["", "", "3100 Massachusetts Avenue NW", "Washington", "DC",
  "20008", "USA"]

Example:
["tel", { "type": ["voice", "text", "cell"], "pref": "1" }, "uri",
  "tel:+1-202-555-1000"]
["tel", { "type": ["fax"] }, "uri", "tel:+1-202-555-1001"]

Example:
["email", {"type":"work"}, "text", "jqpublic@xyz.example.com"]
["email", {"pref":"1"}, "text", "jane_doe@example.com"]

Example:
["lang", {"type":"work", "pref":"1"}, "language-tag", "en"]
["lang", {"type":"work", "pref":"2"}, "language-tag", "fr"]
["lang", {"type":"home"}, "language-tag", "fr"]

Example:
["tz", {}, "text", "Raleigh/North America"]

Example:
["geo", {}, "uri", "geo:37.386013,-122.082932"]

Example:
["title", {}, "text", "Research Scientist"]

Example:
["role", {}, "text", "Project Leader"]

Example:
["logo", {}, "uri", "http://www.example.com/abccorp-512x512.jpg"]

["logo", {}, "uri", "data:image/jpeg;base64,MIICajCCAdOgAwIBAgIC
      AQEEBQAwdzELMAkGA1UEBhMCVVMxLDAqBgNVBAoTI05ldHNjYXBlIENvbW11bm
      ljYXRpb25zIENvcnBvcmF0aW9uMRwwGgYDVQQLExNJbmZvcm1hdGlvbiBTeXN0
      <...the remainder of base64-encoded data...>"]

Example:
["org", {}, "text", "ABC\, Inc.;North American Division;Marketing"]

Example:
["categories", {}, "text", "TRAVEL AGENT"]

["categories", {}, "text", "INTERNET,IETF,INDUSTRY"]

Example:
["note", {}, "text", "This fax number is operational 0800 to 1715
             EST\, Mon-Fri."]

Example:
["sound", {}, "uri", "https://www.example.com/pub/logos/abccorp.mp3"]

["sound", {}, "uri", "data:audio/basic;base64,MIICajCCAdOgAwIBAgICBE
      AQEEBQAwdzELMAkGA1UEBhMCVVMxLDAqBgNVBAoTI05ldHNjYXBlIENvbW11bm
      ljYXRpb25zIENvcnBvcmF0aW9uMRwwGgYDVQQLExNJbmZvcm1hdGlvbiBTeXN0
      <...the remainder of base64-encoded data...>"]

Example:
["uid", {}, "uri", "urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"]

Example:
["url", {}, "uri", "https://example.org/french-rest/chezchic.html"]

Example:
["version", {}, "text", "4.0"]

9. Extension of jCard

Part of the intent of using jCard is to leverage its extensibility to define new properties to relay new information related to a caller. This capability is inherently supported as part of standard extensibility. However, usage of those new properties should be published and registered following [RFC7095], Section 3.6 or new specifications.¶

10. IANA Considerations

  +--------------+----------------+-------------------+------------+
  | Header Field | Parameter Name | Predefined Values | Reference  |
  +--------------+----------------+-------------------+------------+
  | Call-Info    | purpose        | Yes               | [this RFC] |
  +--------------+----------------+-------------------+------------+

  +--------------+----------------+-------------------+------------+
  | Header Field | Parameter Name | Predefined Values | Reference  |
  +--------------+----------------+-------------------+------------+
  | Call-Info    | call-reason    | No                | [this RFC] |
  +--------------+----------------+-------------------+------------+

11. Security Considerations

Revealing information such as the name, location, and affiliation of a person necessarily entails certain privacy risks. The SIP Call-Info header field has no particular confidentiality requirement, as the information sent in SIP is in the clear anyway. Transport-level security can be used to hide information from eavesdroppers, and the same confidentiality mechanisms would protect any Call-Info or jCard information carried or referred to in SIP.¶

The security framework of signing and providing integrity to this data [I-D.ietf-stir-passport-rcd] should be followed, and the use of constraints and other certificate-based associations should be considered. This includes considerations for information about the calling party, which is generally constant, versus per-call data, which is more transient. This also includes the relationship that certificates with constraints presents to how they relate to each other and how that information is managed, protected, and associated with the correct call corresponding to a calling party.¶

Acknowledgements

We would like to thank David Hancock, Alec Fenichel, and other members of the SIPCORE and STIR working groups for their helpful suggestions and comments during the creation of this document.¶

Authors' Addresses