Internet Engineering Task Force (IETF) D. York

Internet Engineering Task Force (IETF) D. York
Request for Comments: 8496 Individual
Category: Informational T. Asveren
ISSN: 2070-1721 Ribbon Communications
October 2018

P-Charge-Info: A Private Header Field (P-Header) Extension to the
Session Initiation Protocol (SIP)

Abstract

This text documents the current usage of P-Charge-Info, an existing
Session Initiation Protocol (SIP) private header field (P-Header)
used to convey billing information about the party to be charged.
This P-Header is currently used in production by several equipment
vendors and carriers and has been in use since at least 2007. This
document details the registration of this header field with IANA.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see 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
https://www.rfc-editor.org/info/rfc8496.

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Copyright Notice

Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) 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
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. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Purpose of This Document . . . . . . . . . . . . . . . . . . 4
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. The P-Charge-Info Header . . . . . . . . . . . . . . . . . . 5
5.1. Applicability Statement for the P-Charge-Info Header
Field . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2. Usage of the P-Charge-Info Header Field . . . . . . . . . 5
5.2.1. Procedures at the UA . . . . . . . . . . . . . . . . 5
5.2.2. Procedures at the Proxy . . . . . . . . . . . . . . . 6
5.3. Use-Case Example . . . . . . . . . . . . . . . . . . . . 6
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7.1. Header Field . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8.1. Trust Relationship . . . . . . . . . . . . . . . . . . . 7
8.2. Untrusted Peers . . . . . . . . . . . . . . . . . . . . . 8
8.2.1. Ingress from Untrusted Peers . . . . . . . . . . . . 8
8.2.2. Egress to Untrusted Peers . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Alternatives . . . . . . . . . . . . . . . . . . . . 10
A.1. P-Charging-Vector . . . . . . . . . . . . . . . . . . . . 10
A.2. P-DCS-Billing-Info . . . . . . . . . . . . . . . . . . . 10
A.3. P-Asserted-Identity . . . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11

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1. Overview

In certain network configurations, several network entities have
found it useful to decouple the identity of the caller (what is
normally thought of as "Caller ID") from the identity/number used for
billing purposes. This document records the current usage of
P-Charge-Info, a private SIP header field, to provide simple billing
information and details the registration of this header field with
IANA as required by Section 4 of [RFC5727].

In a typical configuration, the identity of the caller, commonly
referred to as "Caller ID" by end users, is derived from one of the
following SIP header fields:

o P-Asserted-Identity

o From (in the absence of P-Asserted-Identity)

(NOTE: Some service providers have also used the Remote-Party-ID
header field, but this was never standardized and was replaced by
P-Asserted-Identity in [RFC3325].)

This identity/number is typically presented to the receiving user
agent (UA), where it is usually displayed for the end user. It is
also typically used for billing purposes by the network entities
involved in carrying the session.

However, in some network configurations, the "Caller ID" presented to
the receiving UA may be different from the number to be used for
billing purposes.

In this case, there exists a need for a way to pass an additional
billing identifier that can be used between network entities in order
to correctly bill for services.

Several carriers, application providers, and equipment providers have
been using the P-Charge-Info header field since at least 2007 as a
simple mechanism to exchange this billing identifier.

This document specifies the use of the P-Charge-Info header field in
INVITE requests. The header field might be useful in other SIP
messages, but such use is beyond the scope of this document.

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2. Requirements Language

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.

The key words describe requirements needed to interoperate with
existing usage.

3. Purpose of This Document

This document has been prepared to document the existing deployed
usage of the P-Charge-Info header field and to comply with Section 4
of [RFC5727] in registering this header field with IANA. It is noted
that RFC 5727 specifically deprecates new usage of "P-" header
fields, but P-Charge-Info has been in deployment since before 2007
and predates RFC 5727. Given this, the authors believe that
P-Charge-Info is a "grandfathered case" per Section 4 of RFC 5727.

4. Use Cases

The simplest use case for P-Charge-Info is an enterprise environment
where each SIP endpoint has a direct number that is passed by the
enterprise SIP proxy across to a SIP proxy at a SIP service provider
who provides connectivity to the Public Switched Telephone Network
(PSTN). Rather than cause the SIP service provider to have to track
each individual direct number for billing purposes, the enterprise
SIP proxy sends, in the P-Charge-Info header field, a single billing
identifier that the SIP service provider uses for billing purposes.

As another example, a hosted telephony provider or hosted voice-
application provider may have a large SIP network with customers who
are distributed over a very large geographic area and use local-
market PSTN numbers, although the network has only a very few actual
PSTN interconnection points.

The customers may all have local phone numbers, yet outgoing calls
are actually routed across a SIP network and out specific PSTN
gateways or across specific SIP connections to other SIP service
providers. The hosted provider may want to pass a billing identifier
to its SIP service providers either for the purpose of simplicity in
billing or to obtain better rates from the SIP service providers.

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5. The P-Charge-Info Header

5.1. Applicability Statement for the P-Charge-Info Header Field

The P-Charge-Info header field is applicable within a single private
administrative domain or between different administrative domains
where there is a trust relationship between the domains.

5.2. Usage of the P-Charge-Info Header Field

The P-Charge-Info header field is used to convey information about
the identity of the party to be charged. The P-Charge-Info header
field is typically inserted into a SIP request, usually an INVITE, by
one of the following:

o the SIP proxy on the originating network;

o a PSTN gateway acting as a SIP UA; or

o an application server generating billing information.

P-Charge-Info is to be used by the SIP entity that provides billing
services for a session. This could be an entity that is generating
billing records or another entity interacting with it. Upon receipt
of an INVITE request with the P-Charge-Info header field, such an
entity MAY use the value present in P-Charge-Info as indicating the
party responsible for the charges associated with the session. This
decision, for example, could be based on local policy.

5.2.1. Procedures at the UA

The P-Charge-Info header field may be inserted by PSTN gateways or
application servers acting as a SIP UA.

The P-Charge-Info header field is ignored by an end-user UA and
should not normally be received by such a UA. It MUST NOT be sent to
an end-user UA, as this would provide information to the UA about the
party to be charged; providing such information may cause security-
related issues; for example, calling-party information would be known
by the UA for an otherwise anonymous call. A UA SHOULD ignore it if
it receives this header. Similarly, an end-user UA originating a SIP
message SHOULD NOT insert this header field.

A PSTN gateway or application server acting as a UA MAY use the
content of the P-Charge-Info header field present in an INVITE
request it received as the identity to be charged for the session for
billing-related procedures, e.g., in a billing record or during
interaction with another entity generating billing records. A PSTN

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gateway or application server acting as a UA MAY use the content of
the P-Charge-Info header field to populate information about the
identity of the party to charge in another type of signaling, such as
ISDN User Part (ISUP).

5.2.2. Procedures at the Proxy

A SIP proxy that supports this extension and receives a request,
typically a SIP INVITE, MAY insert a P-Charge-Info header field. The
contents of the inserted header field may be decided based on local
policy or by querying an external entity to determine the identity of
the party to be charged.

When a proxy receives an INVITE request, it MAY use the content of
the P-Charge-Info header field contained in the request for billing-
related procedures, e.g., in a billing record or during interaction
with another entity that is generating billing records.

A SIP proxy that does not support this extension will pass any
received P-Charge-Info header field unmodified, in compliance with
RFC 3261.

A proxy supporting this extension MUST remove the P-Charge-Info
header field before sending a request to a UA that is not acting as a
PSTN gateway or appropriate application server, if the role of the UA
is known.

5.3. Use-Case Example

The content of the P-Charge-Info header field is typically just a
SIP/tel URI used as a billing indicator. An example could be as
simple as one of:

P-Charge-Info: <sip:[email protected];user=phone>

P-Charge-Info: <sip:[email protected]>

P-Charge-Info: <sips:[email protected]>

P-Charge-Info: <tel:+14075551234>

Any other applicable SIP URI could be used.

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6. Formal Syntax

This RFC contains the definition of one or more SIP header fields
that allow choosing between addr-spec and name-addr when constructing
header-field values. [RFC8217] prohibits the use of addr-spec if its
value would contain a comma, semicolon, or question mark.

The private header field specified here is described in both prose
and an augmented Backus-Naur Form (BNF) defined in [RFC5234].
Further, several BNF definitions are inherited from SIP and are not
repeated here. Implementors need to be familiar with the notation
and contents of [RFC3261] and [RFC5234] to understand this document.

The syntax of the P-Charge-Info header field is described as follows:

P-Charge-Info = "P-Charge-Info" HCOLON (name-addr / addr-spec)
; name-addr and addr-spec are specified in RFC 3261

The SIP URI contained in the name-addr/addr-spec is the billing
indicator that is passed between the parties.

7. IANA Considerations

This specification registers a new proprietary SIP header field
according to the procedures defined in [RFC5727].

7.1. Header Field

The P-Charge-Info private header field has been registered in the
"Header Fields" subregistry of the "Session Initiation Protocol (SIP)
Parameters" registry as follows:

Header Field Name: P-Charge-Info

Compact Form: none

Reference: RFC 8496

8. Security Considerations

8.1. Trust Relationship

Given that the information contained in the P-Charge-Info header
field will be used for billing purposes, the proxies and other SIP
entities that share this information MUST have a trust relationship.

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If an untrusted entity were inserted between the trusted entities, it
could potentially interfere with the billing records for the call.
If the SIP connections are not made over a private network, a
mechanism for securing the confidentiality and integrity of the SIP
connection MUST be used to protect the information. One such
mechanism could be TLS encryption of the SIP signaling stream.

8.2. Untrusted Peers

8.2.1. Ingress from Untrusted Peers

If the P-Charge-Info header field was accepted by a SIP entity from
an untrusted peer, there is the potential for fraud if the untrusted
entity sent incorrect information, either inadvertently or
maliciously.

Therefore, a SIP entity MUST remove and ignore the P-Charge-Info
header field when it is received from an untrusted entity.

8.2.2. Egress to Untrusted Peers

If the P-Charge-Info header field was sent by a SIP entity to an
untrusted peer, there is potential for exposure of network
information that is internal to a trust domain. For instance, the
untrusted entity may learn the identities of public SIP proxies used
within the trust domain, which could then potentially be directly
attacked.

If an implementation does not strip P-Charge-Info from the message
where specified in this document, it introduces serious privacy
risks. Examples include revealing third-party billing relationships
that might be sensitive, as well as unmasking the identity of callers
who wish to remain anonymous. Depending on circumstances, the latter
case may result in unwanted harassment and even physical harm to the
calling party.

Therefore, a SIP entity MUST remove the P-Charge-Info header field
when it is sent to an untrusted entity.

9. References

9.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.

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[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,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC5727] Peterson, J., Jennings, C., and R. Sparks, "Change Process
for the Session Initiation Protocol (SIP) and the Real-
time Applications and Infrastructure Area", BCP 67,
RFC 5727, DOI 10.17487/RFC5727, March 2010,
<https://www.rfc-editor.org/info/rfc5727>.

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

[RFC8217] Sparks, R., "Clarifications for When to Use the name-addr
Production in SIP Messages", RFC 8217,
DOI 10.17487/RFC8217, August 2017,
<https://www.rfc-editor.org/info/rfc8217>.

9.2. Informative References

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
DOI 10.17487/RFC3325, November 2002,
<https://www.rfc-editor.org/info/rfc3325>.

[RFC5503] Andreasen, F., McKibben, B., and B. Marshall, "Private
Session Initiation Protocol (SIP) Proxy-to-Proxy
Extensions for Supporting the PacketCable Distributed Call
Signaling Architecture", RFC 5503, DOI 10.17487/RFC5503,
March 2009, <https://www.rfc-editor.org/info/rfc5503>.

[RFC7315] Jesske, R., Drage, K., and C. Holmberg, "Private Header
(P-Header) Extensions to the Session Initiation Protocol
(SIP) for the 3GPP", RFC 7315, DOI 10.17487/RFC7315, July
2014, <https://www.rfc-editor.org/info/rfc7315>.

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Appendix A. Alternatives

A.1. P-Charging-Vector

P-Charging-Vector is defined in Section 4.6 of [RFC7315] and used by
the 3GPP to carry information related to the charging of a session.
There are, however, some differences in the semantics associated with
P-Charging-Vector and P-Charge-Info. P-Charging-Vector is mainly
used to carry information for correlation of multiple charging
records generated for a single session. On the other hand,
P-Charge-Info is used to convey information about the party to be
billed for a call. Furthermore, P-Charging-Vector has a mandatory
icid-value parameter that is a globally unique value to identify the
session for which the charging information is generated. Such a
globally unique identifier is not necessary when carrying information
about the user to be billed when it is attached to the corresponding
session-related signaling.

A.2. P-DCS-Billing-Info

P-DCS-Billing-Info is defined in Section 7 of [RFC5503] and used for
passing billing information between trusted entities in the
PacketCable Distributed Call Signaling Architecture. For many
billing situations, particularly the very large-scale residential
telephone networks for which this header field is designed,
P-DCS-Billing-Info is an excellent solution. However, this ability
to address a range of situations adds complexity. According to RFC
5503, the following information is mandatory to include in each use
of the P-DCS-Billing-Info header field:

o Billing-Correlation-ID, a globally unique identifier

o Financial-Entity-ID

o RKS-Group-ID (record-keeping server)

The P-DCS-Billing-Info header field may also include a variety of
additional parameters.

While this may work well in many billing scenarios, there are other
billing scenarios that do not need this level of complexity. In
those simpler scenarios, all that is needed is a number to use for
billing. P-Charge-Info provides this simple solution for simple
billing scenarios.

Additionally, according to Section 7.3 of RFC 5503, it is mandatory
for a UA to create a Billing-Correlation-ID and insert this into the
P-DCS-Billing-Info header field (along with the other required

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information) sent in the initial SIP INVITE. This again makes sense
for the residential-telephone-service environment for which this
header field is designed. In contrast, P-Charge-Info is designed to
be used among proxies and not at all by normal user agents.
(P-Charge-Info may, though, be used by user agents associated with
PSTN gateways.)

A.3. P-Asserted-Identity

Early reviewers of this document asked why the P-Asserted-Identity
header field documented in [RFC3325] could not be used. As mentioned
in the use-case example above, P-Asserted-Identity is used to
indicate the identity of the calling party. However, in this
instance, the requirement is to provide an additional identity of the
SIP-to-PSTN interconnect point.

It would be typical to find both P-Asserted-Identity and
P-Charge-Info used in a SIP exchange. P-Asserted-Identity would be
used to provide the caller identity that would be displayed to the
end user as "Caller ID", while P-Charge-Info would provide the
billing identifier used for the billing associated with the call.

Acknowledgements

The authors thank the following people for their comments: Keith
Drage, Miguel Garcia, Sumit Garg, John Haluska, Juha Heinanen,
Christer Holmberg, Paul Kyzivat, Adam Roach, Jonathan Rosenberg,
Henning Schulzrinne, Tom Taylor, and Glen Wang.

Authors' Addresses

Dan York
Individual
Keene, NH
United States of America

Email: [email protected]

Tolga Asveren
Ribbon Communications
3 Paragon Way, Suite 100
Freehold, NJ 007728
United States of America

Email: [email protected]

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