Network Working Group J. Haluska
Internet Draft Telcordia
Intended status: Informational January 13, 2010
Expires: June 2010
The isup-oli SIP URI Parameter
draft-haluska-dispatch-isup-oli-01.txt
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Abstract
A SIP URI parameter "isup-oli" is being used for interworking the
ISUP Originating Line Information parameter or equivalent PSTN
signaling information with SIP. This parameter has been also been
discussed in various documentation, but nowhere is it formally
documented. This document formally documents the usage, syntax, and
semantics of this parameter, providing a reference for discussion of
this parameter. It does not seek to achieve standardization of this
parameter.
Table of Contents
1. Introduction...................................................2
2. Usage..........................................................3
3. Formal Syntax..................................................4
4. Semantics......................................................4
5. Security Considerations........................................5
6. IANA Considerations............................................5
7. References.....................................................6
7.1. Normative References......................................6
7.2. Informative References....................................6
8. Acknowledgments................................................6
1. Introduction
In the North American PSTN, information describing the class of
service for a call, which effectively characterizes the originator,
is carried in signaling messages. In ANSI ISUP [ANSI-ISUP], this is
carried in the North American specific Originating Line Information
(OLI) parameter. In MF signaling, it is carried in the ANI II
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digits. In either case the parameter carries a two-digit value; a
few examples include 07- "Special Operator Handling Required", 29-
"prison/inmate service", 34- "Telco Operator Handled Call"..
The isup-oli SIP URI parameter is being used for interworking the
OLI information between PSTN signaling and SIP at a gateway. There
is currently no formal documentation of this parameter. This
document intends to provide a description of the parameter. It does
not aim to achieve standardization of this parameter. It describes
the formal syntax, usage, and semantics of the parameter as
currently implemented.
2. Usage
PSTN gateways are used to interwork between PSTN signaling and SIP
[RFC3261]. In North America, ANSI ISUP and various flavors of MF
signaling are the most commonly used PSTN signaling protocols.
The Originating Line Information conveys information describing the
class of service for a call, which effectively characterizes the
originator. It is signaled in ANSI ISUP using the OLI parameter, and
in MF it is signaled in the ANI II digits. In either case, the
information is conveyed using a two digit value.
The OLI information from the PSTN signaling is mapped to the isup-
oli SIP URI parameter. Since the PSTN OLI provides information about
the originating line, the isup-oli SIP URI parameter is also used to
describe the originator; e.g., in the From header of the INVITE.
For interworking from ISUP to SIP, if the incoming IAM contains an
OLI parameter, and if the gateway populates the From header of the
corresponding SIP INVITE message with a SIP URI based on the ISUP
Calling Party Number parameter, then the gateway also includes the
isup-oli SIP URI parameter populated with the numeric value of the
received ISUP OLI parameter.
For example, if the gateway receives an incoming IAM message with
calling party number 732-758-5735, and OLI value of "29", then the
gateway includes in the outgoing SIP INVITE message the following
From header:
From: sip:+1-732-758-5735@example.com;user=phone;isup-oli=29
Similarly, if incoming MF signaling includes ANI II information, it
is mapped to the isup-oli SIP URI parameter in the From header of
the corresponding INVITE.
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Likewise, when a gateway receives SIP signaling containing the isup-
oli SIP URI parameter, it is mapped to the appropriate PSTN
signaling, i.e., the ISUP OLI parameter, or MF II digits.
3. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) as described in [RFC5234].
The isup-oli parameter is a SIP URI parameter. Per the ABNF in RFC
3261 Section 25.1 it is a "uri-parameter". The formal syntax is:
isup-oli = ";isup-oli=" oli-value
oli-value = 2*(DIGIT)
The "isup-oli" SIP URI parameter is optional. It can be included at
most once in a SIP URI.
4. Semantics
When a SIP URI describing the originator of a session contains an
isup-oli parameter, the intended semantic is that the "class of
service" defined for this session by that isup-oli value applies to
the entity described by that SIP URI. For example, if a From header
contains a SIP URI with an isup-oli parameter with value "29"
indicating "prison/inmate service", this means that the call/session
is characterized as originating from a prison/inmate access. This
follows directly from the mapping of the OLI information in the PSTN
signaling to the SIP signaling.
Population and interpretation of "oli-value" is taken from the ANI
II values registry maintained by the North American Numbering Plan
Administration (NANPA) at
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http://www.nanpa.com/number_resource_info/ani_ii_assignments.html.
That is, the value is in the format of exactly two digits, with the
interpretation as given in the registry.
In PSTN signaling, the OLI provides information about the class of
service at the originating access for a call, which effectives
characterizes the originator. Thus, the isup-oli SIP URI parameter
is only meaningful when associated with the originator of a session.
5. Security Considerations
This document describes a parameter currently in use; it does not
define a new protocol mechanism. This section is provided to
identify potential risks with the use of this parameter.
The isup-oli parameter conveys information of a private nature. In
the PSTN this information is not shared with end users; similar
steps should be taken with the isup-oli parameter to prevent
disclosure to unintended recipients.
In the PSTN, OLI information is populated by trusted network
equipment, not by end user equipment. This should also apply to SIP
signaling.
If the isup-oli value can be tampered with, a caller may be able to
access services which would otherwise be unavailable. For example if
an isup-oli for a call from a prison/inmate access could be changed
to another value, or removed completely, then the appropriate
restrictions would not be applied to the call. Likewise an attacker
could modify the OLI value to one with greater restrictions,
limiting the services that the caller would otherwise legitimately
access. Restricting the use of this parameter between domains with
an appropriate trust level and using security mechanisms which
provide message integrity, such as TLS, can mitigate these risks.
6. IANA Considerations
This document does not request any action of IANA. It simply
documents the current usage of the isup-oli SIP URI parameter.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
7.2. Informative References
[ANSI-ISUP] American National Standards Institute, "Signaling System
No. 7; ISDN User Part", ANSI T1.113, January 1995,
8. Acknowledgments
The author would like to thank Renee Berkowitz, Martin Dolly, Wesley
Downum, Gary Munson, and the members of the Operator Services
Technical Forum.
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Authors' Addresses
John Haluska
Telcordia Technologies, Inc.
331 Newman Springs Road
Red Bank, NJ 07701
Email: jhaluska@telcordia.com
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