ISDN User Part (ISUP) Cause Location Parameter for the SIP Reason Header Field
updates
- rfc-3326 — The Reason Header Field for the Session Initiation Protocol (SIP)
Extracted elements (10)
The location parameter is primarily intended for use in IMS (IP Multimedia Subsystem) networks defined by 3GPP, but is open to any network that includes ISUP interworking gateways and uses Q.850 reason codes. It allows the ISUP location of call release to be transported from the originating PSTN entity to the SIP entity via a response or BYE message.
The interworking from the PSTN needs only to include the location received by the interworking gateway; the full ISUP location chain need not be reconstructed. RFC 3398 describes detailed security considerations for ISUP-SIP interworking that remain applicable.
As defined by RFC 6432, any SIP response message, with the exception of 100 (Trying), MAY contain a Reason header field with a Q.850 cause code, and this specification adds the location parameter to such uses.
The UAC or UAS SHALL include the location parameter in a request or response when setting up the Reason header field with a Q.850 cause when the ISUP Q.850 location is available.
The use of the location parameter is restricted to Q.850 cause values. Other values MUST be ignored if present.
The ISUP location value does not reveal the identity of the originating or terminating party, nor the physical location of the caller or callee. It shows only the ISUP network location of the device that released the call, providing little more exposure than the Q.850 cause code itself.
The 'location' parameter is added to the SIP Reason header field's reason-extension, carrying the Q.850 ISUP location value as a named token. The location is a 4-bit value identifying where in the ISUP network a call was released.
IANA registered a new SIP header parameter 'location' in the 'Header Field Parameters and Parameter Values' subregistry of the 'Session Initiation Protocol (SIP) Parameters' registry, for the Reason header field, with predefined values, per RFC 3968 guidelines.
This document does not change the security considerations of RFC 3326. The Q.850 location value is very imprecise and does not add major security constraints or extend fraud scenarios beyond those already described in RFC 3398 for ISUP-SIP interworking.
The isup-cause-location ABNF extends reason-extension with 'location' EQUAL isup-location-value, where isup-location-value is one of 16 named tokens (U, LPN, LN, TN, RLN, RPN, LOC-6, INTL, LOC-8, LOC-9, BI, LOC-11, LOC-12, LOC-13, LOC-14, LOC-15) corresponding to 4-bit Q.850 location codes.