6.8 Procedures at the MCPTT gateway
24.3793GPPMission Critical Push To Talk (MCPTT) call controlProtocol specificationRelease 18TS
6.8.1 General
As described in clause 5.5, the MCPTT gateway servers are inserted in the path between MCPTT functions that reside in MCPTT systems from different trust domains.
This clause specifies the behavior of an MCPTT gateway server that acts as an exit point from an MCPTT system or as an entry point in an MCPTT system.
Local policies enforcement covers a wide variety of actions that are left to implementation. An example of local policies enforcement is given in clause 6.8.4.
6.8.2 MCPTT gateway server acting as an exit point from an MCPTT system
When acting as an exit point from a local MCPTT system to an interconnected MCPTT system, the MCPTT gateway server receives SIP requests and SIP responses intended for the controlling, non-controlling or participating function in the interconnected MCPTT system.
When receiving an outgoing SIP message, the MCPTT gateway server acting as an exit point:
1) shall identify the MCPTT system identity of the interconnected MCPTT system from the Request-URI of the outgoing SIP messages or other information elements from the outgoing SIP message;
2) may enforce local policy, and if local policy enforcement results in rejecting a SIP request (e.g. not having a mutual aid relationship), the MCPTT gateway shall reject the request by sending back a SIP 403 (Forbidden) response including a warning text "179 service not authorized with the interconnected system", and the MCPTT gateway server shall not continue with the rest of the steps;
3) may replace in the outgoing SIP message any addressing information linked to the local MCPTT system topology with its own addressing information. This includes:
a) the P-Asserted-Identity header field may be set to the MCPTT gateway server’s own URI; and
b) the Request-URI may be set to the public service identity of the targeted function in the interconnected MCPTT system, or to the URI of the MCPTT gateway server that acts as an entry point in the interconnected MCPTT system; and
NOTE: How the MCPTT gateway server determines the public service identity of the targeted MCPTT function in the interconnected MCPTT system or the URI of the MCPTT gateway server in the interconnected MCPTT system is out of the scope of the present document.
4) shall forward the outgoing SIP message according to 3GPP TS 24.229 [4]
6.8.3 MCPTT gateway server acting as an entry point in an MCPTT system
When acting as an entry point in an MCPTT system from an interconnected MCPTT system, the MCPTT gateway receives SIP requests and SIP responses intended for the controlling, non-controlling or participating function in the local MCPTT system.
When receiving an incoming SIP message, the MCPTT gateway server acting as an entry point:
1) shall identify the MCPTT system identity of the interconnected MCPTT system from the P-Asserted-Identity header field of the incoming SIP messages;
2) may enforce local policy, and if local policy enforcement results in rejecting a SIP request (e.g. not having a mutual aid relationship), the MCPTT gateway shall reject the request by sending back a SIP 403 (Forbidden) response including a warning text "180 service not authorized by the interconnected system", and the MCPTT gateway server shall not continue with the rest of the steps;
3) should replace in the incoming SIP message its own addressing information with the addressing information of the targeted MCPTT function in the local MCPTT system:
a) the Request-URI should be set to the public service identity of the targeted MCPTT function in the local MCPTT system; and
NOTE: How the MCPTT gateway server determines the public service identity of the targeted MCPTT function in the local MCPTT system is out of the scope of the present document.
4) shall forward the incoming SIP message according to 3GPP TS 24.229 [4]
6.8.4 Local policies enforcement
Below is one example of local policy enforcement that can be handled by an MCPTT gateway server.
If an MCPTT gateway server acting as an exit point receives a SIP request or a SIP response that contains sensitive information that cannot be exposed to the targeted partner system based on local policies but does not prevent the service from being delivered (e.g. a functional alias), the MCPTT gateway server can remove that information from the outgoing SIP message before forwarding it.