For interconnection with PSTN, a transparent SIP trunking service from the SIP Proxy to a SIP enabled PSTN gateway must be setup.

The authorization of SIP requests is based on transitive trust. The SIP Proxy has a trust relationship with each SIP subscriber and the SIP gateway has a trust relation with the SIP Proxy.

The trust relationship between SIP subscribers and SIP Proxy is based on DIGEST algorithm, both have a database with shared credentials.

The trust relation between the SIP Proxy and the SIP gateway can be realized either by IP addresses or by the use of TLS protocol combined with digital certificates signed by the same Certificate Authority. The SIP gateway may
not use DIGEST authentication in the relation with the SIP Proxy because it does not have access to the SIP accounts database of the SIP Proxy.

Gateway requirements

Ideally, the PSTN gateway must obey to the following specifications:

 

- Support for RFC 3261 and RFC 3263 standards
- Support for SIP extensions for caller id and privacy (P headers)
- RTP active mode (send RTP data as soon as call setup is completed)
- Use public routable IP addresses for both signaling and media
- ENUM lookups

For outbound calls from the SIP Proxy to the gateway, the SIP Proxy performs the authentication for all SIP requests using the DIGEST algorithm. This check makes sure that the caller has a valid account in the SIP Proxy database and does not impersonate other users.

 

Once the SIP request is authenticated, the SIP Proxy authorizes the request based on the rights associated with the subscriber account and decides whether a SIP session to the gateway is allowed or not. If the session is allowed, the SIP Proxy asserts an identity header belonging to the SIP account, which is a telephone number presented as caller ID to the destination, locates a gateway for the dialed number and sends out the request to the gateway.

For inbound calls from the gateway to the SIP Proxy, the gateway must initiate sessions to the SIP Proxy IP address or one of its domain names. Ideally, the SIP gateway should perform ENUM lookups in the DNS server and contact the SIP Proxy by using the SIP URI of the SIP account where the ENUM mapping points to. In practice, today not many gateways support ENUM lookups, in this case the SIP Proxy will perform ENUM queries and translate the telephone number from the username part of the request URI into a SIP address. The format of the request URI must be setup in the SIP Proxy siteconfig configuration files.