For a typical SIP call, SIP client must register with the SIP registrar by composing a REGISTER request and sending it to the SIP registrar. The NetScaler appliance’s SIP ALG intercepts the request, replaces the IP address and port number in the request with the LSN pool IP address and port number provided in the LSN configuration, and forwards the request to the SIP registrar. The SIP ALG then opens a pinhole in the NetScaler configuration to allow further SIP communication between the SIP client and the SIP registrar. The SIP registrar sends a 200 OK response to the SIP client over the LSN pool IP address and port number. The NetScaler appliance captures this response in the pinhole, and the SIP ALG replaces the SIP header, putting the original Contact, Via, Route, and Record-Route SIP fields back into the message. The SIP ALG then forwards the message to the SIP client. The following figure shows how SIP ALG uses LSN in a SIP call registration flow.

A SIP call is initiated with a SIP INVITE message sent from the internal to the external network. The SIP ALG performs NAT on the IP addresses and port numbers in the Via, Contact, Route, and Record-Route SIP header fields, replacing them with the LSN pool IP address and port number. LSN stores these mappings for subsequent SIP messages in the SIP call. The SIP ALG then opens separate pinholes in the NetScaler configuration to allow SIP and media through the NetScaler appliance on the dynamically assigned ports specified in the SDP and SIP headers. When a 200 OK message arrives at the NetScaler, it is captured by one of the created pinholes. The SIP ALG replaces the SIP header, restoring the original Contact, Via, Route, and Record-Route SIP fields, and then forwards the message to the internal SIP client.

A SIP incoming call is initiated with a SIP INVITE message from the external client to the internal network. The SIP registrar forwards the INVITE message to the SIP client in the internal network, using the pinhole that was created when the Internal SIP client registered with the SIP registrar.

The SIP ALG performs NAT on the LSN IP addresses and port numbers in the Via, Contact, Route, and Record-Route SIP header fields, translating them to the IP address and port number of the internal SIP client, and forwards the request to the SIP client. When the 200 OK response message sent by the internal SIP client arrives at the NetScaler appliance, the SIP ALG performs NAT on the IP addresses and port numbers in the Via, Contact, Route, and Record-Route SIP header fields, translating them to the LSN pool IP address and port number, forwards the response message to the SIP registrar, and then opens a pinhole in the outbound direction for further SIP communication.

The BYE message terminates a call. When the device receives a BYE message, it translates the header fields in the message just as it does for any other message. But because a BYE message must be acknowledged by the receiver with a 200 OK, the ALG delays call teardown for 15 seconds to allow time for transmission of the 200 OK.

When both client A and client B in the same network initiate a call, the SIP messages are routed through the SIP proxy in the outside network. The SIP ALG processes the INVITE from client A as a normal outgoing call. Since client B is in the same network, the SIP proxy sends the INIVITE back to the NetScaler appliance. The SIP ALG examines the INIVITE message, determines that it contains the NAT IP address of client A, and replaces that address with the private IP address of client A before sending the message to client B. Once the call is established between the clients, the NetScaler is not involved in the media transmission between the clients.