Tham khảo tài liệu 'voip technologies part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | VoIP System for Enterprise Network 141 6. Open VPN In order to realize high security to connect multiple Asterisks located in different Intranets we have implemented VPN capability. In this section the development process is described. 1 REGISTER 1 REGISTER Asterisk Server1 . 136 24 SIP Server 24 5 OK SDP 6 OK SDP 8 VPN is established Asterisk Server2 . 28 24 4 OpenVPN session is started 7 OpenVPN is started as client based on SDP 3 INVITE 2 INVITE Fig. 26. VPN establishing procedure shows the procedure to establish VPN between two Asterisk servers by using OpenVPN http based on the regular SIP sequence. To realize this procedure we have developed a program . sip_app to have SIP client function with the function to invoke the external application. It is developed by using oSIP2 http software osip and eXosip2 http as en libraries in GNU and has the SIP client function SDP control function and the function to invoke the external process as child process. In the Asterisk server1 OpenVPN is registered as the external process and sip_app send the REGISTER message to SIP server 1 . In the Asterisk server2 sip_app send the REGISTER message to SIP server 1 and send INVITE message to the Asterisk server1 2 3 . Asterisk server1 invoke the OpenVPN as the server mode 4 and reply 200 OK after inserting the necessary connection information into a record in SDP 5 6 . Asterisk server2 invoke OpenVPN as the client mode after getting the necessary information from a record in SDP 7 . OpenVPN in the Asterisk server2 communicate with OpenVPN in the Asterisk server1 and VPN between two servers has been established 8 . Table 1 shows the values of SDP at the process 6 in . Record m shows media type . application VPN and the kind of protocol . OpenVPN . Record a is used by sip_app to control external process invoke. IP4 in Table1 is the IP address of the
