Cisco SPA962-EU Administration Guide - Page 27
SIP-NAT Interoperation, communications with the SIP entities outside the private network. - change background
UPC - 745883578504
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Chapter 1 Introducing Linksys 900 Series IP Phones Technology Background Step 2 Step 3 2. On the administration web server, System tab, set to the IP address and port number of your syslog server. Note that this address and port number has to be reachable from the SPA900 Series IP phone. 3. Set to 3, but do not change the value of the parameter. 4. To capture SIP signaling messages, under the Line tab, set to Full. The output is named syslog.514.log. To determine the type of NAT your router is using set to yes. View the syslog messages to determine if your network uses symmetric NAT. SIP-NAT Interoperation In the case of SIP, the addresses where messages/data should be sent to a 900 Series phone system are embedded in the SIP messages sent by the device. If the 900 Series phone system is sitting behind a NAT device, the private IP address assigned to it is not usable for communications with the SIP entities outside the private network. Note If the ITSP offers an outbound NAT-Aware proxy, this discovers the public IP address from the remote endpoint and eliminates the need to modify the SIP message from the UAC. The 900 Series phone system must substitute the private IP address information with the proper external IP address/port in the mapping chosen by the underlying NAT to communicate with a particular public peer address/port. For this, the 900 Series phone system must perform the following tasks: • Discover the NAT mappings used to communicate with the peer. This can be done with the help of an external device, such as a STUN server. A STUN server responds to a special NAT-Mapping-Discovery request by sending back a message to the source IP address/port of the request, where the message contains the source IP address/port of the original request. The 900 Series phone system can send this request when it first attempts to communicate with a SIP entity over the Internet. It then stores the mapping discovery results returned by the server. • Communicate the NAT mapping information to the external SIP entities. If the entity is a SIP Registrar, the information should be carried in the Contact header that overwrites the private address/port information. If the entity is another SIP UA when establishing a call, the information should be carried in the Contact header as well as in the SDP embedded in SIP message bodies. The VIA header in outbound SIP requests might also need to be substituted with the public address if the UAS relies on it to route back responses. • Extend the discovered NAT mappings by sending keep-alive packets. Because the mapping is alive only for a short period, the 900 Series phone system continues to send periodic keep-alive packets through the mapping to extend its validity as necessary. Document Version 3.0 Linksys 900 Series IP Phone Administrator Guide 1-13