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Linksys IP Phone Administrator Guide
Firmware Version 5.1
Chapter 1 Introducing Linksys IP Phones
Technology Background
SIP-NAT Interoperation
In the case of SIP, the addresses where messages/data should be sent to a Linksys IP phone system are
embedded in the SIP messages sent by the device. If the Linksys IP phone system is located 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 The ITSP might offer an outbound NAT-Aware proxy, which discovers the public IP address from the
remote endpoint and eliminates the need to modify the SIP message from the UAC.
The Linksys IP 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 Linksys IP 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 Linksys IP 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 Linksys IP phone system continues to send
periodic keep-alive packets through the mapping to extend its validity as necessary.