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Chapter 1
Introducing Linksys 900 Series IP Phones
2.
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Step 2
To determine the type of NAT your router is using set <STUN Test Enable> to yes.
Step 3
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.
If the ITSP offers an outbound NAT-Aware proxy, this discovers the public IP address from the remote
Note
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:
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Document Version 3.0
On the administration web server, System tab, set <Debug Server> 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.
Set <Debug level> to 3, but do not change the value of the <syslog server> parameter.
To capture SIP signaling messages, under the Line tab, set <SIP Debug Option> to Full. The output
is named syslog.514.log.
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.
Linksys 900 Series IP Phone Administrator Guide
Technology Background
1-13
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