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Chapter 8 The NAT Configuration Screens
User’s Guide
96
1 Jane requests a file from the Real Audio server (port 7070).
2 Port 7070 is a “trigger” port and causes the WiMAX Modem to record Jane’s
computer IP address. The WiMAX Modem associates Jane's computer IP address
with the "incoming" port range of 6970-7170.
3 The Real Audio server responds using a port number ranging between 6970-7170.
4 The WiMAX Modem forwards the traffic to Jane’s computer IP address.
5 Only Jane can connect to the Real Audio server until the connection is closed or
times out. The WiMAX Modem times out in three minutes with UDP (User
Datagram Protocol), or two hours with TCP/IP (Transfer Control Protocol/Internet
Protocol).
Two points to remember about trigger ports:
1 Trigger events only happen on data that is coming from inside the WiMAX Modem
and going to the outside.
2 If an application needs a continuous data stream, that port (range) will be tied up
so that another computer on the LAN can’t trigger it.
8.5 ALG
Some applications, such as SIP, cannot operate through NAT (are NAT un-
friendly) because they embed IP addresses and port numbers in their packets’
data payload.
Some NAT routers may include a SIP Application Layer Gateway (ALG). An
Application Layer Gateway (ALG) manages a specific protocol (such as SIP, H.323
or FTP) at the application layer.
A SIP ALG allows SIP calls to pass through NAT by examining and translating IP
addresses embedded in the data stream.