Alg - ZyXEL Communications MAX-306M1 User Manual

Chapter 10 The NAT Configuration Screens
Jane requests a file from the Real Audio server (port 7070).
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Port 7070 is a "trigger" port and causes the WiMAX Modem to record Jane's
2
computer IP address. The WiMAX Modem associates Jane's computer IP address
with the "incoming" port range of 6970-7170.
The Real Audio server responds using a port number ranging between 6970-7170.
3
The WiMAX Modem forwards the traffic to Jane's computer IP address.
4
Only Jane can connect to the Real Audio server until the connection is closed or
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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:
Trigger events only happen on data that is coming from inside the WiMAX Modem
1
and going to the outside.
If an application needs a continuous data stream, that port (range) will be tied up
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so that another computer on the LAN can't trigger it.

10.5 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.
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User's Guide