Advanced Settings
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Wireless Cable Voice Gateway CG3700EMR-1CMNDS
as FTP and IRC servers) send replies to multiple port numbers. Using the port triggering
function of your gateway, you can tell the gateway to open more incoming ports when a
particular outgoing port originates a session.
An example is Internet Relay Chat (IRC). Your computer connects to an IRC server at
destination port 6667. The IRC server not only responds to your originating source port, but
also sends an “identify” message to your computer on port 113. Using port triggering, you can
tell the gateway, “When you initiate a session with destination port 6667, you must allow
incoming traffic also on port 113 to reach the originating computer.” Using steps similar to the
preceding example, the following sequence shows the effects of the port triggering rule you
have defined:
1.
You open an IRC client program to start a chat session on your computer.
2.
Your IRC client composes a request message to an IRC server using a destination port
number of 6667, the standard port number for an IRC server process. Your computer then
sends this request message to your gateway.
3.
Your gateway creates an entry in its internal session table describing this communication
session between your computer and the IRC server. Your gateway stores the original
information, performs Network Address Translation (NAT) on the source address and port,
and sends this request message through the Internet to the IRC server.
4.
Noting your port triggering rule and having observed the destination port number of 6667,
your gateway creates an additional session entry to send any incoming port 113 traffic to
your computer.
5.
The IRC server sends a return message to your gateway using the NAT-assigned source
port (as in the previous example, say port 33333) as the destination port. The IRC server
also sends an identify message to your gateway with destination port 113.
6.
When your gateway receives the incoming message to destination port 33333, it checks its
session table for an active session for port number 33333. Finding an active session, the
gateway restores the original address information that is replaced by NAT and sends this
reply message to your computer.
7.
When your gateway receives the incoming message to destination port 113, it checks its
session table and finds an active session for port 113 associated with your computer. The
gateway replaces the destination IP address of the message with the IP address of your
computer and forwards the message to your computer.
8.
When you finish your chat session, your gateway eventually senses a period of inactivity in
the communications. The gateway then removes the session information from its session
table, and incoming traffic is no longer accepted on ports 33333 or 113.
To configure port triggering, you need to know which inbound ports the application needs.
Also, you need to know the number of the outbound port that triggers the opening of the
inbound ports. You can usually find this information by contacting the publisher of the
application or user groups or newsgroups.
Note:
Only one computer at a time can use the triggered application.
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