Control Access to the Internet
40
VDSL Voice Gateway
5. Select the Port Triggering radio button.
6. In the Port Triggering Timeout field, enter a value up to 9999 minutes.
This value controls how long the inbound ports stay open when the gateway detects no
activity
. This value is required because the gateway cannot detect when the application
terminates.
Application Example: Port Triggering for Internet Relay Chat
Some application servers, such as FTP and IRC servers, send replies to multiple port
numbers. Using port triggering, you can tell the gateway to open more incoming ports when a
particular outgoing port starts 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 also allow
incoming traffic on port 113 to reach the originating computer.” The following sequence shows
the effects of this port triggering rule:
1. Y
ou open an IRC client program to start a chat session on your computer.
2. Y
our 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. Y
our 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 observing the destination port number of 6667, your
gateway creates another session entry to send any incoming port 1
13 traffic to your
computer.
5. The IRC server sends a return message to your gateway using the NA
T-assigned source
port (for example, port 33333) as the destination port and sends an “identify” message to
your gateway with destination port 113.
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