Alg Technical Reference - ZyXEL Communications ZYWALL USG 20 Manual

Table 98 Configuration > Network > ALG (continued)
LABEL
Additional FTP
Signaling Port
for
Transformations
Apply
Reset

19.3 ALG Technical Reference

Here is more detailed information about the Application Layer Gateway.
ALG
Some applications cannot operate through NAT (are NAT un-friendly) because
they embed IP addresses and port numbers in their packets' data payload. The
ZyWALL examines and uses IP address and port number information embedded in
the VoIP traffic's data stream. When a device behind the ZyWALL uses an
application for which the ZyWALL has VoIP pass through enabled, the ZyWALL
translates the device's private IP address inside the data stream to a public IP
address. It also records session port numbers and allows the related sessions to
go through the firewall so the application's traffic can come in from the WAN to the
LAN.
ALG and Trunks
If you send your ALG-managed traffic through an interface trunk and all of the
interfaces are set to active, you can configure routing policies to specify which
interface the ALG-managed traffic uses.
You could also have a trunk with one interface set to active and a second interface
set to passive. The ZyWALL does not automatically change ALG-managed
connections to the second (passive) interface when the active interface's
connection goes down. When the active interface's connection fails, the client
needs to re-initialize the connection through the second interface (that was set to
passive) in order to have the connection go through the second interface. VoIP
clients usually re-register automatically at set intervals or the users can manually
force them to re-register.
FTP
File Transfer Protocol (FTP) is an Internet file transfer service that operates on the
Internet and over TCP/IP networks. A system running the FTP server accepts
ZyWALL USG 20/20W User's Guide
DESCRIPTION
If you are also using FTP on an additional TCP port number, enter it
here.
Click Apply to save your changes back to the ZyWALL.
Click Reset to return the screen to its last-saved settings.
Chapter 19 ALG
357