Alg Technical Reference - ZyXEL Communications ISG50-ISDN User Manual

Table 106 Configuration > Network > ALG (continued)
LABEL
Additional FTP
Signaling Port for
Transformations
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20.3 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 ISG50 examines and uses IP
address and port number information embedded in the VoIP traffic's data stream. When a device
behind the ISG50 uses an application for which the ISG50 has VoIP pass through enabled, the
ISG50 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 ISG50 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 commands from a system running an
FTP client. The service allows users to send commands to the server for uploading and downloading
files.
H.323
H.323 is a standard teleconferencing protocol suite that provides audio, data and video
conferencing. It allows for real-time point-to-point and multipoint communication between client
computers over a packet-based network that does not provide a guaranteed quality of service.
NetMeeting uses H.323.
ISG50 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 ISG50.
Click Reset to return the screen to its last-saved settings.
Chapter 20 ALG
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