ZyXEL Communications 5 Series User Manual page 532

Chapter 30 ALG Screen
ALG and the Firewall
The ZyWALL uses the dynamic port that the session uses for data transfer in creating an
implicit temporary firewall rule for the session's traffic. The firewall rule only allows the
session's traffic to go through in the direction that the ZyWALL determines from its
inspection of the data payload of the application's packets. The firewall rule is automatically
deleted after the application's traffic has gone through.
ALG and Multiple WAN
When the ZyWALL has two WAN interfaces and uses the second highest priority WAN
interfaces as a back up, traffic cannot pass through when the primary WAN connection fails.
The ZyWALL does not automatically change the connection to the secondary WAN
interfaces.
If the primary WAN connection fails, the client needs to re-initialize the connection through
the secondary WAN interfaces to have the connection go through the secondary WAN
interfaces.
When the ZyWALL uses both of the WAN interfaces at the same time, you can configure
routing policies to specify the WAN interfaces that the connection's traffic is to use.
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.
If the FTP server is located on the LAN, you must also configure NAT port forwarding and
firewall rules if you want to allow access to the server from the WAN.
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.
RTP
When you make a VoIP call using H.323 or SIP, the RTP (Real time Transport Protocol) is
used to handle voice data transfer. See RFC 1889 for details on RTP.
H.323 ALG Details
• The H.323 ALG supports peer-to-peer H.323 calls.
• The H.323 ALG handles H.323 calls that go through NAT or that the ZyWALL routes.
You can also make other H.323 calls that do not go through NAT or routing. Examples
would be calls between LAN IP addresses that are on the same subnet.
• The H.323 ALG allows calls to go out through NAT. For example, you could make a call
from a private IP address on the LAN to a peer device on the WAN.
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ZyWALL 5/35/70 Series User's Guide