ZyXEL Communications ZyWall ATP series User Manual page 352

Chapter 14 ALG
ALG
Some applications cannot operate through NAT (are NAT unfriendly) because they embed IP addresses
and port numbers in their packets' data payload. The Zyxel Device examines and uses IP address and
port number information embedded in the VoIP traffic's data stream. When a device behind the Zyxel
Device uses an application for which the Zyxel Device has VoIP pass through enabled, the Zyxel Device
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 security policy 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
Zyxel Device 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.
SIP
The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol that handles the
setting up, altering and tearing down of voice and multimedia sessions over the Internet. SIP is used in
VoIP (Voice over IP), the sending of voice signals over the Internet Protocol.
SIP signaling is separate from the media for which it handles sessions. The media that is exchanged
during the session can use a different path from that of the signaling. SIP handles telephone calls and
can interface with traditional circuit-switched telephone networks.
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.
ZyWALL ATP Series User's Guide
352