Chapter 24 ALG Screen

24.2 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. The FTP ALG allows TCP packets with a port 21 destination to pass through. 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.

24.3 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.

24.4 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.

24.4.1H.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.

•You must configure the firewall and port forwarding to allow incoming (peer-to-peer) calls from the WAN to a private IP address on the LAN (or DMZ or WLAN). The following example shows H.323 signaling (1) and audio (2) sessions between H.323 devices A and B.

Figure 268 H.323 ALG Example

•The H.323 ALG operates on TCP packets with a port 1720 destination.

•The ZyWALL allows H.323 audio connections.

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