H.323 Alg Overview - D-Link DFL-1600 User Manual

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signal channel is opened between two H.323 endpoints or between a H.323
endpoint and a gatekeeper. For communication between two H.323
endpoints, TCP 1720 is used. When connecting to a gatekeeper, UDP port
1719 (H.225 RAS messages) are used.
H.245 Media Control and Transport
The H.245 protocol provides control of multimedia sessions established
between two H.323 endpoints. The most important task for this protocol is
to negotiate opening and closing of logical channels. A logical channel is,
for instance, an audio channel used for voice communication. Video and
T.120 channels are also called logical channels during negotiation.
T.120
The T.120 standard is made up of a suite of communication and
application protocols. Depending on the type of H.323 product, the T.120
protocol can be used for application sharing, file transfer and conferencing
features such as whiteboards.
18.4.4

H.323 ALG Overview

The H.323 ALG is a flexible application layer gateway that allows H.323
devices such as H.323 phones and applications to make and receive calls
between each other while connected to private networks secured by D-Link
Firewalls.
The H.323 specification was not designed to handle NAT, as IP addresses
and ports are sent in the payload of H.323 messages. The H.323 ALG
modifies and translates H.323 messages to make sure that H.323 messages
will be routed to the correct destination and allowed through the firewall.
The H.323 ALG has the following features:
H.323 version 5 (H.225.0 v5, H.245 v10)
Application sharing (T.120)
Gatekeeper support
NAT, SAT
Chapter 18. Application Layer Gateway (ALG)
D-Link Firewalls User's Guide