D-Link NetDefend DFL-210 User Manual page 227

6.2.8. The H.323 ALG
Gatekeepers
Multipoint Control Units
H.323 Protocols
The different protocols used in implementing H.323 are:
H.225 RAS signalling and Call
Control (Setup) signalling
H.245 Media Control and
Transport
T.120
H.323 ALG features
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 when connected via 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 D-Link
Firewall.
The H.323 ALG has the following features:
• The H.323 ALG supports version 5 of the H.323 specification. This specification is built upon
H.225.0 v5 and H.245 v10.
• In addition to support voice and video calls, the H.323 ALG supports application sharing over
the T.120 protocol. T.120 uses TCP to transport data while voice and video is transported over
required for communication between two H.323 terminals.
The Gatekeeper is a component in the H.323 system which is
used for addressing, authorization and authentication of
terminals and gateways. It can also take care of bandwidth
management, accounting,
gatekeeper may allow calls to be placed directly between
endpoints, or it may route the call signalling through itself to
perform functions such as follow-me/find-me, forward on
busy, etc. It is needed when there is more then one H.323
terminal behind a NATing device with only one public IP.
MCUs provide support for conferences of three or more
H.323 terminals. All H.323 terminals participating in the
conference call have to establish a connection with the MCU.
The MCU then manages the calls, resources, video and audio
codecs used in the call.
Used for call signalling. It is used to establish a connection
between two H.323 endpoints. This call 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.
Provides control of multimedia sessions established between
two H.323 endpoints. Its most important task is to negotiate
opening and closing of logical channels. A logical channel
could be, for example, an audio channel used for voice
communication. Video and T.120 channels are also called
logical channels during negotiation.
A suite of communication and application protocols.
Depending on the type of H.323 product, T.120 protocol can
be used for application sharing, file transfer as well as for
conferencing features such as whiteboards.
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Chapter 6. Security Mechanisms
billing and charging. The