H.323 Inspection - Cisco FirePOWER ASA 5500 series Configuration Manual

Chapter 25 Configuring Application Layer Protocol Inspection

H.323 Inspection

This section describes the H.323 application inspection. This section includes the following topics:
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H.323 Inspection Overview
H.323 inspection provides support for H.323 compliant applications such as Cisco CallManager and
VocalTec Gatekeeper. H.323 is a suite of protocols defined by the International Telecommunication
Union for multimedia conferences over LANs. The security appliance supports H.323 through Version
4, including H.323 v3 feature Multiple Calls on One Call Signaling Channel.
With H323 inspection enabled, the security appliance supports multiple calls on the same call signaling
channel, a feature introduced with H.323 Version 3. This feature reduces call setup time and reduces the
use of ports on the security appliance.
The two major functions of H.323 inspection are as follows:
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How H.323 Works
The H.323 collection of protocols collectively may use up to two TCP connection and four to six UDP
connections. FastConnect uses only one TCP connection, and RAS uses a single UDP connection for
registration, admissions, and status.
An H.323 client may initially establish a TCP connection to an H.323 server using TCP port 1720 to
request Q.931 call setup. As part of the call setup process, the H.323 terminal supplies a port number to
the client to use for an H.245 TCP connection. In environments where H.323 gatekeeper is in use, the
initial packet is transmitted using UDP.
H.323 inspection monitors the Q.931 TCP connection to determine the H.245 port number. If the H.323
terminals are not using FastConnect, the security appliance dynamically allocates the H.245 connection
based on the inspection of the H.225 messages.
Within each H.245 message, the H.323 endpoints exchange port numbers that are used for subsequent
UDP data streams. H.323 inspection inspects the H.245 messages to identify these ports and dynamically
creates connections for the media exchange. RTP uses the negotiated port number, while RTCP uses the
next higher port number.
The H.323 control channel handles H.225 and H.245 and H.323 RAS. H.323 inspection uses the
following ports.
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OL-10088-01
H.323 Inspection Overview, page 25-37
How H.323 Works, page 25-37
Limitations and Restrictions, page 25-38
Configuring H.323 and H.225 Timeout Values, page 25-41
Verifying and Monitoring H.323 Inspection, page 25-41
NAT the necessary embedded IPv4 addresses in the H.225 and H.245 messages. Because H.323
messages are encoded in PER encoding format, the security appliance uses an ASN.1 decoder to
decode the H.323 messages.
Dynamically allocate the negotiated H.245 and RTP/RTCP connections.
1718—Gate Keeper Discovery UDP port
1719—RAS UDP port
Cisco Security Appliance Command Line Configuration Guide
H.323 Inspection
25-37