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[ScreenOS] How to turn Off h.323 ALG's for VoIP

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This article provides information on how to turn off the h.323 ALG's for VoIP in ScreenOS.
h.323 Avaya 2640 phone registers "No Signalling connection" Error - Invalid TPKT value (1066)


At times, you may have to disable the ALG for VoIP in ScreenOS. Some VoIP phones produce fragmented packets that are difficult for ScreenOS to handle.  As a workaround, disable the ALG for VoIP and communications should succeed. ScreenOS does a better job at handling fragmentation.

H323 is a set of standards from the ITU-T, which defines a set of protocols to provide audio and visual communication over a computer network. H323 works with a lot of protocols for setting up a call or video connection and tearing down the session. So to disable the h323 ALG completely, we need to disable ras, q931, and h.245 protocols as well.

To disable the ALG, apply the following commands from the CLI (either console, telnet, or SSH):

To disable H323 alg completely we need to run the following commands.

Unset alg h323 enable

Unset alg ras
The Registration, Admission, and Status (RAS) channel is used to carry messages used in the gatekeeper discovery and endpoint registration processes, which associates an endpoint's alias address with its call signalling channel transport address. The RAS channel is an unreliable channel.
Unset alg q931

Q.931 is a connection control (similar to TCP) protocol which manages call setup, maintenance, and breakdown.
Unset alg h245

H.245 is used for receiving and transmitting capabilities as well as mode preference from the receiving end, logical channel signalling, and Control and Indication.

Acknowledged signalling procedures are specified to ensure reliable audiovisual and data communication. H.245 messages are in ASN.1 syntax. They consist of an exchange of messages. Multimedia, System Control Message message types can be defined as request, response, command, and indication messages.

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