H.245 Tunneling

The H.245 protocol is a media control signaling protocol that is a part of H.323 protocol suite. The H.245 protocol is used primarily to exchange and negotiate end to end H.245 control messages between communicating H.323 endpoints/terminals. These H.323 endpoints exchange terminal capabilities and logical channel manipulations (open, close, modify). The H.245 messages can be encapsulated and carried between H.225 controlled endpoints within H.225 messages. This way of "piggy-backing" an H.245 message to an H.225 message is referred to as H245 Tunneling. The H.245 Tunneling method is optional and negotiable between communicating H.323 endpoints. If both endpoints support this option, usually the H.245 Media Controlled messages are exchanged via the Tunneling method. 

Enabling

The H.245 Tunneling feature is by default set to Enabled in the H.323 Profile object. The functionality is Enabled and Disabled through the drop down menu of the H.225 Tunneling field. The H.323 Profile object can then be linked to communicate with each individual gateway through the External Gateway object.  Refer to the Configure H.323 without GateKeeper and Configure H.323 with GateKeeper topics for information on configuring the H.323 functionality. Refer to screen capture below.

Open

Functionality

The tunneling feature relies on H.225 endpoint-to-endpoint connectivity (via TCP) to pass H.245 messages. The Tunneling feature uses the H.225 communication channel without creating a separate TCP socket connection (per the H.323 call) for media control. This approach allows the following:

• Faster call setup because there is no need to establish a new socket via a three way handshake

• Preservation of TCP socket resources within the operating system

While using the H.245 tunneling feature, a single TCP socket is created and used per H.323 call. An H.323 call setup that does not use tunneling uses two sockets. The H.245 messages are encapsulated into H.225 messages within a special header called H245Control as described by ASN.1. The H245 Control header can be added to any message except initial Setup message. However, if there are no other messages to be shipped out from an endpoint, a designated Facility message is used. (H.323 V2 accepts and defines Empty Facility message) as transport vehicle to carry on H.245 contents.