H.245 Tunneling


The H.245 protocol is a media control protocol that is a part of H.323 protocol suite. The H.245 protocol is used primarily to negotiate master-slave relationship between communicating endpoints. These 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 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.



You enable inbound tunneling when you configure H.323 Signaling with the H.323 Signaling pane. If inbound tunneling is enabled, you can enable outbound tunneling in the IP Bearer Traffic Profile used for outbound calls.  



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:


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 H245Control 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.


Related Topics:

H323 Overview

Configuring H.323 Signaling