using H245 tunneling flag in H323V3

Tue Sep 16 11:01:29 EDT 2003


> Following has been added in H323 Version-4:
> If the called entity does not yet know if H.245 tunneling can be
> supported, it shall include the provisionalRespToH245Tunneling flag. This
> may happen, for example, when a Gatekeeper is responding to a calling
> entity with a message such as Call Proceeding before the called endpoint
> responds to the h245Tunneling flag. The provisionalRespToH245Tunneling
> effectively eliminates the meaning of the h245Tunneling flag in a message
> and the flag shall thus be ignored by the receiving endpoint.
> I want to know how to take care of the same in h323 version-3.
> The Scenario is as follows:
> 1. Calling endpoint sends a Setup (h245 tunneling = TRUE) message to a
> gatekeeper.
> 2. Gatekeeper forwards the Setup (h245 tunneling = TRUE) message to the
> called endpoint and sends a CallProceeding (h245 tunneling = TRUE) message
> to the calling endpoint.

It would not be strictly valid for the GK to set the tunneling flag to TRUE
or FALSE, really.  This is the problem.  It really does not know!  Of the
Gatekeeper sends a Call Proceeding, it should probably wait to get a Call
Proceeding from the upstream entity to ensure that the flag is properly set.
The problem, of couse, is that takes time.

So, I'd take the same approach and set the flag to true and what I would do
is have the Gatekeeper tunnel the H.245-- even if it does not use H.245
tunneling to the destination endpoint.

> 3. Called endpoint sends a Alerting (h245 tunneling = False) to the
> gatekeeper, and gatekeeper forwards this message to the calling endpoint.

That would certainly turn off tunneling.

> H245 signaling is direct between the endpoints.
> After step # 3, what should be the behaviour of the Calling Endpoint? How
> the H245 signaling would be completed?

If you perform step 3, the calling endpoint shall not transmit any
additional H.245 messages and shall establish a separate connection for
H.245 messages from that point forward.  If it knows the h245Address, then
it may initiate a TCP connection.  If it does not, it may send a Facility
with startH245.


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