Reply intended for the list unintentionally sent to Paul personally, so here you all are! Chris Chris Wayman Purvis wrote:

Paul,

First of all, sorry for the fact that this seems to be a week-old mail I'm finally responding to...

I may have misunderstood, but I don't understand why this is necessary. If both endpoints have expressed willingness to use tunnelled H.245, why can an endpoint not simply start sending H.245 messages (of COURSE starting with TCS, and potentially tunnelled in purpose-built FACILITY messages) when it wishes to start a tunnelled H.245 session? I would have thought that would be a pretty clear indication of intent to start a tunnelled H.245 session!

FACILITY/reason=startH245 is required for separate H.245 connections because an endpoint may not know an address at its interlocutor to use to initiate an H.245 connection. This problem doesn't exist in tunnelled H.245, so I see no necessity for the extra interpretation!

We may want some extra text specifying behaviour at the start of an H.245 session, to ensure that the destination of the initial tunnelled H.245 message actually does something constructive (sends a TCS) rather than non-constructive (merely Acking the TCS it's been sent and not advancing anything useful).

Regards, Chris

Paul Long wrote:

...

How does an EP force the remote EP to bring up a tunneled H.245 session?

Consider this call scenario: Fast Connect was enough to establish the desired media between two EPs. Both support H.245 Tunneling but neither has initiated an H.245 session because neither had any need to. One EP now needs to use H.245 procedures. It doesn't matter, but reasons include DTMF transmission via UII and requesting a different video format or different audio bit-rate via RequestMode. That EP can initiate its own H.245 procedures, e.g., TCS and MSD, but how does it force the other EP to bring up an H.245 session?

H.323 and H.225.0 say that Facility/reason=startH245 is just for switching from Fast Connect or H.245 Tunneling to a separate H.245 channel, not for initiating an H.245 session via tunneling. Is it just hoped that when an EP sees that the other EP is bringing up an H.245 session, it will do the same? Seems like a pretty weak assumption. It would be nice if Facility/reason=startH245 without an h245Address meant to bring up an H.245 session on whatever transport is already established, i.e., tunneled or separate channel. Seems like we could do this in v4 because startH245 without an h245Address currently has no meaning. Whadya think?

-- Dr Chris Purvis -- Development Manager ISDN Communications Ltd, The Stable Block, Ronans, Chavey Down Road Winkfield Row, Berkshire. RG42 6LY ENGLAND Phone: +44 1344 899 007 Fax: +44 1344 899 001