H.323 Fast Connect and Versioning

paulej at PACKETIZER.COM paulej at PACKETIZER.COM
Tue Sep 9 15:31:49 EDT 2003


Folks,

Today, I was exchanging e-mail with somebody over the fax version number issue and the different syntax that is used (1998 vs 2002).

If we open H.245 and exchange a full set of capabilities, and H.323 endpoint could determine the version supported by the other side and open a channel supporting that particular version.  However, I don't think any text is explicitly clear on that.

Another scenario-- and one I have more trouble with-- is Fast Connect.  If a calling endpoint populates the fastStart element with "version 2" proposals, for example, the called side (say, a version 0 device) might accept the proposal and return the response.  However, it is not allowed to modify the version field.  The reason is that Fast Connect proposals are not ordered in a way such that replies must be ordered the same way.  Rather, the calling device determines which proposals are accepted based on characteristics of the proposals returned (e.g., codec type, samples per packet, or other information).  In some cases, a calling endpoint will actually not try to "match" the proposal returned, but just accept it as a proposal and run with it.

The problem is that if a calling device proposes version 2 and the called device returns version 2 (but is actually a v0 device), then the wrong syntax will be transmitted on the wire.  Thus, the text needs to state somewhere one of these options (or something similar):
  1.. The calling device must offer a proposal for each version it wants to potentially use and the called device must accept the first proposal it can accept (in order of the proposals) and the called device must not accept any proposal for a version it does not support
  2.. The calling device must wait for capability exchange to complete to determine the actual supported version of the other device
Alternatively, we could make an allowance for the endpoint to change the version number in the Fast Connect proposal, but I don't think that's a good idea, as it would quite possibly break interoperability with some devices.

What would a version 0 device do today if it received a Fast Connect proposal advertising version 2?  Would it accept it?  I suspect so and I'm afraid that we might have some interop problems regardless of the direction we go.

Perhaps we can require the calling device to not transmit any data until it receives at least one IFP packet from the called side and determines the ASN.1 version used to encode the message.  As much as we can push onto the shoulders of a v2 device, the better, as I don't think we have any real deployments in the field (yet)... might be wrong, but I think it would be a far less significant impact on that side.

I'm open to suggestions.  Perhaps this issue is even addressed and I've simply overlooked it.

Thanks,
Paul

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