Francois,
At worst, it will indeed "'break'" things" ("...a RELEASE COMPLETE message...shall be returned"); at best, the behavior will be implementation-dependent because H.323 disallows it ("...shall not include both a fastStart element and encapsulated H.245 messages...").
(sigh) H.323 was intended to be extensible and backwards compatible. All versions of H.323 are inherently compatible with all other versions of H.323. Every single change that is considered for a new version of the Recommendation must be considered in this light. So far, we've done a pretty good job. Let's not create v4 as an isolated Recommendation that is not backward compatible.
Setup is unique in that the transmitting EP cannot know the version of the receiving EP before it sends it. Therefore, we cannot define purely version-specific behavior for Setup. For other messages, e.g., Alerting and OpenLogicalChannel, we can because, by the time these messages can be transmitted, the transmitting EP knows the version of the receiving EP. That's how we are generally able to make version-specific changes to H.225.0 and H.245.
BTW, relying on version-specific behavior creates insurmountable problems with GKs that use the routed model, i.e., all GKs. Over time, as there are more and more entities of different versions out in the field, and the versioning-GW aspect of a GK becomes more critical, we will find that we have painted ourselves into a corner unless we address the problem now. The only solution I can see is to _never_ define purely version-specific behavior--always signal/negotiate it. (Paul?)
Paul Long Smith Micro Software, Inc.
-----Original Message----- From: Francois Audet [mailto:audet@NORTELNETWORKS.COM] Sent: Thursday, June 01, 2000 7:49 AM To: ITU-SG16@MAILBAG.INTEL.COM Subject: Re: On TD26 - Fast TCS and M/S negotiation in H.323v4
Paul,
We agreed at the Osaka meeting to allow a SETUP to contain both the fastStart and h245Control, but it has not been officially approved by the ITU.
As it stands right now, I don't think I can agree to include it in H.323v4 on the grounds that it breaks backward compatibility with V2-- I just don't see a clean solution here.
It doesn't "break" anything. The call will still proceed.
I'm opening to hearing more suggestions, but as you pointed out, there are two issues:
- H.323v2 states explicitly that it is illegal
There are a lot of things that changed from v2 to v4. That's the role of
versions.
- There is no way to know what version to destination EP is before sending this illegal message
That would apply to any single change that we do on H.225.0 and H.245.
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