This one concerns me a great deal.
H.323 has always required that an EP send TCS as the very first H.245 message regardless of whether the EP is the calling or called EP (8.2/H.323v4: "...the H.245 terminalCapabilitySet message ... shall be the first H.245 message sent."). No exceptions. (There are two sections labeled "8.2.3", so I'll call the second one, "8.2.3+".) And 8.2.3+/H.323v4 even reiterates this ("As detailed in section 8.2, the first two H.245 messages sent by an endpoint on the H.245 Control Channel are the terminalCapabilitySet message and the masterSlaveDetermination messages."). However, 8.2.3+/H.323v4 goes on to say that the called EP sends TCSAck first, which clearly contradicts 8.2 and itself! Therefore, in order to resolve this contradiction and restore backwards compatibility with previous versions of H.323, the fix is to simply say that _TCS_ is the first message sent by the called EP and not _TCSAck_. Here are the three necessary mods ("[...]" means insert; "<...>" means delete).
1. 8.2.3+/H.323v4: "To indicate that the called entity understands the earlyH245Control field, the first H.245 message that the called entity sends shall be the terminalCapabilitySet<Ack> message tunneled in the H.225.0 Call Signalling Channel."
2. Table 41/H.323v4: <TerminalCapabilitySetAck(calling endpoint) TerminalCapabilitySet(called endpoint)> [TerminalCapabilitySet(called endpoint) TerminalCapabilitySetAck(calling endpoint)]
3. 8.2.3+/H.323v4: "The calling entity shall recognize that the earlyH245Control field was not understood when either it receives a Connect message and still has not received a< response to the initial> terminalCapabilitySet message, the first H.245 message received from the called entity is not a tunneled terminalCapabilitySet<Ack> message, or fastStart or fastConnectRefused is received and< no response has been received for the>[ a] terminalCapabilitySet message [has not been received]."
Paul Long ipDialog
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