TCS is always first message, not TCSAck

[Paul Long]

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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