Where'd You Come From? (Q18)

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For various reasons, a registered EP must determine whether an incoming call uses the GK-routed or direct call model. About the only reliable and practical way to do this is to listen on two ports for call-signaling messages. One is the well-known call-signaling port, 1720; the other is a dynamic port that the EP previously registered with its GK. If the EP gets a Setup message on the well-known port, the call uses the direct model; if from the dynamic port, it uses the GK-routed model. There is a small risk in doing this, however. The remote EP could have found out the dynamic port via an LRQ sent to the GK or from a previous GK-routed call and then used that port for a direct call. In that case, the EP would mistakenly believe that the call is using the GK-routed model.