Presumably you could have a situation where there's deep-packet inspection on the traffic which would only allow a "handshake response" to come back through, an

Presumably you could have a situation where there's deep-packet inspection on the traffic which would only allow a "handshake response" to come back through, and drop your attempt at an initialisation. I doubt that happens much, and I assume you'll fall back hapilly enough to the second time the "client" sends an init packet and then you simply respond with the response packet.