Which event is always triggered when the client sends data to a virtual server using TCP.

HTTP_DATA is not a valid F5 iRule event name and does not exist in the iRules event model.

The CLIENT_DATA event is triggered at the TCP layer every time a client transmits data to the virtual server, making it the correct event for inspecting raw incoming client payload in iRules. It operates independently of any application-layer protocol, so it fires for any TCP-based traffic regardless of whether HTTP or another protocol is in use.

HTTP_REQUEST fires only after the HTTP profile has fully parsed an HTTP request, not generically whenever any TCP data segment arrives from a client.

VS_CONNECTED is not a valid F5 iRule event; the correct event for a newly accepted client TCP connection is CLIENT_ACCEPTED, not VS_CONNECTED.