What make you think you need to see the TCP headers? You can't deduce how long it took to full fill the request from the TCP headers.

send a request and determine its success

That's a GET request.

determine how long the request took to full fil

That requires a clock of some sorts.

send multiple requests without waiting the first responce

That's several GET requests, in parallel.

send requests only after the previous request has been returned or fullfilled.

That's several GET requests, in series.

So, what makes you think that GET requests won't do?

Abigail