Protocol analyzers (or sniffers) enable you to capture some or all of the traffic on a network and display it in some meaningful way (this last part is called a decode). Because they typically are quite expensive (into the tens of thousands of dollars), they are not often used except in large networking shops or by expensive consultants brought in to solve a thorny problem.
Well, this is not entirely true. Tethereal will not display timestamps when capturing data; it will display timestamps when displaying a previously captured file, though.
By the way, if you look closely at this screen, you'll notice that I cheated. I saved a copy of the file so that I could go back to it. This copy has been put up at http://www.networkinglinuxbook.com, along with the other materials for the book.
This is a simple form of benchmarking.