The TCP Header/Segment

Introduction

This page will introduce several new concepts, nothing of great difficulty, but essential for you to understand. We will learn what a TCP segment is, analyse it and start to explore the guts of TCP :)

So buckle up and get ready. It's all really simple, you just need to clear your mind and try to see things in the simplest form and you will discover how easy and friendly TCP really is. You can only feel comfortable with something once you get to know it.

TCP Header and TCP Segment

If we wanted to be more accurate with the terms we use, then perhaps we would title this page as "Analysing A TCP Segment". Why? Well, that's what it's called in the networking world so we need to know it by the correct term.

This of course leads us to another new definition, a TCP segment:

The unit of transfer between the TCP software on to machines is called a TCP segment.

If your expression has transformed itself to resemble a confused person, then don't worry, just keep reading...

Understanding this term is easier than you thought 5 seconds ago, just take a good look at the diagram below:

Now you see that a TCP segment is basically the TCP header plus the data that's right behind it and, of course, the data belongs to the upper layers (5,6,7).

The data contents could be part of a file transfer, or the response from a http request, the fact is that we really are not interested in the data's contents, but only in the fact that it's part of the TCP segment.

The screen shot below was taken from my packet sniffer, and it shows the DATA portion belonging to the TCP Header:
 

If you tried to capture a similar packet from any packet sniffer, it is more likely to display the Data portion within the TCP header, just as the screen shot on the left.

So the question is whether a TCP header and a TCP segment are basically the same thing.

Even though it might seem they are, in most cases, when referring to the TCP header, we are talking about the header without the data, whereas a TCP segment includes the data.

 
 

Getting Ready To Analyse The TCP Header

We are now ready to begin examining the structure of the TCP header. However, be sure to keep in mind that the 'TCP Header' is the same thing as a 'TCP Segment', meaning it's the TCP header information plus the Data, just as the diagrams above show.

The last screen shot certainly gives out a fair bit of information, but there is still much that hasn't been revealed, not to mention nothing's really been analysed as yet :)
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