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Analysing The TCP Header
Analysing
The TCP Header
Introduction
A fair amount of time was spent trying to figure out which way to analyse the TCP header. Most websites and other resources mention the protocol's main characteristics
with a bit of information attached, leaving the reader with
a lot of questions and making it difficult to comprehend how certain aspects of the protocol
works.
For this reason
a different approach was selected. Our method certainly gets right into
the protocol's guts and contains a lot of information which some of you might
choose to skip, but it is guaranteed to satisfy you by giving a thorough understanding of what is going on.
Get
Ready.... Here It Comes!
For those who
skipped the first introduction page of the protocol, you will be happy
to find out that the tcp quick-overview page contains a brief
summary of the protocol's main characteristics to help refresh your
memory. If you need to dive into the details at any point, simply return
to this page!
The diagram
below shows the TCP header captured from a packet that I was running
on the network. We'll be using it to help us through our step by step analysis
of TCP. |
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As you can see, the
TCP header has been completely expanded to show us all the fields the
protocol contains. The numbers on the right are each field's length
in bits. This is also
shown in the quick TCP overview
page.
Since much time was spent to ensure our analysis was complete in all aspects, be sure that by the end of it, you will understand each field's purpose and how it works.
We should also point
out that when the packet in our example arrives to its destination, only
section 7 (the last one) is sent
to the upper OSI layers because it contains the data it is waiting for.
The rest of the information (including the MAC header, IP Header and TCP header)
is overhead which serves the purpose of getting the packet to its destination and allowing the
receiving end to figure out what to do with the packet, e.g. send
the data to the correct local application.
Now you're starting
to understand the somewhat complex mechanisim involved in determing how data gets from one point to another!
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Since you have made it this far, you can select
the section you want to read about by simply clicking on the coloured
area on the above packet, or by using the menu below. It is highly recommended that you start from the first section and slowly progress to the final one. This will avoid confusion and limit the case of you scratching your head halfway through any of the other sections:
Next
- Section 1: Source & Destination Port Number
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