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:
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Back |
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Next - Section
1: Source & Destination Port Number |
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