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Quick Overview Of TCP

Posted in TCP Protocol Analysis

As previously mentioned on a number of occasions, TCP is one of the two protocols that lives at the Transport layer and is used to carry data from one host to another. What makes TCP so popular is the way it works when sending and receiving data. Unlike UDP, TCP will check for errors in every packet it receives in an endless struggle to avoid data corruption.

Some common protocols that use TCP are: FTP, Telnet, HTTP, HTTPS, DNS, SMTP and POP3. Let's have a closer look at the main characteristics of this wonderful protocol.

When people refer to "TCP/IP" remember that they are talking about a suite of protocols and not just one protocol, like most people think. TCP/IP is not one protocol. Please see the Protocols section for more information.

Main Features

Here are the main features of the TCP that we are going to analyse:

  • Reliable Transport
  • Connection-Oriented
  • Flow Control
  • Windowing
  • Acknowledgements
  • More overhead

Reliable Transport

It's a reliable transport because of the different techniques it uses to ensure that the data received is error free. TCP is a robust protocol used for file transfers where data error is not an option. When you decide to download a 50MB file from a website, you wouldn't want to find out after the download is complete that the file has an error! Even though, in reality, this does happen, it just goes to show that you can't always be perfect with certain things.

This picture shows the TCP header within an ethernet II frame. Right below this you will find our second diagram that zooms into to the TCP header, displaying the field the protocol contains:


The TCP Header/Segment

Posted in TCP Protocol Analysis

This article shows the TCP Header and Segment. We explain where the TCP Header and Segment are located in an Ethernet frame and also briefly view the available options in the TCP Header. Our easy-to-understand detailed diagrams help ensure all information provided is easily understood.

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 when reading the definition, just take a good look at the diagram below:


In-Depth TCP Header Analysis - Introduction

Posted in TCP Protocol Analysis

This article is an introduction to the 7-page TCP Header analysis section that follows. We briefly view each section of the TCP Header and then move on to its analysis using detailed colourful diagrams that help the learning process become much easier.

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.

tcp-analysis-1As 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!








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:

TCP Source & Destination Port Number - Section 1

Posted in TCP Protocol Analysis

This section contains one of the most well-known fields in the TCP header, the Source and Destination port numbers. These fields are used to specify the application or services offered on local or remote hosts. We explain the importance and functionality of the TCP source and destination ports, alongside with plenty of examples.

You will come to understand how important ports are and how they can be used to gain information on remote systems that have been targetted for attacks. We will cover basic and advanced port communications using detailed examples and colourful diagrams, but for now, we will start with some basics to help break down the topic and allow us to smoothly progress in to more advanced and complex information.



When a host needs to generate a request or send data, it requires some information:

1) IP Address of the desired host to which it wants to send the data or request.

2) Port number to which the data or request should be sent to on the remote host. In the case of a request, it allows the sender to specify the service it is intending to use. We will analyse this soon.

1) The IP Address is used to uniquely identify the desired host we need to contact. This information is not shown in the above packet because it exists in the IP header section located right above the TCP header we are analysing. If we were to expand the IP header, we would (certainly) find the source and destination IP Address fields in there.

2) The 2nd important aspect, the port number, allows us to identify the service or application our data or request must be sent to, as we have previously stated. When a host, whether it be a simple computer or a dedicated server, offers various services such as http, ftp, telnet, all clients connecting to it must use a port number to choose which particular service they would like to use.

The best way to understand the concept is through examples and there are plenty of them below, so let's take a look at a few, starting from a simple one and then moving towards something slightly more complicated.


Time To Dive Deeper!

TCP Sequence & Acknowledgement Numbers - Section 2

Posted in TCP Protocol Analysis

This page will closely examine the Sequence and Acknowledgement numbers. The very purpose of their existence is related directly to the fact that the Internet, and generally most networks, are packet switched (we will explain shortly) and because we nearly always send and receive data that is larger than the maximum transmission unit (a.k.a MTU - analysed on sections 5 and 6 ) which is 1500 on most networks.

Let's take a look at the fields we are about to analyse:


As you can see, the Sequence number proceeds the Acknowledgement number.

We are going to explain how these numbers increment and what they mean, how various operating systems handle them in a different manner and lastly, what way these numbers can become a security hazard for those who require a solid secure network.


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