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TCP Header Length Analysis - Section 3

Posted in TCP Protocol Analysis

The third field under close examination is the TCP Header length. There really isn't that much to say about the Header length other than to explain what it represents and how to interpret its values, but this alone is very important as you will soon see.

Let's take a quick look at the TCP Header length field, noting its position within the TCP structure:

tcp-analysis-section-3-1

You might also have seen the Header length represented as "Data offset" in other packet sniffers or applications, this is virtually the same as the Header length, only with a 'fancier' name.

Analysing the Header length

If you open any networking book that covers the TCP header, you will almost certainly find the following description for this particular field:

TCP Flag Options - Section 4

Posted in TCP Protocol Analysis

As we have seen in the previous pages, some TCP segments carry data while others are simple acknowledgements for previously received data. The popular 3-way handshake utilises the SYNs and ACKs available in the TCP to help complete the connection before data is transferred.

Our conclusion is that each TCP segment has a purpose, and this is determined with the help of the TCP flag options, allowing the sender or receiver to specify which flags should be used so the segment is handled correctly by the other end.

Let's take a look at the TCP flags field to begin our analysis:

tcp-analysis-section-4-1

You can see the 2 flags that are used during the 3-way handshake (SYN, ACK) and data transfers.

As with all flags, a value of '1' means that a particular flag is 'set' or, if you like, is 'on'. In this example, only the "SYN" flag is set, indicating that this is the first segment of a new TCP connection.

In addition to this, each flag is one bit long, and since there are 6 flags, this makes the Flags section 6 bits in total.

 

 

 You would have to agree that the most popular flags are the "SYN", "ACK" and "FIN", used to establish connections, acknowledge successful segment transfers and, lastly, terminate connections. While the rest of the flags are not as well known, their role and purpose makes them, in some cases, equally important.

We will begin our analysis by examining all six flags, starting from the top, that is, the Urgent Pointer:

 

1st Flag - Urgent Pointer

TCP Window Size, Checksum & Urgent Pointer - Section 5

Posted in TCP Protocol Analysis

Our fifth section contains some very interesting fields that are used by the TCP transport protocol. We see how TCP helps control how much data is transferred per segment, make sure there are no errors in the segment and, lastly, flag our data as urgent, to ensure it gets the priority it requires when leaving the sender and arriving at the recipient.

So let's not waste any time and get right into our analysis!

tcp-analysis-section-5-1

The fifth section we are analysing here occupies a total of 6 bytes in the TCP header.

These values, like most of the fields in the protocol's header, remain constant in size, regardless of the amount of application data.

This means that while the values they contain will change, the total amount of space the field occupied will not.






 

The Window Flag

Analysing TCP Header Options - Section 6

Posted in TCP Protocol Analysis

The TCP Options (MSS, Window Scaling, Selective Acknowledgements, Timestamps, Nop) are located at the end of the TCP Header which is also why they are covered last. Thanks to the TCP Options field we have been able to enhance the TCP protocol by introducing new features or 'addons' as some people like to call them, defined by their respective RFC's.

As data communication continues to become more complex and less tolerable to errors and latency, it was clear that these new features had to be incorporated to the TCP transport to help overcome the problems created by the new links and speeds available.

To give you an example, Window Scaling, mentioned in the previous pages and elaborated here, is possible using the TCP Options field because the original Window field is only 16 bits long, allowing a maximum decimal number of 65,535. Clearly this is far too small when we want to express 'Window size' values using numbers in the range of thousands to a million e.g 400,000 or 950,000.

Before we delve into any details, let's take a look at the TCP Options field:

tcp-analysis-section-6-1

 

As you can see, the TCP Options field is the sixth section of the TCP Header analysis.

Located at the end of the header and right before the Data section, it allows us to make use of the new enhancements recommended by the engineers who help design the protocols we use in data communications today.







 

TCP Options

TCP Data - Section 7

Posted in TCP Protocol Analysis

Finally, the last page of our incredible TCP Analysis. As most of you would expect, this section is dedicated to the DATA section that follows the TCP Header.

 

The Data Section

The following diagram may have been tiresome, however, it will be displayed one final time to note the data portion of the packet:

tcp-analysis-section-7-1

Your knowledge regarding the procedure followed when the above packet arrives to its destination is assumed. However, a summary is given below to refresh our understanding in order to avoid confusion.

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