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:
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.