# How is TCP packet CheckSum Calculated? - Theory

Posted by Suraj Singh on December 26, 2017 · 5 mins read

Today, I m going to share with you actual method to calculate the checksum of TCP header packet at the binary level.

#### Introduction

Are you working on a networking project where you created TCP/UDP packets from scratch but don't know how to find checksum of from pseudo TCP packets? well, readers, you come to the right place. here, I am going to share with you the actual binary concept to calculate the checksum of pseudo TCP packets. Before starting this tutorial, I want to highlight that for this tutorial you need basic information of binary number operations.

#### How it works?

I am assuming that you already aware of TCP pseudo concept. basically, to calculate the checksum of TCP packet we need to create a TCP pseudo packet to calculate the actual checksum of TCP packet. if you don't know about pseudo concept then, I think you need to take a look here.

Now, let's focus on theory again. first, we have to divide and slice binary data into 16 bits pieces. After slicing in 16-bit pieces, we have to do find out the binary addition of all sliced pieces. After getting the binary sum of 16-bit pieces, there is one more small trick before one's complement of the binary sum. actually, we just need to take a 4-bit number from starting point of binary number and then add it again with the main binary sum. At last, calculate one's complement of the binary sum. done! I hope now you got the basic concept of TCP Packet CheckSum.

#### Example To Understand Theory.

Step 1.

Let assume that We have 4 bytes of pseudo header data (32-bits).

`Binary Representation = 10000000111111100101011111001010`

Step 2.

Divide binary Number Into 16-bit pieces.

`0                16-bit             32-bit    --------------------------------------- 1000000011111110   0101011111001010 ---------------------------------------`

Step 3.

Calculate the sum of all sliced pieces of binary numbers.

`  0-bit           16-bit   1000000011111110    + 0101011111001010 ----------------------   01101100011001000-----------------------`

Step 4.

Add 4-bits of binary number into the calculated binary number. here, these 4 bits are nothing other than starting 4 digits of the same calculated binary number.

` Calculated Binary Numbers =    011011000110010004-bit to add into same binary number = { 0110 } 1100011001000Addtion:  0-bit           16-bit   01101100011001000    +              0110----------------------   01101100011001110-----------------------`

Step 5.

Find One's Complement. (Just Change All 0's Into 1's).

`Binary Number    = 01101100011001110One's Complement = 00010011100110001`

Step 6.

Done! That One's Complemented binary number is our checksum. hence, with this method, you can easily find the correct checksum of any TCP/UDP packets.

`CheckSum = One's Complement = 00010011100110001`

I hope you enjoyed this tutorial.
if you have any suggestion or query.
comment below

have a nice day.
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