Sequence & Acknowledgement Number
Sequence and acknowledgement fields are another important fields in tcp header.Each of these fields are 32 bits long.
These fields are in the features that make TCP connection oriented. With sequence and acknowledgement numbers, packet reache to the destination without lost. If any of the packet do not reach to the destination, it is known by its sequence number and requested again. This results no loast during the session.
Before data transfer a new TCP connection is created. The host’s first sequence number that is used during starting of the connection is called “Initial Sequence Number(ISN)”.
From the point of security, this is open to the hijacking attacks. It is easy for an hacker to hijack the system. Different operating systems handle sequence and acknowledgement parameters differently. A hacker checks the ISN number and then determines the operating system. After that hijacking is almost completed. He predict the next sequence number and after that the destination do not understand that it is an hacker. Hacker also use flooding towards the sending host in order to be busy. You can check this scenario better with the below picture.
Let’s explain these sequence and acknowledgement numbers with the below picture.
In the third step, host send the next packet with the next sequence number that remote node has wanted.And it also acknowledge the correct receivement of the packet and request the next one. After this packet received by the remote host, 3 way handshake is completed.
After 3 way handshake completed, host send the data packet with the same sequence and acknowledgement numbers. Because the ACK packet do not change the sequence and acknowledgement number values.
TCP Header Length
TCP header lenght in TCP header is also called “data offset” in some sniffers. It is a 4-bit long field in TCP header. It represents the TCP header length as you can understand from its name.
You can reach the TCP Header article series below…