TCP Header : Sequence & Acknowledgement Number

TCP Header seq ack

Sequence & Acknowledgement Number

TCP Sequence Number and TCP Acknowledgement fields are another important fields in TCP Header. Each of these TCP Header Fields are 32 bits long. Below, you can see an example TCP Header to check these fields.


TCP Sequence Number and TCP Acknowledgement fields
Data is generally sent and received with packets larger than MTU. So using tcp sequence number and acknowledgement number getting important during this process.



These fields are in the features that make TCP, connection oriented. With tcp sequence number and acknowledgement numbers, packet reaches to the destination without any 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 lost during the session. And this makes TCP, a reliable transport protocol. At this point, UDP (Ser Datagram Protocol) is not a reliable protocol. You can compare TCP and UDP and learn the key differences of these two protocols.


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 tcp sequence number 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.

manipulating tcp sequence number, session hijacking attacks
Sequence and the acknowledgement numbers are also used to reorder of the packets at the remote node.Because sometimes the packets reaches the destiantion at different times and different order.



Let’s explain these tcp sequence number and tcp acknowledgement numbers with the below picture.


TCP Sequence Number and TCP Acknowledgement fields

As mentioned above, firstl the TCP 3 way handshake is completed and TCP connection established. After that the data is sent. Think this example as requesting a webpage. Firstly host send a packet with SYN flag set and the sequence number (ISN number) is filled. In the second step the remote node get the apcket and reply it with its own ISN number as a sequence number, and acknowledge the packet come with requesting next one in its ACK value. Be careful, there are two ACK here. One is 3 way handsake ACK and the other is acknowledgement number. One is flag and the other is number.


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.


TCP Header Length, 4 bits long
When the packet goes to the receiver, the receiver check its header length and after that it understand where the data section begins.


Lesson tags: tcp, tcp header
Back to: CCNA 200-301 > TCP and UDP
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CCNA 200-301