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What is the purpose of the sequence number in the TCP header?
At offset 32 into the TCP header is the sequence number. The sequence number is a counter used to keep track of every byte sent outward by a host. If a TCP packet contains 1400 bytes of data, then the sequence number will be increased by 1400 after the packet is transmitted. At offset 64 is the acknowledgement number.
Why initial sequence number is random?
An ISN is designed to randomly select a sequence number for the first byte of data transmitted in a new TCP connection. The ISN can be any number from 0 to 4,294,967,295. Each byte can select any ISN unless it is in use not by a current connection.
How are the TCP seq and Ack numbers related?
The TCP seq and ack numbers are coordinated with one another and are key values during the TCP handshake, TCP close, and, of course, while data is transferred between the client and server. [By default, Wireshark converts all sequence and acknowledgement numbers into relative numbers.
Why is the sequence number included in the acknowledgement number?
This sequence number is included on each transmitted packet, and acknowledged by the opposite host as an acknowledgement number to inform the sending host that the transmitted data was received successfully.
How many sequence numbers does ACK segment consume?
Note that the ACK segment does not consume any sequence numbers if it does not carry data, but some implementations allow this third segment in the connection phase to carry the first chunk of data from the client (emphasis mine). In this case, the segment consumes as many sequence numbers as the number of data bytes.
What does a sequence number of zero mean in TCP?
The server responds to the client with a sequence number of zero, as this is its first packet in this TCP session, and a relative acknowledgement number of 1. The acknowledgement number is set to 1 to indicate the receipt of the client’s SYN flag in packet #1.