Which of the following is not a field in TCP header ?

Answer: Fragment offset is not a field in the TCP header; it belongs to the IP header.

Why: Fragment offset is used for IP fragmentation and reassembly (it is part of the IPv4 header along with flags and identification). TCP operates above IP and does not include fragmentation controls in its header.

Quick reference — the listed fields and their locations:

• Sequence number: a 32-bit TCP header field used to order bytes and support reliable delivery.

• Checksum: a 16-bit TCP header field that verifies integrity of the TCP header and payload (with a pseudo-header).

• Window size: a 16-bit TCP header field used for flow control, indicating how many bytes the receiver can accept.

• Fragment offset: an IP header field (not in TCP) used to record a fragment's position within the original IP datagram.

Common TCP header fields (for context):

• Source port and Destination port

• Sequence number and Acknowledgment number

• Data offset, Reserved bits, Control flags (e.g., SYN, ACK), Window size, Checksum, Urgent pointer, and Options