TCP Sequence Numbers

Understanding TCP sequence numbers in networking interviews

This question tests your grasp of foundational TCP mechanics—specifically, how the protocol guarantees ordered delivery of data across unreliable networks. It's the kind of question that sounds simple but rewards a precise, layered answer. Interviewers ask it to confirm you understand not just the name of the field, but why it exists and what problem it solves.

The sequence number field is central to TCP's reliability model. To answer well, think about what happens when packets arrive out of order, get duplicated, or are lost in transit. The sequence number is how the receiver reconstructs the original byte stream and how the sender knows which data has been acknowledged. This underpins flow control, retransmission logic, and the entire connection state machine.

• Ordered delivery and reassembly
• Duplicate detection
• Acknowledgment and retransmission
• Connection state synchronization (SYN, FIN)