Computer Networks · TEE · practice

TCP sequence and ACK arithmetic · example 1

Unit 5 · Process delivery, UDP, TCP and SCTP · optional practice

Optional extension · no matching calculation in the two supplied papers

Optional worked extension. This calculation format was not found in the supplied December 2025 final or February 2026 re-exam. The numbers and practice marks are authored. Study the core concept and actual-paper questions first; this is extra explanation, not a TEE prediction.

Practice question. A client SYN has Seq=1000; the server SYN has Seq=5000. No handshake data is sent. The client then sends 200 bytes in one segment and immediately finishes its sending direction. Find handshake ACKs, data-byte range, data ACK and FIN ACK. [5 practice marks]

Answer

Given

Client ISN=1000; server ISN=5000; data=200 B; no sequence wrap or loss.

To find

Each sequence/acknowledgement boundary.

Formula and why

\mathrm{first}=ISN+1,\quad \mathrm{next}=\mathrm{first}+L

SYN and FIN consume one sequence value each; data consumes one value per byte; an empty ACK consumes none.

Step-by-step solution

Step 1. Acknowledge SYNs

Server ACK=1001; client ACK=5001.

Step 2. Number the data

First byte=1001; last=1001+200−1=1200.

Step 3. Acknowledge data

Next expected byte=1200+1=1201; receiver sends ACK=1201.

Step 4. Acknowledge FIN

Client FIN Seq=1201; FIN ACK=1202.

Final answer: Handshake ACKs 1001 and 5001; data 1001…1200; data ACK 1201; FIN ACK 1202.