Computer Networks · TEE · practice

TCP sequence and ACK arithmetic · example 2

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=99; the server SYN has Seq=700. No handshake data is sent. The client then sends 500 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=99; server ISN=700; data=500 B; no sequence wrap or loss.

To find

Each sequence/acknowledgement boundary.

Formula and why

\mathrm{first}=ISN+1,\quad \mathrm{next}=\mathrm{first}+LLaTeX: \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=100; client ACK=701.

Step 2. Number the data

First byte=100; last=100+500−1=599.

Step 3. Acknowledge data

Next expected byte=599+1=600; receiver sends ACK=600.

Step 4. Acknowledge FIN

Client FIN Seq=600; FIN ACK=601.

Final answer: Handshake ACKs 100 and 701; data 100…599; data ACK 600; FIN ACK 601.