This toy exchange shows byte counting only, not the rest of TCP behavior.
What this toy example shows
This book shows one post-handshake data segment and one ACK. It only models byte-counting arithmetic.
payload bytes=2
Checked values
Client sequence 1001 plus payload length 2 gives ACK 1003.
1001+2=1003
What this does not model
NOTE: toy post-handshake byte-counting only; not TCP checksum, retransmission, sliding window, delayed ACKs, segmentation offload, congestion control, or real capture timing.
toy data exchange=1
Summary
TCP counts bytes in the stream; this tiny ACK asks for byte 1003 next.
next byte=1003