TCP Receive Window Update: Computing the Window Before and After an ACK

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Nov 4, 2026

In TCP, the receiver’s advertised window limits which byte sequence numbers the sender may transmit next without overrunning the receiver’s buffer. The receiver communicates this via a cumulative acknowledgment (ACK) plus a window advertisement (often called the receive window).

The key idea is:

The left edge of the receive window is the ACK number (the next byte the receiver expects).
If the advertised window is $W$ bytes, the receive window covers $W$ bytes starting at the ACK, i.e., from ACK to ACK + W − 1 (inclusive).

We will apply this to compute the window “before” and “after” the given update.

Given

Previous ACK: $22001$
Previous advertised window: $10000$ bytes
New ACK: $24001$
New advertised window: $12000$ bytes

Draw the window before and after using byte ranges

1
Step 1
Left edge = previous ACK = $22001$ . Right edge = $22001 + 10000 - 1 = 32000$ .
2
Step 2
Previous window spans sequence numbers $[22001, 32000]$ (inclusive).
3
Step 3
Left edge = new ACK = $24001$ . Right edge = $24001 + 12000 - 1 = 36000$ .
4
Step 4
New window spans sequence numbers $[24001, 36000]$ (inclusive).
5
Step 5
The new window overlaps the old window from $24001$ to $32000$ ; it advances the left edge by $2000$ bytes and extends the right edge by $4000$ bytes.

Window diagrams (byte sequence number space)

Before (ACK=22001, window=10000)

Left edge = $22001$
Right edge = $22001 + 10000 - 1 = 32000$

A more explicit number-line style:

After (ACK=24001, window=12000)

Left edge = $24001$
Right edge = $24001 + 12000 - 1 = 36000$

Receive window edges: before vs after (inclusive byte ranges)

Left edge = ACK; right edge = ACK + window - 1.

What changed?

ACK advanced from $22001$ to $24001$ → the receiver has cumulatively received $24001 - 22001 = 2000$ more bytes.
Advertised window increased from $10000$ $10000$ to $12000$ $12000$ :
- Old right edge: $32000$
- New right edge: $36000$
- Extension: $36000 - 32000 = 4000$ more bytes buffered beyond the old right edge.

sliding window behavior is reflected by the left edge moving with cumulative ACK and the right edge moving with the window advertisement.

Quick rule for exams

Compute ranges as $[ACK,\ ACK+W-1]$ using the advertised receive window $W$ in bytes.

Byte-accuracy matters

ACK in TCP is for the next expected byte (cumulative). Always use $ACK + W - 1$ for the inclusive right edge—forgetting “−1” shifts your diagram by one byte.

Common interpretation checks

TCP window evolution around an ACK update

Old receive window

Before segment processed

Covers bytes $22001..32000$ (ACK=22001, W=10000)."

New ACK arrives in the update

Receiver advances cumulative ACK

Left edge moves to 24001 because bytes up to 24000 are now acknowledged."

New advertised window

Updated flow control

Right edge becomes 36000 since W increases to 12000."

Knowledge Check

Question 1 of 4

Q1Single choice

Given ACK=22001 and advertised window W=10000 bytes, what is the inclusive right edge of the receive window?

32000

32001

31000

22010

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