Advertised Window TCP Calculator

Advanced TCP Window Inputs

Receive buffer size

Receive buffer unit

Unread queued data

Queued data unit

Window scale shift count

Round trip time

Milliseconds

Target bandwidth

Mbps

MSS size

Bytes per segment

Safety margin

Percent reserved from free space

Formula Used

Raw advertised window = receive buffer size − unread queued data.

Safe advertised window = raw advertised window × (1 − safety margin ÷ 100).

Scale factor = 2^{window scale shift}.

TCP window field = ceiling(safe advertised window ÷ scale factor). The field is capped at 65,535.

Effective advertised window = TCP window field × scale factor.

Bandwidth delay product = bandwidth × round trip time ÷ 8.

Throughput estimate = effective advertised window × 8 ÷ round trip time.

How to Use This Calculator

Enter the receive buffer size assigned to the TCP socket.
Enter unread queued data already held in the receive buffer.
Select the window scale shift negotiated by the connection.
Add round trip time and target bandwidth for path analysis.
Enter MSS to estimate how many full segments fit.
Add a safety margin when the receiver needs spare buffer space.
Press calculate to view the result above the form.
Use CSV or PDF export for reports and records.

Example Data Table

Scenario	Receive Buffer	Queued Data	Scale Shift	RTT	Bandwidth	Expected Note
Local service	64 KiB	8 KiB	0	5 ms	100 Mbps	Small RTT needs modest window.
Regional transfer	1 MiB	128 KiB	4	40 ms	500 Mbps	Scaling helps express larger space.
Long distance path	8 MiB	1 MiB	7	120 ms	1000 Mbps	BDP coverage becomes important.

TCP Advertised Window Overview

A TCP advertised window shows how much receive space is available. The receiver sends this value to the sender. It protects the receiving host from overload. A larger window can permit more data in flight. A smaller window slows delivery before buffers overflow.

This calculator models that window from simple inputs. It starts with the receive buffer. It subtracts unread bytes already waiting for the application. The remaining space becomes the raw advertised window. A safety margin can reduce that number. This is useful when traffic is bursty. It gives the system extra room.

Why Window Scale Matters

The classic TCP window field has a limited size. Window scaling lets both endpoints express larger receive windows. The scale value shifts the advertised field by a power of two. This page estimates the field value placed on the wire. It also shows the effective value after scaling. If the field would exceed its limit, the result warns you.

Round trip time also matters. A sender needs enough usable window to cover the bandwidth delay product. The bandwidth delay product is the amount of data that can fill the path. When the advertised window is below that amount, throughput may be limited. When it is above that amount, the path has a better chance to stay busy.

Practical Planning Notes

Use realistic buffer values. Operating systems may apply autotuning, memory pressure rules, and socket limits. Network devices may also shape traffic. Treat the output as a planning guide, not a packet trace. For exact behavior, compare these estimates with live captures.

The segment estimate divides the usable window by MSS. It helps you see how many full segments can fit. The throughput estimate divides window size by round trip time. It is a simple upper bound. Real transfers may be lower because of congestion control, loss, delayed acknowledgments, and application delays.

Try several scenarios. Increase RTT for distant links. Increase queued data when applications read slowly. Adjust the safety margin for busy servers. A good configuration leaves buffer headroom and still supports the target path rate. Document each tested case. Share the exported rows with teammates. Recheck values after kernel tuning, route changes, or workload shifts later too.

FAQs

What is a TCP advertised window?

It is the receive space a TCP receiver tells the sender is available. The sender uses it to limit how much unacknowledged data can be in flight.

Why does queued unread data reduce the window?

Unread data already occupies receive buffer space. Less free buffer space means the receiver should advertise a smaller window to avoid overflow.

What does window scaling do?

Window scaling expands the useful receive window beyond the 16-bit TCP window field. It uses a shift count negotiated during connection setup.

What is the TCP window field value?

It is the value stored in the TCP header. With scaling enabled, the effective window equals this field multiplied by the scale factor.

Why is bandwidth delay product included?

It estimates how much data can fill the network path. A window below this value can restrict throughput on high latency or high speed links.

What does the throughput estimate mean?

It is a simple upper estimate based on window size and round trip time. Real throughput may be lower due to loss, congestion, and application behavior.

What MSS value should I enter?

Use the connection MSS when known. A common Ethernet TCP payload value is 1460 bytes, but tunnels and options can reduce it.

Can this replace packet capture analysis?

No. It is a planning and estimation tool. Use packet captures, socket statistics, and system metrics when exact connection behavior is required.