TCP ACK and delayed ACK in Windows 11 25H2
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Short answer
Section titled “Short answer”TcpAckFrequency and TcpDelAckTicks control TCP acknowledgements at the level of a specific interface. The parameter reads and ranges are confirmed; shortening the ACK delay does not guarantee a gain and increases the number of return packets.
What was checked
Section titled “What was checked”We checked the parameter path for a network interface, the default values, the ranges and the effect on TCP acknowledgement rules.
Scope of research
Section titled “Scope of research”Windows 11 25H2 build 26200.9168, interface-level TCP parameters. Networks with packet loss, Wi-Fi, VPN and different servers were not part of the controlled measurements.
Methodology
Section titled “Methodology”Analysis of the TCP components and a system trace, verification of value conversion and cross-checking against Microsoft’s public documentation.
Canonical path
Section titled “Canonical path”HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{Interface-GUID}
| Value | Type | Default | Effective range | Role |
|---|---|---|---|---|
TcpAckFrequency |
REG_DWORD |
2 |
0..255 |
number of full-sized segments before ACK |
TcpDelAckTicks |
REG_DWORD |
2 |
0..6 |
delayed ACK delay in 100-ms ticks |
How ACK policy works
Section titled “How ACK policy works”TcpAckFrequency=1 sets sending an ACK for every full-sized segment. The value 2 keeps the normal TCP delayed acknowledgement rules. TcpDelAckTicks is measured in steps of 100 ms: the default value 2 sets a limit of about 200 ms, while 0 removes the additional timer wait. Other TCP rules can still trigger an immediate acknowledgement.
The TCP stack uses the values read for a specific network interface. Writing to the global registry key does not replace the interface setting. A new VPN, vEthernet or temporary adapter gets its own GUID and keeps the default values until the parameters are set specifically for it.
Usefulness
Section titled “Usefulness”Shortening the ACK delay may help applications sensitive to small-packet processing and delayed acknowledgements. At the same time, the number of return packets and the load on the network card and CPU grow. The result depends on the other side of the connection, congestion control and MTU, so a reduction in network latency is not guaranteed.
What is confirmed
Section titled “What is confirmed”- The parameter path for an interface with its GUID.
- The default values and effective ranges of
0..255and0..6. - The global registry key does not replace the setting of a specific interface.
- A new VPN or temporary adapter gets its own GUID and default values.
What is not confirmed
Section titled “What is not confirmed”- A reduction in ping or application latency.
- Behaviour with specific servers, routes and packet loss.
- A gain on Wi-Fi, VPN and in games.
Practical conclusion
Section titled “Practical conclusion”Leave the default values. Reduce the ACK delay only with a reproducible problem involving small packets, and monitor the number of return packets and the network load.
Restoring the state
Section titled “Restoring the state”Return the interface values 2 or delete the entries. The changes apply to new connections.
Limitations
Section titled “Limitations”The check applies to Windows 11 25H2. The parameter reads, default values and ranges are confirmed. The final gain was not measured in controlled tests with different servers, Wi-Fi and Ethernet, MTU and packet loss. Offloading part of the processing to the network card does not cancel the TCP acknowledgement rules. However, batch processing, VPN and the behaviour of the other side of the connection can hide or change the observed effect.
The article contains no dynamic observations of its own: the reading of values by the network stack in a running system and the actual acknowledgement intervals in a controlled scenario were not measured. The statements about the reads and ranges rely on an analysis of the TCP components and Microsoft’s public documentation, not on our own measurements.
For how to perform the dynamic part of the check yourself, see How to check it yourself.
The research and the tools used belong to the BoosterX developer, so the developer has a direct interest in the results. The methodology and the limits of applicability are described above, and the conclusions can be verified against open data and the listed public sources.
Sources
Section titled “Sources”- TCP/IP registry values, Microsoft Learn, checked 2026-09-01.
- TCP delayed acknowledgement, Microsoft Learn, checked 2026-09-01.
Public sources checked: 2026-09-02.
Change history
Section titled “Change history”- 2026-09-20: added a conflict-of-interest disclaimer and a link to the independent dynamic check in the methodology.
- 2026-09-19: explicitly added to the limitations the absence of our own dynamic observations and ACK interval measurements.
- 2026-09-02: first publication; the path, defaults and ranges were confirmed, and the limits of the effect on latency were added.