1. Tcp Receive Window Auto Tuning
2. Tcp Receive Window Auto-tuning Linux 10
3. Tcp Receive Window Auto-tuning Linux Free
4. Linux Tcp Window Full
5. Tcp Receive Window Auto-tuning

Applies to: Windows Server 2019, Windows Server 2016, Windows Server (Semi-Annual Channel)

Use the information in this topic to tune the performance network adapters for computers that are running Windows Server 2016 and later versions. If your network adapters provide tuning options, you can use these options to optimize network throughput and resource usage.

Oct 05, 2012 The Receive Window Auto-Tuning feature lets the operating system continually monitor routing conditions such as bandwidth, network delay, and application delay. Therefore, the operating system can configure connections by scaling the TCP receive window to maximize the network performance.

• In this case, the devil is the operating system that has a hard limit on the TCP window size that an application can use. It looks like in this case the limit is 416 Kbytes. These limits exist for good reasons. Larger TCP windows take more system memory and if you have multiple applications running, using large windows they may bog down the system.
• Dec 23, 2019 TCP receive window autotuning enables these scenarios to fully use the network. For a TCP receive window that has a particular size, you can use the following equation to calculate the total throughput of a single connection. Total achievable throughput in bytes = TCP receive window size in bytes. (1 / connection latency in seconds).

The correct tuning settings for your network adapters depend on the following variables:

• The network adapter and its feature set
• The type of workload that the server performs
• The server hardware and software resources
• Your performance goals for the server

The following sections describe some of your performance tuning options.

Enabling offload features

Turning on network adapter offload features is usually beneficial. However, the network adapter might not be powerful enough to handle the offload capabilities with high throughput.

Important

Do not use the offload features IPsec Task Offload or TCP Chimney Offload. These technologies are deprecated in Windows Server 2016, and might adversely affect server and networking performance. In addition, these technologies might not be supported by Microsoft in the future.

For example, consider a network adapter that has limited hardware resources.In that case, enabling segmentation offload features might reduce the maximum sustainable throughput of the adapter. However, if the reduced throughput is acceptable, you should go ahead an enable the segmentation offload features.

Note

Some network adapters require you to enable offload features independently for the send and receive paths.

Enabling receive-side scaling (RSS) for web servers

RSS can improve web scalability and performance when there are fewer network adapters than logical processors on the server. When all the web traffic is going through the RSS-capable network adapters, the server can process incoming web requests from different connections simultaneously across different CPUs.

Important

Avoid using both non-RSS network adapters and RSS-capable network adapters on the same server. Because of the load distribution logic in RSS and Hypertext Transfer Protocol (HTTP), performance might be severely degraded if a non-RSS-capable network adapter accepts web traffic on a server that has one or more RSS-capable network adapters. In this circumstance, you should use RSS-capable network adapters or disable RSS on the network adapter properties Advanced Properties tab.

To determine whether a network adapter is RSS-capable, you can view the RSS information on the network adapter properties Advanced Properties tab.

RSS Profiles and RSS Queues

The default RSS predefined profile is NUMAStatic, which differs from the default that the previous versions of Windows used. Before you start using RSS profiles, review the available profiles to understand when they are beneficial and how they apply to your network environment and hardware.

For example, if you open Task Manager and review the logical processors on your server, and they seem to be underutilized for receive traffic, you can try increasing the number of RSS queues from the default of two to the maximum that your network adapter supports. Your network adapter might have options to change the number of RSS queues as part of the driver.

Increasing network adapter resources

For network adapters that allow you to manually configure resources such as receive and send buffers, you should increase the allocated resources.

Some network adapters set their receive buffers low to conserve allocated memory from the host. The low value results in dropped packets and decreased performance. Therefore, for receive-intensive scenarios, we recommend that you increase the receive buffer value to the maximum.

Note

If a network adapter does not expose manual resource configuration, either it dynamically configures the resources, or the resources are set to a fixed value that cannot be changed.

Enabling interrupt moderation

To control interrupt moderation, some network adapters expose different interrupt moderation levels, different buffer coalescing parameters (sometimes separately for send and receive buffers), or both.

You should consider interrupt moderation for CPU-bound workloads. When using interrupt moderation, consider the trade-off between the host CPU savings and latency versus the increased host CPU savings because of more interrupts and less latency. If the network adapter does not perform interrupt moderation, but it does expose buffer coalescing, you can improve performance by increasing the number of coalesced buffers to allow more buffers per send or receive.

Performance tuning for low-latency packet processing

Many network adapters provide options to optimize operating system-induced latency. Latency is the elapsed time between the network driver processing an incoming packet and the network driver sending the packet back. This time is usually measured in microseconds. For comparison, the transmission time for packet transmissions over long distances is usually measured in milliseconds (an order of magnitude larger). This tuning will not reduce the time a packet spends in transit.

Following are some performance tuning suggestions for microsecond-sensitive networks.

• Set the computer BIOS to High Performance, with C-states disabled. However, note that this is system and BIOS dependent, and some systems will provide higher performance if the operating system controls power management. You can check and adjust your power management settings from Settings or by using the powercfg command. For more information, see Powercfg Command-Line Options.

• Set the operating system power management profile to High Performance System.

  Note

  This setting does not work properly if the system BIOS has been set to disable operating system control of power management.

• Enable static offloads. For example, enable the UDP Checksums, TCP Checksums, and Send Large Offload (LSO) settings.

• If the traffic is multi-streamed, such as when receiving high-volume multicast traffic, enable RSS.

• Disable the Interrupt Moderation setting for network card drivers that require the lowest possible latency. Remember, this configuration can use more CPU time and it represents a tradeoff.

• Handle network adapter interrupts and DPCs on a core processor that shares CPU cache with the core that is being used by the program (user thread) that is handling the packet. CPU affinity tuning can be used to direct a process to certain logical processors in conjunction with RSS configuration to accomplish this. Using the same core for the interrupt, DPC, and user mode thread exhibits worse performance as load increases because the ISR, DPC, and thread contend for the use of the core.

System management interrupts

Many hardware systems use System Management Interrupts (SMI) for a variety of maintenance functions, such as reporting error correction code (ECC) memory errors, maintaining legacy USB compatibility, controlling the fan, and managing BIOS-controlled power settings.

The SMI is the highest-priority interrupt on the system, and places the CPU in a management mode. This mode preempts all other activity while SMI runs an interrupt service routine, typically contained in BIOS.

Unfortunately, this behavior can result in latency spikes of 100 microseconds or more.

If you need to achieve the lowest latency, you should request a BIOS version from your hardware provider that reduces SMIs to the lowest degree possible. These BIOS versions are frequently referred to as 'low latency BIOS' or 'SMI free BIOS.' In some cases, it is not possible for a hardware platform to eliminate SMI activity altogether because it is used to control essential functions (for example, cooling fans).

Note

The operating system cannot control SMIs because the logical processor is running in a special maintenance mode, which prevents operating system intervention.

Performance tuning TCP

You can use the following items to tune TCP performance.

TCP receive window autotuning

In Windows Vista, Windows Server 2008, and later versions of Windows, the Windows network stack uses a feature that is named TCP receive window autotuning level to negotiate the TCP receive window size. This feature can negotiate a defined receive window size for every TCP communication during the TCP Handshake.

In earlier versions of Windows, the Windows network stack used a fixed-size receive window (65,535 bytes) that limited the overall potential throughput for connections. The total achievable throughput of TCP connections could limit network usage scenarios. TCP receive window autotuning enables these scenarios to fully use the network.

For a TCP receive window that has a particular size, you can use the following equation to calculate the total throughput of a single connection.

Total achievable throughput in bytes = TCP receive window size in bytes * (1 / connection latency in seconds)

For example, for a connection that has a latency of 10 ms, the total achievable throughput is only 51 Mbps. This value is reasonable for a large corporate network infrastructure. However, by using autotuning to adjust the receive window, the connection can achieve the full line rate of a 1-Gbps connection.

Tcp Receive Window Auto Tuning

Some applications define the size of the TCP receive window. If the application does not define the receive window size, the link speed determines the size as follows:

• Less than 1 megabit per second (Mbps): 8 kilobytes (KB)
• 1 Mbps to 100 Mbps: 17 KB
• 100 Mbps to 10 gigabits per second (Gbps): 64 KB
• 10 Gbps or faster: 128 KB

For example, on a computer that has a 1-Gbps network adapter installed, the window size should be 64 KB.

This feature also makes full use of other features to improve network performance. These features include the rest of the TCP options that are defined in RFC 1323. By using these features, Windows-based computers can negotiate TCP receive window sizes that are smaller but are scaled at a defined value, depending on the configuration. This behavior the sizes easier to handle for networking devices.

Note

You may experience an issue in which the network device is not compliant with the TCP window scale option, as defined in RFC 1323 and, therefore, doesn't support the scale factor. In such cases, refer to this KB 934430, Network connectivity fails when you try to use Windows Vista behind a firewall device or contact the Support team for your network device vendor.

Review and configure TCP receive window autotuning level

You can use either netsh commands or Windows PowerShell cmdlets to review or modify the TCP receive window autotuning level.

Note

Unlike in versions of Windows that pre-date Windows 10 or Windows Server 2019, you can no longer use the registry to configure the TCP receive window size. For more information about the deprecated settings, see Deprecated TCP parameters.

Note

For detailed information about the available autotuning levels, see Autotuning levels.

To use netsh to review or modify the autotuning level

To review the current settings, open a Command Prompt window and run the following command:

The output of this command should resemble the following:

To modify the setting, run the following command at the command prompt:

Note

In the preceding command, <Value> represents the new value for the auto tuning level.

For more information about this command, see Netsh commands for Interface Transmission Control Protocol.

To use Powershell to review or modify the autotuning level

To review the current settings, open a PowerShell window and run the following cmdlet.

The output of this cmdlet should resemble the following.

To modify the setting, run the following cmdlet at the PowerShell command prompt.

Note

In the preceding command, <Value> represents the new value for the auto tuning level.

For more information about these cmdlets, see the following articles:

Autotuning levels

You can set receive window autotuning to any of five levels. The default level is Normal. The following table describes the levels.

If you use an application to capture network packets, the application should report data that resembles the following for different window autotuning level settings.

• Autotuning level: Normal (default state)

• Autotuning level: Disabled

• Autotuning level: Restricted

• Autotuning level: Highly restricted

• Autotuning level: Experimental

Deprecated TCP parameters

The following registry settings from Windows Server 2003 are no longer supported, and are ignored in later versions.

• TcpWindowSize
• NumTcbTablePartitions
• MaxHashTableSize

All of these settings were located in the following registry subkey:

HKEY_LOCAL_MACHINESystemCurrentControlSetServicesTcpipParameters

Windows Filtering Platform

Windows Vista and Windows Server 2008 introduced the Windows Filtering Platform (WFP). WFP provides APIs to non-Microsoft independent software vendors (ISVs) to create packet processing filters. Examples include firewall and antivirus software.

Note

A poorly-written WFP filter can significantly decrease a server's networking performance. For more information, see Porting Packet-Processing Drivers and Apps to WFP in the Windows Dev Center.

For links to all topics in this guide, see Network Subsystem Performance Tuning.

The TCP window scale option is an option to increase the receive window size allowed in Transmission Control Protocol above its former maximum value of 65,535 bytes. This TCP option, along with several others, is defined in IETF RFC 1323 which deals with long fat networks (LFNs).

TCP windows[edit]

The throughput of a communication is limited by two windows: the congestion window and the receive window. The congestion window tries not to exceed the capacity of the network (congestion control); the receive window tries not to exceed the capacity of the receiver to process data (flow control). The receiver may be overwhelmed by data if for example it is very busy (such as a Web server). Each TCP segment contains the current value of the receive window. If, for example, a sender receives an ack which acknowledges byte 4000 and specifies a receive window of 10000 (bytes), the sender will not send packets after byte 14000, even if the congestion window allows it.

Theory[edit]

TCP window scale option is needed for efficient transfer of data when the bandwidth-delay product (BDP) is greater than 64K. For instance, if a T1 transmission line of 1.5 Mbit/second was used over a satellite link with a 513 millisecond round trip time (RTT), the bandwidth-delay product is ${\displaystyle (1,500,000\ast 0.513)=769,500}$ bits or about 96,187 bytes. Using a maximum buffer size of 64 KiB only allows the buffer to be filled to (65,535 / 96,187) = 68% of the theoretical maximum speed of 1.5 Mbits/second, or 1.02 Mbit/s.

Tcp Receive Window Auto-tuning Linux 10

By using the window scale option, the receive window size may be increased up to a maximum value of 1,073,725,440 ((2^16-1)*(2^14) or 65,535 x 16,384)) bytes. This is done by specifying a two byte shift count in the header options field. The true receive window size is left shifted by the value in shift count. A maximum value of 14 may be used for the shift count value. This would allow a single TCP connection to transfer data over the example satellite link at 1.5 Mbit/second utilizing all of the available bandwidth.

Essentially, not more than one full transmission window can be transferred within one round-trip time period. The window scale option enables a single TCP connection to fully utilize an LFN with a BDP of up to 1 GB, e.g. a 10 Gbit/s link with round-trip time of 800 ms.

Possible side effects[edit]

Because some firewalls do not properly implement TCP Window Scaling, it can cause a user's Internet connection to malfunction intermittently for a few minutes, then appear to start working again for no reason. There is also an issue if a firewall doesn't support the TCP extensions.^[1]

Configuration of operating systems[edit]

Windows[edit]

TCP Window Scaling is implemented in Windows since Windows 2000.^[2][3] It is enabled by default in Windows Vista / Server 2008 and newer, but can be turned off manually if required.^[4]Windows Vista and Windows 7 have a fixed default TCP receive buffer of 64 kB, scaling up to 16 MB through 'autotuning', limiting manual TCP tuning over long fat networks.^[5]

Linux[edit]

Tcp Receive Window Auto-tuning Linux Free

Linux kernels (from 2.6.8, August 2004) have enabled TCP Window Scaling by default. The configuration parameters are found in the /proc filesystem, see pseudo-file /proc/sys/net/ipv4/tcp_window_scaling and its companions /proc/sys/net/ipv4/tcp_rmem and /proc/sys/net/ipv4/tcp_wmem (more information: man tcp, section sysctl).^[6]

Scaling can be turned off by issuing the command sysctl -w 'net.ipv4.tcp_window_scaling=0' as root.To maintain the changes after a restart, include the line 'net.ipv4.tcp_window_scaling=0' in /etc/sysctl.conf (or /etc/sysctl.d/99-sysctl.conf as of systemd 207).

FreeBSD, OpenBSD, NetBSD and Mac OS X[edit]

Default setting for FreeBSD, OpenBSD, NetBSD and Mac OS X is to have window scaling (and other features related to RFC 1323) enabled.
To verify their status, a user can check the value of the 'net.inet.tcp.rfc1323' variable via the sysctl command:

A value of 1 (output 'net.inet.tcp.rfc1323=1') means scaling is enabled, 0 means 'disabled'. If enabled it can be turned off by issuing the command:

This setting is lost across a system restart. To ensure that it is set at boot time, add the following line to /etc/sysctl.conf:net.inet.tcp.rfc1323=0

Sources[edit]

Linux Tcp Window Full

1. ^'Network connectivity may fail when you try to use Windows Vista behind a firewall device'. Support.microsoft.com. Retrieved July 11, 2019.
2. ^'Description of Windows 2000 and Windows Server 2003 TCP Features'. Support.microsoft.com. Retrieved July 11, 2019.
3. ^'TCP Receive Window Size and Window Scaling'. Archived from the original on January 1, 2008.
4. ^'Network connectivity fails when you try to use Windows Vista behind a firewall device'. Microsoft. July 8, 2009.
5. ^'MS Windows'. Fasterdata.es.net. Retrieved July 11, 2019.
6. ^'/proc/sys/net/ipv4/* Variables'.

Tcp Receive Window Auto-tuning