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Table Of Contents
Related Features and Technologies
Supported Standards, MIBs, and RFCs
Verifying the Window Scaling Configuration
TCP Window Scaling
Feature History
This document describes the TCP Window Scaling feature and includes the following sections:
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Supported Standards, MIBs, and RFCs
Feature Overview
The TCP Window Scaling feature adds support for the Window Scaling option in RFC 1323. A larger window size is recommended to improve TCP performance in network paths with large bandwidth, long-delay characteristics that are called Long Fat Networks (LFNs) . This TCP Window Scaling enhancement provides that support.
The window scaling extension in Cisco IOS software expands the definition of the TCP window to
32 bits and then uses a scale factor to carry this 32-bit value in the 16-bit window field of the TCP header. The window size can increase to a scale factor of 14. Typical applications use a scale factor of 3 when deployed in LFNs.Benefits
The Cisco IOS window scaling feature complies with RFC 1323, TCP Extensions for High Performance. The maximum window size has been increased to 1,073,741,823 bytes. The larger scalable window size will allow TCP to perform better over LFNs.
Related Features and Technologies
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Related Documents
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Supported Platforms
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Determining Platform Support Through Cisco Feature Navigator
Cisco IOS software is packaged in feature sets that support specific platforms. To get updated information regarding platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.
Cisco Feature Navigator is a web-based tool that enables you to quickly determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. Under the release section, you can compare releases side by side to display both the features unique to each software release and the features in common.
Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:
Supported Standards, MIBs, and RFCs
Standards
No new or modified standards are supported by this feature.
MIBs
No new or modified MIBs are supported by this feature.
To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:
http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml
RFCs
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Prerequisites
Both sides of the link must be configured to support window scaling or the default of 65,535 bytes will apply as the maximum window size.
Configuration Tasks
See the following sections for configuration tasks for the TCP Window Scaling feature. Each task in the list is identified as either required or optional.
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Setting the TCP Window Size
To set the TCP window size and enable window scaling, use the following command in global configuration mode:
Verifying the Window Scaling Configuration
Enter the show running-config EXEC command to determine if TCP window scaling is enabled. In the following example output—abbreviated to show only the window size configuration—the TCP window size is set to a number greater than 65,535, indicating that the TCP Window Scaling feature is enabled:
Router# show running-configip tcp window-size 750000Troubleshooting Tips
Use the debug ip tcp Winscale EXEC command to enable diagnostic output concerning various events relating to the operation of the TCP Window Scaling feature to be displayed on a console. The debug ip tcp Winscale command is intended only for troubleshooting purposes because the volume of output generated by the software when it is used can result in severe performance degradation on the router.
Configuration Examples
The following configuration example shows a TCP window size of 750,000 bytes being configured:
ip tcp window-size 750000Command Reference
This section documents the modified ip tcp window-size command. All other commands used with this feature are documented in the Cisco IOS Release 12.2 command reference publications.
ip tcp window-size
To alter the TCP window size, use the ip tcp window-size command in global configuration mode. To restore the default value, use the no form of this command.
ip tcp window-size bytes
no ip tcp window-size
Syntax Description
bytes
Window size (in bytes). An integer from 0 to 1,073,741,823. The default value is 4128 bytes. Window scaling is enabled when the window size is greater than 65,535 bytes.
Defaults
The default window size is 4128 bytes when window scaling is not enabled. If only one neighbor is configured for the window scaling extension, the default window size is 65,535 bytes.
Command Modes
Global configuration
Command History
9.1
This command was introduced.
12.2(8)T
Default window size and maximum window scaling factor were increased.
Usage Guidelines
Do not use this command unless you clearly understand why you want to change the default value.
To enable window scaling to support Long Fat Networks (LFNs), the TCP window size must be more than 65,535 bytes. The remote side of the link also needs to be configured to support window scaling. If both sides are not configured with window scaling, the default maximum value of 65,535 bytes is applied.
The scale factor is automatically calculated based on the window-size you configure. You cannot directly configure the scale factor.
Examples
The following example sets the TCP window size to 750,000 bytes:
ip tcp window-size 750000Glossary
LFN—Long Fat Networks. Large bandwidth, long-delay networks where the throughput is high and the transmission distance is long. Networks with satellite connections are one example of an LFN. Satellite links always have high propagation delays and typically have high bandwidth.
TCP—Transmission Control protocol. Connection-oriented transport layer protocol that provides reliable full-duplex data transmission. TCP is part of the TCP/IP protocol stack.