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Automatic Tuning of TCP Buffers

Note: This mechanism is sometimes referred to as "Dynamic Right-Sizing" (DRS).

The issues mentioned under "Large TCP Windows" are arguments in favor of "buffer auto-tuning", a promising but relatively new approach to better TCP performance in operating systems. See the TCP auto-tuning zoo reference for a description of some approaches.

Microsoft introduced (receive-side) buffer auto-tuning in Windows Vista. This implementation is explained in a TechNet Magazine "Cable Guy" article.

FreeBSD introduced buffer auto-tuning as part of its 7.0 release.

Mac OS X introduced buffer auto-tuning in release 10.5.

Linux auto-tuning details

Some automatic buffer tuning is implemented in Linux 2.4 (sender-side), and Linux 2.6 implements it for both the send and receive directions.

In a post to the web100-discuss mailing list, John Heffner describes the Linux 2.6.16 (March 2006) Linux implementation as follows:

For the sender, we explored separating the send buffer and retransmit queue, but this has been put on the back burner. This is a cleaner approach, but is not necessary to achieve good performance. What is currently implemented in Linux is essentially what is described in Semke '98, but without the max-min fair sharing. When memory runs out, Linux implements something more like congestion control for reducing memory. It's not clear that this is well-behaved, and I'm not aware of any literature on this. However, it's rarely used in practice.

For the receiver, we took an approach similar to DRS, but not quite the same. RTT is measured with timestamps (when available), rather than using a bounding function. This allows it to track a rise in RTT (for example, due to path change or queuing). Also, a subtle but important difference is that receiving rate is measured by the amount of data consumed by the application, not data received by TCP.

Matt Mathis reports on the end2end-interest mailing list (26/07/06):

Linux 2.6.17 now has sender and receiver side autotuning and a 4 MB DEFAULT maximum window size. Yes, by default it negotiates a TCP window scale of 7.

4 MB is sufficient to support about 100 Mb/s on a 300 ms path or 1 Gb/s on a 30 ms path, assuming you have enough data and an extremely clean (loss-less) network.


  • TCP auto-tuning zoo, Web page by Tom Dunegan of Oak Ridge National Laboratory, PDF
  • A Comparison of TCP Automatic Tuning Techniques for Distributed Computing, E. Weigle, W. Feng, 2002, PDF
  • Automatic TCP Buffer Tuning, J. Semke, J. Mahdavi, M. Mathis, SIGCOMM 1998, PS
  • Dynamic Right-Sizing in TCP, M. Fisk, W. Feng, Proc. of the Los Alamos Computer Science Institute Symposium, October 2001, PDF
  • Dynamic Right-Sizing in FTP (drsFTP): Enhancing Grid Performance in User-Space, M.K. Gardner, Wu-chun Feng, M. Fisk, July 2002, PDF. This paper describes an implementation of buffer-tuning at application level for FTP, i.e. outside of the kernel.
  • Socket Buffer Auto-Sizing for High-Performance Data Transfers, R. Prasad, M. Jain, C. Dovrolis, PDF
  • The Cable Guy: TCP Receive Window Auto-Tuning, J. Davies, January 2007, Microsoft TechNet Magazine
  • What's New in FreeBSD 7.0, F. Biancuzzi, A. Oppermann et al., February 2008, ONLamp
  • How to disable the TCP autotuning diagnostic tool, Microsoft Support, Article ID: 967475, February 2009

– Main.SimonLeinen - 04 Apr 2006 - 21 Mar 2011

– Main.ChrisWelti - 03 Aug 2006

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