Window Autotuning / Auto-Scaling - Supported?

[Christian Engelbrecht]

Hey Guys,

It's my first question within the support forum as I'm not able to gather any more information from Cisco release notes and how to do's so hopefully you can give me a hint.

It's a very general question and I want to add some more information to it:

One of our server colleagues within the company asked if the network infrastructure supports TCP Window Autotuning! The server guys observe performance problems having window autotuning enabled on their machines. According to his information this problem can be caused because of two certain things:

Either the filer, netapp, unix server is having problems

Or the network components do not support Window Autotuning

He refers to this link within Microsoft Supportforum:

http://blogs.msdn.com/b/wndp/archive/2007/07/05/receive-window-auto-tuning-on-vista.aspx

"

First, make sure that your firewall and router can support window scaling. Some devices from Linksys, Cisco, NetApp, SonicWall, Netgear, Checkpoint, D-Link were reported as having problems with window scaling.

"

Unfortunatly I'm not able to guide him as this kind of option is not mentioned within any kind of IOS version release note (e.g. we are using 6k5 with sup720: s72033-ipservicesk9_wan-mz.122-33.SXH8a and Nexus 5k having NX-OS software n5000-uk9.5.0.2.N2.1).

Does anyone of you guys have information how Window Autotuning is supported by Cisco? Is it even supported? Do I have some kind of possibility from network point of view to influence the way autotuning works?

Awaiting your kind feedback / help.

Regards,

Christian

[Sandeep Choudhary]

Hi Christian,

I am not much aware about about this but i have this document which shows.that (Not all) cisco devices supporets the TCP window scaling:

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t8/feature/guide/tcpwslfn.html

Regards

[Christian Engelbrecht]

I already came across the same document ... but according to this thread https://supportforums.cisco.com/thread/206824 this command only have an impact on traffic destined to the router -> Not on the application itself .... so I don't think that this is solving the issue / having the impact that I want to have.

[Joseph W. Doherty · Accepted solution]

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Posting

Windows auto-tuning transiting a Cisco router or switch is invisible to the device.  However, in certain topologies it's possible auto-tuning will burst beyond what's typical for older TCP implementations which could result in buffer drops on the Cisco device.  (Windows auto-tuning uses TCP window scaling dynamically.)

For special network appliances, like firewalls, proxies, accelerators, etc., they might "transparently" sit in the middle of a TCP flow, and if they don't support TCP window scaling (used by Windows auto-tuning), auto-tuning cannot function as intended.

[Christian Engelbrecht]

Hi JosephDoherty,

Thanks for your explanation.

So basically I as a network engineer have no influence on how window autotuning is transiting a Cisco device? Basically if problems occure auto tuning must be adjusted on the server / client itself to get rid of any kind of issue right?

Regards,

Christian

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

So basically I as a network engineer have no influence on how window autotuning is transiting a Cisco device? Basically if problems occure auto tuning must be adjusted on the server / client itself to get rid of any kind of issue right?

Yes and no.  Again, depends on what kind of network device.  For pure switches and routers that just forward transit traffic, some buffer tuning might be needed.  (NB: one other problem, many switches do not support buffer sizes that could, in theory, be used by large scaled TCP windows.  This issue is often avoided by having sufficient bandwidth.)  For network appliances that "do something" to the transit packets, you may need to upgrade/replace them.

Also keep in mind that large TCP window scaling is only of benefit when the BDP (bandwidth delay product) exceeds the capacity of the original maximum TCP window of 64 KB.

[ROBERTO TACCON]

For example on Cisco ASA Firewall you can configure

tcp-options window-scale allow

http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/conns_tcpnorm.html

also check:

http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a0080742d6e.shtml

[Joseph W. Doherty]

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Nice references that show how two of Cisco security appliances can impact TCP window scaling, and how they may need to have their software upgraded and/or configuration changed.