When to use the TCP Adjust MSS feature

[Rod Brownjohn]

I've been advised by the WLC Config Analyzer tool to enable the TCP Adjust MSS feature on my access points.

I can understand how this could be helpful if the CAPWAP tunnel exceeds the path MTU between the access point and controller.

I wouldn't want to enable a feature unnecessarily though, so I have the following questions:

Should TCP Adjust MSS always be enabled for Local mode access points?

If so how do I select a suitable MSS value?

If not, what are the circumstances that would require the feature?

[Sandeep Choudhary]

Hi Rod,

My view: I would leave this as default

Should TCP Adjust MSS always be enabled for Local mode access points?

No its not Mandatory to be enabled.

*** One of the case i have seen:  One of the client was using anchored WLAN, and WAN over VPN. This provoke long packets to be dropped with default MTU value. So they configured this value to 1300 and then don’t have troubles anymore.

Regards

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[Rod Brownjohn]

Thanks Sandeep.

I've taken some packet captures and can see that without TCP Adjust MSS enabled, any packet larger than 1,430 bytes is being fragmented by the controller or access point and then being reassembled at the other end.

This is transparent to users, but I'm concerned about possible performance impact.

As an exercise I connected a client and ran some speed tests, with and without TCP Adjust MSS enabled.

Results weren't completely consistent, but my feeling was that I had marginally better performance without TCP Adjust MSS enabled, but I was only testing with a single client.  It is possible that results might vary as the number of users and amount of traffic increases, but for now I will leave the feature disabled.