Applicable to Both in LB and Gateway Scenarios

When back-end server advertises a WindowSize ( [WINSIZE] X [2^ WSCALE])  less than MSS, client traffic gets held up on ADC and is not forwarded to back-end server

ADC uses several optimizations to enable best possible handling of TCP traffic,  among other things, the “max packet per MSS” setting when configured, is used as a multiplier value to determine the no of packets to be sent to the back-end server, it serves as a multiplier to (window size / MSS), now in case where the servers sends window size  < MSS (which is not normally seen) the (window size / MSS) comes to zero causing the resultant value of packets to be sent to be zero so ADC does not forward any traffic to the Server.

By Setting “max packet per MSS” to Zero we bypass certain optimizations and also the above calculations thus making this scenario work.

Note: Ideally it is recommended to solve such issues by fixing the backend server behavior (should send winsize> MSS instead of winsize < MSS)  rather than making changes on the TCP profile.

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Problem Cause

When back-end server advertises window < MSS. and TCP profile applied to the Vserver has the “Max packet per MSS” set to a non-zero value, this problem can happen.