MSS setting - what will the size be?

[matsan]

Hello,
found out that our performance problems were due to VLAN tagging and hardware that did not support the tags natively. We thus changed the MTU to 1482 but still got problems. Experimenting with MSS and setting that to 1442 we finally got the performance expected. Also tried with 1482 with good performance.

My question is now how the MSS is calculated.

On the interface page, there is a text under the MSS field:
"If you enter a value in this field, then MSS clamping for TCP connections to the value entered above minus 40 (TCP/IP header size) will be in effect. "

Should I enter 1442 or 1482 in the MSS field to get optimum performance?

Thanks,
/Mattias

[robi]

What kind of NICs are there, having such problems?

[doktornotor]

MSS should be MTU -20 (due to some whacky code logic the text in the GUI is misleading as it is…)

[matsan]

@robi:

What kind of NICs are there, having such problems?

It's an APU1D box with the Realtek 8111E GB interfaces.
The unit is connected to a Cisco 2960 (trunk-only port). I maxed out at 85 MBps on a 100 MBps Internet line before changing the MTU - then I got 98 MBps throughput without changing anything else, leading me to suspect MTU problems due to the VLAN tags.