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MSS Clamping

by on under blog

Normally speaking when dealing with your home xDSL CPE we have to set up the correct WAN MTU in order to avoid fragmentation. This is becuase of PPP overhead, which takes 8 bytes. So it comes that you have a LAN MTU of 1500 bytes and a WAN/xDSL of 1492. This will force your to outbound traffic (LAN to WAN) to be fragmented and it will be done all by CPU.

Since most of the modern home CPE does not have any dedicated ASIC to perform traffic forwarding, this is will be done by CPU. Therefore would be best practice to avoid extra CPU burden (read “fragmentation”). Here is where MSS clamping comes to help you. What if you can force the Maximum Segment Size between you and your server to the perfect value that suits your xDSL capacity? When you apply the command

ip tcp adjust-mss 1452

On your WAN interface, your Cisco CPE will snoop&spoof the bidirectional three-way-handshake and set the best MSS size (+20B TCP header + 20B IP header). This will let the end to end communication fragmentation-less.



© 2018 Matteo Malvica. Illustrations by Sergio Kalisiak.