Tcp-mss is negotiated as part of the TCP 3-way handshake. It limits the maximum size of a TCP segment to better fit the maximum transmission unit (MTU) limits on a network.
This is especially important for VPN traffic as the IPsec encapsulation overhead, along with the IP and frame overhead, can cause the resulting Encapsulating Security Payload (ESP) packet to exceed the MTU of the physical interface, thus causing fragmentation. Fragmentation increases bandwidth and device resources and is always best avoided.
Note that the value of 1350 is a recommended starting point for most Ethernet-based networks with an MTU of 1500 or greater. This value may need to be altered if any device in the path has lower MTU, or if there is any added overhead such as Point-to-Point Protocol (PPP), Frame Relay, etc.
As a general rule, you may need to experiment with different tcp-mss values to obtain optimal performance.
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