What is the function of a mangle rule that changes MSS in MikroTik RouterOS?

A mangle rule that changes the Maximum Segment Size (MSS) in MikroTik RouterOS serves the purpose of limiting the size of TCP packets. The MSS defines the maximum amount of data that a TCP segment can contain, which is especially significant in the context of network devices that might impose limits on packet sizes.

When segments of data exceed the MSS, they can be fragmented, leading to inefficiencies and possible transmission issues. Adjusting the MSS through a mangle rule ensures that TCP connections are optimized for the path they traverse, particularly in scenarios involving Virtual Private Networks (VPNs) or Network Address Translation (NAT), where TCP segments may need to be smaller to prevent fragmentation and ensure smooth connectivity.

By setting an appropriate MSS, it helps to avoid issues caused by packet fragmentation, which can degrade performance or result in connection drops.