MTU

TCP’s Maximum Segment Size option advertises the receiving endpoint’s maximum TCP data size under the option’s defined accounting rules. Each direction has its own advertisement; the two values need not match. A router can reduce those advertisements while forwarding SYN messages so later data segments fit a constrained path. That changes TCP behavior for the affected connections, not the underlying link MTU itself. ...

The path MTU is the smallest link MTU along a particular route. A sender cannot determine it solely from its own interface’s MTU because an intermediate link may carry smaller packets. Classic IPv4 Path MTU Discovery sends packets with Don’t Fragment set and uses ICMP fragmentation-needed feedback to reduce the estimated packet size. A working route and a working size estimate are separate requirements. ...