Originally published at https://blog.pathvector.dev/protocol-lab-mss-37/ — part of the free Protocol Lab series.
This post is part of Protocol Lab, a free, hands-on series for learning networking protocols by building and breaking them in a container lab. All the lab material — topologies, configs, and scripts — lives in the repo: github.com/pathvector-studio/protocol-lab.
In Lab #25, we watched Path MTU Discovery find a smaller link via ICMP — and saw how it black-holes when that ICMP is filtered. This lab is the operational fix: MSS clamping. A router on the path rewrites the MSS in passing TCP SYNs so both endpoints agree, up front, to send segments that fit the narrowest link — no reliance on PMTUD working.
Reading guide: rfc-notes/mss-clamping.md
Prerequisite: Lab 25: MTU and Path MTU Discovery
