Hold your horses. It's not MSS clamping per se. It's the middle box MSS clamping. Say a middle box rewrites my SYN packet from my phone to have smaller MSS, fro

Hold your horses. It's not MSS clamping per se. It's the middle box MSS clamping. Say a middle box rewrites my SYN packet from my phone to have smaller MSS, from 1500 to say 1344. The other party (server) responds with 1500 MSS, and both parties successfully exchange TCP handshake assuming PMTU of 1344. Now say there is a 1300 link between. And say my phone sends large packet. My phone will happily transmit 1344 byte packet. Say TCP, with DF flag.... the intermediate router with small MTU will stop it, and return back ICMP PMTU message. The point I'm making - the middle box doing clamping, usually don't do much about this ICMP. They drop it. What they should do is to send this ICMP back to my phone, but that's not what I saw in my tests. Or maybe I just tested some buggy middle boxes.