But IPv4 headers could have a variable length too, it's just that we don't encounter that much in the wild. And what about the AH header, GRE and IPIP tunnel headers, 6rd/6in4 tunnel headers, etc. Filtering at fixed offsets sounds very brittle to me. Isn't the real problem here that network providers and administrators appropriate the right to filter on OSI layers they shouldn't be touching? This problem sounds like the exact reason why Google insisted on having its QUIC headers ciphered: so network equipment can't pull of this kind of misbehaviour.