3 ms·
PMTU discovery on the Internet is generally unreliable. Very few people understand that it exists, and even fewer understand how it actually works. Most ADSL
by minimax 14y ago
PMTU discovery on the Internet is generally unreliable. Very few people understand that it exists, and even fewer understand how it actually works. Most ADSL (PPPoE) providers rewrite the TCP MSS on TCP SYN packets traveling over their network to account for the PMTU discovery brokenness. [1] You see the same thing happen with VPN connections where the PMTU is effectively reduced by the size of the overhead for the encapsulation protocol.
1. http://www.cisco.com/en/US/docs/ios/12_2t/12_2t4/feature/guide/ft_admss.html#wp1062184 http://www.cisco.com/en/US/docs/ios/12_2t/12_2t4/feature/gui...
- gonzo 14y agojust because people don't understand doesn't give them latitude to break the RFCs.
- minimax 14y agoI agree, and when I worked in that world it was a huge pain in the ass. Users don't care about RFCs though, they just want the Internet to work, so you end up doing something that's pragmatic but kludgey. As I pointed out in another comment, Google is responding with a TCP MSS of 1430 (so assuming a PMTU of 1470). It's just what you do so you can get on with your business.