Exploring Usable Path MTU in the Internet

Abstract

To optimise their transmission, Internet endpoints need to know the largest size of packet they can send across a specific Internet path, the Path Maximum Transmission Unit (PMTU). This paper explores the PMTU size experienced across the Internet core, wired and mobile edge networks. Our results show that MSS Clamping has been widely deployed in edge networks, and some webservers artificially reduce their advertised MSS, both of which we expect help avoid PMTUD failure for TCP. The maximum packet size used by a TCP connection is also constrained by the acMSS. MSS Clamping was observed in over 20% of edge networks tested. We find a significant proportion of webservers that advertise a low MSS can still be reached with a 1500 byte packet. We also find more than half of IPv6 webservers do not attempt PMTUD and clamp the MSS to 1280 bytes. Furthermore, we see evidence of black-hole detection mechanisms implemented by over a quarter of IPv6 webservers and almost 15% of IPv4 webservers. We also consider the implications for UDP - which necessarily can not utilise MSS Clamping. The paper provides useful input to the design of a robust PMTUD method that can be appropriate for the growing volume of UDP-based applications, by determining ICMP quotations can be used as to verify sender authenticity.