Inferring and Debugging Path MTU Discovery Failures

If a host can send packets larger than an Internet path can forward, it relies on the timely delivery of Internet Control Message Protocol (ICMP) messages advising that the packet is too big to forward. An ICMP Packet Too Big message reports the largest packet size - or Maximum Transmission Unit (MTU) - that can be forwarded to the next hop. The iterative process of determining the largest packet size supported by a path by learning the next-hop MTU of each MTU-constraining link on the path is known as Path MTU Discovery (PMTUD). It is fundamental to the optimal operation of the Internet. There is a perception that PMTUD is not working well in the modern Internet due to ICMP messages being firewalled or otherwise disabled due to security concerns. This paper provides a review of modern PMTUD failure modes. We present a tool designed to help network operators and users infer the location of a failure. The tool provides fairly detailed information about each failure, so the failure can be resolved. Finally, we provide data on the failures that occurred on a large jumbo-capable network and find that although disabling ICMP messages is a problem, many other failure modes were found.