A PMTUD black hole can cause a particularly subtle set of issues in hybrid cloud-based environments where the cloud resources are connected to a corporate office or other datacenter via IPSec tunnels. PMTUD black holes basically cause certain (but not all) traffic to not make it through the tunnel.
Before we get down and dirty into the problems with PMTUD, let’s quickly go over what PMTUD is.
PMTUD stands for Path Maximum Transmission Unit Discovery and it is a protocol/algorithm defined in RFC 1191 that determines the best packet size for IPv4 datagrams flowing between any two given hosts. In this way, it attempts to optimize traffic through the Internet by using the largest possible datagram that doesn’t require intermediary routers to fragment the traffic (since fragmentation and, more importantly, reassembly are expensive operations for routers to perform).