These asumptions frame the rest of this article:
- Many applications recover gracefully from path MTU discovery (PMTUD) blackholes
- A non-trivial portion of applications don’t
- MTU dysfunction abounds in modern networks
- There are valid use-cases for jumbo Ethernet
- PMTUD is preferable to manually decreasing interface MTU
- Maximizing operational stability and efficiency in delivery of IT sericees is desirable.
- The cure should never be worse than the disease
- Packet loss on IP networks leads to application performance problems, which adversely impacts operational stability
- MTU mismatches are an easily remediated cause of undetected packet loss
- Path MTU Discovery (PMTU) provides a mechanism for two hosts on separate subnetes/broadcast domains with an MTU mismatch between them to compensate and avoid adverse impact
- But PMTUD “in real life” is a fragile framework
- Ethernet and IP networks represents the overwhelming majority of enterprise networks
- Commonly adopted networks design practices result in widespread MTU mismatch
- Modern public/private cloud hosting architectures often involve multiple layers of nested encapsulation as part of recursive underlay/overlay services
- Remediating MTU mismatch introduces its own risks to operational stability
- Most devices on enterprise networks have explicitly configured network interface MTU values
- Most devices on enterprise networks consume DHCP services for IP address assignment