Vertical Interface Design in Message-Oriented Protocols
Does not expose the interface’s MPU to upstack-adjacent interfaces
Does not consume the downstack-adjacent interface’s MPU to infer its own MTU
This breaks the value of layered architectures\, forcing us to go outside the stack to get L( n-1 ) information in order to configure L( n )
IP overlays / “In-network-tunneling” Break PMTUD for the Overlay
It’s literally not possible for an IP-tunneling interface to infer enough from an underlay PTB message to translate it into the overlay network
Common Design Practices Exacerbate MTU Issues
Stacked overlays
Encapsulating protocols with variable header lengths
Heterogenous L( n) MTUs in the same L( n) broadcast domain