Network Connect / Pulse Desktop prior to 5.2R2
Calculation of the MTU size for the virtual adapter is determined by the local physical interface of the client machine. The virtual adapter will be 100 bytes less than the local physical adapter.
If the local physical adapter is 1500 bytes, then the virtual adapter will be 1400 bytes.
Pulse Secure Desktop 5.2R2 and above
Pulse Secure Desktop 5.2R2 retrieves the negotiated MSS value from the TCP control channel and uses it to calculate a more accurate MTU for the virtual adapter. This improvement was made to help avoid UDP ESP fragmentation when traversing networks as Pulse is able to evaluate the max MTU size across the network instead of evaluating only the local interface.