MTU stands for Maximum Transmission Unit
and is a characteristic of the hardware (physical layer).
MTU sizes are specified in octets (8-bit values).
The MTU size defines the maximum size that a packet or frame
sent onto the network or received from the network can be.
If the application sends a small block of data that fits in
one frame then only one frame will be sent onto the network.
If the application sends a block of data that is larger than
the MTU size the data will be split into multiple packets,
each of which will create a frame of less than or equal to
the MTU size.
In FreeRTOS+TCP the MTU is specified in bytes and set by the
ipconfigNETWORK_MTU setting in
FreeRTOSIPConfig.h. Check the specification for your MAC or
Ethernet hardware to find the correct setting for your system.
Some MAC devices are restricted to 1400.