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MTU stands for Maximum Transmission Unit and is a characteristic of the hardware (physical layer). MTU sizes are specified in octets (8-bit values).

MTU Size

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-Plus-TCP the MTU is specified in bytes and set by the ipconfigNETWORK_MTU setting in FreeRTOSIPConfig.h. If ipconfigNETWORK_MTU is not defined in FreeRTOSIPConfig.h, the default value of 1500 defined in FreeRTOSIPConfigDefaults.h will be used. Check the specification for your MAC or Ethernet hardware to find the correct setting for your system. Some MAC devices are restricted to 1400.

See also MSS.

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