Jumbo Frames

The CDC-ECM protocol (and other networking over USB protocols) describe procedures to transport 802.3-like frames over USB. Standard physical Ethernet frames have an MTU (maximum transfer unit) of 1500 bytes, but there is no such limitation in USB. USB transfers can be up to 64KB in size.

In the world of physical Ethernets, Gigabit Ethernets also see the 1500 limit as insufficient, and so there is provision in the network stacks of Windows, (embedded) Linux and other operating systems for jumbo frames. The actual MTU is a property of the network card and its driver. There is a procedure for two ends of an Ethernet link to discover the actual MTU in effect.

The Gadget implementation of CDC-ECM (the g_ether module) can be readily patched to support larger MTU’s.