According how the USB system works, each packet is sent as a transfer, a series of maximum-length packets terminated by a short packet or a special ZLP (zero-length packet). The problem is that Ethernet frames are about 1500 bytes in size-about 3 USB 2.0 packets, and 23 USB 1.1 packets. This was a good strategy for USB 1.1 systems (current when the protocol was issued) with 64 byte packets but not for USB 2.0 systems which use 512 byte packets. Of these protocols ECM is by far the simplest-frames are simply sent and received without modification one at a time. Remarkably, even some of the most modern platforms need minor accommodations and therefore support for these subsets is still needed. However, minor modifications of the standard, so-called subsets, make practical implementations possible on such platforms. Regarding de facto standards, some standards, such as ECM, specify use of USB resources that early systems did not have. The RNDIS specification is available from Microsoft's web site.
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They are available for download from the USB-IF (see below). The latter three are part of the larger Communications Device Class (CDC) group of protocols of the USB Implementers Forum (USB-IF).
The main industry protocols are (in chronological order): Remote NDIS (RNDIS, a Microsoft vendor protocol), Ethernet Control Model (ECM), Ethernet Emulation Model (EEM), and Network Control Model (NCM).
Even though USB is not a physical Ethernet, the networking stacks of all major operating systems are set up to transport IEEE 802.3 frames, without caring much what the underlying transport really is. The main motivation for these protocols is to allow application-independent exchange of data with USB devices, instead of specialized protocols such as video or MTP. There are numerous protocols for Ethernet-style networking over USB.
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