Ethernet over USB research
There are 4 protocols implementing Ethernet-over-USB:
- RNDIS - Microsoft's proprietary protocol, not covered here as there are open alternatives
- ECM - Ethernet Control Module
- EEM - Ethernet Emulation Model
- NCM - Network Control Model
ECM, EEM and NCM protocols are USB-IF managed open standards (part of USB CDC).
ECM is the oldest one, and according to Wikipedia article the simplest protocol of these three, its main advantage is simplicity - it has 2 communication channels: first channel is used for transferring control messages like link status, etc. - requires 3 endpoints: 1 for control transfers, 1 bulk in, and 1 bulk out - second channel is used for sending raw, unmangled Ethernet frames.
The main disadvantage of this protocol is its poor performance, maximum packet size is 64 bytes, Ethernet packets larger than that require few USB transfers. Every Ethernet frame must be sent in a single USB transfer (1 USB transfer may consist of many packets), it is not possible to bundle many Ethernet frames into one USB transfer.
EEM protocol improves performance while still being relatively simple. EEM brings following improvements - no more 64 byte limit, can have any max packet size (usually 512 bytes for USB 2.0) - small Ethernet frames may be packed into a single USB packet, thus improving performance - CDC EEM 1.0 Specification, section 2 - Management Overview
Unlike CDC ECM, EEM does not extend an interface across the USB bus but instead considers the USB bus to be a vehicle for moving Ethernet packets. - designed for local-only connection over USB emulated Ethernet, where ECM is designed for connecting to a real network over Ethernet-to-USB modem. - Requires only 2 endpoints (bulk in and bulk out)
NCM is the latest protocol designed for high throughput, like ECM it is designed to control hardware with a real, physical Ethernet link, it is designed for controlling modern, complex hardware, thus protocol itself is very complex.
I consider EEM protocol to be the best choice - it is designed for what we need - emulated Ethernet with no physical layer - better performance than ECM - ability to pack multiple Ethernet frames into one USB transfer reduces the amount of time the driver needs to spend handling interrupts, thus offloading CPU - EMM requires only 2 endpoints, so more endpoints are available for other potential protocols we may implement in the future
Ethernet implementation in software
USB driver for NRF52 is already provided by nrf-hal project.
Implementations of EEM protocol exist, but these are mostly written in C. I haven't found a single Rust implementation of EEM protocol (nor ECM), so a custom implementation is needed.
Besides the EEM driver, we need an actual Ethernet driver, additionally we may need IP + UDP support for CoAP protocol, this may not necessarily be a requirement as CoAP protocol (used by CHARRA) doesn't seem to depend on UDP or any other transport layer, yet implementations do depend and removing that dependence could require more work than getting UDP support.
There is one particularly interesting project called smoltcp, it is a library providing implementation of TCP/IP stack. These are the most important features: - provides support for IP + TCP + UDP and others - support for individual protocols may be disabled at compile time so we can remove unneeded features like TCP (we can rely on UDP since USB already provides reliable transmission) - no unsafe code inside library
Looks like this is the only TCP/IP stack written in Rust and compatible with