How is linux cdc_ecm driver related to rndis protocol? Is cdc_ecm based on rndis specification from microsoft in any way?
From this wiki page https://en.wikipedia.org/wiki/RNDIS
The USB Implementers Forum (USB-IF) defines at least three non-proprietary USB communications device class (USB CDC) protocols with comparable "virtual Ethernet" functionality; one of them (CDC-ECM) predates RNDIS and is widely used for interoperability with non-Microsoft operating systems, but does not work with Windows.
seems cdc_ecm to be based on microsoft rndis.
CDC-ECM is a standard that was created by USB-IF. It's not related to RNDIS at all. As your quote says, it predates RNDIS. This means it was created before RNDIS existed, so it couldn't be based on RNDIS unless a time machine enters the picture. And if you look at the protocol, it's clearly different.
CDC-ECM works out of the box on Linux hosts, MacOS, ChromeOS, and certain (mainly Samsung) Android phones.
Microsoft decided to create a new Ethernet on USB protocol instead of using the standard that already existed (CDC-ECM). That's just how Microsoft is. So they created RNDIS which isn't a standard and wasn't published. It's a hack based on NDIS, which is an old DOS and Windows 3.11 era network protocol from Microsoft. That's also just how Microsoft is. They like to base stuff on top of older MS software, which is based on even older MS software, going all the way back to DOS. That makes it really complex and hard for anyone else to be compatible with.
Microsoft doesn't include CDC-ECM drivers in Windows, but does have RNDIS drivers.
One can't get Windows to load the included RNDIS drivers, without a INF file, using just the vendor and product ID, device class, etc. There is a non-standard USB device identification descriptor that Microsoft created (see the pattern here) that must be used to identify a RNDIS device to Windows so it will load the generic driver.
Linux has a CDC-ECM / RNDIS combo gadget that is useful for wider compatibility. This appears as a two function USB device. One function is RNDIS and Windows will use this. The other function is CDC-ECM and everyone else will use that. It's not "ECM with RNDIS support". It's two different functions, only one of which can be used at once, that are both created by the same gadget.