I hadn't heard of Extprot before so I took a quick look. It appears to be heavily inspired by Protobuf, having almost exactly the same arrangement on the wire,

I hadn't heard of Extprot before so I took a quick look. It appears to be heavily inspired by Protobuf, having almost exactly the same arrangement on the wire, adding a few tweaks but keeping many idiosyncrasies. The Cap'n Proto documentation compares it extensively with Protobuf and most of those comparisons probably apply to Extprot as well. (Disclosure: I am the author of Cap'n Proto and also Protobuf v2.)