IIRC, the "zsync" protocol used by koreader is actually a combination of filemq[1,2] and zyre[3]. Where zyre is for peer discovery and filemq handles the actual file transferring.
[1]
http://rfc.zeromq.org/spec:19
[2]
https://github.com/zeromq/filemq
[3]
https://github.com/zeromq/zyre