Comment 6 for bug 988144

> the remote 'rsync' process takes too long to send back the
> list of block checksums to the local 'rsync' in some situations

As to that a quick web search indicates previous reports:

https://unix.stackexchange.com/questions/68775/rsync-timed-out 3
«It looks like the server times out on the SSH connection level.»
«Since the timeout appears to happen when rsync is creating the checksums»

https://bugzilla.samba.org/show_bug.cgi?id=7195
«it seems - in my case at least - to be related to the delta-xfer algorithm»