In a recent case from the field we've seen XDCR holding 70,000 sockets that do not have a process on the other side of the connection. When lsof is run, these sockets show up as follows:
In this case the user had set the file descriptor limit to 70k and so at this point XDCR is unable to create new connections. This issue was previously tracked in
MB-44182 and believed to be fixed, but it seems that the core issue hasn't been completely fixed.