If the remote host advertises a zero window size for its input window and the local host has data in its output window, a window probe is sent at intervals limited by CFG_WIN_PROBE_MIN and CFG_WIN_PROBE_MAX. The window probes are sent until space opens up in the remote hosts input window. A window probe consists of the one data byte past the end of the window. Since a host is required to respond to data sent outside of its window, a window probe forces the remote host to send back a packet with its updated input window size.

Left TCP

TCP Keep Alive Packets

Down TCP Delayed Ack