Direction for protocol in commit message.

Each side sends a UDP packet at a regular interval (n ms).

The packet contains whatever data the application have accumulated for
sending since the last sending; the data are grouped into units (like
frames, except I can't use "frames" since that's an Ethernet thing).
Each unit is numbered monotonically increasing, and the numbers are
included in the packet.

The packet also contains the highest unit number we've seen from the
other side; when the other side reads the packet, they can drop units up
to and including that number for future sending.