add notes about unix dgrams..

NOTES.md

@@ -40,6 +40,20 @@ to ignore these cleanups.
 LANG=C sed -i '' -e 's/[       ]*^M$//' $(find stm32/ -type f)
 ```
 
+Unix dgram sockets
+------------------
+
+By default, unix dgram sockets are not bound, which means that when
+a client sends to a server, there is no return path.  A client can
+bind to a local address, and this address will be received by the
+server allowing the server to reply.
+
+Unix dgrams sockets do not support broadcast, so a server would have
+to keep track of all the clients, and send any packets to each client
+individually, which is fine for a small number of clients.
+
+Using broadcast or multicast IP pushed this work to the IP layer.
+
 Running gdb
 -----------