Update readme with nc examples

Update readme with nc examples and clarify semantics of nc command line
arguments.

Author: progbits

README.md

@@ -74,3 +74,56 @@ PING 10.0.0.2 (10.0.0.2) 56(84) bytes of data.
 64 bytes from 10.0.0.2: icmp_seq=2 ttl=64 time=0.035 ms
 64 bytes from 10.0.0.2: icmp_seq=3 ttl=64 time=0.021 ms
 ```
+
+## New System Calls
+
+The implementation of network stack adds 7 new system calls: `socket`, `bind`,
+`connect`, `listen`, `accept`, `send `and `recv`. As only UDP is currently
+supported, the `listen` and `accept` system calls are currently nops.
+
+- `socket` - Creates a new socket of the specified type (currently UDP only)
+- `bind` - Associates a socket with a local address and port
+- `connect` - Associates a socket with a remote address and port
+- `listen` - Not implemented
+- `accept` - Not implemented
+- `send` - Send data to a remote socket
+- `recv` - Receive data from a remote socket
+
+## Using the Network Stack
+
+An example netcat like userspace program, `nc`, is provided to exercise the
+network stack.
+
+The `nc` program operates in two modes, client or server:
+
+```shell
+nc [-c|-s] [address] [port]
+```
+
+In client mode (`-c`), the program will send data from stdin to the specified
+port of the host located at `address`. For example, to send data to `10.0.0.1` on port `4444`:
+
+```shell
+$ nc -c 10.0.0.1 4444
+$ hello, world!
+```
+
+In server mode (`-s`), the program will listen for data on the specified port.
+To listen to data on port `5555`:
+
+```shell
+$ nc -s 10.0.0.2 5555
+```
+
+As the network interface has a fixed local address (`10.0.0.2`), the `address`
+argument is currently ignored.
+
+## Notes
+
+- The network interface is assigned a fixed address of `10.0.0.2`
+- The connect(...) system call is blocking on establishing the ARP resolution
+  of the hardware address of the remote host.
+- The send(...) system call is blocking on the successful write of a transmit
+  descriptor to the network device.
+- The recv(...) system call is non-blocking, returning immediately if no data
+  is available. This is until the functionality of proc.c is ported to Rust.

nc.c

@@ -3,12 +3,12 @@
 
 enum mode { MODE_SEND, MODE_LISTEN, MODE_UNKNOWN };
 
-const char *usage = "usage: nc [-s|-l] [destination] [port]\n";
+const char *usage = "usage: nc [-c|-s] [address] [port]\n";
 
 enum mode parse_mode(char *mode) {
-  if (mode[1] == 's') {
+  if (mode[1] == 'c') {
     return MODE_SEND;
-  } else if (mode[1] == 'l') {
+  } else if (mode[1] == 's') {
     return MODE_LISTEN;
   }
   return MODE_UNKNOWN;