Add basic usage info

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/README.md b/README.md
index 248eda0..df77951 100644
--- a/README.md
+++ b/README.md
@@ -29,3 +29,47 @@ If you possess an EV certificate for kernel mode code signing you should switch
 Modify the `<CrossCertificateFile>` to contain the full path to the cross-signing certificate of CA that issued your certificate. You should be able to find its `.crt` file in `C:\Program Files (x86)\Windows Kits\10\CrossCertificates`. Note that the `$(WDKContentRoot)` expands to `C:\Program Files (x86)\Windows Kits\10\`.
 
 If you already have `wintun.vcxproj.user` file, just add the `<PropertyGroup>` section.
+
+## Usage
+
+After loading the driver and creating a network interface the typical way using [SetupAPI](https://docs.microsoft.com/en-us/windows-hardware/drivers/install/setupapi), open `\\.\Device\WINTUN%d` as Local System, where `%d` is the [LUID](https://docs.microsoft.com/en-us/windows/desktop/api/ifdef/ns-ifdef-_net_luid_lh) of the network device. You may then [`ReadFile`](https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-readfile) and [`WriteFile`](https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-writefile) bundles of packets of the following format:
+
+```
++------------------------------+
+| size_0                       |
+|   4 bytes, native endian     |
++------------------------------+
+|                              |
+| padding                      |
+|   12 bytes, all zero         |
+|                              |
++------------------------------+
+|                              |
+| packet_0                     |
+|   size_0 bytes               |
+|                              |
+~                              ~
+|                              |
++------------------------------+
+| padding                      |
+|   16-((4+size_0)&15) bytes,  |
+|   all zero                   |
++------------------------------+
+| size_1                       |
+|   4 bytes, native endian     |
++------------------------------+
+|                              |
+| padding                      |
+|   12 bytes, all zero         |
+|                              |
++------------------------------+
+|                              |
+| packet_1                     |
+|   size_1 bytes               |
+|                              |
+~                              ~
+```
+
+Each packet segment should contain a layer 3 IPv4 or IPv6 packet. Up to 256 packets may be read or written during each call to `ReadFile` or `WriteFile`.
+
+It is advisable to use [overlapped I/O](https://docs.microsoft.com/en-us/windows/desktop/sync/synchronization-and-overlapped-input-and-output) for this. If using blocking I/O instead, it may be desirable to open separate handles for reading and writing.