Add JAGGRAB documentation

Signed-off-by: Graham <gpe@openrs2.org>

share/doc/protocol/jaggrab.md

@@ -0,0 +1,58 @@
+# JAGGRAB protocol
+
+JAGGRAB is a very simple protocol used by the loader to fetch the game's code.
+It is only used as a fallback if fetching the code via HTTP fails. It is very
+similar to [HTTP/0.9][http09] - the first version of HTTP.
+
+The loader opens a connection to the secondary game server port (443) and sends
+the 'switch to JAGGRAB mode' packet. As this packet has no payload, it only
+contains a single opcode byte: `17`.
+
+The loader then writes the following string:
+
+    JAGGRAB /<file name>
+
+where `<file name>` is replaced with the name of the file to fetch. The string
+is followed by two line feed characters.
+
+The server responds with the requested file and then closes the connection.
+
+The file names are suffixed with their CRC-32 checksum (for example,
+`unpackclass.pack` -> `unpackclass_-1911426584.pack`). The same file names are
+used when the client requests the files over HTTP, so the suffixes are
+presumably for cache busting. The Old School RuneScape servers do not require
+the checksum suffix to be present or correct.
+
+In build 550, the following files may be requested with JAGGRAB:
+
+| Remote file name       | Local file name        | Description                                        |
+|------------------------|------------------------|----------------------------------------------------|
+| `unpackclass.pack`     | `game_unpacker.dat`    | packclass unpacker                                 |
+| `runescape.pack200`    | `main_file_cache.dat0` | SD client (pack200 format)                         |
+| `runescape.js5`        | `main_file_cache.dat1` | SD client (packclass format)                       |
+| `runescape_gl.pack200` | `main_file_cache.dat3` | HD client (pack200 format)                         |
+| `runescape_gl.js5`     | `main_file_cache.dat4` | HD client (packclass format)                       |
+| `jaggl.pack200`        | `main_file_cache.dat5` | OpenGL bindings (pack200 format)                   |
+| `jaggl.js5`            | `main_file_cache.dat6` | OpenGL bindings (packclass format)                 |
+| `jaggl_0_0.lib`        | `jaggl.dll`            | OpenGL native library (Windows i386)               |
+| `jaggl_1_0.lib`        | `libjaggl.so`          | OpenGL native library (Linux i386)                 |
+| `jaggl_1_1.lib`        | `libjaggl_dri.so`      | DRI hack native library (Linux i386)               |
+| `jaggl_2_0.lib`        | `libjaggl.jnilib`      | OpenGL native library (macOS PowerPC)              |
+| `jaggl_3_0.lib`        | `libjaggl.jnilib`      | OpenGL native library (macOS i386)                 |
+| `jaggl_4_0.lib`        | `jaggl.dll`            | OpenGL native library (Windows amd64)              |
+| `jagmisc_0.lib`        | `jagmisc.dll`          | Miscellaneous native library (Windows i386)        |
+| `jagmisc_1.lib`        | `jagmiscms.dll`        | Miscellaneous native library (Windows i386, MSJVM) |
+| `jagmisc_2.lib`        | `jagmisc64.dll`        | Miscellaneous native library (Windows amd64)       |
+
+Note that OpenRS2's client patcher changes the list of supported platforms:
+
+* MSJVM support is removed.
+* PowerPC support is removed.
+* amd64 support is added to macOS and Linux.
+
+This causes some differences to the list of supported files.
+
+It also normalises some of the local file names (for example, `jagmisc64.dll` ->
+`jagmisc.dll`, `libjaggl.jnilib` -> `libjaggl.dylib`).
+
+[http09]: https://www.w3.org/Protocols/HTTP/AsImplemented.html