Add JS5 protocol documentation

Signed-off-by: Graham <gpe@openrs2.org>
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Graham 4 years ago
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# JS5 protocol
## Upstream
| Opcode | Description |
|-------:|--------------------------|
| 0 | Request group (prefetch) |
| 1 | Request group (urgent) |
| 2 | Logged in |
| 3 | Logged out |
| 4 | Rekey |
| 6 | Connected |
| 7 | Disconnect |
All upstream packets are exactly 4 bytes long, including the opcode. Unused
payload bytes are set to zero.
### Request group (prefetch/urgent)
| Data type | Description |
|---------------|-------------|
| UnsignedByte | Archive ID |
| UnsignedShort | Group ID |
Requests a group. Urgent requests have a higher priority than prefetch requests,
as the client needs the group immediately. Prefetch requests are used to
prepopulate most of the cache in the background.
### Rekey
| Data type | Description |
|---------------|----------------------|
| UnsignedByte | Key |
| UnsignedShort | Unused (set to zero) |
Sent to set the encryption key.
### Logged in/out
Sent whenever the player logs in or out of the game. Consensus in the community
is that the logged in/out state is probably used for prioritisation, much like
the distinction between prefetch/urgent requests.
### Connected
| Data type | Description |
|----------------|-----------------------------|
| UnsignedMedium | Unknown (always set to `3`) |
Sent immediately after the JS5 connection is established. Its purpose is not
known.
### Disconnect
Requests that the server closes the connection. Sent by the `::serverjs5drop`
command.
## Downstream
There is only a single type of response packet, and as such, there are no
response opcodes. The packet contains a group requested by the client.
| Data type | Description |
|----------------|-----------------------------------------------------------|
| UnsignedByte | Archive ID |
| UnsignedShort | Group ID |
| UnsignedByte | Compression type (ORed with `0x80` for prefetch requests) |
| Int | Compressed length |
| Int (optional) | Uncompressed length (present iff the group is compressed) |
| Byte[] | Compressed data (with `0xFF` markers, see below) |
If encryption is enabled, all downstream bytes are XORed with the encryption
key. The client enables encryption if the checksum of a group does not match the
expected checksum in the master index or index.
After the first 512 bytes of a response, every subsequent block of 511 bytes is
prefixed with a single `0xFF` byte.
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