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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