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85 lines
3.1 KiB
85 lines
3.1 KiB
# JS5 protocol
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The JS5 remote protocol is used by the client to download groups of files from
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the server as they are required, as well as prefetching certain groups in
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advance. Groups contain the client's assets and scripts, and are stored in the
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cache.
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## Upstream
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| Opcode | Description |
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|-------:|--------------------------|
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| 0 | Request group (prefetch) |
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| 1 | Request group (urgent) |
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| 2 | Logged in |
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| 3 | Logged out |
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| 4 | Rekey |
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| 6 | Connected |
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| 7 | Disconnect |
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All upstream packets are exactly 4 bytes long, including the opcode. Unused
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payload bytes are set to zero.
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### Request group (prefetch/urgent)
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| Data type | Description |
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|---------------|-------------|
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| UnsignedByte | Archive ID |
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| UnsignedShort | Group ID |
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Requests a group. Urgent requests have a higher priority than prefetch requests,
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as the client needs the group immediately. Prefetch requests are used to
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prepopulate most of the cache in the background.
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### Rekey
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| Data type | Description |
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|---------------|----------------------|
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| UnsignedByte | Key |
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| UnsignedShort | Unused (set to zero) |
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Sent to set the encryption key.
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### Logged in/out
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Sent whenever the player logs in or out of the game. Consensus in the community
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is that the logged in/out state is probably used for prioritisation, much like
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the distinction between prefetch/urgent requests.
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### Connected
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| Data type | Description |
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|----------------|-----------------------------|
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| UnsignedMedium | Unknown (always set to `3`) |
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Sent immediately after the JS5 connection is established. Its purpose is not
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known.
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### Disconnect
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Requests that the server closes the connection. Sent by the `::serverjs5drop`
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command.
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## Downstream
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There is only a single type of response packet, and as such, there are no
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response opcodes. The packet contains a group requested by the client.
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| Data type | Description |
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|----------------|-----------------------------------------------------------|
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| UnsignedByte | Archive ID |
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| UnsignedShort | Group ID |
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| UnsignedByte | Compression type (ORed with `0x80` for prefetch requests) |
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| Int | Compressed length |
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| Int (optional) | Uncompressed length (present iff the group is compressed) |
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| Byte[] | Compressed data (with `0xFF` markers, see below) |
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If encryption is enabled, all downstream bytes are XORed with the encryption
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key. The client enables encryption if the checksum of a group does not match the
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expected checksum in the master index or index.
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The encryption scheme is probably used to defend against middle boxes tampering
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with the plaintext traffic.
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After the first 512 bytes of a response (including the header), every subsequent
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block of 511 bytes is prefixed with a single `0xFF` byte.
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