Open-source multiplayer game server compatible with the RuneScape client
https://www.openrs2.org/
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71 lines
2.5 KiB
71 lines
2.5 KiB
# Protocol overview
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The RuneScape protocol runs on top of the [Transmission Control Protocol
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(TCP)][tcp]. As TCP is stream-oriented, a framing scheme is required to
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determine where one packet ends and where the next begins. RuneScape primarily
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uses a [type-length-value (TLV)][tlv] scheme for framing, though some exceptions
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do exist - for example, the JAGGRAB and JS5 protocols use different framing
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schemes.
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A RuneScape connection may be in several states, each of which has a different
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set of opcodes (types in TLV parlance). Opcodes may have different meanings in
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different states. Opcodes may be optionally encrypted with
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the [ISAAC stream cipher][isaac].
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All connections start in the [login](login.md) state.
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## Types
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There are three types of packets:
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* Fixed length - supports any payload length, including no payload.
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* Variable length (byte) - supports up to 255 bytes of payload.
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* Variable length (short) - supports up to 65,535 bytes of payload in theory,
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though in practice the client's receive buffer limits the payload to 5,000
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bytes in the 550 build. This limit is raised by OpenRS2's client patcher.
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They are structured as follows:
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### Fixed length
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| Data type | Description |
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|--------------|-------------|
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| UnsignedByte | Opcode |
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| Byte\[len\] | Payload |
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### Variable length (byte)
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| Data type | Description |
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|--------------|--------------|
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| UnsignedByte | Opcode |
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| UnsignedByte | Length (len) |
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| Byte\[len\] | Payload |
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### Variable length (short)
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| Data type | Description |
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|---------------|--------------|
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| UnsignedByte | Opcode |
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| UnsignedShort | Length (len) |
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| Byte\[len\] | Payload |
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## Opcode encryption
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Opcodes are encrypted by adding the opcode to the next 32-bit output from the
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ISAAC random number generator and truncating the result to 8 bits. Decryption is
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the same process in reverse: the 32-bit output is subtracted from the encrypted
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opcode, and then the result is truncated to 8 bits.
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There are two ISAAC random number generators: one used for upstream (client ->
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server) traffic and one used for downstream (server -> client) traffic. Both are
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initialised with a seed consisting of four 32-bit integers.
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The upstream seed is the same as the downstream seed, except each integer
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component is incremented by 50.
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The seed is negotiated during the login process. Before it is negotiated,
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opcodes are not encrypted.
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[isaac]: https://burtleburtle.net/bob/rand/isaacafa.html
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[tcp]: https://en.wikipedia.org/wiki/Transmission_Control_Protocol
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[tlv]: https://en.wikipedia.org/wiki/Type-length-value
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