Documentation: extensions: updated description of steam broker protocol

Documentation/extensions/steam-broker.md

@@ -1,8 +1,139 @@
-# Steam broker protocol
-`sb_connect <ip:port> <server's steam id> <secure> <challenge>` - used for obtaining auth ticket during connection to GoldSrc server
+# Steam API Broker Protocol Specification
 
-`sb_disconnect <ip:port> <challenge>` - used for signaling broker about disconnecting from GoldSrc server
+## Overview
 
-`sb_gamedir <gamedir>` - used for signaling broker about game startup and for announcing started mod, so broker could choose proper AppID for Steam API initialization
+Due to proprietary nature of Steamworks SDK, it cannot be run on same amount
+of platforms supported by Xash3D FWGS, neither we can link library directly due to
+GNU GPLv3 license. 
 
-`sb_terminate` - used for signaling broker about game shutdown, ideally broker should be terminated too (and later restarted automatically and waiting for sb_gamedir message)
+However, here comes the broker, by running it (in trusted
+network, preferrably) on a machine that has Steam client installed, the
+engine can communicate with it, acquiring needed information to log-in into
+Steam protected multiplayer servers.
+
+## Key Characteristics
+
+Broker uses a simple TCP-based binary protocol with the following properties:
+
+- Single active session per connection: once a session is activated, the broker will reject all other TCP connections until the session is terminated.
+- All messages follow a consistent structure with a header, length, and payload: this is called a "frame".
+- Stateful interactions: commands affect the session state, and certain commands are only valid in specific states.
+- Numeric parameters are encoded in little-endian format for compatibility with majority of platforms.
+---
+
+## Frame Format
+
+All communication uses a fixed frame structure:
+
+| Field | Size | Type | Description |
+|-------|------|------|-------------|
+| Header | 4 bytes | ASCII | Frame start signature |
+| Length | 2 bytes | uint16_t (LE) | Size of payload in bytes |
+| Payload | N bytes | Binary | Command string or response data |
+
+**Notes**:
+- Header must be exactly `SBRK` (`53 42 52 4B` in hexadecimal form)
+- Maximum frame size: soft limit is 4096 bytes, hard limit is 65535 bytes (due to uint16_t usage)
+
+---
+
+## Session Lifecycle
+
+```
+┌──────────────┐
+│     IDLE     │ ← Initial state, accepts new TCP connections
+└──────────────┘
+       │ sb_gamedir
+       ↓
+┌──────────────┐
+│    ACTIVE    │ ← Session established, rejecting all of other TCP connections
+└──────────────┘
+       │ sb_connect
+       ↓
+┌──────────────┐
+│    ACTIVE    │ ← Client getting auth ticket to connect to game server
+└──────────────┘
+       │ sb_disconnect
+       ↓
+┌──────────────┐
+│     ...      │ ← Client announcing disconnect from game server, could connect somewhere again
+└──────────────┘
+       │ 
+       ↓
+┌──────────────┐
+│    ACTIVE    │ ← Client shutting down game, terminating broker to reset Steam client state
+└──────────────┘
+       │ sb_terminate
+       ↓
+┌──────────────┐
+│  RESTARTING  │
+└──────────────┘
+```
+
+---
+
+## Commands
+
+All commands listed below are meant to be sent from client to broker as payload of a frame. The broker will parse the command string and respond accordingly.
+
+### Session activation
+
+Activates the session and signaling game startup.
+
+**Format**: `sb_gamedir <gamedir>`
+
+**Response**: None
+
+---
+
+### Authentication ticket request
+
+Requests authentication ticket for a game server.
+
+**Precondition**: Session must be active
+
+**Format**: `sb_connect <ip:port> <server_steamid> <secure> <challenge>`
+
+**Parameters**:
+
+| Name | Description |
+|------|------------------|
+| `<ip:port>` | Game server address (e.g., `127.0.0.1:27015`) |
+| `<server_steamid>` | Server SteamID |
+| `<secure>` | 0 = insecure, 1 = secure |
+| `<challenge>` | Random value for reply prevention |
+
+
+**Response Format**:
+
+| Field | Size | Type | Description |
+|-------|------|------|-------------|
+| Header | 11 bytes | string | `"sb_connect\n"` |
+| Challenge | 4 bytes | int32_t (LE) | Echo of challenge from request |
+| SteamID | 8 bytes | uint64_t (LE) | Client SteamID from configuration |
+| Size | 4 bytes | uint32_t (LE) | Length of ticket in bytes |
+| Ticket | N bytes | - | Authentication ticket data |
+
+---
+
+### Server disconnection announcement
+
+Notifies broker of disconnection from game server.
+
+**Precondition**: Session must be active
+
+**Format**: `sb_disconnect <ip:port> <challenge>`
+
+**Response**: None
+
+---
+
+### Session termination
+
+Signaling game shutdown to broker and resetting Steam client state.
+
+**Format**: `sb_terminate`
+
+**Response**: None
+
+---