Create EN-US version of muxcool.md

Author: oweid

docs/en_US/developer/protocols/muxcool.md

@@ -1,3 +1,115 @@
-# Mux.Cool
+# Mux.Cool Protocol
 
-The documentation for this page is missing. Please submit a [pull request](https://github.com/v2fly/v2fly-github-io/pulls) or refer to the Chinese documentation.
+Mux.Cool protocol is a multiplexing transport protocol used to transmit multiple independent data streams within a single established data stream.
+
+## Version
+
+Current version is 1 Beta.
+
+## Dependencies 
+
+### Underlying Protocol
+
+Mux.Cool must run on top of an established reliable data stream.
+
+## Communication Process
+
+A Mux.Cool connection can transmit multiple sub-connections, each with an independent ID and state. The transmission process consists of frames, with each frame transmitting data for a specific sub-connection.
+
+### Client Behavior
+
+When a connection is needed and no existing connections are available, the client initiates a new connection to the server, referred to as the "main connection".
+
+1. A main connection can be used to send multiple sub-connections. The client can independently determine the number of sub-connections the main connection can carry.
+2. For a new sub-connection, the client must send a `New` state to notify the server to establish the sub-connection, then use the `Keep` state to transmit data.
+3. When a sub-connection ends, the client sends an `End` state to notify the server to close the sub-connection.
+4. The client can decide when to close the main connection but must ensure the server maintains the connection simultaneously.
+5. The client can use the KeepAlive state to prevent the server from closing the main connection.
+
+### Server Behavior
+
+When the server receives a new sub-connection, it should handle it as a normal connection.
+
+1. Upon receiving an `End` state, the server can close the upstream connection to the target address.
+2. In server responses, the same ID as the request must be used to transmit sub-connection data.
+3. The server cannot use the `New` state.
+4. The server can use the KeepAlive state to prevent the client from closing the main connection.
+
+## Transport Format
+
+Mux.Cool uses a symmetric transport format, meaning clients and servers send and receive data in the same format.
+
+### Frame Format
+
+| 2 bytes | L bytes | X bytes |
+|---------|---------|---------|
+| Metadata Length L | Metadata | Extra Data |
+
+### Metadata
+
+Metadata has several types, distinguished by state S. All metadata types include ID and Opt fields:
+
+* ID: Unique identifier for the sub-connection
+* Opt:
+  * D(0x01): Has extra data
+
+When option Opt(D) is enabled, the extra data format is:
+
+| 2 bytes | L bytes |
+|---------|---------|
+| Length L | Data |
+
+#### New Sub-connection (New)
+
+| 2 bytes | 1 byte | 1 byte | 1 byte | 2 bytes | 1 byte | X bytes |
+|---------|---------|---------|---------|---------|---------|---------|
+| ID | 0x01 | Option Opt | Network Type N | Port | Address Type T | Address A |
+
+Where:
+
+* Network Type N:
+  * 0x01: TCP, indicating sub-connection traffic should be sent to target as TCP
+  * 0x02: UDP, indicating sub-connection traffic should be sent to target as UDP
+* Address Type T:
+  * 0x01: IPv4
+  * 0x02: Domain name
+  * 0x03: IPv6
+* Address A:
+  * When T = 0x01: 4-byte IPv4 address
+  * When T = 0x02: 1-byte length (L) + L-byte domain name
+  * When T = 0x03: 16-byte IPv6 address
+
+When creating a new sub-connection with Opt(D) enabled, the frame's data must be sent to the target host.
+
+#### Keep Sub-connection (Keep)
+
+| 2 bytes | 1 byte | 1 byte |
+|---------|---------|---------|
+| ID | 0x02 | Option Opt |
+
+When keeping a sub-connection with Opt(D) enabled, the frame's data must be sent to the target host.
+
+#### Close Sub-connection (End)
+
+| 2 bytes | 1 byte | 1 byte |
+|---------|---------|---------|
+| ID | 0x03 | Option Opt |
+
+When closing a sub-connection with Opt(D) enabled, the frame's data must be sent to the target host.
+
+#### Keep Connection (KeepAlive)
+
+| 2 bytes | 1 byte | 1 byte |
+|---------|---------|---------|
+| ID | 0x04 | Option Opt |
+
+For keep-alive:
+
+* When Opt(D) is enabled, the frame's data must be discarded
+* ID can be a random value
+
+## Application
+
+The Mux.Cool protocol is independent of the underlying protocol and can theoretically use any reliable stream connection to transmit Mux.Cool protocol data.
+
+In destination-oriented protocols like Shadowsocks and VMess, a specified address must be included when establishing a connection. For compatibility, the Mux.Cool protocol specifies the address as "v1.mux.cool". When the main connection's target address matches this, forwarding proceeds in Mux.Cool mode; otherwise, traditional forwarding is used.