mKCP Protocol

mKCP is a stream transport protocol, modified from the KCP Protocol, capable of transmitting arbitrary data streams in order.

Version

mKCP has no version number and does not guarantee compatibility between versions.

Dependencies

Underlying Protocol

mKCP is a UDP-based protocol; all communication is transmitted using UDP.

Functions

• fnv: FNV-1a hash function
  • Input parameter is a string of arbitrary length;
  • Outputs a 32-bit unsigned integer;

Communication Process

1. mKCP splits the data stream into several packets for transmission. A data stream has a unique identifier to distinguish different data streams. Every packet in a data stream carries the same identifier.
2. mKCP has no handshake process. When a packet is received, the identifier carried by it determines whether it is a new call or an ongoing one.
3. Each packet contains several segments. Segments are divided into three categories: Data, ACK (Acknowledgment), and Ping (Heartbeat). Each segment needs to be processed separately.

Data Format

Packet

4 Bytes	2 Bytes	L Bytes
Authentication A	Data Length L	Segment Part

Where:

• Authentication A = fnv(Segment Part), big endian;
• The Segment Part may contain multiple segments;

Data Segment

2 Bytes	1 Byte	1 Byte	4 Bytes	4 Bytes	4 Bytes	2 Bytes	Len Bytes
Identifier Conv	Command Cmd	Option Opt	Timestamp Ts	Sequence Sn	Unacknowledged Una	Length Len	Data

Where:

• Identifier Conv: The identifier of the mKCP data stream.
• Command Cmd: Constant 0x01.
• Option Opt: Optional values are:
  • 0x00: Empty option.
  • 0x01: The peer has sent all data.
• Timestamp Ts: The time when the current segment was sent from the remote end, big endian.
• Sequence Sn: The position of this data segment in the data stream. The sequence number of the starting segment is 0, and each subsequent new segment increases by 1 in order.
• Unacknowledged Una: The smallest Sn that the remote host is sending and has not yet received an acknowledgment for.

ACK Segment

2 Bytes	1 Byte	1 Byte	4 Bytes	4 Bytes	4 Bytes	2 Bytes	Len * 4 Bytes
Identifier Conv	Command Cmd	Option Opt	Window Wnd	Next Receive Sn	Timestamp Ts	Length Len	Received Sns

Where:

• Identifier Conv: The identifier of the mKCP data stream.
• Command Cmd: Constant 0x00.
• Option Opt: Same as above.
• Window Wnd: The maximum sequence number the remote host can receive.
• Next Receive Sn: The smallest sequence number among the data segments not yet received by the remote host.
• Timestamp Ts: The timestamp of the latest data segment received by the remote host, used for calculating latency.
• Received Sns: Each is 4 bytes, indicating that data with this sequence number has been confirmed as received.

Note:

• The remote host expects to receive data within the sequence number range [Sn, Wnd).

Ping (Heartbeat) Segment

2 Bytes	1 Byte	1 Byte	4 Bytes	4 Bytes	4 Bytes
Identifier Conv	Command Cmd	Option Opt	Unacknowledged Una	Next Receive Sn	Latency Rto

Where:

• Identifier Conv: The identifier of the mKCP data stream.
• Command Cmd: Optional values are:
  • 0x02: Remote host forcibly terminates the session.
  • 0x03: Normal heartbeat.
• Option Opt: Same as above.
• Unacknowledged Una: Same as Una in the Data Segment.
• Next Receive Sn: Same as Sn in the ACK Segment.
• Latency Rto: The latency calculated by the remote host itself.