RakNet

RakNet^[1] is a networking library used by Bedrock Edition. This documentation is based on the old Pocket Edition protocol and may be incomplete or outdated. The source code for the RakNet Library can be found here. RakNet uses UDP as its networking protocol. The key differences from TCP are that not all packets are guaranteed to reach the destination at all or in order (it isn't reliable) and it uses messages with a specified length instead of streams of data. RakNet compensates for the out-of-order delivery and makes it reliable.

NOTE: You may want to use an existing library if one exists for your language of choice. Here is an incomplete list. Not all of these are maintained or complete. If these are outdated you may need to change the protocol version to 11 before it can work. The protocol doesn't seem to have had any significant changes since the official implementation was discontinued in 2014, so as long as you bump the protocol version older libraries are likely to still work.

Note that some types and fields are encoded in network byte order (big-endian, see wikipedia), and decoding them in the wrong way could cause unexpected behaviour.

1. C→S: Open Connection Request 1
2. S→C: Open Connection Reply 1
3. C→S: Open Connection Request 2
4. S→C: Open Connection Reply 2

From here on, the RakNet connection is established and all RakNet messages are contained in a Frame Set Packet.

1. C→S: Connection Request
2. S→C: Connection Request Accepted
3. C→S: New Incoming Connection

Next packets should be Game Packets.

This is the first RakNet message sent by the client to a RakNet server. 0x01 is sent almost every second if the client is on the server list menu and every 4-5 seconds if the client is in-game. 0x02 is only replied to if there are open connections to the server. Note: as of 0.15.6 it seems that the game refreshes it's LAN world list every 4-5 seconds, however it seems the game will still ping in 1 second intervals.
This packet should be responded to with unconnected Pong.

Unconnected Pong message is sent by the server to the client in response to the Unconnected Ping message. Time shall be set to the time received in the Unconnected Ping as it will probably be used to calculate ping. The Server ID String contains various data that is sent to the client such as MOTD, number of players, max player count, etc. The format is given below:

Server ID string format

For Minecraft Bedrock this is separated by semicolons and uses the following format:
Edition (MCPE or MCEE for Education Edition);MOTD line 1;Protocol Version;Version Name;Player Count;Max Player Count;Server Unique ID;MOTD line 2;Game mode;Game mode (numeric);Port (IPv4);Port (IPv6);
Example:
MCPE;Dedicated Server;390;1.14.60;0;10;13253860892328930865;Bedrock level;Survival;1;19132;19133;
Result:
👁 Image

The Game mode and Game mode (numeric) values seem to not be used by the client.

This is the first packet sent in the Encapsulated Frame Set Packet by the client to the server. It is sent with the reliability type Unreliable so it can be handled immediately by the server. It is used for calculating the average ping between the client and the server.

Connected Pong message is sent in response to the Connected Ping message with reliability type set to Unreliable so it can be handled immediately by the client.

The first packet in the handshake sequence, sent by the client to the server.

The client sends these to the target server with ever decreasing MTU until the server responds. This is used to discover the MTU size for the connection. According to the official docs, "The MTU size is the maximum size of a packet RakNet will generate", and "if you set the MTU size larger than your or any router along the network path takes, then the network will split the packet at best, or drop it at worst", so this process of sending decreasingly padded packets (compensating for the size of the packet itself and UDP overhead, which together make 46 bytes) seem to be used to discover the maximum packet size the network can handle. See the official documentation page for more info about the MTU size. If the RakNet protocol does not match your own, respond with the Incompatible protocol packet.

You should always respond to the first of these packets that you receive with Open Connection Reply 1, containing an MTU size of the amount of padding you received in bytes plus 46.

The second packet in the handshake sequence, sent by the server to the client.

The third packet in the handshake sequence, sent by the client to the server.

The fourth packet in the handshake sequence, sent by the server to the client.

Note 1: there can be multiple frames in each Frame Set Packet.
Note 2: A packet is a Frame Set Packet if (packet_ID & 0x80) == 0x80 and (packet_ID & 0x60) == 0

The reliability types are as follows:

Sequenced implies ordered.

In Minecraft Bedrock Edition, the game packet contains bytes encoded according to the Bedrock Edition Protocol.

A NACK is sent by either the client or the server periodically to request packets that were not received. They are sent back only if they were marked with reliable (see Frame Set Packet).

An ACK sould be sent periodically to acknowledge every packet that was received.

Note: for both ACK and NACK packets, there can be multiple records.