RakNet

RakNet is the networking protocol that Roblox uses. It uses UDP as a base, and provides out-of-order delivery compensation and reliable transmission.

Thank you to https://wiki.vg for hosting this information (I largely got it from [1])

Data Types

RakNet has specific data types. Some of them you may recognize, but there are unique types that RakNet will use

Type	sizeof(Type)	Notes
byte	1 byte
bool	1 byte	True is 0x01, false is 0x00
long	8 bytes
magic	16 bytes	OFFLINE_MESSAGE_DATA_ID (00ffff00fefefefefdfdfdfd12345678?)
short/unsigned short	2
string	sizeof(unsigned short) + strlen(string) bytes	Strings are perl-style, which means they have a short at the beginning of the string which indicates the size of said string, and then after that contains string data
address	7 bytes	First byte encodes the IP Version (4 or 6), followed by the IP address stored in bytes
uint24le	3 bytes	3-byte little-endian unsigned integer

Connection Handshake

1. Client sends the server a Open Connection Request 1.
2. Server sends the client a Open Connection Reply 1.
3. Client sends the server a Open Connection Request 2.
4. Server sends the client a Open Connection Reply 2.

The RakNet connection is established at this point, and all RakNet messages are encoded within a Frame Set Packet.

1. Client sends the server a Connection Request.
2. Server sends the client a Connection Request Accepted.
3. Client sends the server a New Incoming Connection.

Packets afterwards are Game Packets.

Packets

Open Connection Request 1 (Packet 0x05)

Field Name	Field Type	Notes
Magic	magic
Version	byte	I dont know what it is on Roblox
MTU	Padding	The server will send a MTU back around the size of this MTU's padding?