7. Protocol Overview

Data Transfer

| Header |            Content            |
| A | B  | C | D | E | F | G | H | I | … |

All fields (header and content) are written in little-endian (a.k.a. Intel's byte order). This includes both numeric and string fields.

Header

The only field in a packet's header is a 2-byte unsigned integer, specifying the packet's total size.

| Header |            Content            |
|  Size  |                               |
| A | B  | C | D | E | F | G | H | I | … |

Thus, the largest packet size is 64K (65535 bytes), with 2 bytes reserved for size. The smallest valid packet is 2 bytes long and has no real meaning (as TCP makes keep-alive redundant).

Content

The packet's content is what server emulators typically call 'a packet'. The content starts with a unique dynamic-size prefix identifying the type of the packet, followed by the packet's actual content.

| Header |                Content                |
|  Size  |       Opcode(s)       |      Data     |
| A | B  | C | D | E | F | G | H | I | J | K | … |

Opcodes

If we put protocol versions that predate Prelude BETA (336) aside, then each 'packet' (emulator-wise) starts with 1 to 3 opcodes, where the 1st opcode is a single byte, 2nd – two bytes and 3rd – four bytes.

| Header |                       Content                       |
|  Size  | Opcode1 |  Opcode2  |    Opcode3    |      Data     |
| A | B  |    C    |  D  |  E  | F | G | H | I | J | K | L | … |

All transmitted data is enciphered. There are different protocol encryption schemes for login and for game server communications.