Sunken records

S2

StarCraft II's native service connection: records packed bit by bit, with no length field.

Every fact on this page comes from one source, so it is all ⚠️ single source until checked elsewhere.

Sunken is a plain TCP connection, separate from Front. After sign-in hands over to it, it's encrypted with RC4, with a separate key state for each direction. Once decrypted, it's a stream of bit-packed records.

Reading bits

The decoder keeps one cursor for the whole record and reads each byte from bit 0 (lowest) to bit 7, then moves to bit 0 of the next byte. A value can cross a byte boundary. When it does, the bits read first become the higher-order part of the value.

read(width):
    value = 0
    while width > 0:
        take  = min(width, bits left in current byte)
        chunk = (current byte >> current bit) & ((1 << take) - 1)
        value = (value << take) | chunk
        advance cursor by take; width -= take

The route: what every record starts with

Bits Field Meaning
6 Command The command within the service.
1 Service flag 1: a service record follows. 0: a command response.
4 Service slot Only when the flag is 1. Which service, such as slot 5 for Chat.

So a service record's route is 11 bits, and its payload starts at bit 3 of the second byte.

A command response has no slot and no payload: 6 command bits, the zero flag, then a 9-bit result. That's exactly 16 bits, always two bytes.

Where a record ends

Records have no length field. The route, together with the direction, selects the payload's schema, and decoding that schema moves the cursor to the end of the payload. Then:

  1. Skip zero bits up to the next byte boundary. They must be zero.
  2. The next byte starts the next record.
Payload part Bits it uses
Fixed-width value The width given by its schema
Array, string or blob A count, then that many elements or bytes. Strings and blobs may first align to a byte.
Optional value One presence bit, then the value if present
Choice A selector, then the chosen value
Structure Each member in turn, including any filler bits

Counts are stored relative to their minimum. A count allowed to be 1 to 64 is stored as the count minus 1, in 6 bits. A count with a fixed value uses no bits at all.

Because the schema decides where a record ends, a client can't skip a record it doesn't understand. StarCraft II's schemas are compiled into the game, using Blizzard's BSN format. SC2Docs documents about 592 of these types.

Worked example: leaving a chat channel

The client sends Chat command 2, LeaveRequest, for channel 6. It's two bytes: 42 35.

Where Bits Value Meaning
Byte 0, bits 0–5 6 2 Command 2
Byte 0, bit 6 1 1 A service slot follows
Byte 0, bit 7, then byte 1, bits 0–2 1 + 3 0 then 101 = 5 Slot 5: Chat
Byte 1, bits 3–5 3 110 = 6 channelIndex = 6
Byte 1, bits 6–7 2 00 Padding to the byte boundary

Route (11 bits) + payload (3 bits) + padding (2 bits) = 16 bits = 2 bytes.

Sources and credits

Categories: Battle.net 2.0
Last edited September 16, 2026 by John Leighow.
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