import struct

LIMIT = 1024

def encode(payload):
    if len(payload) > LIMIT:
        raise ValueError("frame too large")
    return struct.pack("!I", len(payload)) + payload

def decode(buffer):
    if len(buffer) < 4:
        return None
    size = struct.unpack("!I", buffer[:4])[0]
    if size > LIMIT:
        raise ValueError("frame too large")
    if len(buffer) < 4 + size:
        return None
    return buffer[4:4 + size], buffer[4 + size:]

assert decode(encode(b"hello")) == (b"hello", b"")
assert decode(encode(b"hello")[:6]) is None