if (closed) throw IOException("closed")

    // Disable the timeout to read the first byte of a new frame.
    val b0: Int
    val timeoutBefore = source.timeout().timeoutNanos()
    source.timeout().clearTimeout()
    try {
      b0 = source.readByte() and 0xff
    } finally {
      source.timeout().timeout(timeoutBefore, TimeUnit.NANOSECONDS)
    }

    opcode = b0 and B0_MASK_OPCODE