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This article discusses the benefits and inner workings of the Windows NT 4.0 Remoteboot utility.
The Remoteboot ProcessWhen you start a computer, the operating system is loaded into memory. When a Remote Initial Program Load (RPL) ROM chip is placed on a network interface card (NIC), the client computer will retrieve startup and configuration software from the server when the client starts. This RPL ROM interrupts the computer's standard startup process and redirects the calls for the operating system to the NIC. The RPL ROM also establishes a basic network connection between the workstation and server during the startup process. This process is known as booting remotely, or the remoteboot process.
How Remoteboot WorksThe server provides two kinds of services to the remoteboot client. The first service is called a Boot Block, which contains all the information needed to start the client. The second service is called the remoteboot profile. This profile defines the client's operating system environment after the initial startup process. Both services are sent across the network in frames. These frames consist of a boot request, boot acknowledgment, boot block request, and the boot block.
When the client computer is switched on, the RPL ROM initializes and broadcasts a Boot Request frame (FIND frame), which contains the adapter's media access control address (adapter ID). The adapter ID is assigned by the NIC's manufacturer, and is a unique 12-digit hexadecimal number. The first six digits identify the adapter and the manufacturer. When the server receives the FIND frame, it compares the adapter ID to the database of known adapters. If the server does not recognize the adapter ID or if the adapter ID does not have an assigned profile, the server records the information temporarily in its database but does not respond to the client's request. The client continues to send FIND frames until it receives an acknowledgment (ACK).
If the server has a match for the adapter ID with a valid profile, it responds with an acknowledgment (FOUND frame). This frame contains the server's adapter ID and the client's adapter ID. At this point, the DLC protocol is being used for all communications between the client and the server. If multiple Remoteboot servers reply to the initial FIND broadcast, the client only responds to the first received ACK.
After the client receives the ACK, it then sends a request for the boot block request (SEND.FILE.REQUEST frame) to the first server that replied. The server receives the request and replies with a boot block (FILE.DATA.RESPONSE frame) telling the client where the boot image file is located.
After the client receives this information, it passes control from the RPL ROM and starts running the entry point of the boot block. At this point the operating system is being loaded into memory, and the system switches from DLC to the real-mode network drivers (usually NetBEUI).
The Windows 95 Remote Boot ProcessWindows 95 uses the same process, except for a few differences.
Windows 95 creates a RAM disk to act as a hard disk drive, and then copies the real-mode files from the server to the RAM disk. After the information has been loaded into the RAM disk, it loads the real-mode drivers and connects to the server, which contains a server-based setup and computer directory for Windows 95.
Why Use Remoteboot?Remoteboot allows for a secured network by eliminating the need for hard or floppy disk drives. Users cannot load programs or infect the network with viruses. It also allows greater control over the distribution of software. In addition, centralized software updates reduce costs.