Bootstrap Protocol

The BOOTP was first defined in September 1985 by Bill Croft and John Gilmore as a replacement for the Reverse Address Resolution Protocol (RARP), published in June 1984. The primary motivation for replacing RARP with BOOTP is that RARP was a link layer protocol. This made implementation difficult on many server platforms, and required that a server be present on each individual IP subnet. BOOTP introduced the innovation of relay agents, which forwarded BOOTP packets from the local network using standard IP routing, so that one central BOOTP server could serve hosts on many subnets. An increasing set of BOOTP vendor information extensions was defined to supply BOOTP clients of relevant information about the network, like default gateway, name server IP address, the domain name, etcetera. With the advent of the Dynamic Host Configuration Protocol, the BOOTP vendor information extensions were incorporated as DHCP option fields, to allow DHCP servers to also serve BOOTP clients. ==Operation==

Case 1: Client and server on same network When a BOOTP client is started, it has no IP address, so it broadcasts a message containing its MAC address onto the network. This message is called a “BOOTP request”, and it is picked up by the BOOTP server, which replies to the client with the following information that the client needs: • The client's IP address, subnet mask, and default gateway address. • The IP address and host name of the BOOTP server. • The IP address of the server that has the boot image, which the client needs to load its operating system. When the client receives this information from the BOOTP server, it configures and initializes its TCP/IP protocol stack, and then connects to the server on which the boot image is shared. The client loads the boot image and uses this information to load and start its operating system. The Dynamic Host Configuration Protocol (DHCP) was developed as an extension of BOOTP. BOOTP is defined in Requests for Comments (RFC) 951 and 1084. Case 2: Client and server on different networks • Problem with the bootp request is that the request is broadcast. A broadcast IP datagram cannot pass through any router. The router discards this packet. • To solve this problem, there is a need for an intermediary (relay). • One of the host or router can be configured at application layer to operate as relay agent. • The relay agent knows the uni-cast address of bootp server and listens for broadcast message on port 67. • When it receives this broadcast packet, it encapsulates the message in unicast datagram and sends request to bootp server. • The packet carrying a unicast destination address is routed by any router and reaches the bootp server. • The relay agent, after receiving the reply, sends it to bootp client. ==IETF standards documentation==