Determining an Internet Address at Startup Chapter 6

Introduction Application programs specify a destination using the IP address Usually a computer’s IP address is kept on its secondary storage where the operating system finds it at startup What if a machine does not have a disk? Workstations that store files on a remote server? Such machines need an IP address before they can use TCP/IP file transfer protocols to get their initial boot image

Introduction Bootstrapping code found in ROM is usually built so that the same image can run on many machines Therefore, an IP address will not be put into the operating system code The bootstrapping code uses the network to contact a server and get the computer’s IP @ The machine can use its hardware @ to get this information from its own network When it has an IP @ it can reach an internet

Reverse ARP Problem: Given a hardware address, allow a server to map it to an internet address Uses the same message format in Figure 5.3, encapsulated in the data portion of a frame-Fig 2.7 E dest E src RARP msg preamble 803516 ...

Reverse ARP The sender broadcasts a RARP request and puts its hardware @ in the target hardware @ field Those authorized will reply (RARP servers) See Figure 6.1 Servers answer requests by filling in the target protocol @ field, and change the message type to reply

Reverse ARP What happens if the RARP is lost? Retry Announce failure after a short time to avoid annoying broadcasts

Primary and Backup RARP Servers Having several RARP servers makes the system more reliable If one is unavailable or busy, another could respond Disadvantage - many respond, much traffic Solution 1: Use a primary server which responds first; if machine times out and tries again, other (non-primary) servers respond to second request Solution 2: Non-primary servers respond after random time; usually respond after primary and not together

Summary At system startup, a computer without its own permanent storage must contact a server to find its IP @ before it can communicate with TCP/IP The computer can communicate on the local network using its hardware @ Using RARP, a server supplies an internet address Once it receives the IP @, it stores it in memory and does not use RARP again until it reboots

For Next Time Read Chapter 7 Make the equivalent of 15-20 slides outlining Chapter 7 Quiz over Chapter 7