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Determining an Internet Address at Startup

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

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Determining an Internet Address at Startup

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  1. Determining an Internet Address at Startup Chapter 6

  2. 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

  3. 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

  4. 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 ...

  5. 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

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

  7. 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

  8. 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

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

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