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
The machine can use its to get this information from its own network
When it has an 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 in the target field
Those authorized will reply (RARP servers)
See Figure 6.1
Servers answer requests by filling in the target 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 before it can communicate with TCP/IP
The computer can communicate on the local network using its
Using RARP, a server supplies an internet address
Once it receives the 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 slides outlining Chapter 7
Quiz over Chapter 7