Transcript Chapter 6

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
preamble E dest
E src
803516
RARP msg
...
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 (nonprimary) 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