Download Chapter 5

Mapping IP Addresses to
Hardware Addresses
Chapter 5
Introduction
• Two machines on a physical network can
communicate only if they know each other’s
hardware address
• Network connections are given IP addresses
• How does a host or a router map an IP address to a
hardware address so that a packet can be sent
across a network?
Address Resolution
• Generally, machine A (or a user on machine A)
knows the IP address of machine, but not the
hardware address
• The hardware addresses must be found for:
– the destination machine
– any intermediate routers along the path
• Mapping high-level (IP) addresses to hardware
addresses is called address resolution
Address Resolution
• How can address resolution be done?
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keep lookup tables in each machine
encode hardware addresses in high-level addresses
direct mapping
dynamic binding
Direct Mapping
• Recall proNet which had an interface board with 8
switches that could be set to values (addresses)
from 0 to 255
– 0 was not used
– 255 was for broadcasting
• Network administrators could assign their own
hardware addresses and have them correspond to
the last decimal digit in the IP address
– 129.7.61.15 corresponds to the machine with hardware
address of 15
Direct Mapping
• Choosing a numbering scheme means selecting a
function f that maps IP addresses to hardware
addresses
• On connection-oriented networks such as ATM,
one or more computers (servers) store pairs of
addresses as a table in memory for quick searching
• Software can use a hashing function to search the
table
Dynamic Binding
• TCP/IP’s approach is to use the Address
Resolution Protocol (ARP)
– When A wants to communicate with B in the same
network, A broadcasts a packet that asks the host with
IP address IB to respond with its hardware address PB
– All hosts , including B hear the request, but only B
replies with its hardware address
– When A receives the reply, it plugs the hardware
address in and sends the packet to B
• Why not just broadcast the message?
ARP Cache
• Recently acquired IP to hardware address bindings
are stored in a cache on each computer. Why?
• Thus, when two computers on a network
communicate:
– ARP request
– ARP reply
– repeated transfer of packets
• The term soft state describes the situation in which
information becomes incorrect, or not helpful
– information is deleted after a time
ARP Cache
• Advantages
– ?
• Disadvantages
– Delay of knowing that a machine is not responding
ARP Refinements
• If A requests a hardware address for B, B will
likely need a hardware address for A
– Have A send its hardware address to B
– Put A’s information in the cache as well
• All computers on the network can update their
caches when they see the mappings on the line
• A newly added computer can do an ARP broadcast
when it boots, to notify others
ARP Implementation
• Two functions
– Mapping an IP @ to a hardware @
– Answering requests from other machines
• Steps:
– Upon reception of request, check cache
– If in cache, place data in a frame and send the frame
– If not in cache, broadcast ARP request, wait for reply
ARP Implementation Problems
• The target machine could be down, or too busy to
respond. What happens?
• A binding has been made for a machine, but then
it goes down. This is why cache is short-lived.
• Delays
• See bottom of p. 83:
“the ARP software removes packets from the queue,
places each packet in a frame, and …”
ARP Encapsulation
• When ARP messages are sent from one machine
to another, they are sent in frames
• What are frames? See Figure 2.7-Frame type 0806
ARP message
frame header
frame data area
ARP Protocol Format
• See ARP message format in Figure 5.3
• This shows the 28-octet format used on Ethernet
hardware (48 bits) when resolving IP addresses
(32 bits)
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Hardware type: 1 for Ethernet
Protocol type: 080016 for IP addresses ...
Sender Hardware Address (6 octets), IP (4 octets)
Target Hardware Address (6 octets), IP (4 octets)
• The length of fields depends on the hardware and
protocol address lengths
Summary
• IP addresses are assigned independent of hardware
addresses
• To send a packet, a computer must know the
other’s hardware address
• ARP broadcasts a message asking for the
computer with a given IP address to respond with
its hardware address
For Next Time
• Read Chapter 6