Answer Posted / bharat bhushan
In an earlier section, there was an example where a chat
program was written to communicate between two servers. To
send data, the user (Tom) would type text into a dialog box,
hit send and the following happened:
1. The program passed Tom's typed text in a buffer, to
the socket.
2. The data was put inside a TCP data packet with a TCP
header added to the data. This header contained a source and
destination port number along with some other information
and a checksum.
3. The TCP packet was be placed inside an IP data packet
with a source and destination IP address along with some
other data for network management.
4. The IP data packet was placed inside an ethernet data
packet. This data packet includes the destination and source
address of the network interface cards (NIC) on the two
computers. The address here is the hardware address of the
respective cards and is called the MAC address.
5. The ethernet packet was transmitted over the network line.
6. With a direct connection between the two computers,
the network interface card on the intended machine,
recognized its address and grabbed the data.
7. The IP data packet was extracted from the ethernet
data packet.
8. The TCP data packet was extracted from the IP data packet.
9. The data was extracted from the TCP packet and the
program displayed the retrieved data (text) in the text
display window for the intended recipient to read.
In step 4 above, the IP data was going to be placed inside
an ethernet data packet, but the computer constructing the
packet does not have the ethernet address of the recipient's
computer. The computer that is sending the data, in order to
create the ethernet part of the packet, must get the
ethernet hardware (MAC) address of the computer with the
intended IP address. This must be accomplished before the
ethernet packet can be constructed. The ethernet device
driver software on the receiving computer is not programmed
to look at IP addresses encased in the ethernet packet. If
it did, the protocols could not be independent and changes
to one would affect the other. This is where address
resolution protocol (ARP) is used. Tom's computer sends a
network broadcast asking the computer that has the
recipient's IP address to send it's ethernet address. This
is done by broadcasting. The ethernet destination is set
with all bits on so all ethernet cards on the network will
receive the data packet. The ARP message consists of an
ethernet header and ARP packet. The ethernet header contains:
1. A 6 byte ethernet destination address.
2. A 6 byte ethernet source address.
3. A 2 byte frame type. The frame type is 0806
hexadecimal for ARP and 8035 for RARP
The encapsulated ARP data packet contains the following:
1. Type of hardware address (2 bytes). 1=ethernet.
2. Type of protocol address being mapped( 2 bytes). 0800H
(hexadecimal) = IP address.
3. Byte size of the hardware address (1 byte). 6
4. Byte size of the protocol address (1 byte). 4
5. Type of operation. 1 = ARP request, 2=ARP reply,
3=RARP request, 4=RARP reply.
6. The sender's ethernet address (6 bytes)
7. The sender's IP address (4 bytes)
8. The recipient's ethernet address (6 bytes)
9. The recipient's IP address (4 bytes)
When the ARP reply is sent, the recipient's ethernet address
is left blank.
In order to increase the efficiency of the network and not
tie up bandwidth doing ARP broadcasting, each computer keeps
a table of IP addresses and matching ethernet addresses in
memory. This is called ARP cache. Before sending a
broadcast, the sending computer will check to see if the
information is in it's ARP cache. If it is it will complete
the ethernet data packet without an ARP broadcast. Each
entry normally lasts 20 minutes after it is created. RFC
1122 specifies that it should be possible to configure the
ARP cache timeout value on the host. To examine the cache on
a Windows, UNIX, or Linux computer type "arp -a".
If the receiving host is on another network, the sending
computer will go through its route table and determine the
correct router (A router should be between two or more
networks) to send to, and it will substitute the ethernet
address of the router in the ethernet message. The encased
IP address will still have the intended IP address. When the
router gets the message, it looks at the IP data to tell
where to send the data next. If the recipient is on a
network the router is connected to, it will do the ARP
resolution either using it's ARP buffer cache or broadcasting.
Reverse Address Resolution Protocol (RARP)
As mentioned earlier, reverse address resolution protocol
(RARP) is used for diskless computers to determine their IP
address using the network. The RARP message format is very
similar to the ARP format. When the booting computer sends
the broadcast ARP request, it places its own hardware
address in both the sending and receiving fields in the
encapsulated ARP data packet. The RARP server will fill in
the correct sending and receiving IP addresses in its
response to the message. This way the booting computer will
know its IP address when it gets the message from the RARP
server.
| Is This Answer Correct ? | 11 Yes | 2 No |
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