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Packets
It
turns out that everything you do on the Internet involves
packets. For example, every Web page that you receive comes
as a series of packets, and every e-mail you send leaves as
a series of packets. Networks that ship data around in small
packets are called packet switched networks.
On the
Internet, the network breaks an e-mail message into parts of
a certain size in bytes. These are the packets. Each packet
carries the information that will help it get to its
destination -- the sender's IP address, the intended
receiver's IP address, something that tells the network how
many packets this e-mail message has been broken into and
the number of this particular packet. The packets carry the
data in the protocols that the Internet uses: Transmission
Control Protocol/Internet Protocol (TCP/IP). Each packet
contains part of the body of your message. A typical packet
contains perhaps 1,000 or 1,500 bytes.
Each
packet is then sent off to its destination by the best
available route -- a route that might be taken by all the
other packets in the message or by none of the other packets
in the message. This makes the network more efficient.
First, the network can balance the load across various
pieces of equipment on a millisecond-by-millisecond basis.
Second, if there is a problem with one piece of equipment in
the network while a message is being transferred, packets
can be routed around the problem, ensuring the delivery of
the entire message.
Depending on the type of network, packets may be referred to
by another name:
frame
block
cell
segment
Most packets are split into three parts:
header - The header contains instructions about the data
carried by the packet. These instructions may include:
Length of packet (some networks have fixed-length packets,
while others rely on the header to contain this information)
Synchronization (a few bits that help the packet match up to
the network)
Packet number (which packet this is in a sequence of
packets)
Protocol (on networks that carry multiple types of
information, the protocol defines what type of packet is
being transmitted: e-mail, Web page, streaming video)
Destination address (where the packet is going)
Originating address (where the packet came from)
payload - Also called the body or data of a packet. This is
the actual data that the packet is delivering to the
destination. If a packet is fixed-length, then the payload
may be padded with blank information to make it the right
size.
trailer - The trailer, sometimes called the footer,
typically contains a couple of bits that tell the receiving
device that it has reached the end of the packet. It may
also have some type of error checking. The most common error
checking used in packets is Cyclic Redundancy Check (CRC).
CRC is pretty neat.
Here
is how it works in certain computer networks: It takes the
sum of all the 1s in the payload and adds them together. The
result is stored as a hexadecimal value in the trailer. The
receiving device adds up the 1s in the payload and compares
the result to the value stored in the trailer. If the values
match, the packet is good. But if the values do not match,
the receiving device sends a request to the originating
device to resend the packet.
As an
example, let's look at how an e-mail message might get
broken into packets. Let's say that you send an e-mail to a
friend. The e-mail is about 3,500 bits (3.5 kilobits) in
size. The network you send it over uses fixed-length packets
of 1,024 bits (1 kilobit). The header of each packet is 96
bits long and the trailer is 32 bits long, leaving 896 bits
for the payload. To break the 3,500 bits of message into
packets, you will need four packets (divide 3,500 by 896).
Three packets will contain 896 bits of payload and the
fourth will have 812 bits. Here is what one of the four
packets would contain:
Each
packet's header will contain the proper protocols, the
originating address (the IP address of your computer), the
destination address (the IP address of the computer where
you are sending the e-mail) and the packet number (1, 2, 3
or 4 since there are 4 packets). Routers in the network will
look at the destination address in the header and compare it
to their lookup table to find out where to send the packet.
Once the packet arrives at its destination, your friend's
computer will strip the header and trailer off each packet
and reassemble the e-mail based on the numbered sequence of
the packets.
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