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IP Overview and Key Operational Characteristics
(Page 1 of 2)
The Internet Protocol (IP) is the
core of the TCP/IP protocol suite and its main protocol at the network
layer. The network layer is primarily
concerned with the delivery of data, not between devices on the same
physical network, but between devices that may be on different networks
that are interconnected in an arbitrary manner: an internetwork.
IP is the mechanism by which this data is sent on TCP/IP networks. (It
does have help from other protocols at the network layer too, of course!)
at the TCP/IP layer model and consider
what IP does from an architectural standpoint. As the layer three protocol,
it provides a service to layer four in the TCP/IP stack, represented
mainly by the TCP
and UDP protocols. This service is to
take data that has been packaged by either TCP or UDP, manipulate it
as necessary, and send it out. This service is sometimes called internetwork
datagram delivery, as shown in Figure 54.
As we will see, there are many details to how exactly this service is
accomplished, but in a nutshell, that's what IP does: sends data from
point A to point B over an internetwork of connected networks.
Figure 54: The Main Function of IP: Internetwork Datagram Delivery
The fundamental job of the Internet Protocol is the delivery of datagrams from one device to another over an internetwork. In this generic example, a distant client and server communicate with each other by passing IP datagrams over a series of interconnected networks.
Key Concept: While the Internet Protocol has many functions and characteristics, it can be boiled down to one primary purpose: the delivery of datagrams across an internetwork of connected networks.
Key IP Characteristics
Of course there are a myriad of ways
in which IP could have been implemented in order to accomplish this
task. To understand how the designers of TCP/IP made IP work, let's
take a look at the key characteristics used to describe IP and the general
manner in which it operates. The Internet Protocol is said to be:
- Universally-Addressed: In order to send
data from point A to point B, it is necessary to ensure that devices
know how to identify which device is point B. IP defines
the addressing mechanism for the network and uses these addresses for
- Underlying-Protocol Independent: IP is
designed to allow the transmission of data across any type of underlying
network that is designed to work with a TCP/IP stack. It includes provisions
to allow it to adapt to the requirements of various lower-level protocols
such as Ethernet
802.11. IP can also run on the special
data link protocols SLIP and PPP that
were created for it. An important example is IP's ability to fragment
large blocks of data into smaller ones to match the size limits of physical
networks, and then have the recipient reassemble the pieces again as
- Delivered Connectionlessly: IP is a connectionless
protocol. This means that when A wants to send data to B, it doesn't
first set up a connection to B and then send the datait just makes
the datagram and sends it. See
the topic in the networking fundamentals section on connection-oriented
and connectionless protocols for more
information on this.
- Delivered Unreliably: IP is said to be
an unreliable protocol. That doesn't mean that one day your
IP software will decide to go fishing rather than run your network.
J It does mean
that when datagrams are sent from device A to device B,
device A just sends each one and then moves on to the next. IP
doesn't keep track of the ones it sent. It does not provide reliability
or service quality capabilities such as error protection for the data
it sends (though it does on the IP header), flow control or retransmission
of lost datagrams.
For this reason, IP is sometimes called a best-effort protocol.
It does what it can to get data to where it needs to go, but makes
no guarantees that the data will actually get there.
- Delivered Without Acknowledgments: In
a similar manner to its unreliable nature, IP doesn't use acknowledgements.
When device B gets a datagram from device A, it doesn't
send back a thank you note to tell A that the datagram
was received. It leaves device A in the dark so to
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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