Today, the world of computer networking uses one networking model: TCP/IP (TransmissionControl Protocol/Internet Protocol).
The TCP/IP model both defines and references a large collection of protocols that allow computers
to communicate. To define a protocol, TCP/IP uses documents called Requests for
Comments (RFC). (You can find these RFCs using any online search engine.)
To understand a networking model, each model breaks the functions into a small
number of categories called layers. Each layer includes protocols and standards that relate to
that category of functions. TCP/IP actually has two alternative models, as shown
in Figure 1
The top two layers focus more on the applications that need to send and
receive data. The bottom layer focuses on how to transmit bits over each individual link, with
the internet layer focusing on delivering data over the entire path from the original sending
computer to the final destination computer.
Many of you will have already heard of several TCP/IP protocols, like the examples listed in
Figure 2. Most of the protocols and standards in this table will be explained in more detail as
you work through this book. Following the table, this section takes a closer look at the layers of
the TCP/IP model.
Lets see each layer :
HTTP Application Layer
Berners-Lee gave HTTP functions to ask for the contents of web pages, specifically
by giving the web browser the ability to request files from the server, and giving the server a
way to return the content of those files.
TCP/IP Transport Layer
Although many TCP/IP application layer protocols exist, the TCP/IP transport layer includes a
smaller number of protocols. The two most commonly used transport layer protocols are the
Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).
To appreciate what the transport layer protocols do, you must think about the layer above the
transport layer, the application layer. Why? Well, each layer provides a service to the layer
above it, like the error-recovery service provided to application layer protocols by TCP.
TCP/IP needs a mechanism to guarantee delivery of data across a network. Because many application
layer protocols probably want a way to guarantee delivery of data across a network, the
creators of TCP included an error-recovery feature. To recover from errors, TCP uses the concept
of acknowledgments.
Very Important also!
TCP/IP Network Layer
IP provides several features, most importantly, addressing and routing. This section begins by
comparing IP’s addressing and routing with another commonly known system that uses addressing
and routing: the postal service.
Imagine that you just wrote two letters: one to a friend on the other side of the country and one
to a friend on the other side of town. You addressed the envelopes and put on the stamps, so
both are ready to give to the postal service. Is there much difference in how you treat each letter?
Not really. Typically, you would just put them in the same mailbox and expect the postal
service to deliver both letters.
The postal service, however, must think about each letter separately, and then make a decision
of where to send each letter so that it is delivered. For the letter sent across town, the people in
the local post office probably just need to put the letter on another truck.
For the letter that needs to go across the country, the postal service sends the letter to another
post office, then another, and so on, until the letter gets delivered across the country. At each
post office, the postal service must process the letter and choose where to send it next.
The TCP/IP application and transport layers act like the person sending letters through the postal
service. These upper layers work the same way regardless of whether the endpoint host computers
are on the same LAN or are separated by the entire Internet.
To send a message, these upper layers ask the layer below them, the network layer, to deliver the message.
The lower layers of the TCP/IP model act more like the postal service to deliver those messages
to the correct destinations. To do so, these lower layers must understand the underlying physical
network because they must choose how to best deliver the data from one host to another.
Internet Protocol Addressing Basics
IP defines addresses for several important reasons. First, each device that uses TCP/IP—each
TCP/IP host—needs a unique address so that it can be identified in the network. IP also defines
how to group addresses together, just like the postal system groups addresses based on postal
codes (like ZIP codes in the United States).
IP Routing Basics
Step 1, on the left of Figure 6, begins with Larry being ready to send an IP packet. Larry’s IP
process chooses to send the packet to some router—a nearby router on the same LAN—with
the expectation that the router will know how to forward the packet. (This logic is much like
you or me sending all our letters by putting them in a nearby post office box.) Larry doesn’t
need to know anything more about the topology or the other routers.
At Step 2, router R1 receives the IP packet, and R1’s IP process makes a decision. R1 looks at
the destination address (2.2.2.2), compares that address to its known IP routes, and chooses to
forward the packet to router R2. This process of forwarding the IP packet is called IP routing
(or simply routing).
At Step 3, router R2 repeats the same kind of logic used by router R1. R2’s IP process will compare
the packet’s destination IP address (2.2.2.2) to R2’s known IP routes and make a choice to
forward the packet to the right, on to Bob.
OSI MODEL
Even today, networking documents often describe TCP/IP protocols and standards by referencing
OSI layers, both by layer number and layer name. For example, a common description of a
LAN switch is “layer 2 switch,” with “layer 2” referring to OSI layer 2. Because OSI did have a
well-defined set of functions associated with each of its seven layers, if you know those functions,
you can understand what people mean when they refer to a product or function by its OSI
layer.
The only difference in these bottom four layers is the name of OSI Layer 3 (network)
compared to the original TCP/IP model (internet). The upper three layers of the OSI reference
model (application, presentation, and session—Layers 7, 6, and 5) define functions that all map
to the TCP/IP application layer.
OSI Reference Model Layer Definitions
Finally
While networking models use layers to help humans categorize and understand the many functions
in a network, networking models use layers for many reasons. For example, consider
another postal service analogy. A person writing a letter does not have to think about how the
postal service will deliver a letter across the country.
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