Networking 4 - TCP/IP Model

To understand TCP/IP Model you need to understand some basics.

Protocol - A protocol is a set of rules that govern how systems communicate. For networking they govern how data is transferred from one system to another.

Protocol Suite -  A protocol suite is a collection of protocols that are designed to work together.

Protocol Stacks - It is possible to write a single protocol that takes data from one computer application and sends it to an application on another computer.- A Single stack Protocol

The problem with this approach is that it very inflexible, as any changes require changing the entire application and protocol software.

The approach used in networking is to create layered protocol stacks.

Each level of the stack performs a particular function and communicates with the levels above and below it.

This layered arrangement is not confined to networking, and how it works is probably best understood if you compare it to real life example.

Lets take an example of a parcel service between two offices.

The task is simple – send parcels between people in each office.

We will divide the task into two distinct processes as follows:

1    Take a package, wrap it and address it.

2     Send it to the destination

At the receiving end

1   Receive the package

2  Deliver it to the recipient

    Typically you would have an internal mail man that:

    Collects the parcels from the senders and takes then to a mail dispatch room.

    The parcels are placed in a van by the dispatcher and then driven to the remote office.

At the remote office

1 The parcels are received by the dispatcher and placed into a tray for the mail man

2.The mail man collects the parcels and delivers them to the recipients.

The question really is what is the advantage of splitting the task into different layers/tasks?

The answer is that any of the layers/tasks can be changed without affecting the other layers.

So if for example, if we decide to use a train instead of a van to transport the messages between the offices we could do so without affecting the mail man.

In fact the mail man doesn’t know, and doesn’t care, how the parcels are transported between the offices, as all he does is collect them, and pass them to the delivery man.

Although this appears very simple, and maybe trivial, it does illustrate some very important points that are crucial when it comes to understanding networking protocols and how they are organised.

Layer 1, the physical layer, defines electrical aspects of activating and maintaining physical links in networks. The physical layer represents the basic network hardware, such as switches and routers.

Layer 2, the Data link layer, provides a reliable synchronization and transfer of information across the physical layer for accessing the transmission medium. Layer 2 specifies how packets access links and are attached to additional headers to form frames when entering a new networking environment, such as a LAN. Layer 2 also provides error detection and flow control.

Layer 3, the network layer (IP) specifies the networking aspects. This layer handles the way that addresses are assigned to packets and the way that packets are supposed to be forwarded from one end point to another.

Layer 4, the transport layer, lies just above the network layer and handles the details of data transmission. Layer 4 is implemented in the end-points but not in network routers and acts as an interface protocol between a communicating host and a network. Consequently, this layer provides logical communication between processes running on different hosts.

Layer 5, the application layer, determines how a specific user application should use a network. Among such applications are the Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP), and the World Wide Web (WWW).

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The transmission of a given message between two users is carried out by (1) flowing down the data through each and all layers of the transmitting end, (2) sending it to certain layers of protocols in the devices between two end points, and (3) when the message arrives at the other end, letting the data flow up through the layers of the receiving end until it reaches its destination. A message is transmitted from host 1 to host 2, and, all five layers of the protocol model participate in making this connection. The data being transmitted from host 1 is passed down through all five layers to reach router R1. Router R1 is located as a gateway to the operating regions of host 1 and therefore does not involve any tasks in layers 4 and 5. The same scenario is applied at the other end: router R2. Similarly, router R2, acting as a gateway to the operating regions of host 2, does not involve any tasks in layers 4 and 5. Finally at host 2, the data is transmitted upward from the physical layer to the application layer.

The main idea of the communication protocol stack is that the process of communication between two end points in a network can be partitioned into layers, with each layer adding its own set of special related functions. a different way of realizing protocol layers used for two hosts communicating through two routers. This figure illustrates a structural perspective of a communication set-up and identifies the order of fundamental protocol layers involved