How Data Travels Between Devices Through the OSI Model

To demonstrate how a packet travels from Machine A’s OSI Model to Machine B’s OSI Mel, we want Machine A to contact the web service HTTP on Machine B. This is equivalent to opening a web browser and typing in the address of the website we’re trying to connect to e.g., http://www.nemyenterprises.com. This address is translated into the corresponding IP address of the form: A.B.C.D. by the Domain Name Server (DNS).

Alternately, we can establish the connection directly by using the IP address since IP addresses are the only way that computers and network devices can connect to one another. In our demonstration, we will request the IP address of the website that is hosted on Machine B (10.1.1.2). Here is what happens when the user on Machine A requests the website on Machine B by entering the following (Uniform Resource Locator) URL into his web browser: http://10.1.1.2.

Exploring the OSI Model from Machine A

Point of View:

1. When the user submits the request using a web browser, the application (a Web browser) passes the request to the Presentation Layer.
2. The Presentation Layer decodes the information that the user requested into a format that other layers will understand and then passes it on to the Session Layer.
3. The Session Layer creates a new session for this connection. Sessions are kept separate so connections don’t interact with each other; even if you have multiple requests for the same destination there will always be a separate session for each request. The session information will be passed on to the Transport Layer.
4. The Transport Layer checks the protocol used to initiate the request and the port numbers of the source and destination.

The protocol that was used to initiate the request is TCP. For the Transport Layer, the protocol that was used will be passed on to it by default because it is associated with the web browser software.

In our case, we are requesting a webpage over HTTP, so the Transport Layer will have Port 80 set for the destination port. For the source port, it could be any port that is available and not reserved for services; the operating system takes care of that so you don’t have to worry about it. For the sake of discussion, we will assume that the port number of the source Machine A is 44872. Once that information is set, the Transport Layer will pass the information on to the network layer.

• The Network Layer examines the IP addresses of the source (Machine A) and the destination (Machine B) and assigns the source IP address 10.1.1.1 and the destination

10.1.1.2. Always associate the source IP with the source port and the destination IP with the destination port, so we’ll associate IP address 10.1.1.1 with 44872 and IP address 10.1.1.2 with 80. Some engineers associate the IP address with the port in the following format: IP Address: Port Number. You should always associate IP Addresses with ports because they are always set together. When the destination IP receives the request, it forwards it to the destination Port 80 and so on.

Figure 1-5.             Packet traveling from one host to another using the OSI Model

• The Data Link Layer logs the source MAC address of Machine A and the destination MAC address of Machine B. Some might ask why you would need a MAC address if both machines can connect through the IP address. The answer will be explained and discussed later. For now, know that the MAC addresses of both the source and destination hosts must be logged. All of the information will be passed down to the last layer, the Physical Layer.
• The Physical Layer is the messenger that carries all of information that was passed down to it and delivers it to the destination IP address. The Physical Layer will contact the destination IP address and establish connection with it after examining the packet’s information. If the packet is allowed to pass through the firewall or any other device that inspects it, then it will pass to Machine B’s Physical Layer.

Second Stage: Machine B OSI Model

1. Machine B’s Physical Layer is the first layer in the OSI Model to receive the packet. This is why you need to make sure that cables and media carriers (physical layers) are always in a good shape (a well installed and maintained network cable will last for approximately 10 years). The Physical Layer passes the information up to the Data Link Layer.
2. The Data Link Layer accepts the packet (since the destination address matches it’s MAC Address) and forwards the traffic to the Network Layer.
3. The Network Layer accepts the traffic coming to the destination IP address (since it matches its IP address) and forwards the traffic to the Transport Layer.
4. The Transport Layer inspects the received packet and determines the protocol that was used inside it. Once it determines the packet’s TCP protocol, it will ask which port the traffic came over. When the packet shows Port 80 as the destination port, the transport later will decide that the service requested is HTTP. Before forwarding the traffic, the Transport Layer has to know the port number that will receive the traffic in the response back to Host A. The port that will receive the traffic back is 44872. Machine A will tell Machine B’s transport layer that any traffic you want to send back, please send it to Port 44872 on my end. This makes sense because if another port was chosen to send the traffic back, the newly selected port information won’t be logged into the Transport Layer, and thus the connection won’t be successful. The Transport Layer will pass the information to the Session Layer.
5. The Session Layer will establish a new session for the current packet and forward the information to the Presentation Layer.
6. The Presentation Layer decodes the received packet into a form that is understandable by the Application Layer and then it forwards the information to the Application Layer.
7. The Application Layer receives the packet information that requests a HTTP service and then forwards this service to the web server. The web server on Machine B will send an Acknowledgement to Machine A and wait for Machine A to Acknowledge back; then the connection will be established between the two hosts.

Take a moment to think about what we just talked about. This process happens hundreds of times in the background when two devices try to communicate. The big surprise is how fast it happens.