I’ve decided to group the top three layers together into one post. This is because these are more related to the data to be transmitted across the network, rather than the underlying transport mechanisms themselves. These three layers deal with the semantics of the communication, such as who the data will be sent to, the format of the data and the etiquette to be adhered too between the communicating nodes.
The Pirate Ship: As with most technical concepts, analogies can help us understand the underpinning processes which are happening as part of the communication. For this series, I’m going to use the following analogy: I work in an office in Manchester and I’d like to send a pirate ship made of Lego to a friend, Rich, who works in an office in London. In our day to day lives, that’s a pretty simple concept that requires a couple of addresses and a postal service. Communicating data across a network can occur in much the same way. Let’s step through the network stack to see how.
Application Layer (OSI Layer 7)
The application layer is the piece of the puzzle that is going to sit closest to our end user application. It is worth mentioning here that when we are referring to the services in this layer we are not referring to the actual application being used by the end-user. To expand on this, an example application being used might be the AnyCo ERP solution. That ERP solution may provide the capability to send reports via “email”. So it’s actually the email service which fits into the application layer, not AnyCo ERP. AnyCo ERP would sit outside of the OSI stack in an upper out of scope layer. Other application services you might find in the application layer, might be “File Transfer”, “Web Access” or “Network Management” services.
This layer is primarily responsible for determining suitable communication partner nodes and their identities. It also responsible for ensuring that the relevant resources are available to send the transmission. It’s in this layer that the aforementioned X.400 protocol exists. Synchronisation of communication is also dealt with at this level.
The Pirate Ship: In our scenario, The Layer 7 service we want to use is Lego Sending. I have established that Rich is a suitable communication partner as he has advertised that he likes Lego and can accept that type of toy. I’ve also established that the postal services have capacity and is suitable for sending the pirate ship.
The Presentation Layer (OSI Layer 6)
Now that we have established a suitable place to send my data and that the relevant network resources are in place to do that. We need to look at what exactly we are going to send. The Presentation layer deals with the format of the data, it is there to abstract the meaning of the data as the application sees it into a standardised format that can be used by the underlying network layers. Where an application may be providing freeform text, the network needs a way of encoding it. An example of a protocol working at this level is XML. Encryption may also happen at this level.
The Pirate Ship: Let’s think of the presentation of our pirate ship as a set of Lego bricks stuck together in a specific arrangement. The bricks are of standard sizes, colours and shapes. It’s those attributes that make up the format of the data.
The Session Layer (OSI Layer 5)
This is the layer responsible for setting up and tearing down the connection that will be used to transmit the data. It should be thought of as something that is more persistent than a single transmission of data. It is not responsible for actually sending the data. It simply executes the steps required to set up and maintain a connection. These steps might be simple requests for resources or handshakes between devices to be traversed. During a session, you might for example authenticate with a website and create a session. From there you may download lots of different files using the same session. For our purposes, we’ll keep it simple.
The Pirate Ship: I call my postal service to tell them I’m going to send a package to my friend. They verify my account number and then book slots for the package on all the the vehicles which will be traversed between my office and Rich’s.
So far, via analogy, we have established the objective of our application and the Layer 7 application service (Lego Sending). We have found the identities of our destination communication partner, ensure that the relevant resources are in place to send our lego and called the postal service to set-up the relevant connections to start the communication. Next well see what happens when we actually start sending the Lego.