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23 April 2012

A Guide To OSI Model

Credits: prtablegm[HF]

This guide is not meant to be all encompassing. It is a reference to give someone who is interested in network security, but does not know where to start, some guidance of how to begin learning about the vast field of network security and where to look for information. The important thing to keep in mind is stay focused and learn little by little. It is easy to become intimidated when looking at the big picture and all that it entails. Focus on each section and gradually increase your knowledge base.

[Image: osi_model_1.JPG]

OSI Layers Explained

The Open Systems Interconnection model (OSI model) is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a way of sub-dividing a communications system into smaller parts called layers. A layer is a collection of similar functions that provide services to the layer above it and receives services from the layer below it. (That part is a direct quote from Wikipedia for I could not say it better my self)

Physical Layer: The physical layer is at the bottom of this data networking model. It deals with crude data that is in the form of electrical signals. The data bits are sent as 0's and 1's. 0's correspond to low voltage signals and 1's correspond to high voltage signals. The mechanical aspects of communication, such as wires or connectors come under this layer. The physical layer also deals with how these wires, connectors, and voltage electrical signals work. Also, the process that is required for these physical aspects are taken into account in this layer itself. Read more on ethernet cable wiring.

The Data Link Layer: The transmission of the data over the communication medium is the responsibility of this layer. The 0's and 1's that are used in the communication are grouped into logical encapsulation. This encapsulation is called frames. The data is transported in frames. The responsibility of these frames is that of the data link layer.

Network Layer: All over the world, there are many different types of ethernets. These networks are connected to each other through various media. When a data packet wants to reach a particular destination, it has to traverse through these networks. Essentially, there are lot of operations that are taking place between the connected networks. Also, the packet data which is traversing, has to choose an optimum route, and the addressing of these packets has to be proper. The various operations between the networks, packet data issues, addressing and routing are handled by this network layer.

Transport Layer: The transport layer ensures quality and reliability of the communication. The data packet switching is entirely handled by the transport layer. There are basically two types of packet switching. They are connectionless packet switching and connection oriented packet switching. In connectionless packet switching, the packet data is allowed to choose the route in which it is going to reach the destination. Obviously, the packet in itself cant do this. Physical devices like routers are mainly responsible for the behavior of packets, but the packets formed from the same datum can reach their destination in different ways. Whereas, in connection oriented packet switching, once the route is decided, then all the packets have to follow the same route. Examples of connectionless packet switching are text messages in mobile phones, and the example of connection oriented switching is a direct voice call.

The Sessions Layer: The sessions layer is mainly responsible for creating, maintaining and destroying the communication link. PDU (Protocol Data Unit), in which various protocols are defined, that have to be followed during communication, are the responsibility of the sessions layer. The applications that use RPC's (remote procedure calls) are taken care of by the sessions layer.

Presentation Layer: There are various techniques of data compression which are used to send and receive the optimized data. For example, if certain data is repeating itself for a number of times, then it is logical to send the data only once, and specify the number of times it is repeated. This bundling of the repeated data is one of the techniques of compressions. The compression and decompression of the data is handled by the presentation layer. Also, encryption and decryption techniques used to thwart malicious attacks on data are handled by the presentation layer.

Application Layer: This is the topmost layer of the OSI reference model. This layer comes into picture when there is a process to process communication. Whenever a user invokes any application, all the associated processes are run. Many a times, when an application wants to communicate with another application, then there has to be communication between these associated processes. The application layer is responsible for this interprocess communication.

Thank you Keith Barker for this story.

King invites King to lunch.

Once upon a time, there were two Kings. King A and King B. They lived in separate kingdoms, kingdom A and kingdom B.

One day, King A, decided to invite King B to a lunch. No telephones were available, and King A doesn't have to ride a horse all the way over to kingdom B to personally invite King B, as both kings have a huge staff of 7 workers. So here is what happens for the invitation.

King A calls for staff member 7, the scribe (application layer), and the scribe writes down the message indicating that the local King A, wants to invite the remote King B to a lunch. The scribe puts this message into a container, such as an envelope. Staff member 7, the scribe, then hands it to the next staff member 6, the translator.

Staff member 6, the translator (presentation layer), simply ensures that the language that the scribe used, is appropriate for the language understood by King B?s staff on remote side. The translator puts the results in another envelope, and hands it to staff member 5, the negotiator.

Staff member 5, the negotiator (session layer), knows how to deal with people, and the rules for engagement for communicating with other kingdoms. He adds his 2 cents, and puts the results in yet another envelope. He hands this envelope to staff member 4, to the middle manager.

Staff member 4, the middle manager (transport layer) is very concerned about making sure that the envelope it received makes it all the way to kingdom B. In fact, if the envelope is quite large, the middle manager, staff member 4, may divide the envelope into 2, and label them 1 of 2, and 2 of 2 with the intent of getting an acknowledgment from the middle manager on the remote side (kingdom b), to verify that all the pieces got there. If the messages are not too important, the middle manager may just send the information, and not even ask for an acknowledgment. Depends on the rules for the specific message. Either way, the middle manager, hands the envelope(s) down to staff member 3, which is Google maps guy, who only has the ability to the world from a 5,000 foot view, and no closer.

Staff member 3, looking at the view of the countryside from 5,000 feet (network layer), and seeing that the envelope is to be delivered to kingdom b, sees that kingdom b is absolutely remote, (not local), and adds information regarding whether the message needs to travel North, South, East or West. In this example, lets say that staff member 3 made the decision that the message needs to go east. After adding that, he hands the envelope to staff member 2, which is the mail room guy.

Staff member 2, the mail room guy (data link layer), knowing that the message needs to go east, takes a look at all of his carriers, (FedEx, UPS, USPS, Carrier Pigeon, etc), and if there is only 1 that is going east, he prepares the message for that carrier. It if is FedEx, staff member 2 puts the message in the appropriate FedEx envelope (frame), and puts it in the pile for FedEx pickup.

FedEx isn't really a staff member, but if they were, they would be staff member 1 (physical layer), who takes the data and causes it to actually leave kingdom A, on its way (finally) to kingdom B.

Now, let's go on a road trip, and see what happens in Kingdom B, when the message arrives there.

The package, finally arrives at kingdom b, and is received at kingdom B's mail room and follows this process:

Mail room receives L1 via FedEx, or possibly another carrier that FedEx handed off to in transit.

Mail room (L2)verifies that the package is for us, at this castle based on the house (castle) number, and hands it to google maps guy at L3.

Google maps guy (L3), looks at the map and says, yup, this envelope doesn't need to be forwarded to any remote location, as it is destined to us. I will just remove the google map info, and hand it up to L4. (Note, if the message needed to go to a different street , the google maps guy would add new information, such as north, south, east or west, and push it back down to the mail room to be sent.)

Middle manager, (L4) is nervous, as the envelope says 1 of 2, and he wants to keep his job. He puts a sticky note on his monitor, that says look for envelope labeled 2 of 2, and if that second envelope doesn't arrive, he may take the initiative to send a little message back to the middle manager at kingdom A, asking for a resend of that information. As data does arrive, middle manager removes any information applied by the remote middle manager, (such as information regarding 1 of 2, and 2 of 2), and pushed the info up to L5.

The negotiator, (L5), understanding the politics and rules for engagement, accepts the contents of the envelope, using any special rules that the negotiator on the remote side may have imposed or recommended. Removing any L5 specific information, he pushes the rest of the data to L6.

The translator (L6) sees that the sender used English, and our local kingdom understands English. He also noticed that there was some short hand used by L6 on the remote side, so the local side decompresses that, and hands the results up to L7.

The scribe (L7) takes the message, approaches King B, and reads? King A, invites you to lunch!?

All that for a lunch invitation!

Any ideas on what process happens if King B responds?

Same logic on virtually all media types.

[Image: telecommunication_osi.gif]

Ways to remember the OSI Model

Please Do Not Trouble Steve's Pet Alligator
Please Do Not Throw Sausage Pizza Away
All People Seem To Need Dominos Pizza
American Pu**y Seems To Need Deeper Penetration
All People Seem To Need Data Processing
All Prostitutes Seem To Need Double Penetration

Yes it just keeps going. Whatever it takes to remember the OSI model.

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