OSI Model Flashcards
What is the OSI Model
Open Systems Interconnection Model.
The OSI model consists of seven layers. Each layer has a different set of responsibilities and is arranged from Layer 7 to Layer 1.
7. Application
6. Presentation
5. Session
4. Transport
3.Network
2.Data Link
1. Physical
What does the Application layer do?
The application layer is the layer in which protocols and rules are in place to determine how the user should interact with data sent or received.
Everyday applications such as email clients, browsers, or file server browsing software such as FileZilla provide a friendly, Graphical User Interface (GUI) for users to interact with data sent or received. Other protocols include DNS (Domain Name System), which is how website addresses are translated into IP addresses.
What does the Presentation Layer do?
This layer acts as a translator for data to and from the application layer (layer 7). The receiving computer will also understand data sent to a computer in one format destined for in another format. For example, when you send an email, the other user may have another email client to you, but the contents of the email will still need to display the same.
Security features such as data encryption (like HTTPS when visiting a secure site) occur at this layer.
What does the Session Layer do
Once data has been correctly translated or formatted from the presentation layer (layer 6), the session layer (layer 5) will begin to create a connection to the other computer that the data is destined for. When a connection is established, a session is created. Whilst this connection is active, so is the session.
The session layer (layer 5) synchronises the two computers to ensure that they are on the same page before data is sent and received. Once these checks are in place, the session layer will begin to divide up the data sent into smaller chunks of data and begin to send these chunks (packets) one at a time. This dividing up is beneficial because if the connection is lost, only the chunks that weren’t yet sent will have to be sent again — not the entire piece of the data (think of it as loading a save file in a video game).
What is worthy of noting is that sessions are unique — meaning that data cannot travel over different sessions, but in fact, only across each session instead
What does the Transport layer do
Layer 4 of the OSI model plays a vital part in transmitting data across a network and can be a little bit difficult to grasp. When data is sent between devices, it follows one of two different protocols that are decided based upon several factors:
TCP
UDP
What is TCP
The Transmission Control Protocol (TCP). Potentially hinted by the name, this protocol is designed with reliability and guarantee in mind. This protocol reserves a constant connection between the two devices for the amount of time it takes for the data to be sent and received.
Not only this, but TCP incorporates error checking into its design. Error checking is how TCP can guarantee that data sent from the small chunks in the session layer (layer 5) has then been received and reassembled in the same order.
TCP is used for situations such as file sharing, internet browsing or sending an email. This usage is because these services require the data to be accurate and complete (no good having half a file!).
What is UDP
User Datagram Protocol (or UDP for short). This protocol is not nearly as advanced as its brother - the TCP protocol. It doesn’t boast the many features offered by TCP, such as error checking and reliability. In fact, any data that gets sent via UDP is sent to the computer whether it gets there or not. There is no synchronisation between the two devices or guarantee; just hope for the best, and fingers crossed
What are the advantages of TCP
Guarantees the accuracy of data.
Capable of synchronising two devices to prevent each other from being flooded with data.
Performs a lot more processes for reliability
What are the disadvantages of TCP
Requires a reliable connection between the two devices. If one small chunk of data is not received, then the entire chunk of data cannot be used and must be re-sent.
A slow connection can bottleneck another device as the connection will be reserved on the other device the whole time.
TCP is significantly slower than UDP because more work (computing) has to be done by the devices using this protocol.
What are the advantages of UDP
UDP is much faster than TCP.
UDP leaves the application layer (user software) to decide if there is any control over how quickly packets are sent.
UDP does not reserve a continuous connection on a device as TCP does.
What are the disadvantages of UDP
UDP doesn’t care if the data is received.
It is quite flexible to software developers in this sense.
This means that unstable connections result in a terrible experience for the user.
What is the Network Layer
Whilst some protocols at this layer determine exactly what is the “optimal” path that data should take to reach a device, we should only know about their existence at this stage of the networking module. Briefly, these protocols include OSPF (Open Shortest Path First) and RIP (Routing Information Protocol). The factors that decide what route is taken is decided by the following:
What path is the shortest? I.e. has the least amount of devices that the packet needs to travel across.
What path is the most reliable? I.e. have packets been lost on that path before?
Which path has the faster physical connection? I.e. is one path using a copper connection (slower) or a fibre (considerably faster)?
At this layer, everything is dealt with via IP addresses such as 192.168.1.100. Devices such as routers capable of delivering packets using IP addresses are known as Layer 3 devices — because they are capable of working at the third layer of the OSI model.
Think packets.
What is the Data Link Layer
The data link layer focuses on the physical addressing of the transmission. It receives a packet from the network layer (including the IP address for the remote computer) and adds in the physical MAC (Media Access Control) address of the receiving endpoint. Inside every network-enabled computer is a Network Interface Card (NIC) which comes with a unique MAC address to identify it.
MAC addresses are set by the manufacturer and literally burnt into the card; they can’t be changed – although they can be spoofed. When information is sent across a network, it’s actually the physical address that is used to identify where exactly to send the information.
Additionally, it’s also the job of the data link layer to present the data in a format suitable for transmission.
Think Frames
What is the Physical Layer
This layer is one of the easiest layers to grasp. Put simply, this layer references the physical components of the hardware used in networking and is the lowest layer that you will find. Devices use electrical signals to transfer data between each other in a binary numbering system (1’s and 0’s). Think Bits and Bits
What is a frame
A frame is at layer 2 - the data link layer, meaning there is no such information as IP addresses.
TCP/IP packets contain various sections of information known as headers that are added from encapsulation. Name them
Source Port, Destination Port, Source IP, Destination IP, Sequence Number, Acknowledgement Number, Checksum, Checksum, and Flag
As a TCP/IP Header, what is a Source Port
This value is the port opened by the sender to send the TCP packet from. This value is chosen randomly (out of the ports from 0-65535 that aren’t already in use at the time).
As a TCP/IP Header, what is a Destination Port
This value is the port number that an application or service is running on the remote host (the one receiving data); for example, a webserver running on port 80. Unlike the source port, this value is not chosen at random.
As a TCP/IP Header, what is a Source IP
This is the IP address of the device that is sending the packet.
As a TCP/IP Header, what is a sequence number
When a connection occurs, the first piece of data transmitted is given a random number. We’ll explain this more in-depth further on.
As a TCP/IP Header, what is an Acknowledgement Number
After a piece of data has been given a sequence number, the number for the next piece of data will have the sequence number + 1. We’ll also explain this more in-depth further on.
