The OSI Model is the architectural model for all network communications. The seven layers are representative of the different protocols and services that are necessary to establish end-to-end connectivity between hosts. You have to have a thorough understanding of the OSI Model in order to establish a root-cause and troubleshoot connectivity errors.
Application Layer 7
This is where end-user interaction takes place. It serves as the bridge between the user and the Presentation Layer. This is where services, such as http, ssh, and ftp, are run. Do note, this doesn’t include things you might traditionally think of as ‘applications’ such as your browser. The OSI Model only goes as far as the background services that those ‘applications’ rely on to function.
Presentation Layer 6
Here, code is translated so that it can be interpreted by the Application Layer. For example, EBCDIC would be translated to ASCII. This layer also handles data compression and encryption. Protocols such as SSL or TLS would be associated with layer 6, but protocol suites like IPsec or OpenVPN would belong to layer 3.
Session Layer 5
Host to host communication is handled at the Session Layer. Sessions are created, managed, and ended at this layer. This is where the configuration of simplex, half-duplex, and full-duplex is applied. Session-layer tools, such as NetBIOS, TCP/IP Sockets, and remote procedure calls, are able provide communication to higher layer protocols.
Transport Layer 4
The protocols in the Transport Layer are responsible for packaging and unpacking the data as its sent and received. These protocols provide end-to-end transport services between hosts. The most common of these protocols are TCP and UDP. TCP is considered connection-oriented because it sends a Request, receives an Acknowledgement/Request, and sends an Acknowledgment with a remote host in order to establish a reliable connection. Part of these requests set up Window Size (how big of packets can be sent) and Flow Control (how groups of packets will be sent and how missing packets will be addressed). UDP, on the other hand, is connectionless. Packets are sent with no regard as to whether they are all received. This creates a much faster and unhindered connection that services such as voice and video are heavily reliant on.
Network Layer 3
The Network Layer handles host addressing using either the IP, or IPv6, protocols. Routers, and particular switches, function at layer 3. They put data into packets so that they can be addressed to another host, either over the local network or across the internet. Other closely related protocols that function on this layer are IPsec and ICMP.
Data Link Layer 2
The Data Link Layer controls interface addressing across the local network using a MAC address. This ensures that the data frame leaving the host’s interface is sent to the appropriate interface on the local router. This layer also provides error notification and flow control. It consists of two sublayers; the Media Access Control that defines how packets are placed on the media, and the Logical Link Control that identifies and encapsulates Network Layer protocols.
Physical Layer 1
The physical layer is responsible for translating everything into 1s and 0s to be sent across a physical medium. These mediums can include copper wires, fiber optics, or even audible tones. It’s also responsible for converting received bits back into data.