OSI Model

---

In this blog:

• What is the OSI model?

• Layer 7 - Application.

• Layer 6 - Presentation.

• Layer 5 - Session.

• Layer 4 - Transport.

• Layer 3 - Network.

• Layer 2 - Datalink ➡️

Logical Link Control (LLC).

Media Access Control (MAC).

• Layer 1 - Physical.

---

This blog includes TCP and UDP.

➡️ See my blog on Wireshark for security for more information on TCP and UDP.

---

What is the OSI model?

The Open Systems Interconnect model, or OSI model, is a layered framework for the travel of data from one system, or network, to the other.

The model defines the method with which information and data is transported along the network.

There exists channels through which information is secured before it is exchanged between the layers and protocols which ensure smooth transfer of data with minimal loss or downtime.

The top 3 layers (Application, Presentation, Session) belong to the web interface where data is passed on without any specific headers being added.

The bottom 3 layers (Network, datalink, Physical) belong to the network interface.

The Transport layer acts as a mediator.

---

Layer 7 - Application

Teams chats (typical application layer service)

~ Sent as data.

~ The top layer of the OSI model provides access to users by defining the UI design that users interact with.

~ The layer provides network protocols to the application processes, including telnet, FTP (file server or HTTP/S (web server).

~ For example, Microsoft Outlook is an application that works with the mail server protocols SMTP, IMAP and POP3.

~ Various applications such as Microsoft Edge or Safari use protocols to communicate with the lower levels of the OSI model.

---

Layer 6 - Presentation

Symmetric encryption.

~ Sent as data.

~ This layer ensures that data passing through is in the appropriate format/ syntax for the recipient application.

~ Checks for OS compatibility, encryption and compression of data.

Data compression for an image.

~ For example, MP3 data is able to be streamed on an iPhone from an Apache server running on Linux because the presentation layer negotiates the syntax that should be used so that both sides understand.

---

Layer 5 - Session

~ Sent as data.

~ The session layer establishes, manages, and terminates the connections between the local and remote application.

~ Establishes SIDs for the current session. ~ Examples of protocols at this layer include Remote procedure calls (RPC) and Network file system (NFS). These exist on Windows.

NetBIOS from Windows.

~ For example, netBIOS from Windows allows for the point-to-point tunnelling protocol (PPTP) which provides name recognition, security and logging services for two application processes attempting to establish a secure session on the network.

PPTP methodology.

---

Layer 4 - Transport

~ Sent as segments.

~ Handles transportation issues between hosts.

~ The layer establishes, maintains and terminates virtual circuits.

~ Also has Transport layer headers – based on TCP or UDP.

TCP handshake.

~ Examples of protocols used are TCP and UDP. TCP allows for flow control and consists of a 3-way handshake (syn, syn ack, syn). ~ UDP does not ensure reliability and does not retransmit lost packet data.

~ Converts data from session layer into segments of a limited size and segment number for easy reassembly at the other end.

Data converted to segments of a limited size.

See my blog on Wireshark for security for more information on TCP and UDP.

---

Layer 3 - Network

~ Sent as packets.

~ This layer takes care of data delivery and routes data packets, adding an IP header to the data – This is like having a from and to address on a letter and therefore converting the segmented data into a packet of data.

~ For example, Microsoft Outlook is an application that works with the protocols SMTP, IMAP and POP3.

~ Various applications such as Microsoft Edge or Safari use protocols to communicate with the lower levels of the OSI model.

~ Routing of packets on the Network layer is taken care of by the router (connects different networks)

---

Layer 2 - Datalink

Logical Link Control (LLC)

~ Ensures reliable data transmission by sending an acknowledgement for each received packet and appending a sequence number to each transmitted packet before sending.

Media Access Control (MAC)

~ Sent as frames.

~ Interacts with the physical layer below.

~ It is responsible for the creation of frames from the packets from the network layer above and appending a frame header and frame trailer.

~ Two physical/ virtual devices come into play at the datalink layer. The switch is a unicast device that works based on the mac address (Unique number assigned to each system in the network interface card (NIC) and ensures point to point delivery of data.

~ The hub is a broadcast device, so the data reaching the hub is broadcast to all devices in the network.

---

Layer 1 - Physical

~ Sent as bits.

~ The frame is converted into analogue or digital signals (bitstream).

~ The bitstream is transported to its destination wirelessly or via physical wires.

~ It is transported to devices within the same network (Baseband communication) via digital signals and to devices outside of the local area network (broadband communication) via analogue signals.

~ The corresponding device receives the data in bitstream and all layers of the OSI model are verified and the application layer can now display the message on the other device.

---