A deep-dive into the OSI model

Hello!

In this post, I will be exploring the Open Systems Interconnection (OSI) model for implementing computer networks and each of its layers in detail.

It is important to note that while the TCP/IP model is more commonly used, both the OSI and TCP/IP model provide exactly the same set of services. The only difference between them is how responsiblities are distributed across layers.

OSI vs TCP/IP

[Image credit: guru99.com]

Layer 1 (Physical)

The lowest layer of the OSI model is responsible for carrying raw bits on a physical medium. The physical medium could be: electrical wire, radio waves or optical fibre. Transmission medium standards such as types of connectors, wire specifications and signalling modes are all defined at this layer.

Data unit: Bits
Delivery type: Local

Physical

[Image credit: web.stanford.edu]

Layer 2 (Data Link)

The second layer of the OSI model is responsible for providing local delivery of frames between two directly-connected devices. The Data Link layer has two components:

Media Access Control (MAC)

The MAC component controls the transmission and reception from the physical medium of layer 1. It employs either the Carrier Sense Multi Access with Collision Detection (CSMA/CD) or Carrier Sense Multi Access with Collision Avoidance (CSMA/CA) algorithm for this purpose.

Its main responsibilites include:
1. Check whether the phyiscal link is free. If yes, transmit, otherwise receive.
2. Check if any collision occoured while transmitting. If yes, it sends a jam signal to instruct other hosts to stop transmitting, it then times out for a random period of time and then attempts retransmission.
Logical link Control (LLC)

The LLC component provides the following capabilities:
1. Flow control - this prevents a fast sender from overwhelming a slow receiver. Note that in actual network implementations, this LLC component is rarely used. Flow control is generally provided by the Transport layer of the OSI model.
2. Error checking - this allows retransmission of corrupt frames. Like flow control, this is also rarely used. In actual networks, error checking and retransmission is usually done in an end-to-end manner between two hosts. However, in case of highly-lossy links such as WiFi, error checking of the LLC component is commonly used.
3. Multiplexing of layer 3 protocols - this allows identifying which layer 3 protocol is encapsulated in a frame’s data unit.

Data unit: Frames
Delivery type: Local

Here’s a flowchart showing the CSMA/CD algorithm in detail:

CSMA/CD

[Image credit: orca-project.eu]

Layer 3 (Network)

Perhaps the most important layer of the OSI model, the third layer is responsible for providing global delivery of datagrams between two devices, that are not necessarily directly connected in a network. The two devices may be in different networks with multiple routers and switches in between them, but as long as there is a single path available the Network layer can try (best-effort) to deliver datagrams.

The network layer provides a best-effort delivery of datagrams with no gurantees on the following:

Ordering - datagrams may not necessarily be delivered in the same order as they were sent.
Integrity - datagrams may be corrupt.
Reliability - datagrams may be dropped and hence never make it to the receiver.

Other than datagram forwarding, the Network layer is also responsible for host addressing. In the Internet Protocol (IP) each device on a network is identified using a 32-bit address (IPv4) or a 128-bit address (IPv6).

Data unit: Datagrams
Delivery type: Global

IP Header

[Image credit: juniper.net]

Layer 4 (Transport)

The fourth layer of the OSI model is responsible for providing global delivery of segments between two devices, that are not necessarily directly connected in a network.

The network layer can provide both of the following:

Best-effort delivery of segments with no gurantees on ordering, integrity, reliability and with no congestion and flow control. This is done using the User Datagram Protocol (UDP).
Reliable, in-order, intact delivery of segments with both, flow and congestion control. This is done using the Transmission Control Protocol (TCP).

While UDP and TCP are the most common level 4 protocols, there are other lesser-known transport protocols such as Stream Control Transmission Protocol (SCTP) which is used in cellular networks.

Data unit: Segments
Delivery type: Global

Layer 5 (Session)

The fifth layer of the OSI model is responsible for setting up, breaking down and managing a communication session for communication between two hosts.

The fifth layer controls the following session-specific parameters:

Communication mode (full-duplex or half-duplex)
Checkpointing interval (for fault-tolerance)
Authentication (for session security)

It is important to note that in the TCP/IP model, the Session layer responsibilites are bundled into the Application later.

Data unit: Application Data
Delivery type: Global

Layer 6 (Presentation)

The sixth layer of the OSI model, also commonly known as the Syntax layer, is responsible for managing how application data is formatted and serialized in a packet.

The Presentation layer manages the following data presentation parameters:

Encryption/Decryption (for security of data)
Compression (for bandwidth management)
Serialization (for managing how complex multi-dimensional data structures are to be represented as a linear array of 1s and 0s)

Data unit: Application Data
Delivery type: Global

Like Layer 5, the Presentation layer responsibilites are bundled into the Application layer in the TCP/IP model.

Layer 7 (Application)

The top-most layer of the OSI model is responsible for providing network access to end-users. It abstracts away most the technicalities associated with network communication.

Common layer 7 protocols include Hyper Text Transfer protocol (HTTP), File Transfer Protocol (FTP), BitTorrent protocol and many custom application-specific protocols such as those used by instant messaging applications.

Data unit: Application Data
Delivery type: Global