Network Protocols

The most prevalent protocols for network traffic are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

• Historical Context: Developed in the 1980s, their original purpose was to support simple applications like remote terminal logins and file transfers. They were designed for an environment with slow, expensive bandwidth and limited CPU power.
• Modern Performance Issues: While adequate for standard internet Browse, TCP and UDP are often too slow for high-volume traffic in modern distributed environments like data centers. Their legacy design introduces overhead and error control methods that increase latency and reduce the achievable throughput on high-speed links, creating a known performance bottleneck.

OSI and Internet Reference Models

Two primary models describe network communication layers: the OSI model and the TCP/IP model.

The OSI Reference Model

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a telecommunication or computing system into seven abstract layers.

Core Principles

• Abstraction: Each of the seven layers is created for a different level of abstraction.
• Well-Defined Function: Every layer performs a specific, well-defined function.
• Standardization: The function of each layer was chosen with international standard protocols in mind.
• Minimized Information Flow: The design aims to minimize the flow of information across layer boundaries.
• Optimum Number of Layers: The number of layers is intended to be optimal, ensuring neither too many nor too few distinct functions.

Central Concepts

The OSI model is built around three core concepts:

• Services: What a layer provides to the layer above it.
• Interfaces: How the layer above accesses the services of the layer below it.
• Protocols: The rules and conventions used for communication within a layer.

The Seven Layers of OSI

1. Physical Layer
2. Data Link Layer
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

The TCP/IP (IETF/Internet) Reference Model

The TCP/IP model is a more practical model that is specific to the IETF protocol suite used in the modern internet. It consists of four layers.

The Four Layers of TCP/IP

• Link Layer: The lowest layer in the model. It describes how data is physically sent through the network and what links must do to meet the needs of the connectionless Internet Layer. Examples include Ethernet and 802.11.
• Internet Layer: This layer's job is to permit hosts to send packets into any network and have them travel independently to the destination. It defines the official packet format and protocol, called IP (Internet Protocol), and its companion, ICMP (Internet Control Message Protocol).
• Transport Layer: Positioned above the Internet Layer, this layer provides end-to-end communication services. It features two main protocols:
  • TCP (Transmission Control Protocol): A reliable, connection-oriented protocol.
  • UDP (User Datagram Protocol): An unreliable, connectionless protocol.
• Application Layer: This layer contains all the higher-level protocols used by applications. Examples include HTTP, SMTP, and DNS.

Comparison of OSI and TCP/IP Models

The TCP/IP layers align loosely with the OSI model. However, the OSI model is more general and finely grained, whereas the TCP/IP model is specific to its protocol suite. The most notable difference is the absence of the Presentation and Session layers in the TCP/IP model.

OSI Layer #	OSI Layer Name	TCP/IP Layer Name
7	Application	Application
6	Presentation	(Not present in the model)
5	Session	(Not present in the model)
4	Transport	Transport
3	Network	Internet
2	Data Link	Link
1	Physical	(Not present in the model)

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