Question

List the four layers in the Internet software hierarchy and identify a task performed by each layer.

Short answer

The four layers are Application (interfaces, protocols), Transport (reliable delivery), Internet (routing, IP addressing), and Link (local transmission, MAC addresses).

Step-by-step solution

- Application Layer

The application layer is the top layer of the Internet software hierarchy. Its main task is to provide interfaces and communication protocols needed for processes such as email, file transfer, and web browsing.

- Transport Layer

The transport layer is responsible for data delivery between hosts. It performs tasks such as error recovery, data flow control, and ensuring data is delivered error-free and in sequence.

- Internet Layer

The Internet layer, also known as the network layer, primarily handles the routing of data packets across different networks. It determines the path the packets take and manages addressing through assigning IP addresses.

- Link Layer

The link layer, also sometimes referred to as the network access layer, is responsible for moving packets from one node to another over a network segment. It deals with data encapsulation, physical addressing (MAC addresses), and error checking at the local network level.

Key concepts

Application Layer

The Application Layer acts as the topmost layer in Internet communication protocols. This layer is mainly concerned with interactions between software applications and communication interfaces. It serves as the platform for services such as email exchanges, file transfers, and accessible web browsing.
Communication protocols like HTTP, HTTPS, FTP, SMTP, and DNS are pivotal in the Application Layer.

• HTTP/HTTPS: Protocols for transmitting data over the World Wide Web.
• FTP: Used for transferring files over a network.
• SMTP: Facilitates the sending of emails.
• DNS: Translates domain names into IP addresses.

In essence, the Application Layer acts as the interface between network services and end-user processes, ensuring smooth communication between applications.

Transport Layer

The Transport Layer ensures that data is delivered error-free, in order, and that it reaches the correct destination. Its responsibility lies in providing reliable communication between hosts.
The transport layer utilizes protocols such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

• TCP: Ensures complete and reliable data transmission through error detection and correction.
• UDP: Offers faster data transmission without error-checking mechanisms for applications that can tolerate some data loss like video streaming.

Moreover, this layer also handles functions like flow control, which modulates the speed of data transmission to prevent network congestion, and establishing and terminating communication sessions.

Internet Layer

Also known as the Network Layer, the Internet Layer is responsible for routing data packets across different networks. Its main function is to determine the most efficient path for data transfer.
Key protocols include:

• IP (Internet Protocol): Assigns unique addresses to devices (IP addresses) ensuring data reaches the correct source and destination.
• ICMP (Internet Control Message Protocol): Used for sending error messages and operational information.
• ARP (Address Resolution Protocol): Translates IP addresses into physical MAC addresses.

The Internet Layer ensures that data packets are correctly routed by utilizing logical addressing, thereby maintaining a smooth flow across interconnected networks.

Link Layer

The Link Layer, sometimes referred to as the Network Access Layer, manages data transmission within the local network. This layer is crucial for the initial physical connection setup.
It handles processes such as:

• Data Encapsulation: Wrapping data with protocol information before transmission.
• Physical Addressing: Utilizes MAC addresses to identify devices within a network segment.
• Error Checking: Detects and may correct errors found in the physical transmission, ensuring data integrity.

In summary, the Link Layer enables the transfer of packets across a single segment of a network and ensures the deployment of local-network data-link-level protocols.