Transport Service

The Transport Layer (Layer 4 in the OSI model) is responsible for providing end-to-end communication services for applications. It ensures that data is transferred reliably and in the correct sequence, even if the underlying network is unreliable. The transport layer provides services such as error detection, flow control, and multiplexing. The two main transport layer protocols are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Key Points:

Role of the Transport Layer:

• The transport layer establishes logical communication between applications running on different hosts.
• It provides end-to-end communication, ensuring that data is delivered correctly, in order, and without errors.
• It enables multiplexing, allowing multiple applications to use the network simultaneously by assigning unique port numbers to each application.

Types of Transport Services:

• Connection-Oriented Service: This service guarantees reliable data transfer and ensures that packets are delivered in the correct order. It is used by TCP.
• Connectionless Service: This service does not guarantee reliability or order of delivery. It is used by UDP.

Key Transport Services:

• Reliability: Ensures that data is delivered accurately, without errors, and in the correct sequence (provided by TCP).
• Flow Control: Manages the rate of data transmission to prevent congestion in the network.
• Congestion Control: Prevents the network from becoming overwhelmed by too much data.
• Error Detection and Correction: Ensures that corrupted data is detected and retransmitted (for TCP).

Example of Transport Services:

• Scenario: Consider a user accessing a website using a browser.
• TCP (Transmission Control Protocol):
• Connection-Oriented: When the browser sends a request to the web server, TCP establishes a connection using a three-way handshake (SYN, SYN-ACK, ACK).
• Reliability: TCP ensures that the data (e.g., HTML, images) sent from the server to the browser is reliable. If any data is lost or corrupted, TCP will retransmit it.
• Flow Control: TCP adjusts the rate of data transmission to avoid congestion on the network, ensuring that the receiver’s buffer is not overwhelmed.
• Error Detection: TCP checks for errors in the data and requests retransmission if necessary.
• Example: When a user requests a webpage, the browser sends a TCP packet with the request, and the server responds with the necessary data. If any packets are lost or corrupted during the transfer, TCP ensures that they are retransmitted.

UDP (User Datagram Protocol):

• Connectionless: UDP does not establish a connection before sending data. It simply sends packets to the destination without any acknowledgment or error-checking.
• Unreliable: There is no guarantee that the data will arrive at the destination or that it will be in the correct order. UDP does not retransmit lost packets.
• Low Latency: UDP is preferred for real-time applications (e.g., VoIP, video streaming), where low latency is crucial, and occasional packet loss is acceptable.
• Example: A user is making a voice or video call using a real-time application like Skype. UDP is used because it is faster and doesn't require the overhead of establishing a connection or ensuring reliable delivery. Some packet loss is acceptable in real-time communication.