Transport Layer

What is the Transport Layer?

The transport layer is a logical layer within a network communication model that enables communication between processes running on different devices. It is the fourth layer in the open systems interconnection (OSI) network model and the second layer of the TCP/IP network model.

Key Takeaways

In network communication models, the transport layer enables communication between processes running on different devices.
The transport layer is the fourth layer of the OSI model and corresponds to the second level of the TCP/IP network model.
The three main functions of the transport layer are reliable data delivery, message segmentation and reassembly, and flow control.
The transport layer supports both connection-oriented and connectionless protocols.
Connection-oriented protocols like TCP prioritize reliability, while connectionless protocols like UDP prioritize speed.

Table of Contents Table of Contents

What is the Transport Layer?
Key Takeaways
How Transport Layer Works
Transport Layer Functions
Transport Layer in the OSI Model
Transport Layer in TCP/IP Model
Transport Layer Protocols
Transport Layer Characteristics
Transport Layer Security
Transport Layer Pros and Cons
The Bottom Line
FAQs
References

Table of Contents

What is the Transport Layer?
Key Takeaways
How Transport Layer Works

Show Full Guide

Transport Layer Functions
Transport Layer in the OSI Model
Transport Layer in TCP/IP Model
Transport Layer Protocols
Transport Layer Characteristics
Transport Layer Security
Transport Layer Pros and Cons
The Bottom Line
FAQs
References

How Transport Layer Works

Software applications don’t communicate directly over a network. Instead, they rely on network protocols and different parts of the network stack to manage data transmission and routing between devices.

The transport layer’s job is to ensure that data is delivered to the correct process on the receiving device. To identify the specific process, the transport layer uses port numbers. This allows multiple applications on a single device to communicate over the network simultaneously without interfering with each other.

What Services Can the Transport Layer Provide?

The transport layer can directly support multiplexing and demultiplexing services by using port numbers to manage multiple application connections. It can also support error detection and error correction services indirectly by relying on protocols within the transport layer that provide these functionalities.

Transport Layer Functions

The three main functions of the transport layer are reliable data delivery, message segmentation and reassembly, and flow control.

Here’s a simple explanation of the three transport layer functions in the context of a point of sale (POS) system that uses the Transmission Control Protocol (TCP):

Data delivery

Ensures that all the transaction data (like the total amount, item details, and payment confirmation) sent between the POS terminal and the central server arrives completely and without errors. For example, when a customer swipes their credit card, the POS sends payment information to the bank. Reliable data delivery ensures this data isn’t lost or corrupted during transmission so the payment can be processed correctly.

Message segmentation and reassembly

It breaks large amounts of transaction data down into smaller packets for easier transmission and then reassembles them at the receiving end. For example, if a receipt with multiple items needs to be sent to the server, the system will split the receipt into smaller parts to send over the network. At the server, the parts will be reassembled to reconstruct the full receipt.

Flow control

Manages the speed of data transmission to ensure neither the POS terminal nor the server will be overwhelmed. For example, if a POS terminal using TCP processes transactions faster than the server can handle, flow control will ensure the data is sent at a pace the server can manage without dropping or delaying transactions.

In this example, it should be noted that TCP is a connection-oriented protocol (COP). Connectionless protocols do not have built-in flow control mechanisms.

Transport Layer in the OSI Model

The OSI network model divides the tasks involved in moving information between networked computers into seven distinct layers. In this conceptual model, the transport layer is responsible for segmenting and reassembling data and managing flow control between devices.

Transport Layer in TCP/IP Model

The TCP/IP network model organizes networking tasks into a stack consisting of four layers. Just like in the OSI model, the transport layer for TCP/IP is responsible for enabling communication between application processes running on networked devices.

Transport Layer Protocols

Transport layer protocols establish a logical connection between applications running on different devices, even if those devices are separated by numerous networks and routers. They ensure that data is delivered not just to the correct destination machine but also to the specific application process running on that machine.

The two most widely used transport layer protocols are the Transmission Control Protocol and the User Datagram Protocol (UDP). TCP provides reliable, connection-oriented communication, and UDP provides fast, connectionless communication.

Transport Layer Characteristics

The characteristics and behavior of the transport layer depend on whether it is using a connection-oriented protocol or a connectionless protocol.

Connection-oriented protocols like TCP ensure data arrives at the destination accurately and in order. This type of protocol is best suited for use cases like email and financial transactions, where reliability and completeness are priorities.

In contrast, connectionless protocols like UDP ensure fast data transfers and low operational overhead. This type of protocol is ideal for applications like video streaming and online gaming, where speed is more important than guaranteed delivery.

Connection-Oriented Protocols vs. Connectionless Protocols for Network Communication

	Connection-oriented protocols	Connectionless protocols
Connection setup	Establishes a connection before data transfer begins.	No requirement for establishing a connection; data is sent immediately.
Reliability	Ensures reliable delivery with error detection, retransmission, and acknowledgment.	No guarantee of delivery; reliability is not ensured.
Data ordering	Delivers data in the correct sequence.	Data may arrive out of order.
Error correction	Detects and corrects errors.	Detects errors but does not correct them.
Overhead	Higher overhead.	Lower overhead.
Speed	Slower.	Faster.
Flow control	Manages data flow to avoid overwhelming the receiver.	No built-in flow control mechanisms.
Use case	Best for application processes that require reliability, such as email, file transfers, and web browsing.	Ideal for time-sensitive applications like video streaming, VoIP, and online gaming.
Resource consumption	Consumes more memory and processing power.	Is more lightweight and consumes fewer resources.
Example protocol	Transmission Control Protocol.	User Datagram Protocol.

Transport Layer Security

Transport layer security (TLS) is a cryptography protocol that provides secure communication over a network. This protocol, which uses the transport layer to establish a secure channel for communication, ensures that data transmitted between clients and servers is protected from eavesdropping, tampering, and forgery. The presence of TLS in a URL that begins HTTPS helps users trust that they are communicating with a legitimate website or service.

Transport Layer Pros and Cons

The transport layer plays an important role in device-to-device communication over a network, but the benefits can have trade-offs depending on what protocols are being used and the requirements of the application.

Pros

Provides standardized options for transferring data over a network
Breaks down large data chunks into manageable packets for efficient transmission and reassembles them at the receiver
Uses port numbers to handle multiple application processes simultaneously
Supports both connection-oriented (reliable) and connectionless (fast) protocols to suit diverse applications

Cons

Dependent on lower network layers for routing and addressing
Implementing features like error correction and flow control can add complexity to data transmissions
Connection-oriented protocols can introduce latency and consume more memory and processing power
Connectionless protocols can sacrifice reliability and error correction for speed, which can result in data loss

The Bottom Line

Some transfer layer definitions only address how the term is used in the OSI model. It’s important to understand, however, that the transfer layer is a logical layer used in the TCP/IP network model as well.

While the OSI model is a useful conceptual framework for understanding and teaching the fundamental principles of network communication, the Internet is actually built on the TCP/IP model.