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.
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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):
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.