Transport Layer Protocols: TCP, UDP, and NetBIOS Explained

1. Transport Layer Functions

The primary function of the transport layer is to ensure reliable communication between communicating endpoints. This involves managing error detection and flow control (typically between source and destination) to prevent data loss.

In addition to these core functions, the transport layer can multiplex multiple application-level connections over a single network connection. This reduces costs, especially in scenarios where network providers charge based on connection time or the number of connections. To facilitate this, the transport layer maintains a routing protocol, as multiple points may connect simultaneously. This routing is usually achieved using numerical addresses at the transport layer.

2. Comparison Between Link and Transport Layers

The link layer protocol manages connections between two nodes over a direct channel, whereas the transport protocol facilitates communication between an origin and destination across an entire network. Link-layer packets either reach the other end of the cable or are lost. In contrast, transport protocol data units (TPDUs) might temporarily disappear within the subnet and reappear later.

Link-layer connections are straightforward, as the sender is always ready. However, transport layer operations are more complex, requiring the exchange of control information.

A transport layer can multiplex multiple communications within a single network transmission, reducing communication costs when using a subnet.

3. Transport Layer Protocols and Network Services

Transport protocols are designed considering the type of service provided by the underlying network. According to ISO standards, there are three classes of network-level services:

Class A

These network services handle error management, simplifying the design of the transport service level.

Class B

The network layer informs the transport layer of errors but does not correct them.

Class C

The network layer provides an unreliable service, requiring the transport layer to detect and correct all subnet errors.

4. Internet Transport Protocols: TCP and UDP

The Internet transport layer supports two primary protocols for exchanging information: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is connection-oriented and reliable, making its design complex as it operates over IP (Internet Protocol), which is a Class C service. UDP is connectionless and performs no error handling, adding only a small header to IP packets, making its implementation simple.

5. Microsoft Networks Transport Protocols: NetBIOS

Microsoft networks use the NetBIOS protocol at the transport layer for communication between stations. Developed in 1983 by Systec Inc. (now Hughes LAN Systems) for IBM, NetBIOS aimed to create a communication protocol for small local networks. It later became a high-level protocol in Microsoft's DOS and Windows operating systems.

NetBIOS stations are identified by names (up to 16 characters long) and organized in a flat structure, where all devices are at the same level without hierarchy.

Newer versions of Microsoft operating systems do not rely on the NetBEUI protocol for network information transfer. While NetBIOS is not strictly required (TCP/IP and domain names can be used, as in Windows 2000/XP), it is still included for compatibility with older Microsoft networks (Windows 9x/NT). Instead of NetBEUI, NetBIOS relies on other protocols, such as NetBT, to transport information over the network.