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| 6.1 |  | Characteristics of Transport Layer Technologies |
| 6.1.1 |  | Common transport layer technologies |

The transport layer provides end-to-end traffic accountability. Layer 4 technologies ensure reliable data delivery using acknowledgments, sequence numbers, and flow control mechanisms. The transport layer is the first layer that provides end-user functions.

Problems at the transport layer can present symptoms ranging from sub-optimal network operation to complete network communications failure. There are at least 35 recognized transport layer protocols. Some of the more common of these are:

- User Datagram Protocol (UDP)
- Transport Control Protocol (TCP)
- Sequenced Packet Exchange (SPX)
- AppleTalk Transaction Protocol (ATP)
- NetBIOS

This section will discuss the characteristics of these protocols and related transport layer technologies.

A specific network protocol can communicate with another network protocol at the layer above or below it. Within the TCP/IP protocol suite, Layer 4 operations are primarily handled by UDP and TCP. UDP and TCP rely on IP at the network layer and use port numbers to identify what higher layer application traffic is contained in the packet. ICMP is a protocol from the TCP/IP suite that operates at the network layer and it too relies on IP. Unlike UDP and TCP, ICMP does not carry user data. ICMP is primarily used by network devices for self-management and self-tuning functions and by network engineers for troubleshooting network problems. UDP, TCP, and ICMP are all used heavily on the Internet, supporting a wide variety of traffic types and applications.

The Network Basic Input/Output System (NetBIOS) was developed for IBM in 1983 by Sytek Corporation and officially defines a session level interface and a data transport protocol. NetBIOS was extended by IBM in 1985 to create the NetBIOS Extended User Interface (NetBEUI) protocol. NetBEUI supports NetBIOS operations at the network layer. NetBEUI and NetBIOS are commonly used in Microsoft and IBM LANs.

NetBEUI operates at the network layer and interfaces directly with ISO’s Logical Link Control 2 (LLC2) at the data-link layer. NetBIOS interfaces with NetBEUI and with IBM’s Server Message Block (SMB) protocol at the application layer. Together, NetBIOS and NetBEUI can be considered to be operating from the network layer through to the presentation layer. Because both NetBIOS and NetBEUI are non-hierarchical broadcast-based protocols, they depend on other hierarchical protocols, such as IP or IPX, to operate in a routed network.

Novell’s proprietary protocol suite uses Sequenced Packet Exchange (SPX) at the transport layer to implement reliable data delivery. In the early days of local area networking, Novell’s suite of protocols was commonly implemented. Until version 5 of Novell’s network operating system, IPX/SPX was the default protocol suite installed for Novell networks.

Because IPX is not compatible with IP, networks running the Novell protocols are unable to communicate with the Internet without being translated. Due to the growing need for corporate, academic, and government networks to be connected to the Internet, almost all Novell network installations now use the TCP/IP protocol suite. This has led to the steady decline of the number of new network installations using the IPX/SPX protocol suite.

AppleTalk Transaction Protocol (ATP) is used at the transport layer of legacy AppleTalk networks and relies on AppleTalk’s Datagram Delivery Protocol (DDP) at the network layer. Because ATP is incompatible with IP, new Mac networks usually use the TCP/IP protocol suite in preference to using the AppleTalk protocol suite.

Although legacy networks using IPX/SPX and AppleTalk still exist, troubleshooting these protocol suites is not in the scope of this course and will not be discussed further in this curriculum.