Inside TCP/IP, Third Edition -- Ch 3 -- Network Support for TCP/IP Page 1 of 47 Inside TCP/IP, Third Edition - 3 - Network Support for TCP/IP One of the reasons for the popularity of TCP/IP networks is that TCP/IP can run on a variety of networks and computing devices. The TCP/IP protocol suite can run on computers such as main frames, mini computers, workstations, personal computers, hand-held computers and organizers, and even cellular phones! The actual network hardware and software is usually transparent to the user of the network. In fact, most users are not even aware that they are using the TCP/IP protocol let alone the type of network hardware. However, this book is written for you, the network professional, who wants to learn about TCP/IP, the "glue" that ties together the different network components. The networking hardware and how TCP/IP runs on top of it is therefore of concern to you and is discussed in this chapter. Overview of Networking Hardware that Run TCP/IP By networking hardware, what is meant is layers two and one of the OSI model. Recall from the discussion in Chapter 2, "TCP/IP Protocol Layering Concepts," that layer two of the OSI model is the Data Link layer and layer one of the OSI model is the Physical layer. The Data Link layer transmits and receives data units called frames and interacts with the Physical layer that is responsible for sending and receiving the frame over a physical media. Network interface cards are the pieces of network hardware that go inside a computer and connect the computer to the physical network. Network interface cards typically implement the functionality of the Data Link and Physical layers of the OSI model. Before describing the variety of networking hardware available for running TCP/IP applications and protocols, you might ask a very fundamental question: Why do I need so many different types of network hardware to run TCP/IP? The motivation for having so many different types of hardware is that it is difficult for a single type of hardware to satisfy all the requirements of a real-life network. Most organizations use a Local Area Network (LAN) at a given location, because LANs provide high speeds at relatively low implementation costs. However, LANs are restricted to limited distances and cannot span the distances needed by widely separated sites that need to be connected by a network. For spanning long distances, Wide Area Networks (WANs) that connect remote sites are used. Most WANs use a fundamentally different technology fromLANs. As their name implies, they are designed for connecting networks over long distances. While WANs can span longer distances than LANs, they typically have limited speeds and are slower than LANs. So we see that a single LAN or WAN may not meet the needs of an organization. What is needed in this case is a LAN/WAN combination (see fig. 3.1). file://I:\chapters\z\zc648.html 3/21/01 Inside TCP/IP, Third Edition -- Ch 3 -- Network Support for TCP/IP Page 2 of 47 Such a network consists of one or more LAN and WAN networks. The LANs and WANs differ from each other in the distances they can span, the speeds they provide, and the fundamentally different network hardware technologies they use. Networks that use such different network hardware technologies need to be tied together by special "glue" devices. These devices are called bridges, routers, or gateways. (This chapter will discuss the roles of bridges and routers to connect different network hardware technologies. You will learn more about routers, gateways, and Internet routing protocols in later sections of this book.) One of the goals of connecting different types of networks is to provide users access to resources on the network regardless of the location of the user. This access should be provided in a manner that is transparent to the user. In other words, the complexity of the network and its underlying technologies, protocols, hardware devices, and cabling should be hidden from the user. While the network details are hidden from the user, they cannot be hidden from the network designer/implementer. From a network designer/implementer perspective, the network components and resources must interoperate and be easy to manage and maintain. The network designer or implementer needs to be knowledgeable about the network hardware and protocol components to determine which kinds of devices can be connected to each other and how they interoperate. FIGURE 3.1 LAN/WAN combination network. Figure 3.1 shows a network that consists of LANs and WANs. In each of these categories (LANs, WANs), there are several different types of hardware that are available. The network designer or implementer must make this choice based on the business needs of the organization. The following are some of the more common types of LAN technologies: l IEEE LANs l Ethernet l Token Ring l Switched networks l FDDI The following are some of the more common types of technologies: l X.25 l Frame Relay l SMDS l MAN l SLIP/PPP/CSLIP file://I:\chapters\z\zc648.html 3/21/01 Inside TCP/IP, Third Edition -- Ch 3 -- Network Support for TCP/IP Page 3 of 47 l ATM TCP/IP on IEEE Local Area Networks As you work with TCP/IP network hardware, you will encounter references to the Institute of Electrical and Electronic Engineers (IEEE) 802 standards. These are ubiquitous standards for LANs and are closely connected with the first two OSI layers: the Data Link layer and the Physical layer. The IEEE standard defines Ethernet LANs by the name of IEEE 802.3 standard, and Token Ring LANs by the name of IEEE 802.5 standard. These standards constitute a large number of LAN networks that run TCP/IP. A brief background on the evolution of IEEE LAN standards will be helpful here, and this section offers just that. The IEEE undertook Project 802 in February of 1980 to identify and formalize LAN standards for data rates not exceeding 20 megabits per second (Mbps). Standardization efforts resulted in the IEEE 802 LAN standards. The number 802 was chosen to mark the calendar date when IEEE undertook the LAN standardization efforts (80 for 1980, 2 for February). Figure 3.2 shows the IEEE LAN standards in relationship to the OSI model discussed in Chapter 2. You can see that the primary emphasis of the IEEE committee was to standardize the hardware technologies used at the Physical and Data Link layers. This is not surprising considering that networking hardware such as network interface cards and LAN wiring can be modeled completely by the two lower OSI layers. The IEEE standards divide the OSI data link layer into two sublayers: l Media Access Control (MAC) l Logical Link Control (LLC) FIGURE 3.2 Relationship of the IEEE 802 Standard to the OSI model. (Courtesy IEEE Standard 802-1990) Media Access Control (MAC) The MAC layer deals with media access techniques utilized to control access to a shared physical medium. Token Ring and Ethernet have different implementations of the MAC layer because they use different methods to share the physical media. This also means that if your TCP/IP network needs to link Ethernet and Token Ring networks, you must use one of the "glue" devices such as a bridge or router between the Ethernet and Token Ring networks. Logical Link Control (LLC) All IEEE LANs have the same LLC layer as defined by standard 802.2. The advantage of a common sublayer such as the LLC is that upper-layer mechanisms can be the same regardless of what kind of networking hardware you use. Figure 3.2 shows the interface between Upper Layer Protocols and the LLC layer defined by Link file://I:\chapters\z\zc648.html 3/21/01 Inside TCP/IP, Third Edition -- Ch 3 -- Network Support for TCP/IP Page 4 of 47 Service Access Points (LSAPs). LSAPs are logical data link addresses. A single MAC address, such as an Ethernet address, can have multiple LSAP addresses. These multiple addresses allow multiple end-point connections between two nodes on a LAN. The LLC layer is the upper sublayer within the Data Link layer. Recall from the previous chapter that the Data Link layer is responsible for transmission of data between two adjacent nodes on a network. These adjacent nodes have LSAP addresses that are called the Destination Service Access Point (DSAP) and Source Service Access Point (SSAP) within the LLC layer. The LLC layer also provides the options of virtual circuit (connections-oriented) or datagram (connectionless) services or a combination of these two. Datagram Services Datagram services are modeled after postal services. In the datagram approach, every packet contains complete addressing information such as destination and source addresses. No special effort is made to ensure that packets arrive intact or in the correct order. Datagram services are connectionless; there is no attempt to create a connection before transmitting the data. Datagram services may or may not use acknowledgments. Acknowledgments are special frames sent by the receiver that announce to the sender the frames that have been received correctly. They are used to retransmit frames that were not received correctly. This could happen if there are errors in transmission because of physical hardware or interference on the line. Chapter 2 contains a discussion on how the CRC mechanism is used to detect errors in frames. Unacknowledged datagram services means that datagrams are sent but no attempt is made at the Data Link layer to retransmit frames that were incorrectly received.