Models and protocols of computer networks. LAN protocol stacks

Chapter 5

Local network protocols

After reading this chapter and completing the practical exercises, you will be able to:

Ø Explain the following protocols and their use in various network operating systems:

Ø discuss and implement methods to improve the performance of local networks.

At the beginning of the 20th century, sociologist George Herbert Mead, studying the influence of language on people, came to the conclusion that human intelligence primarily developed through language. Language helps us find meaning in the surrounding reality and interpret its details. In networks, a similar role is played by network protocols, which allow diverse systems to find a common environment for interaction.

This chapter describes the protocols most commonly used on local area networks and the network operating systems that use them. You will learn about the advantages and disadvantages of each protocol, which will help you understand their uses. The most popular local network protocol, TCP/IP, is discussed only briefly in this chapter, since it will be described in more detail in Chapter 6. At the end of this chapter, you will be introduced to methods for improving the performance of local networks and selecting the protocols that are needed in a particular situation.

Local network protocolsand their application in networksoperating systems

Network protocols are like a local language or dialect: they enable networks to seamlessly exchange information between connected devices. These protocols are also important for simple electrical signals transmitted over a network communication cable. I protocol network communications would simply be impossible. In order for two computers to communicate freely with each other, they must use the same protocol, just as two people must communicate in the same language. I

In a local network, several protocols can operate individually and in some combinations. Network devices (such as routers) are often configured to automatically recognize and configure different protocols (depending on the operating system used in the router). For example, on a single Ethernet LAN, one protocol might be used to connect to the mainframe, another to work with Novell NetWare servers, and a third to work with Windows servers (for example, running Windows NT Server) (Figure 5.1).

You can install a bridge router that will automatically recognize each protocol and configure itself accordingly, causing it to act as a router for some protocols and as a bridge for others. The presence of several protocols in a network is effective in that such a network can simultaneously perform many functions (for example, provide Internet access to mainframes and servers). The disadvantage of this approach is that some protocols will operate in broadcast mode, that is, they will periodically send packets to identify network devices, generating significant excess traffic.

Some network protocols have become widely used because they are associated with specific network operating systems (for example, Windows systems, IBM mainframes, UNIX servers, and Novell NetWare). It makes sense to study protocols in relation to the operating systems where they are used. In this case, it becomes clear why a specific protocol is needed in a certain type of network. It will also make it easier for you to understand how one protocol (such as NetBEUI) can be replaced by other protocols (such as TCP/IP). However, before learning about protocols and their interrelationships between operating systems, it is important to learn about the general properties of LAN protocols.

General propertieslocal network protocols

In general, local area network protocols have the same properties as other communication protocols, but some of them were developed long ago, during the creation of the first networks, which were slow, unreliable, and more susceptible to electromagnetic and radio interference. Therefore, some protocols are not entirely suitable for modern communications. Disadvantages of such protocols include poor error protection or excessive network traffic. In addition, certain protocols were created for small local networks and long before the advent of modern corporate networks with advanced routing capabilities.

Local network protocols must have the following basic characteristics:

Ensure the reliability of network channels;

Have high performance;

Process source and destination node addresses;

Comply with networking standards, especially IEEE 802.

In general, all the protocols discussed in this chapter meet these requirements, but, as you will learn later, some protocols have more capabilities than others.

In table 5.1 lists local network protocols and operating systems with which these protocols can work. Later in the chapter, protocols and systems (in particular, server operating systems and host computers) will be described in more detail.

4 Table 5.1. Local network protocols and network operating systems

Protocol	Corresponding Operating System
	The first versions of Microsoft Windows operating systems
	UNIX, Novel NetWare, modern versions of Microsoft Windows operating systems, IBM mainframe operating systems
	IBM mainframe and minicomputer operating systems
	Client systems interacting with IBM mainframes configured to work with the SNA protocol

Note

Computer operating system is a set of software that performs two functions on a computer. First, they interact with the computer's hardware and the Basic input/output system (BIOS). Second, they interact with the user interface (for example, the graphical user interface (GUI) on Windows systems or the X Window Subsystem and desktops on UNIX systems). For network computer operating systems There is a third level of interaction in which these systems can communicate with each other over a network using one or more protocols.

ProtocolsIPX/ SPX and systemNovell NetWare

Protocol Internetwork Packet Exchange (IPX) (internetwork packet exchange) was developed by Novell for one of the very first network operating systems that performs server functions, called NetWare. This system was originally intended for Ethernet bus networks, token ring networks, and ARCnet networks, and was designed to work with a single file server. ARCnet is one of the proprietary alternative network technologies that uses special token packets and a mixed topology (bus and star). Currently, the NetWare operating system has become hardware independent and can support various topologies and protocols.

As a prototype for the IPX protocol, Novell used one of the first local network protocols, the IPX protocol. Xerox Network System (XNS), adapting it for its file server operating system NetWare. Xerox Corporation proposed the XNS protocol as a means of transmitting data over Ethernet networks. In the early 1980s, several manufacturers released their own versions of this protocol. Novell's version spawned the IPX protocol for NetWare servers. At the same time, this company developed a companion protocol called Sequenced Packet Exchange (SPX) and focused on working with application programs, such as databases.

The IPX/SPX protocols are widely used in NetWare servers up to and including version 4. Beginning with NetWare 5.0, Novell is encouraging users to migrate to the TCP/IP protocol stack. These protocols are currently the primary protocols for NetWare 6.0 and later, although users may continue to use IPX/SPX protocols, particularly for compatibility with legacy servers and equipment (such as printers).

When IPX/SPX protocols are configured on an Ethernet network based on NetWare servers, four types of Ethernet frames can be used:

o 802 .2 – a relatively new type of frames used in networks based on NetWare servers versions from 3.21 to 4.x;

o 802.3 – an old frame type used on NetWare 286 systems (versions 2.x) and the first versions of the NetWare system and 3.1x);

o Ethernet II – to ensure compatibility with Ethernet II networks and more efficient frame formatting;

o Ethernet SNAP – implementation described in chapter 2 SubNetwork Access Protocol (SNAP), designed for the operation of special networks and applications from manufacturers.

Advantages and disadvantages

The advantage of the IPX protocol (despite its advanced age) compared to other early protocols is the possibility of its routing, i.e., the fact that it can be used to transmit data over many subnets within an enterprise. The disadvantage of the protocol is the additional traffic that occurs due to the fact that active workstations use frequently generated broadcast packets to confirm their presence on the network. With many NetWare servers and hundreds of clients, IPX's "I'm here" broadcasts can generate significant network traffic (Figure 5.2).

Purpose of the SPX protocol

The SPX protocol, a complement to IPX, allows application data to be transferred more reliably than IPX. IPX is slightly faster than its companion protocol, but it uses connectionless services running in the LLC sublayer of the Link Layer. This means that IPX guarantees that the frame will be delivered to its destination with a lower probability. The SPX protocol uses connection-oriented services, which improves the reliability of data transmission. Most often, when referring to both protocols (IPX and SPX), the abbreviation IPX/SPX is used.

The SPX protocol is widely used for transmitting data content over the network. In addition, Novell's Remote Console Utility and Print Services operate based on this protocol. The remote console allows the administrator's workstation to see the same information that is displayed on the NetWare file server console, allowing the user to remotely execute system commands on the server without having to be at the server's keyboard.

Protocol DeploymentIPX/ SPX

To install the IPX/SPX protocols on computers running DOS, special DOS drivers developed for NetWare are used. On 32-bit operating systems (for example, Windows 95 and older), to install protocols, you can run the Novell Client32 program, which provides a command environment for accessing NetWare servers.

To enable computers running Windows systems to access NetWare, you can also use two types of drivers that allow you to work with several protocols: Open Datalink Interface (ODI) and Network Driver Interface Specification (NDIS).

When multiple protocols (such as IPX/SPX and TCP/IP) are deployed on a NetWare network, servers and clients often use a driver Open Datalink Interface, ODI(open channel interface). This driver enables communication with NetWare file servers, mainframes and minicomputers, as well as with the Internet. ODI drivers can be used in network clients running under MS-DOS and Microsoft Windows.

In earlier versions of Windows (Windows 3.11, Windows 95, Windows 98, and Windows NT), Microsoft implemented the GDI driver as a 16-bit application that could not take full advantage of the performance and capabilities of 32-bit Windows 95 and later.

Starting with Windows 95, more advanced solutions from Microsoft are used to connect to NetWare servers via the IPX/SPX protocol - protocol NetWare Link (NWLink) IPX/ SPX and driver Network Driver Interface Specification, NDIS(Network Adapter Standard Interface Specification). Practice Exercises 5-1 and 5-2 show you how to configure Windows 2000 and Windows XP Professional systems to use the NWLink protocol.

As shown in Fig. 5.3, NDIS (Microsoft) and ODI (Novell) drivers operate at the LLC sublayer of the Data Link layer, however, only one of these drivers can be bound to a network adapter at a time.

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EmulationIPX/ SPX

The NWLink protocol emulates IPX/SPX operation, so any Windows system that uses it operates as a computer or device configured for IPX/SPX. NDIS is a driver software specification used by the NWLink protocol that allows it and other network protocols to communicate with a computer's network adapter. This uses a procedure to establish communication between the protocol and the adapter, called binding. Binding Binding of a certain protocol to a specific adapter allows that adapter to operate and provide an interface with the network environment.

Binding to the driverNDIS

The Microsoft NDIS driver can bind one or more protocols to a single network adapter, allowing all of those protocols to work through that adapter. If there are several protocols, then a certain hierarchy is established between them, and if several protocols are deployed on the network, then the network adapter will first try to read the frame or packet using the protocol located at the top level of this hierarchy. If the formatting of the frame or packet corresponds to a different protocol, then the adapter will try to read it using the next protocol specified in the hierarchy, and so on.

Advice

Using the NDIS driver, one protocol can be bound to several network adapters on a computer (for example, on a server). If you have several adapters, you can distribute the network load between them and speed up the server's response to requests when there are a large number of users. In addition, multiple adapters are used if the server also functions as a router. Binding one protocol to multiple adapters also reduces memory footprint because the server does not need to load multiple instances of the same protocol into it.

It should be noted that the user can independently organize the hierarchy of protocols associated with the adapter. This hierarchy is called the binding order. For example, if the first protocol in the hierarchy is IPX/SPX and the second is TCP/IP, then the TCP/IP frame or packet is first interpreted as IPX/SPX data. The network adapter quickly detects the error and rereads the TCP/IP frame or packet, recognizing it correctly.

The protocol binding order can be set in most Microsoft Windows operating systems (for example, Windows 2000 and Windows XP). In Fig. Figure 5.4 shows the binding procedure on a computer running Windows XP Professional. In this figure, the protocols are listed below the line File and Printer Sharing for Microsoft Networks, display nil doc bindings for protocols used to access shared files and printers. Below the line Client for Microsoft Networks shows the order of binding protocols required to access network servers. In Practice Exercises 5-3 and 5-4, you will learn how to set the protocol binding order in Windows 2000 and Windows XP Professional.

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Note

As discussed earlier in this book, it is not recommended to enable RIP on NetWare and Windows 2000/Server 2003 servers because it introduces additional traffic on the network. It is preferable for specialized network routers to perform all routing tasks.

Table 5.2. Protocols used with serversNetWare

Abbreviatura	Full title	Description	LevelmodelsOSI
	Internetwork Packet Exchange	Used as the primary data transfer protocol for Ethernet applications. All frame types can be used: Ethernet 802.2, Ethernet 802.3, Ethernet II and Ethernet SNAP	Network and Transport
	Link Support Layer	Used in conjunction with the ODI driver to support multiple protocols on a single network adapter	Duct
	Multiple Link Interface Driver	Connects two or more channels into one telecommunications line (for example, two ISDN terminal adapters). In Ethernet networks, the MLID protocol in combination with the workstation network adapter allows you to determine the level of conflicts in the network; in networks with a token ring, it coordinates token transfers	Channel (MAC sublayer)
	NetWare Core Protocol	Part of the operating system that facilitates communication between clients and servers when accessing applications or open files located on a NetWare server
	NetWare Link Services Protocol	Provides IPX packets with routing information
	Routing Information Protocol	Collects routing information for servers that provide routing services
	Service Advertising Protocol	Allows NetWare clients to identify the servers and network services running on them. Servers generate SAP broadcast packets every 60 s, and clients use them to locate the nearest server	Session Executive Application
	Sequenced Packet Exchange	Provides application programs with a connection-oriented data transfer mechanism	Transport

ProtocolNetBEUI and serversMicrosoft Windows

Microsoft Windows NT began as a joint project between Microsoft and IBM to develop the LAN Manager server operating system. In the early 1990s, Microsoft transitioned from LAN Manager to Windows NT Server, which later became a widely used operating system.

Based on the Windows NT Server product, Windows 2000 Server and Windows Server 2003 were created. Like modern versions of Novell NetWare, Windows NT systems, Windows 2000 and Windows Server 2003 are compatible with Ethernet and Token Ring local networks, they can scale from small computers with Intel compatible processors to multiprocessor systems. Most often, TCP/IP protocols are used with these systems, but there are still Windows NT Server systems versions 3.51 and 4.0, which implement the native protocol of Windows NT systems - NetBIOS Extended User Interface, NetBEUI. This protocol was created for the LAN Manager and LAN Server operating systems before Windows was introduced. BEUI was implemented in the first versions of Windows NT and is still available in Windows 2000 (although no longer supported on Microsoft systems starting with Windows XP).

Note

On computers running Windows NT and Windows 2000, the NetBEUI protocol is also found under the name NBF (NetBEUI frame). If you use a protocol analyzer to analyze network traffic, then NetBEUI frames will be marked with just such an abbreviation.

StoryNetBEUI

The NetBEUI protocol was originally developed by IBM in 1985 as an improved modification Network Basic Input/ Output System, NetBIOS(basic network input/output system). NetBIOS is not a protocol, but a method for application programs to interact with network devices, as well as a name recognition service used on networks. BIOS names are given to various network objects (such as workstations, servers, or printers). For example, a username can be used to identify his workstation on a network, HPLaser can be used to access a network printer, and a server can be named AccountServer. Such names make it easier to find the necessary network resources. They are translated (converted) into addresses used in network communications using NetBIOS Name Query services.

Application areaNetBEUI

The NetBEUI protocol was developed at a time when computer networks primarily meant local area networks for a relatively small number of computers (from a few to two hundred). The design process did not take into account the features of corporate networks with packet routing. For this reason, the NetBEUI protocol cannot be routed and is best used in small local networks running relatively old operating systems from Microsoft and IBM:

· Microsoft Windows 3.1 or 3.11;

· Microsoft Windows 95;

· Microsoft Windows 98;

· Microsoft LAN Manager;

· Microsoft LAN Manager for UNIX;

· Microsoft Windows NT 3.51 or 4.0

· IBM LAN Server.

When migrating your network from Windows NT Server to Windows 2000 or Windows Server 2003, first configure servers and workstations that use NetBEUI to use TCP/IP. Although Windows 2000 systems support NetBEUI, Microsoft does not recommend using this protocol on later operating systems. However, if the network is small (less than 50 clients) and Internet access is not required, then the NetBEUI protocol may be more efficient than TCP/IP.

NetBEUIand reference modelOSI

The NetBEUI protocol corresponds to several layers of the OSI model. The physical and data link layers are used to interact between network interfaces. Within the Link layer, LLC (Logical Link Control) and MAC (Media Access Control) sublayers are used to control the transmission of encoding and addressing of frames. The protocol also implements functions related to the Transport and Session layers (ensuring transmission reliability, acknowledging the receipt of packets, establishing and terminating sessions).

WhyNetBEUIworks well on networksMicrosoft

There are several reasons for answering the question posed in the section title. First, NetBEUI is easy to install because it does not need to be configured like other protocols (for example, TCP/IP requires an address, and IPX/SPX requires a frame type). Secondly, the protocol allows you to simultaneously support a large number of information exchange sessions on the network (up to 254 in earlier versions of the protocol; in previous versions this limitation was removed). For example, according to Microsoft specifications, a Windows NT server can support 1000 sessions per network adapter (such tests were carried out for Windows 2000 servers). Thirdly, the NetBEUI protocol consumes little RAM and has high performance in small networks. Fourth, it implements reliable mechanisms for detecting and eliminating errors.

FlawsNetBEUI

The inability to route is the main disadvantage of the NetBEUI protocol in medium and large networks, including enterprise networks. Routers cannot forward a NetBEUI packet from one network to another because the NetBEUI frame does not contain information pointing to specific subnets. Another disadvantage of the protocol is that there are few network analyzers available for it (besides those tools that Microsoft has released).

Note

Practice 5-5 shows you how to install the NetBEUI protocol on a Windows 2000 computer.

ProtocolAppleTalk and systemMac OS

Apple has developed a family of protocols AppleTalk for organizing networks based on Macintosh computers running the Mac OS operating system. AppleTalk is a peer-to-peer network protocol, meaning it is designed to exchange data between Macintosh workstations even in the absence of a server. This fact is illustrated in Fig. 5.5, which shows how a switch is used to communicate between Macintosh computers. Novell NetWare, MS-DOS, Microsoft Windows operating systems can work with the AppleTalk protocol 9 x/ M.E. and Windows NT/2000/XP. The first version of the protocol was called AppleTalk Phase I and was released in 1983. In 1989, the version still in use today, AppleTalk Phase II, was developed to enable large numbers of networked computers to operate and interoperate with large, heterogeneous, multi-protocol networks.

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The maximum number of stations in the AppleTalk Phase I network is 254, and for the AppleTalk Phase II network this parameter is several million. Addressing in networks of the first type is carried out using node identification (ID), and in networks of the second type, both the node identifier and the network identifier are used when addressing. The final difference is that the AppleTalk Phase I protocol can only work on networks where there are no other protocols. The AppleTalk Phase II protocol operates on networks with multiple protocols (for example, IPX/SPX and TCP/IP).

Note

Although the AppleTalk protocol was designed as a peer-to-peer protocol, it can be used to exchange data between Mac OS X servers and Windows systems configured to work using this protocol.

ServicesAppleTalk

The AppleTalk protocol includes three basic services:

· remote access to network files using AppleShare File Server programs (in combination with the AppleTalk Filing Protocol);

· Print services based on AppleShare Print Server software (which use the Name Binding Protocol and Printer Access Protocol);

· file services based on AppleShare PC programs for DOS and Windows systems.

AppleTalkand reference modelOSI

In the AppleTalk stack, the original lower-level protocol (according to the OSI model) is the LocalTalk Link Access Protocol, LLAP, operating at the physical and data link layers and providing a legacy access method for data transfer. This uses physical network interfaces designed for the LocalTalk protocol, which can operate in small, slow networks with a maximum number of stations in the network of 32 (for a 300-meter segment with a bus topology). The permissible speed is 230.4 Kbps, which is extremely low for modern network technologies.

The LocalTalk network uses a process called contention to assign addresses. When the Macintosh computer is powered on, it competes with other computers for its address, resulting in a unique host identifier (ID). The next time you turn on the power, the computer may receive a different address.

Access MethodsAppleTalk

Modern AppleTalk Phase II networks use Ethernet or token ring access methods and can use interfaces that are suitable for any other Ethernet or token ring devices. To simplify Ethernet communication, the AppleTalk stack includes a protocol EtherTalk Link Access Protocol, FLAP, operating at the Physical and Data Link levels. With its help, the CSMA/CD access method is implemented in AppleTalk networks with a bus or mixed topology (see chapter 2). Token ring networks use the protocol Token Talk Link Access Protocol, TLAP, also operating at the Physical and Link levels. It uses token passing and a ring/star topology (just like any other token ring network).

Network addressingAppleTalk

Addressing in AppleTalk networks using the ELAP and TLAP protocol is carried out using the protocol AppleTalk Address Resolution Protocol, AARP, which allows you to recognize the physical or MAC addresses of network adapters, so that these addresses can be inserted into AppleTalk frames. (If your Macintosh is configured for AppleTalk and IP, AARP is used to resolve physical and IP addresses.)

Protocols included in the stackAppleTalk

In addition to LLAP, ELAP, TLAP and AARP, there are other protocols that are part of the AppleTalk family. All of them are listed in table. 5.3.

Table 5.3. Protocols included in the stackApple

Abbreviatura	Full title	Description	LevelmodelsOSI
	AppleTalk Address Resolution Protocol	Used to recognize physical (MAC) addresses in Ethernet and Token Ring networks. If IP is used in addition to AppleTalk, AARP resolves computer and domain names to IP addresses	Channel and Network
	AppleTalk Data Stream Protocol	Provides guaranteed transmission of data streams at the receiving node	Session
	AppleTalk Filing Protocol	Allows workstations and servers to communicate with each other at the Application layer	Executive
	AppleTalk Session Protocol	Initiates, maintains and closes connections between stations. Determines the order in which data fragments are transmitted for reliable delivery to the receiving node	Session
	AppleTalk Transaction Protocol	Provides reliable data exchange between two nodes by assigning a connection number to each transaction	Transport
	Datagram Delivery Protocol	Used to deliver and route data between two communicating stations
	EtherTalk Link Access Protocol	Provides Ethernet communications using CSMA/CD access method in bus or mixed topologies	Physical and Channel
	LocalTalk Link Access Protocol	A legacy access method that controls communications at the Physical (via interfaces and cables) and Data Link layers in certain situations (for example, when contention for a unique ID occurs to provide addressing)	Physical and Channel
	Name Binding Protocol	Manages computer names and IP address registration, allowing clients to associate network services and processes with specific computer names	Transport
	Printer Access Protocol	Opens and closes communication sessions and provides network data transfer for print services	Session
	Routing Table Maintenance Protocol	Used to obtain network routing information when updating routing tables
	TokenTalk Link Access Protocol	Provides operation of token networks with ring/star topology	Physical and Channel
	Zone Information Protocol	Maintains a table of zones into which AppleTalk networks are divided and their corresponding routing tables	Session

CompatibilityAppleTalkWith systemsMac OS X,Windows 2000AndNetware

The native server platform for Macintosh computers is Mac OS X Server, which is based on the Mac OS X operating system. It allows you to share files and printers, manage network users and groups, and provide web services. Mac OS X and Mac OS X Server systems support both AppleTalk and TCP/IP.

A NetWare or Windows 2000 server can be used as a server for Macintosh computers if AppleTalk Phase II is available. For example, in order for a Windows 2000 server to be installed on a Macintosh computer network, the following components must be installed on it:

· AppleTalk Phase II;

· File Services for Macintosh;

· Print Services for Macintosh.

Once the AppleTalk protocol is installed, Windows 2000 Server will be able to communicate with Macintosh computers configured to run AppleTalk Phase II. File Services for Macintosh allows you to allocate disk space on a Windows 2000 server on which Macintosh computers can store files using the AppleTalk protocol. Print Services for Macintosh allows Macintosh computers to access network printers supported by a Windows 2000 server.

Practice 5-6 will show you how to install the AppleTalk Phase II protocol, File Services for Macintosh, and Print Services for Macintosh on a Windows 2000 Server system.

Note

The Mac OS X and Mac OS X Server operating systems are based on the UNIX kernel and even have a terminal window mode in which you can run numerous UNIX commands.

TCP/IP protocoland various server systems

Transmission Control Protocol/ Internet Protocol, TCP/ IP(Transmission Control Protocol/Internet Protocol) is the most common protocol stack currently used and is also the Internet Protocol. This section provides only a brief overview of TCP/IP in the context of a general understanding of the most important protocols. The TCP/IP stack is discussed in more detail in Chapter 6.

Most network server and workstation operating systems support TCP/IP, including NetWare servers, all Windows systems, UNIX, recent versions of Mac OS, IBM's OpenMVS and z/OS systems, and DEC's OpenVMS. In addition, network equipment manufacturers create their own TCP/IP system software, including tools to improve device performance. The TCP/IP stack was originally used on UNIX systems and then quickly spread to many other types of networks.

Advantages of TCP/IP

Among the many benefits of the TCP/IP stack are the following:

· it is used in many networks and on the Internet, which makes it the international language of network communications;

· there are many network devices designed to work with this protocol;

· many modern computer operating systems use TCP/IP as the main protocol;

· For this protocol there are many diagnostic tools and analyzers;

· Many network specialists are familiar with the protocol and know how to use it.

Protocols and applications,included in the TCP/IP stack

In table 5.4 lists the protocols and applications included in the TCP/IP stack. Some of them have already been discussed earlier. A more detailed description is available in chapter b, and also in subsequent chapters.

Table 5.4. Protocols and applications included in the TCP/IP protocol stack

Abbreviation	Full title	Description	Model levelOSI
	Address Resolution Protocol	Provides resolution of IP addresses to MAC addresses	Channel and Network
	Domain Name System (application)	Maintains tables that associate computer IP addresses with their names	Transport
	File Transfer Protocol	Used to send and receive files	Session, Executive and Application
	Hypertext Transfer Protocol	Used to transmit data on the World Wide Web	Executive
	Internet Control Message Protocol	Used to generate network error reports, particularly when transmitting data through routers
	Internet Protocol	Controls logical addressing
	Network File System (application)	Used to transfer files over a network (designed for UNIX computers)	Session, Executive and Application
	Open Shortest Path First (protocol)	Used by routers to exchange information (routing data)
	Point-to-Point protocol	Used as a remote access protocol in combination with wide area network technologies
	Routing Information Protocol	Used when collecting routing data to update routing tables
	Remote Procedure Call (application)	Allows a remote computer to execute procedures on another computer (such as a server)	Session
	Serial Line Internet Protocol	Used as a remote access protocol in combination with wide area network technologies
	Simple Mail Transfer Protocol	Used to transmit email	Executive
	Transmission Control Protocol	Connection-oriented protocol that improves data transmission reliability	Transport
	Telecommunications Network (application)	Allows a workstation to emulate a terminal and connect to mainframes, Internet servers and routers	Session, Executive and Application
	User Data Protocol	Connectionless protocol; used as an alternative to TCP in cases where high reliability is not required	Transport

SNA protocol and IBM operating systems

Legacy IBM mainframes typically use stack protocols Systems Network Architecture, SNA, which was originally developed in 1974. In fact, SNA is a set of private protocols that use a token ring as an access method. Many details of the token networks created by IBM were subsequently included in the IEEE 802.5 standard. However, in an SNA network, the cable section is necessarily built on the basis of shielded twisted pair (STP), and the cables have strictly oriented markings (and wiring) (for example, one end of the cable must go to the mainframe, and the other to devices connected to the mainframe, such as controllers of disk drives or communication channels). This means that the SNA network also uses private (proprietary) cable connectors and network interfaces,

Protocol stackSNAand reference modelOSI

The SNA protocol stack is based on a seven-layer model (Table 5.5), reminiscent of the OSI reference model.

Table 5.5. Seven-level modelSNA

LevelSNA	Equivalent levelOSI	Purpose
Transaction Services	Applied	The highest level, manages the services on which application programs depend (for example, distributed databases and applications running simultaneously on multiple mainframes)
Presentation Services	Representative	Controls data formatting and conversion (for example, conversion from ASCII to EBCDIC and vice versa), and also performs data compression (although, unlike the OSI Executive Layer, this layer does not provide data encryption)
Data Flow Control	Session	Establishes and maintains communication channels between nodes, manages data flows and provides recovery from communication errors
Transmission Control	Transport	Ensures the reliability of data transmission from the source node to the receiving node, and also manages data encryption
Path Control		Controls routing and creation of virtual channels, fragments messages into smaller blocks when transmitting data across heterogeneous networks (this task is performed by the OSI Transport Layer)
Data Link Control	Channel channel	Formats data into frames, provides marker access to the network for single-level data exchanges between computers
Physical Device Management (Physical Control)	Physical	Provides generation and encoding of electrical signals, operation of physical interfaces, network topology, and communication media (e.g. cable)

Advantages and Disadvantages of SNA

Like any protocol stack, SNA has both advantages and disadvantages. Noting the advantages, it should be said that the SNA architecture has existed for more than a quarter of a century and provides a reliable and proven means of exchanging data with IBM systems. A significant disadvantage is that SNA is a proprietary protocol stack that requires special devices and additional training in configuration, management and debugging procedures. For these reasons, SNA networks with IBM mainframes usually work very well, but it requires a large investment in staff training and network support.

Physical elements of an SNA network

In a traditional SNA network with IBM computers, terminals are treated as type 2 physical modules. A physical module is some device that can connect to or control access to the mainframe.

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Abbrevia- tour orName

Full title

Description

LevelmodelsSNA

Advanced Peer-to-Peer Networking (Enhanced Peer-to-Peer Networking Protocol)

Provides peer-to-peer interactions between devices such as mainframes, minicomputers, gateways, and cluster controllers

Transmission Control

Customer Information Control System (subscriber information management system)

Data Flow Management and Representative Services

Distributed Data Management

Programs that provide remote access to information stored on IBM mainframes (for example, via a remote connection from another mainframe located at a distance)

Transaction Services

Information Management System (information management system)

A software environment that provides programmers with basic tools for interacting with the SNA architecture (including secure access, file and storage management). An alternative to IMS is CICS

Data Flow Management Representative Services

Network Control Program

Provides physical device addressing and additional logical addressing, as well as routing. Used for and management of SNA gateway communications (must be installed on any SNA gateway in order for workstations to access the mainframe through the gateway; see chapter 1 and 4, where gateways are discussed in more detail)

Channel Control and Route Control

Synchronous Data Link Control

Creates logical connections (virtual channels) in a network cable and coordinates data transfer over these connections, provides half-duplex and full-duplex communication in channels

Physical Device Management and Channel Management

SNA Distributed Services

Software tools that control the transfer of documents. Used by email systems to transmit messages to specified addresses

Transaction Services

System Services Control Point

Software that controls VTAM

Transmission Controls

Access method used by SNA networks

Physical device management Channel management

Virtual Telecommunications Access Method (virtual telecommunications access method)

Controls data transfer on an SNA network (for example, using flow control techniques). Provides digital data exchange

Transmission Control

DLC protocol for accessing IBM operating systems

If you are using Windows computers to access the mainframe running SNA 9 x, Windows NT and Windows 2000, then an alternative to the SNA gateway is to install the protocol Data Link Control, DLC. This protocol emulates SNA, and it can also be used to connect to some legacy network printers that can only work with it (for example, older Hewlett-Packard printers).

Advice

The DLC protocol is not supported on Windows XP. If you are considering upgrading to this system, please note that you will not be able to use the DLC to access IBM mainframes and may need an SNA gateway.

Basically, the DLC protocol is an alternative to TCP/IP in cases where some host uses SNA communications. The disadvantage of this protocol is that it is not routable. Additionally, it is not really designed for peer-to-peer communications between workstations, but only serves to connect to older IBM mainframes (eg ES9000) or IBM minicomputers (eg AS/400). Practice 5-7 shows how to install DLC on Windows 2000.

ProtocolDNAfor operating systemscomputersDigital (Compaq)

Architecture created in 1974 Digital Network Architecture (DNA) is the same age as SNA. DNA was used in the first networks of the Digital Equipment Corporation (DEC) and was otherwise called DECnet. Then this protocol stack was used much less frequently.

The DNA architecture provides for the use of Ethernet II frames (or DIX - an abbreviation for the names of the development companies Digital, Intel and Xerox) in a bus topology. One of the strengths of DNA is that from the very beginning, the architecture closely followed the OSI reference model. The disadvantage of DNA is that this architecture is private. Additionally, after Compaq acquired DEC, the original DEC computers and DNA networks became less popular. Even once-famous DEC Alpha-based computers are increasingly being replaced by Compaq-branded workstations and servers using Intel Itanium processors.

As DNA becomes less common in networks, the likelihood that you will encounter this architecture in practice decreases. However, for a general presentation in table. Section 5.7 lists some of the protocols and applications that make up the DNA stack.

Table 5.7. Protocols and applications included in the protocol stack

Abbreviation	Full title	Description	Model levelOSI
	Connectionless-Mode Network Service	Provides connectionless services (see chapter 2), as well as routing
	Connection Oriented Network Service	Provides connection-oriented services for routing and routing error control
	Digital Data Communications Message Protocol	Ensures that services operate with connection establishment and error control. At the level of electrical signals, it allows for half-duplex and full-duplex communication	Physical Channel (LLC sublayer)
	File Transfer, Access, and Management (file transfer, access and management)	Allows you to transfer files with text and binary content	Applied
	High-Level Data Link Control	Creates logical connections (virtual channels) in a network cable and coordinates data transfer between them. Controls the formatting of frames	Physical and Channel
		Complies with X.400 standard for postal services	Applied
	Naming Service	Provides network devices with naming services that translate a device's address into its name and vice versa (making it easier for users to work with devices)	Applied
	Network Virtual Terminal (network virtual terminal service)	Translates characters between Service terminals, DNA networks and host computers	Executive and Application

Improving the performance of local networks

The easiest way to improve network performance is to reduce the number of protocols sent through each router. This reduces the workload on routers, allowing them to process network traffic faster. With fewer protocols, there is also less unnecessary traffic generated on the network.

Issues for discussion

When choosing the protocols to use on your network, consider the following questions.

· Should packets be routed?

· What size is the network – small (less than 100 nodes), medium (100 – 500 nodes) or large (over 500 nodes)?

· What servers are used and what protocols do they require?

· Are there mainframes and what protocols do they require?

· Is there direct access to the Internet or connection to intranet applications using web technologies (virtual private network)?

· What speed is required for connections to the global network?

· Are there critical applications?

If frames need to be routed (for example, on a corporate network), then the best protocol to use is TCP/IP, since it is routing-oriented and common in many networks. For small and medium-sized non-routable networks (less than 200 nodes) based on Windows NT servers and in the absence of an Internet connection, the NetBEUI protocol remains the best choice, providing fast and reliable communications. On NetWare networks (with servers earlier than 5.0), you can use IPX/SPX, although on a mixed network with older NetWare servers and newer Windows 2000 servers, you may need IPX/SPX and TCP/IP protocols. The NWLink protocol is a good way to connect Windows 9x/NT/2000 systems to older NetWare servers.

Communication channel problem

Having a connection to the Internet or web services requires TCP/IP to be deployed, and FTP services can be used to transfer files. TCP/IP is also best used for communications with temporary mainframes and UNIX computers, since connecting to a mainframe or to an application running on a UNIX computer may require Telnet terminal emulation. You can also use the DLC protocol to connect to IBM mainframes and minicomputers (if they are running in an SNA environment). Finally, DNA protocol may still be needed on a network containing older DEC computers (eg DEC VAX).

Note

TCP/IP is the best protocol for medium and large networks. It is routable, robust for mission-critical applications, and has a robust error control mechanism. In such networks, it is important to have network monitoring and fault analysis tools. As stated in chapter 6, the TCP/IP stack has the protocols necessary to solve such problems.

In many cases, different network applications require different LAN protocols. Sometimes in modern networks, TCP/IP, NetBEUI, IPX/SPX, SM and even DNA protocols are used in any combination. As you already know, the protocols deployed are related to the type of operating systems used. Their choice is also influenced by the availability of connections to global networks (for example, to access the Internet you need the TCP/IP protocol, which may also be required to connect local networks to each other via a global network). If, say, TCP/IP is used by servers on one LAN, and workstations on another network must access those servers, then both LANs and the connecting WAN must support TCP/IP protocol transmission.

Removing unnecessary protocols

Sometimes workstations on a network remain configured to use multiple protocols even after all hosts and servers have been converted to TCP/IP. In this case, you can easily improve network performance by removing unnecessary protocols from workstations. Practice Exercise 5-8 teaches you how to remove DLC from Windows 2000, and Practice 5-9 teaches you how to remove Client Service for NetWare (and NWLink IPX/SPX) from Windows 2000 and Windows XP Professional.

Summary

· To a large extent, the architecture of networks is determined by protocols. Many networks use multiple protocols to access the various operating systems of network servers and host computers.

· Generally, the LAN protocols used are determined by the type of network server operating system used on a particular network. One of the oldest network systems is NetWare, which works with the IPX/SPX protocol stack and provides data transfer between older versions of NetWare servers and workstations (as well as other servers) connected to the servers. The IPX/SPX protocol is implemented in thousands of local networks, since NetWare is one of the common network operating systems. However, nowadays, due to the fact that many networks are connected to the Internet, new versions of NetWare (5.0 and higher) are focused on working with the more universal TCP/IP protocol stack.

· The native protocol for Windows NT Server systems is NetBEUI, the emergence of which is associated with the development of the LAN Manager network operating system, which Microsoft began jointly with IBM. Medium and large networks with Windows NT servers often use the TCP/IP stack. With the advent of Windows 2000 and Windows Server 2003, TCP/IP replaced NetBEUI due to the requirements of the Active Directory service and the need for Internet access.

· AppleTalk is a protocol used by Macintosh computers running the Mac OS and Mac OS Server operating systems. Windows NT, Windows 2000, Windows Server 2003, and Novell NetWare also support AppleTalk.

· Some network server operating systems (in particular, UNIX) were initially designed to work with the TCP/IP stack (as well as the Internet). Other network operating systems (such as NetWare, Windows NT, and Mac OS Server) implemented the TCP/IP stack after those systems were created.

· Early IBM systems used the SNA protocol stack, which provided data exchange between mainframes (minicomputers) and terminals, controllers and printers, as well as between different computers. Windows operating systems have the ability to install the DLC protocol to emulate SNA communications.

· The DNA protocol stack was designed for use on DEC computer-based networks, but is rarely used today as the number of such computers on networks has decreased significantly.

· A simple and effective way to improve the performance of a local network is to periodically analyze the protocols used and remove those protocols that are no longer used. For access to computers and printers.

· Up until the early 1990s, networking technologies primarily focused on local area network protocols. Currently, the architecture of these protocols has found its logical conclusion in the TCP/IP stack, and private protocols (such as IPX/SPX and NetBEUI) are used less frequently.

3.1.1. General characteristics of local network protocols

When organizing the interaction of nodes in local networks, the main role is given to the link layer protocol. However, in order for the link layer to cope with this task, the structure of local networks must be quite specific, for example, the most popular link layer protocol - Ethernet - is designed for parallel connection of all network nodes to a common bus for them - a piece of coaxial cable or hierarchical tree structure of segments formed by repeaters. The Token Ring protocol is also designed for a very specific configuration - connecting computers in the form of a logical ring.

This approach, which consists in using simple structures of cable connections between computers on a local network, corresponded to the main goal that the developers of the first local networks set for themselves in the second half of the 70s. This goal was to find a simple and cheap solution for connecting several dozen computers located within the same building into a computer network. The solution had to be inexpensive, since inexpensive computers were connected into the network - mini-computers that appeared and quickly spread at that time costing $10,000-20,000. The number of them in one organization was small, so a limit of several dozen (maximum - up to a hundred) computers seemed quite sufficient for the growth of almost any local network.

To simplify and, accordingly, reduce the cost of hardware and software solutions, the developers of the first local networks settled on the joint use

182 Chapter 3 Basic technologies of local networks

using cables by all computers on the network in time sharing mode, that is, TDM mode. The cable sharing mode most clearly manifests itself in classic Ethernet networks, where the coaxial cable physically represents an indivisible piece of cable common to all network nodes. But even in Token Ring and FDDI networks, where each neighboring pair of computers is connected, it would seem, by their own individual sections of cable to a hub, these sections cannot be used by the computers that are directly connected to them at any point in time. These segments form a logical ring, access to which as a single whole can be obtained only by a very specific algorithm in which all computers on the network participate. Using the ring as a common shared resource simplifies the algorithms for transmitting frames over it, since at any given time the ring is occupied by only one computer.

The use of shared media simplifies the logic of the network. For example, there is no need to control the overflow of network nodes with frames from many stations that decided to simultaneously exchange information. In global networks, where sections of cables connecting individual nodes are not considered a common resource, such a need arises, and to solve this problem, very complex frame flow control procedures are introduced into information exchange protocols to prevent overflow of communication channels and network nodes.

The use of very simple configurations (common bus and ring) in local networks, along with positive ones, also had negative consequences, the most unpleasant of which were limitations on performance and reliability. The presence of only one path for transmitting information, shared by all network nodes, in principle limited the network capacity to the throughput of this path (which was divided on average by the number of computers on the network), and the reliability of the network - to the reliability of this path. Therefore, as the popularity of local networks increased and their scope of application expanded, special communication devices - bridges and routers - began to be used more and more, which significantly removed the limitations of a single shared data transmission medium. Basic configurations in the form of a common bus and ring have evolved into elementary local network structures that can now be connected to each other in more complex ways, forming parallel primary or backup paths between nodes.

However, within the underlying structures, the same shared single media protocols that were developed more than 15 years ago still operate. This is due to the fact that the good speed and reliability characteristics of local network cables have satisfied for all these years users of small computer networks, who could build a network without high costs using only network adapters and cables. In addition, the colossal installation base of equipment and software for Ethernet and Token Ring technologies has contributed to the development of the following approach: within small segments, old protocols are used in their unchanged form, and the integration of such segments into a common network occurs using an additional and rather complex equipment.

In the last few years, there has been a movement towards the abandonment of shared data transmission media in local networks and the transition to the use of active switches.

3.1. Protocols and standards of local networks 183

tori to which end nodes are connected by individual communication lines. In its pure form, this approach is offered in ATM (Asynchronous Transfer Mode) technology, and in technologies bearing traditional names with the switched prefix (switched): switched Ethernet, switched Token Ring, switched FDDI, a mixed approach is usually used, combining shared and individual transmission media data. Most often, end nodes are connected into small, separable segments using repeaters, and the segments are connected to each other using individual switched links.

There is also a fairly noticeable tendency towards the use of so-called microsegmentation in traditional technologies, when even end nodes are immediately connected to the switch by individual channels. Such networks are more expensive than shared or mixed networks, but their performance is higher.

When using switches, traditional technologies have a new mode of operation - full-duplex. In a shared segment, stations always operate in half-duplex mode, since at every moment of time the station’s network adapter either transmits its own data or receives someone else’s, but never does it at the same time. This is true for all LAN technologies, since shared environments are supported not only by the classic LAN technologies Ethernet, Token Ring, FDDI, but also by all the new ones - Fast Ethernet, lOOVG-AnyLAN, Gigabit Ethernet.

In full duplex mode, the network adapter can simultaneously transmit its data to the network and receive other people's data from the network. This mode is easily achieved with a direct connection to a bridge/switch or router, since the input and output of each port of such a device operate independently of each other, each with its own frame buffer.

Today, every LAN technology is designed to operate in both half-duplex and full-duplex modes. In these modes, the restrictions imposed on the total length of the network are significantly different, so that the same technology can allow the construction of very different networks depending on the selected operating mode (which depends on whether repeaters or switches are used to connect nodes) . For example, Fast Ethernet technology allows for half-duplex mode to build networks with a diameter of no more than 200 meters, and for full-duplex mode there are no restrictions on network diameter. Therefore, when comparing different technologies, it is necessary to take into account the possibility of their operation in two modes. This chapter focuses on the half-duplex mode of operation of protocols, and the full-duplex mode is discussed in the next chapter, in conjunction with the study of switches.

Despite the emergence of new technologies, the classic Ethernet and Token Ring local network protocols, according to experts, will be widely used for at least another 5-10 years, and therefore knowledge of their details is necessary for the successful use of modern communication equipment. In addition, some modern high-performance technologies, such as Fast Ethernet, Gigabit Ethernet, largely maintain continuity with their predecessors. This once again confirms the importance of studying classic local network protocols, naturally, along with studying new technologies.

184 Chapter 3 Basic technologies of local networks

3.1.2. Structure of IEEE 802.x standards

In 1980, the IEEE Institute established Committee 802 on the standardization of local area networks, which resulted in the adoption of the IEEE 802.x family of standards, which contain recommendations for the design of lower layers of local area networks. Later, the results of the work of this committee formed the basis of a set of international standards ISO 8802-1.„5. These standards were created based on the very common proprietary Ethernet networking standards, ArcNet and Token Ring.

In addition to IEEE, other organizations also took part in the work on standardizing local network protocols. Thus, for networks operating on optical fiber, the American standardization institute ANSI developed the FDDI standard, providing a data transfer rate of 100 Mb/s. Work on standardization of protocols is also carried out by the ECMA association, which adopted the ECMA-80, 81, 82 standards for a local Ethernet network and subsequently the ECMA-89, 90 standards for the token passing method.

The IEEE 802.x family of standards covers only the lower two layers of the seven-layer OSI model - physical and data link. This is due to the fact that these levels most reflect the specifics of local networks. The senior levels, starting with the network level, largely have common features for both local and global networks.

The specifics of local networks are also reflected in the division of the data link layer into two sublevels, which are often also called levels. The Data Link Layer is divided into two sublevels in local networks:

Logical Data Transfer (Logical Link Control, LLC);

Media Access Control (MAC).

MAC level appeared due to the existence of a shared data transmission medium in local networks. It is this level that ensures the correct sharing of the common medium, placing it at the disposal of one or another network station in accordance with a certain algorithm. After access to the medium is obtained, it can be used by a higher level - the LLC level, which organizes the transfer of logical units of data, frames of information, with different levels of quality of transport services. In modern local networks, several MAC level protocols have become widespread, implementing various algorithms for accessing the shared medium. These protocols completely define the specifics of such technologies as Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, FDDI, lOOVG-AnyLAN.

LLC level is responsible for transmitting data frames between nodes with varying degrees of reliability, and also implements the functions of the interface with the adjacent network layer. It is through the LLC layer that the network protocol requests from the data link layer the transport operation it needs with the required quality. At the LLC level, there are several operating modes that differ in the presence or absence of procedures for restoring frames at this level in the event of their loss or distortion, that is, differing in the quality of transport services at this level.

The MAC and LLC layer protocols are mutually independent - each MAC layer protocol can be used with any LLC layer protocol, and vice versa.

This structure is the result of a lot of work carried out by the 802 committee to highlight common approaches and common functions across different proprietary technologies, as well as harmonize the styles of their description. As a result, the data link layer was divided into the two mentioned sublayers. The description of each technology is divided into two parts: a description of the MAC layer and a description of the physical layer. As can be seen from the figure, for almost every technology, a single MAC layer protocol corresponds to several variants of physical layer protocols (in order to save space, in the figure, in order to save space, only Ethernet and Token Ring technologies are shown, but everything said is also true for other technologies, such as ArcNet, FDDI, lOOVG-AnyLAN).

Above the data link layer of all technologies is the common LLC protocol, which supports several operating modes, but is independent of the choice of a specific technology. The LLC standard is overseen by the 802.2 subcommittee. Even non-802-standardized technologies rely on the LLC protocol defined by the 802.2 standard, such as the ANSI-standardized FDDI protocol.

The standards developed by the 802.1 subcommittee stand apart. These standards are common to all technologies. The 802.1 subcommittee developed general definitions of local networks and their properties, and defined the connection between the three layers of the IEEE 802 model and the OSI model. But the most practically important

186 Chapter 3 Basic technologies of local networks

These are 802.1 standards, which describe how different technologies interact with each other, as well as standards for building more complex networks based on basic topologies. This group of standards is collectively called internetworking standards. This includes such important standards as the 802.ID standard, which describes the logic of the bridge/switch, the 802.1H standard, which defines the operation of the broadcast bridge, which can combine Ethernet and FDDI networks, Ethernet and Token Ring, etc. without a router. Today, a set of standards developed by the 802.1 subcommittee continues to grow. For example, it recently added the important 802.1Q standard, which defines a way to build VLANs in switch-based networks.

The 802.3, 802.4, 802.5 and 802.12 standards describe local network technologies that emerged as a result of improvements in the proprietary technologies that formed their basis. Thus, the 802.3 standard was based on Ethernet technology developed by Digital, Intel and Xerox (or Ethernet DIX), the 802.4 standard appeared as a generalization of Datapoint Corporation's ArcNet technology, and the 802.5 standard mainly corresponds to IBM's Token Ring technology.

The original proprietary technologies and their modified versions - the 802.x standards - in some cases existed in parallel for many years. For example, ArcNet technology has not been fully brought into compliance with the 802.4 standard (now it is too late to do this, since somewhere around 1993 the production of ArcNet equipment was curtailed). Discrepancies between Token Ring technology and the 802.5 standard also arise periodically, as IBM regularly makes improvements to its technology and the 802.5 committee reflects these improvements in the standard with some delay. The exception is Ethernet technology. The last proprietary Ethernet standard, DIX, was adopted in 1980, and no proprietary development of Ethernet has been attempted since then. All innovations in the Ethernet family of technologies are made only as a result of the adoption of open standards by the 802.3 committee.

Later standards were initially developed not by one company, but by a group of interested companies, and then submitted to the appropriate IEEE 802 subcommittee for approval. This happened with Fast Ethernet, lOOVG-AnyLAN, and Gigabit Ethernet technologies. A group of interested companies first formed a small association, and then, as the work progressed, other companies joined it, so the process of adopting the standard was open.

Today, the 802 Committee includes the following series of subcommittees, which include those already mentioned and some others:

802.1 - Internetworking - networking;

802.2 - Logical Link Control, LLC - control of logical data transfer;

802.3 - Ethernet with CSMA/CD access method;

802.4 - Token Bus LAN - local networks with the Token Bus access method;

802.5 - Token Ring LAN - local networks with the Token Ring access method;

802.6 - Metropolitan Area Network, MAN - networks of megacities;

802.7 - Broadband Technical Advisory Group - technical advisory group for broadband transmission;

3.1. Protocols and standards of local networks 187

802.8 - Fiber Optic Technical Advisory Group - technical advisory group on fiber-optic networks;

802.9 - Integrated Voice and data Networks - integrated voice and data networks;

o 802.10 - Network Security - network security;

802.11 - Wireless Networks - wireless networks;

802.12 - Demand Priority Access LAN, lOOVG-AnyLAN - local networks with a demand access method with priorities.

» When organizing the interaction of nodes in local networks, the main role is given to the classic technologies Ethernet, Token Ring, FDDI, developed more than 15 years ago and based on the use of shared media.

Shared environments are supported not only by the classic technologies of Ethernet local networks, Token Ring, FDDI, but also by new ones - Fast Ethernet, lOOVG-AnyLAN, Gigabit Ethernet.

The modern trend is a partial or complete rejection of shared environments: connecting nodes with individual connections (for example, in ATM technology), widespread use of switched connections and micro-segmentation. Another important trend is the emergence of a full-duplex operating mode for almost all local network technologies.

The IEEE 802.x Committee develops standards that contain recommendations for the design of the lower layers of local networks - physical and data link. The specifics of local networks are reflected in the division of the data link layer into two sublevels - LLC and MAC.

The standards of the 802.1 subcommittee are common to all technologies and are constantly updated. Along with the definition of local networks and their properties, standards for internetworking, and a description of the logic of the bridge/switch, the results of the committee’s work also include the standardization of the relatively new technology of virtual local area networks VLAN.

» The 802.2 subcommittee developed and maintains the LLC standard. Standards 802.3, 802.4, 802.5 describe local network technologies that emerged as a result of improvements in proprietary technologies that formed their basis, respectively Ethernet, ArcNet, Token Ring.

Later standards were initially developed not by one company, but by a group of interested companies, and then submitted to the appropriate IEEE 802 subcommittee for approval.

188 Chapter 3 Basic technologies of local networks

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The TCP/IP protocol stack is a set of protocols, its name comes from the two most important protocols that are the basis of communication on the Internet. The TCP protocol breaks the transmitted information into portions (packets) and numbers them. Using the IP protocol, all packets are transmitted to the recipient. Next, using the TCP protocol, it is checked whether all packets have been received. When receiving all the portions, TCP places them in the required order and assembles them into a single whole. There are two versions of this protocol used on the Internet:

• Routed network protocol IPv4. In this version of the protocol, each network node is assigned an IP address 32 bits long (i.e. 4 octets or 4 bytes).
• IPv6 allows you to address a significantly larger number of nodes than IPv4. Internet Protocol version 6 uses 128-bit addresses, and can define significantly more addresses.

Note

IPv6 IP addresses are 128 bits long and therefore four times longer than IPv6 IP addresses. v6 IP addresses are written as follows: X:X:X:X:X:X:X:X, where X is a 4-digit hexadecimal number (16 bits), and each number is 4 bits long . Each number ranges from 0 to F. Here is an example of a version 6 IP address: 1080:0:0:0:7:800:300C:427A. In such a record, insignificant zeros can be omitted, so the address fragment: 0800: is written as 800:.

ARP

For network devices to interact with each other, it is necessary that the sending device have the recipient's IP and MAC addresses. The TCP/IP protocol suite includes a special protocol called ARP (Address Resolution Protocol), which allows you to automatically obtain a MAC address from known IP addresses

DHCP protocol

The distribution of IP addresses for connecting to the Internet is carried out by providers, and in local networks - by system administrators. Assigning IP addresses to network nodes when the network size is large is a very tedious procedure for the administrator. Therefore, to automate the process, the Dynamic Host Configuration Protocol (DHCP) was developed, which frees the administrator from these problems by automating the process of assigning IP addresses to all network nodes.

HTTP protocol

The HTTP protocol is used to transfer hypertext, i.e. to transfer Web pages from one computer to another. The basis of HTTP is the client-server technology, that is, it assumes the existence of consumers (clients) who initiate a connection and send a request, and providers (servers) who wait for a connection to receive a request, perform the necessary actions and return a message with the result.

FTP protocol

FTP is a protocol for transferring files from a special file server to the user’s computer. Having established a connection with a remote computer, the user can copy a file from the remote computer to his own or copy a file from his computer to the remote one.

POP protocol

POP standard protocol receiving a mail connection. POP servers process incoming mail, and the POP protocol is designed to handle mail requests from client mail programs.

SMTP protocol

SMTP is a protocol that defines a set of rules for sending mail. The SMTP server returns either an acknowledgment or error message, or requests additional information.

IP address via IPv4 protocol

One of the most important topics when considering TCP/IP is IP addressing. IP address is a numeric identifier assigned to each computer on an IP network and designating the location on the network of the device to which it is assigned. An IP address is a software address, not a hardware address. The host IP address identifies the IP module's access point to the network interface, not the entire machine.

IP address - network (software) address of a node in a computer network built using the IP protocol.

Each of the 4 octets of the decimal notation of an IP address can take a value in the range from 0 to 255, and in theory such an address in decimal notation can be in the range from 0.0.0.0 to 255.255.255.255. IP address is a binary number, but for a person instead of writing it in 32

As was shown earlier, when exchanging information on a network, each layer of the OSI model responds to its own header. In other words, there is interaction between the same levels of the model in various subscriber computers. Such interaction must follow certain rules.

Protocol-- a set of rules that determine the interaction of two levels of the same name in the open systems interaction model in various subscriber computers.

A protocol is not a program. The rules and sequence of actions during information exchange, defined by the protocol, must be implemented in the program. Typically, the functions of protocols at various levels are implemented in drivers for various computer networks.

In accordance with the seven-level structure of the model, we can talk about the need for the existence of protocols for each level.

The open systems concept involves the development of standards for protocols at various levels. The protocols of the three lower levels of the open systems architecture model are the easiest to standardize, since they define the actions and procedures characteristic of computer networks of any class.

It is most difficult to standardize protocols at the upper levels, especially the application level, due to the multiplicity of application tasks and, in some cases, their uniqueness. If in terms of types of structures, methods of access to the physical transmission medium, used network technologies and some other features one can count about a dozen different models of computer networks, then there are no limits in terms of their functional purpose.

Basic types of protocols

It is easiest to imagine the features of network protocols using the example of link-level protocols, which are divided into two main groups: byte-oriented and bit-oriented.

The byte-oriented protocol ensures the transmission of a message over an information channel in the form of a sequence of bytes. In addition to information bytes

Control and service bytes are also transmitted to the channel. This type of protocol is convenient for a computer, since it is focused on processing data presented in the form of binary bytes. Today, the byte-oriented protocol is less convenient in the communication environment, since dividing the information flow in the channel into bytes requires the use of additional signals, which ultimately reduces the throughput of the communication channel.

The most famous and widespread byte-oriented protocol is the BSC (Binary Synchronous Communication) protocol, developed by IBM. The protocol provides the transmission of two types of frames: control and information. Control and service symbols are transmitted in control frames, and messages (individual packets, a sequence of packets) are transmitted in information frames. The BSC protocol operates in three phases: establishing a connection, maintaining a message transfer session, and disconnecting the connection. The protocol requires for each transmitted frame to send a receipt indicating the result of its reception. Frames transmitted with an error are retransmitted. The protocol defines the maximum number of retransmissions.

Note. A receipt is a control frame that contains confirmation that the message was received (positive receipt) or was rejected due to an error (negative receipt).

Transmission of a subsequent frame is possible only when a positive receipt for receiving the previous one is received. This significantly limits the speed of the protocol and places high demands on the quality of the communication channel.

A bit-oriented protocol provides for the transmission of information in the form of a stream of bits that are not divided into bytes. Therefore, special sequences - flags - are used to separate frames. At the beginning of the frame, an opening flag is placed at the end - a closing flag.

The bit-oriented protocol is convenient in relation to the communication environment, since the communication channel is precisely oriented towards transmitting a sequence of bits. It is not very convenient for a computer, because it is necessary to select bytes from the incoming sequence of bits for subsequent message processing. However, given the speed of the computer, we can assume that this operation will not have a significant impact on its performance. Potentially bit-oriented protocols are faster than byte-oriented ones, which makes them widespread in modern computer networks.

A typical representative of the group of bit-oriented protocols is the HDLC (High-level Data Link Control) protocol and its subsets. The HDLC protocol controls the information channel using special control frames in which commands are transmitted. Information frames are numbered. In addition, the HDLC protocol allows you to transmit up to three to five frames into the channel without receiving a positive receipt. A positive receipt received, for example, on the third frame shows that the previous two were received without errors and it is necessary to repeat the transmission of only the fourth and fifth frames. This operating algorithm ensures the high performance of the protocol.

Among the top-level protocols of the OSI model, the X.400 protocol (email) and FTAM (File Transfer, Access and Management - file transfer, file access and file management) should be noted.

A network protocol is a set of rules that allows connection and data exchange between two or more computers connected to a network. In fact, different protocols often describe only different aspects of the same type of communication; taken together, they form the so-called protocol stack. Titles<протокол>And<стек протоколов>also indicate the software that implements the protocol

Protocol levels

The most common classification system for network protocols is the so-called OSI model. In accordance with it, protocols are divided into 7 levels according to their purpose - from physical (generation and recognition of electrical or other signals) to application (API for transferring information by applications):

• Application layer. The upper (7th) level of the model ensures interaction between the network and the user. The layer allows user applications to access network services such as database query processing, file access, and email forwarding. It is also responsible for transmitting service information, providing applications with information about errors, and generating requests to the presentation layer. Example: HTTP, POP3, SMTP.
• Presentation layer. Layer 6 is responsible for protocol conversion and data encoding/decoding. It converts application requests received from the application layer into a format for transmission over the network, and converts data received from the network into a format that applications can understand. The presentation layer can perform compression/decompression or encoding/decoding of data, as well as redirecting requests to another network resource if they cannot be processed locally.
• Session layer. Level 5 of the model is responsible for maintaining a communication session, which allows applications to interact with each other for a long time. The session layer manages session creation/termination, information exchange, task synchronization, determination of data transfer rights, and session maintenance during periods of application inactivity. Transmission synchronization is ensured by placing checkpoints in the data stream, starting from which the process is resumed if interaction is disrupted.
• Transport layer. The 4th level of the model is designed to deliver data without errors, losses and duplication in the sequence in which they were transmitted. It does not matter what data is transmitted, from where and where, that is, it provides the transmission mechanism itself. It divides data blocks into fragments, the size of which depends on the protocol, combines short ones into one, and splits long ones. Protocols at this level are designed for point-to-point communication. Example: TCP, UDP
• Network layer. Layer 3 of the OSI network model is designed to determine the data transmission path. Responsible for translating logical addresses and names into physical ones, determining the shortest routes, switching and routing, monitoring problems and congestion in the network. A network device such as a router operates at this level.
• Data Link layer. This level is often called the channel level. This layer is designed to ensure the interaction of networks at the physical layer and control errors that may occur. It packs the data received from the physical layer into frames, checks for integrity, corrects errors if necessary, and sends it to the network layer. The data link layer can communicate with one or more physical layers, monitoring and managing this interaction. The IEEE 802 specification divides this layer into 2 sublayers - MAC (Media Access Control) regulates access to the shared physical medium, LLC (Logical Link Control) provides network layer service. Switches and bridges operate at this level. In programming, this level represents the network card driver; in operating systems there is a software interface for the interaction of the channel and network layers with each other; this is not a new level, but simply an implementation of the model for a specific OS. Examples of such interfaces: ODI, NDIS
• Physical layer. The lowest level of the model is intended directly for transmitting the data stream. Transmits electrical or optical signals into a cable or radio broadcast and, accordingly, receives and converts them into data bits in accordance with digital signal encoding methods. In other words, it provides an interface between the network media and the network device. At this level, signal concentrators (hubs), signal repeaters (repeaters) and media converters operate. Physical layer functions are implemented on all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter or serial port.

The TCP/IP protocol is mainly used