Chapter 5 Introduction to OSI Reference Model 1 Chapter 5 Introduction to Open System Interconnection Reference Model Introduction The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process. We will provide you with an understanding of each of the seven layers, including their functions and their relationships to each other. This will provide you with an overview of the network process, which can then act as a framework for understanding the details of computer networking. Finally, this chapter will draw comparisons between the theoretical OSI model and the functional TCP/IP model. Although TCP/IP has been used for network communications before the adoption of the OSI model, it supports the same functions and features in a differently layered arrangement. A networking model offers a generic means to separate computer networking functions into multiple layers.
Chapter 5 Introduction to OSI Reference Model 2 Each of these layers relies on the layers below it to provide supporting capabilities and performs support to the layers above it. Such a model of layered functionality is also called a protocol stack or protocol suite. Protocols, or rules, can do their work in either hardware or software or, as with most protocol stacks, in a combination of the two. The nature of these stacks is that the lower layers do their work in hardware or firmware (software that runs on specific hardware chips) while the higher layers work in software. The Open System Interconnection model is a seven-layer structure that specifies the requirements for communications between two computers. The ISO (International Organization for Standardization) standard 7498-1 defined this model. This model allows all network elements to operate together, no matter who created the protocols and what computer vendor supports them. OSI Model Main Benefits The main benefits of the OSI model include the following: Helps users understand the big picture of networking Helps users understand how hardware and software elements function together Makes troubleshooting easier by separating networks into manageable pieces Defines terms that networking professionals can use to compare basic functional relationships on different networks Helps users understand new technologies as they are developed Aids in interpreting vendor explanations of product functionality OSI Layer Repartition The seven layers of the OSI reference model can be divided into two categories: upper layers and lower layers as shown in Fig. 1.2.2. The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. Both users and application layer processes interact with software applications that contain a communications component. The term upper layer is sometimes used to refer to any layer above another layer in the OSI model. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to
Chapter 5 Introduction to OSI Reference Model 3 the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium. Fig.2: Two sets of layers make up the OSI layers OSI Model and Communication between Systems Information being transferred from a software application in one computer system to a software application in another must pass through the OSI layers. For example, if a software application in System A has information to transmit to a software application in System B, the application program in System A will pass its information to the application layer (Layer 7) of System A. The application layer then passes the information to the presentation layer (Layer 6), which relays the data to the session layer (Layer 5), and so on down to the physical layer (Layer 1). At the physical layer, the information is placed on the physical network medium and is sent across the medium to System B. The physical layer of System B removes the information from the physical medium, and then its physical layer passes the information up to the data link layer (Layer 2), which passes it to the network layer (Layer 3), and so on, until it reaches the application layer (Layer 7) of System B. Finally, the application layer of System B passes the information to the recipient application
Chapter 5 Introduction to OSI Reference Model 4 program to complete the communication process. Fig. 3 The "bottom up" communication process. Interaction between OSI Model Layers A given layer in the OSI model generally communicates with three other OSI layers: the layer directly above it, the layer directly below it, its peer layer in other networked computer systems. The data link layer in System A, for example, communicates with the network layer of System A, the physical layer of System A, and the data link layer in System B. Fig. 4 illustrates this example.
Chapter 5 Introduction to OSI Reference Model 5 Fig. 4 OSI Model Layers Communicate with Other Layers
Chapter 5 (Cont.) Functions of the OSI Layers 6 Chapter 5 (Cont.) Functions of the Open System Interconnection Layers In the following, we introduce the functions of each OSI layer. Layer 1 The Physical Layer The physical layer of the OSI model defines connector and interface specifications, as well as the medium (cable) requirements. Electrical, mechanical, functional, and procedural specifications are provided for sending a bit stream on a computer network. Layer 2 The Data Link Layer Layer 2 of the OSI model provides the following functions: Allows a device to access the network to send and receive messages Offers a physical address so a device s data can be sent on the network Works with a device s networking software when sending and receiving messages Provides error-detection capability
Chapter 5 (Cont.) Functions of the OSI Layers 7 Common networking components that function at layer 2 include: Network interface cards Ethernet and Token Ring switches Bridges NICs have a layer 2 or MAC address. A switch uses this address to filter and forward traffic, helping relieve congestion and collisions on a network segment. Layer 3 The Network Layer Layer 3 of the OSI model provides the following functions: An end-to-end logical addressing system so that a packet of data can be routed across several layer 2 networks (Ethernet, Token Ring, Frame Relay, etc.). Note that network layer addresses can also be referred to as logical addresses. Routers and other networked systems make routing decisions at the network layer:
Chapter 5 (Cont.) Functions of the OSI Layers 8 The Internet Protocol (IP) addresses make networks easier to both set up and connect with one another. The Internet uses IP addressing to provide connectivity to millions of networks around the world. To make it easier to manage the network and control the flow of packets, many organizations separate their network layer addressing into smaller parts known as subnets. Routers use the network or subnet portion of the IP addressing to route traffic between different networks. Each router must be configured specifically for the networks or subnets that will be connected to its interfaces. Routers communicate with one another using routing protocols, such as Routing Information Protocol (RIP) and Open version of Shortest Path First (OSPF), to learn of other networks that are present and to calculate the best way to reach each network based on a variety of criteria (such as the path with the fewest routers). Routers and other networked systems make these routing decisions at the network layer. The network layer accomplishes this via a process known as fragmentation. When passing packets between different networks, it may become necessary to adjust their outbound size to one that is compatible with the layer 2 protocol that is being used. The network layer accomplishes this via a process known as fragmentation. A router s network layer is usually responsible for doing the fragmentation. Diagnostics and Reports All reassembly of fragmented packets happens at the network layer of the final destination system. Two of the additional functions of the network layer are diagnostics and the reporting of logical variations in normal network operation. While the network layer diagnostics may be initiated by any networked system, the system discovering the variation reports it to the original sender of the packet that is found to be outside normal network operation. The variation reporting exception is content validation calculations. If the calculation done by the receiving system does not match the value sent by the originating system, the receiver discards the related packet with no report to the sender. Retransmission is left to a higher layer s protocol. Some basic security functionality can also be set up by filtering traffic using layer 3 addressing on routers or other similar devices. Layer 4 The Transport Layer Layer 4, the transport layer of the OSI model, offers end-to-end communication between end devices through a network. Depending on the application, the transport layer either offers reliable, connection-oriented or connectionless, best-effort communications.
Chapter 5 (Cont.) Functions of the OSI Layers 9 Some of the functions offered by the transport layer include: Application identification Client-side entity identification Confirmation that the entire message arrived intact Segmentation of data for network transport Control of data flow to prevent memory overruns Establishment and maintenance of both ends of virtual circuits Transmission-error detection Realignment of segmented data in the correct order on the receiving side Multiplexing or sharing of multiple sessions over a single physical link The most common transport layer protocols are the connection-oriented TCP transmission Control Protocol (TCP) and the connectionless UDP User Datagram Protocol (UDP). Layer 5 The Session Layer Layer 5, the session layer, provides various services, including tracking the number of bytes that each end of the session has acknowledged receiving from the other end of the session. This session layer allows applications functioning on devices to establish, manage, and terminate a dialog through a network. Session layer functionality includes: Virtual connection between application entities
Chapter 5 (Cont.) Functions of the OSI Layers 10 Synchronization of data flow Creation of dialog units Connection parameter negotiations Partitioning of services into functional groups Acknowledgements of data received during a session Retransmission of data if it is not received by a device Layer 6 The Presentation Layer Layer 6, the presentation layer, is responsible for how an application formats the data to be sent out onto the network. The presentation layer basically allows an application to read (or understand) the message. Examples of presentation layer functionality include: Encryption and decryption of a message for security Compression and expansion of a message so that it travels efficiently Graphics formatting Content translation System-specific translation Layer 7 The Application Layer Layer 7, the application layer, provides an interface for the end user operating a device connected to a network. This layer is what the user sees, in terms of loading an application (such as Web browser or e-mail); that is, this application layer is the data the user views while using these applications. Examples of application layer functionality include:
Chapter 5 (Cont.) Functions of the OSI Layers 11 Support for file transfers Ability to print on a network Electronic mail Electronic messaging Browsing the World Wide Web
Chapter 6 Introduction to Protocols 12 Chapter 6 Introduction to Protocols What is a Network Protocol? A protocol is a set of rules that governs the communications between computers on a network. These rules include guidelines that regulate the following characteristics of a network: access method, allowed physical topologies, types of cabling, speed of data transfer. Protocols define the format, timing, sequence, and error checking used on the network. In other manner, to provide shared access to information and resources, LANs must deliver information that is correct, in proper time sequential order, and understood by the recipient. To accomplish these functions, electrical circuits, error detection systems, error correction systems, information coding, data flow control, data formatting systems, and other hardware/software subsystems must all perform in a cooperative fashion following a set of rules or "protocols". Protocols can be implemented either in hardware or software or a mixture of both. Typically, the lower layers are implemented in hardware, with the higher layers being implemented in software. Protocols could be grouped into suites (or families, or stacks) by their technical functions, or origin of the protocol introduction, or both. A protocol may belong to one or multiple protocol suites, depending on how you categorize it. For example, the Gigabit Ethernet protocol IEEE 802.3z is a LAN (Local Area Network) protocol and it can also be used in MAN (Metropolitan Area Network) communications. The OSI model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. In the context of data networking, a protocol is a formal set of rules and conventions that governs how computers exchange information over a network medium. A protocol implements the functions of one or more of the OSI layers. A wide variety of communication protocols exist. Some of these protocols include LAN protocols,
Chapter 6 Introduction to Protocols 13 WAN protocols, network protocols, and routing protocols. LAN protocols operate at the physical and data link layers of the OSI model and define communication over various LAN media. WAN protocols operate at the lowest three layers of the OSI model and define communication over the various wide-area media. Routing protocols are network layer protocols that are responsible for exchanging information between routers so that the routers can select the proper path for network traffic. Finally, network protocols are the various upper-layer protocols that exist in a given protocol suite. Many protocols rely on others for operation. For example, many routing protocols use network protocols to exchange information between routers. This concept of building upon the layers already in existence is the foundation of the OSI model. Protocols and OSI model The OSI model, and any other network communication model, provides only a conceptual framework for communication between computers, but the model itself does not provide specific methods of communication. Actual communication is defined by various communication protocols. In modern protocol design, protocols are "layered" according to the OSI 7 layer model or a similar layered model. Layering is a design principle which divides the protocol design into a number of smaller parts, each part accomplishing a particular sub-task and interacting with the other parts of the protocol only in a small number of well-defined ways. Layering benefits in networks Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple. Layering also permits familiar protocols to be adapted to unusual circumstances. A multitude of different transport protocols are available on Windows, such as TCP, UDP, IPX, and SPX. Each protocol behaves differently. Some require a connection to be established before sending or receiving data. Others don't guarantee the reliability or integrity of the data. Protocol Key Elements A protocol defines what is communicated, how is it communicated and when is it communicated. The key elements of a protocol are: Syntax, Semantics,
Chapter 6 Introduction to Protocols 14 Timing. Let's explain these elements: Syntax The term syntax refers to the structure or format of the data, meaning the order in which they are presented. Example: A simple protocol might expect the first 8 bits of data to be the address of the sender, the second 8 bits to be the address of the receiver, and the rest of the stream to be the message itself. Protocol Key Elements Syntax Semantics Timing Semantic The term semantic refers to the meaning of each section of bits: How a particular pattern to be interpreted, What action is to be taken based on that interpretation? Example: Does an address identify the route to be taken or the final destination of a message? Timing The term timing refers to two characteristics: When data should be sent, How fast they can be sent.
Chapter 6 Introduction to Protocols 15 Example: If a sender produces at 100 Mbps but the receiver can process data at only 1 Mbps, the transmission will overload the receiver and some data will be lost. Some Protocol Types A protocol provides either connection-oriented services or connectionless services and can be routable and non-routable. Let's see these types: Protocol Types Connection-oriented Routable Connectionless Non-routable o Connection-Oriented protocols In connection-oriented services, a path is established between the two communicating parties before any data is exchanged. This ensures that there is a route between the two parties in addition to ensuring that both parties are alive and responding. In addition, most connection-oriented protocols guarantee delivery, which increases overhead as additional computations are performed to verify correctness. o Connectionless protocols On the other hand, a connectionless protocol makes no guarantees that the recipient is listening. A connectionless service is similar to the postal service: the sender addresses a letter to a particular person and puts it in the mail. The sender doesn't know if the recipient is expecting to receive a letter or if severe storms are preventing the post office from delivering the message.
Chapter 6 Introduction to Protocols 16 o Routable protocols If a protocol is routable, a successful communication path can be set up (either a virtual connectionoriented circuit or a data path for datagram communication) between two workstations, no matter what network hardware lies between them. For example, machine A is on a separate network from machine B. A router linking the two networks separates the two machines. A routable protocol realizes that machine B is not on the same network as machine A; therefore, the protocol directs the data to the router, which decides how to best forward it so that it reaches machine B. Routable protocols, are communications protocol that contains a network address as well as a device address. It allows packets [of information] to be forwarded from one network to another." This means information can be routed anywhere -- it uses both an exterior and interior routing system to transmit data. Non-routable protocols A non-routable protocol is not able to make such provisions; the router drops any packets of nonroutable protocols that it receives. The router does not forward a packet from a non-routable protocol even if the packet's intended destination is on the connected subnet. NetBEUI is the only A Three-Layer Model. Non-routable protocols, which regulate the transfer of data, always use interior routing systems as a way to transmit this data. A non-routable protocol is "a communications protocol that contains only a device address and not a network address. It does not incorporate an addressing scheme for sending data from one network to another." This means that these protocols are very specific about where information can routed, and it must be within the interior network. Some people may find this type of protocol very limited. A three Layer Model In very general terms, communications can be said to involve three agents: applications, computers, networks. One example of an application is a file transfer operation. These applications execute on computers that can often support multiple simultaneous applications. Computers are connected to networks, and the data to be exchanged are transferred by the network from one computer to another. Thus, the transfer of data from one application to another involves first getting the data to the computer in
Chapter 6 Introduction to Protocols 17 which the application resides and then getting it to the intended application within the computer. With these concepts in mind, it appears natural to organize the communication task into three relatively independent layers: Network access layer Transport layer Application layer Fig. 1 illustrates this simple architecture. Let's see these layers. The network access layer The network access layer is concerned with the exchange of data between a computer and the network to which it is attached. The sending computer must provide the network with the address of the destination computer, so that the network may route the data to the appropriate destination. The sending computer may wish to invoke certain services, such as priority, that might be provided by the network. Computer 1 Computer 2 Application Application Protocol Application Transport Transport Protocol Transport Network Access Network Access Protocol Communication Network Network Access Fig. 1: Protocols in a simplified architecture The specific software used at this layer depends on the type of network to be used; different standards have been developed for circuit switching, packet switching, local area networks, and others. Thus, it makes sense to separate those functions having to do with network access into a
Chapter 6 Introduction to Protocols 18 separate layer. By doing this, the remainder of the communications software, above the network access layer, need not be concerned with the specifics of the network to be used. The same higherlayer software should function properly regardless of the particular network to which the computer is attached. The transport layer Regardless of the nature of the applications that are exchanging data, there is usually a requirement that data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. As we shall see, the mechanisms for providing reliability are essentially independent of the nature of the applications. Thus, it makes sense to collect those mechanisms in a common layer shared by all applications; this is referred to as the transport layer. The application layer Finally, the application layer contains the logic needed to support the various user applications. For each different type of application, such as file transfer, a separate module is needed that is peculiar to that application. The following table summarizes the main functions of the three layers. Table 1: The main functions of the three layers Functions Network Layer The network access layer is concerned with the exchange of data between a computer and the network to which it is attached. Transport Layer Application Layer Assures that data be exchanged reliably. That is, we would like to be assured that all of the data arrive at the destination application and that the data arrive in the same order in which they were sent. Contains the logic needed to support the various user applications. Figure 2 illustrate this simple architecture. It shows three computers connected to a network: Each computer contains software at the network access and transport layers and software at
Chapter 6 Introduction to Protocols 19 the application layer for one or more applications. Each computer on the network must have a unique network address; this allows the network to deliver data to the proper computer. Each application on a computer must have an address that is unique within that computer; this allows the transport layer to support multiple applications at each computer. Computer 1 Application Service access point Transport Computer 2 Application Network Access Network address Transport Computer 3 Network Access Application Communications Network Transport Network Access Fig. 2: Protocol architectures and networks
Chapter 6 (Cont.) TCP/IP 20 Chapter 6 (Cont.) Protocol Suite TCP/IP The TCP/IP Protocol Suite The main power of the Internet lies in two protocols: the Transmission Control Protocol (TCP), the Internet Protocol (IP). Because of this, the Internet suite of protocols is referred to as the TCP/IP protocol suite (Fig. 1). TCP is very close to the definition of transport layer of the OSI-RM. It provides a connectionoriented delivery of data between two distributed applications in the Internet. TCP provides this reliable service on the top of an unreliable IP. IP is responsible for the routing and switching of datagrams through the Internet. We summarize these functions in the following table. Protocol TCP IP Functions Provides a connection-oriented delivery of data between two distributed applications in the Internet. Responsible for the routing and switching of datagrams through the Internet.
Chapter 6 (Cont.) TCP/IP 21 Fig.1: TCP/IP Suite Here is a list of acronyms used in Figure 1: FTP: File Transfer Protocol HTTP: Hyper Text Transfer Protocol SMTP: Simple Mail Transfer Protocol. SNMP: Simple Network Management Protocol TCP: Transmission Control Protocol UDP: User Datagram Protocol IP: Internet Protocol ICMP: Internet Control Message Protocol
Chapter 6 (Cont.) TCP/IP 22 A comparison between TCP and IP In the following figure we compare the main characteristics between TCP and IP: TCP IP Connection Oriented Connectionless Reliable Slow Unreliable Fast + + Fig.2: A comparison between TCP and IP A standard practice is to show the protocol stack for the TCP/IP suite and compare it with the OSI- RM. Such a stack is shown in the Figure 3. TCP/IP Compared to the OSI Model To compare TCP/IP to OSI model, we first give the two models as illustrated in figure 3.
Chapter 6 (Cont.) TCP/IP 23 Application layer roughly corresponds to Session, Application, and Presentation layers of OSI Model Transport layer roughly corresponds to Transport layers of OSI Model Internet layer is equivalent to Network layer of OSI Model Network Interface layer roughly corresponds to Data Link and Physical layers of OSI Model The Transmission Control Protocol (TCP) TCP provides a connection-oriented, full-duplex, reliable, streamed delivery service using IP to transport messages between two processes. Reliability is ensured by: Connection-oriented service Flow control Error detection Error control Congestion avoidance algorithms; The data part of the IP packet is the TCP packet. TCP takes care of the imperfections of IP and protocols below IP. As mentioned earlier, the IP protocol may result in packets arriving out-ofsequence at the destination. This is due to the possibility that each IP packet could be taking different route to destination. TCP provides the capability of re-arranging the packets back in sequence. IP packet may get lost and TCP would request a duplicate copy of the lost packet. Besides, error checking in IP is provided only for the packet header. The TCP provides for the error checking of the complete packet. If the reliability functions (in-sequence delivery, error checking and congestion control) are embedded in IP, then the IP would become very reliable. However, the data transfer rate will suffer because IP header is processed at every node. The TCP is an end-to-end protocol and therefore is processed only at the sending and destination computers. TCP is connection-oriented. This means that the protocols on two communicating computers set up a connection before actual data transfer takes place. This is a very desirable property of TCP. However, if the actual data message is short, then connection setting up and error checking adds too much overhead. Besides, if the TCP at the receiving computer finds a packet in error, it requests the TCP at the sending station to retransmit a copy of the packet. This is not a good error recovery mechanism for delay sensitive applications. Due to these two inefficiencies of TCP, the Internet protocol suite includes a connectionless protocol at the same level. It's called the user datagram
Chapter 6 (Cont.) TCP/IP 24 protocol (UDP). UDP is a connectionless protocol and does not require the use of retransmission of packets. However, the capability of error control is included as an option in UDP. Used Addresses in TCP/IP The used Addresses in TCP/IP are (Fig. 5): Port address, Internet address, Physical address The buffer size of these addresses is illustrated in the following table: Buffer size Port Address 16 Internet Address 32 Ethernet Address 48 User Process TCP Port Address IP Internet Address Ethernet Layer Physical Address Fig.5: Three types of addresses used in TCP/IP
Chapter 6 (Cont.) TCP/IP 25 Connection-oriented service TCP performs data communication in full-duplex mode, that is both the sender and receiver processes can send segments simultaneously. For connection establishment in full-duplex mode, a four-way protocol can be used. However, the second and third steps can be combined to form a three-way handshaking protocol. Reliable Communication To ensure reliability, TCP performs flow control, error control and congestion control. Flow control: TCP uses byte-oriented sliding window protocol, which allows efficient transmission of data and at the same time the destination host is not overwhelmed with data. The receiver has a buffer size of 8 K bytes. After receiving 4 K bytes, the window size is reduced to 4 Kbytes. After receiving another 3 K bytes, the window size reduces to 1 K bytes. After the buffer gets empty by 4 K bytes, the window size increases to 7 K bytes. So it may be noted that the window size is totally controlled by the receiver window size, which can be increased or decreased dynamically by the destination. The destination host can send acknowledgement any time. Error Control: Error control in TCP includes mechanism for detecting corrupted segments with the help of checksum field. Acknowledgement method is used to confirm the receipt of uncorrupted data. If the acknowledgement is not received before the time-out, it is assumed that the data or the acknowledgement has been corrupted or lost. It may be noted that there is no negative acknowledgement in TCP. To keep track of lost or discarded segments and to perform the operations smoothly, the following four timers are used by TCP: Retransmission; it is dynamically decided by the round trip delay time. Persistence; this is used to deal with window size advertisement. Kep-alive; commonly used in situations where there is long idle connection between two processes Time-waited; it is used during connection terminations Congestion control: To avoid congestion, the sender process uses two strategies known as slow-start and additive increase, and the send one is known as multiplicative decrease. To start with, the congestion window
Chapter 6 (Cont.) TCP/IP 26 size is set to the maximum segment size and for each segment that is acknowledged, the size of the congestion window size is increased by maximum segment size until it reaches one-half of the allowable window size. Ironically, this is known as slow-start, although the rate of increase is exponential. After reaching the threshold, the window size is increased by one segment for each acknowledgement. This continues till there is no time-out. When a time-out occurs, the threshold is set to one-half of the last congestion window size. TCP/IP Topology The TCP/IP protocol suite can be used on stand-alone LANs and WANs or on complex internetworks created by gluing many networks together. Figure 6 illustrates stand-alone network links. Any hosts that are equipped with TCP/IP can communicate with one another across a LAN, point-to-point line, or wide area packet network. Networks are joined into an internetwork by means of IP routers. Figure 7 shows an internetwork that was created by connecting the stand-alone networks together via IP routers. Fig.6: Stand-alone networks.
Chapter 6 (Cont.) TCP/IP 27 Fig. 7: Gluing networks together with routers.