OSI Model & TCP/IP

1. OSI Model & TCP/IP By Hossein Pour Taheri

2. Out Line • Introduction OSI • OSI History • OSI Layers • Introduction TCP/IP • TCP/IP Layers

3. Introduction OSI • The Open System Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design. • It divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

4. OSI History • In 1978, the International Standards Organization (ISO) began to develop its OSI framework architecture. • OSI has two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols.

5. OSI History • The concept of a 7 layer model was provided by the work of Charles Bachman, then of Honeywell. •  Various aspects of OSI design evolved from experiences with the Advanced Research Projects Agency Network (ARPANET) and the fledgling Internet.

6. OSI Layers PDU

7. Layer1: Physical Layer • The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. • This includes the layout of pin, voltages, cable specification, hubs, repeaters,  network adapters, host bus adapters, and more.

8. Layer1: Physical Layer • The major functions and services performed by the Physical Layer are: • Establishment and termination of a connection to a communication medium. • Participation in the process whereby the communication resources are effectively shared among multiple users. For example, flow control. • Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

9. Layer1: Physical Layer con. • Highly physical network technology dependent • Main tasks • define the signaling protocol • what is the meaning of 1s or 0s • voltages or frequencies • what is bad signals • define the physical connections required • RS232 connectors for RS232 serial line communication • RJ45 or BNC connectors for Ethernet • define the communication media • define the network topology

10. Physical Network Technologies • Circuit-switched network (CS) • connection-oriented network • establish connection before communication • once communication established, a circuit line is reserved for the communicating partners • example: telephone network • Packet-switched network (PS) • store-forward based network • packet sent from a node to another node • the intermediate node stores the packet and decides to forward to another node towards the destination • no circuit line is reserved • example: Ethernet

11. Circuit-switched Packet-switched line resource dedicated shared performance guaranteed averaged cost expensive less adaptive routing not easy easy switch device highly complex simple reliability high higher utilization low higher Comparisons: CS and PS

12. Layer 2: Data Link Layer • The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. • Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. • The data link layer is divided into two sub-layers by IEEE.

13. Layer 2: Data Link Layer • One is Media Access Control (MAC) and another is Logical Link Control (LLC). • Mac is lower sub-layer, and it defines the way about the media access transfer, such as CSMA/CD/CA(Carrier Sense Multiple Access/Collision Detection/Collision Avoidance) • LLC provides data transmission method in different network. It will re-package date and add a new header.

14. Services to the Network Layer (NL) • DLL processes data transfer using a data link protocol. • The actual services can vary from system to system. Three reasonable services to the NL are: 1. Unacknowledged connectionless service. 2. Acknowledged connectionless service. 3. Acknowledged connection-oriented service.

15. 1. Unacknowledged connectionless service • The source machine send frames to the destination machine without having the destination machine acknowledged them. • No logical connection is established beforehand or released afterward. • If a frame is lost due to noise on the line, no attempt is made to detect the loss or recover from it in the DLL. • This class of service is appropriate when the error rate is very low so that recovery task is left for solution to higher layers. • It is also appropriate for real-time traffic, such as voice, in which late data are worse than bad data. • Most LANs use unacknowledged connectionless service in the DLL

16. 2. Acknowledged connectionless service • Is more reliable. • Still no logical connections used, but each frame sent is individually acknowledged. • The sender knows whether a frame has arrived correctly. • If it has not arrived within a specific time interval, it can be sent again. • This service is useful over unreliable channels, such as wireless system. • If the large packet is broken up into frames, If individual frames are acknowledged or retransmitted, entire packets get through much faster than unbroken frame that is lost, it may take a very long time for the packet to get through..

17. 3. ACKed connection-oriented service • The service requires established connection between source/destination machines before data are transferred. • Any frame sent over the connection is numbered, and the DLL guarantees that each frame sent, is received, and are received in the same order. • With connectionless service, in contrast, it is possible that a lost acknowledgement causes a packet to be sent several times and thus received several times. • When connection-oriented service is used, transfers go through 3 distinct phases: • 1. The connection is established and counters needed to keep • track of which frames have been received and which ones • have not. • 2. One or more frames are transmitted and acknowledged. • 3. Connection is released, freeing up the variables - buffers and other resources used to maintain the connection.

18. Layer 3: Network Layer • The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. • The Network layer handles the logical addresses associated with individual machines on a network

19. Layer 3: Network Layer • The Network Layer performs • network routing functions, • perform fragmentation and reassembly, • report delivery errors. • Routers operate at this layer—sending data throughout the extended network and making the Internet possible.

20. Layer 4: Transport Layer • The Transport layer’s name is highly evocative of its function: this layer’s job is to ensure reliable end-to-end transmission of PDUs from sender to receiver • The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. • The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control.

21. Layer 5: Session Layer • The Session Layer controls the dialogues (connections) between computers. • It establishes, manages and terminates the connections between the local and remote application. • It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. 

22. Layer 5: Session Layer • Checkpoints define the last point up to which successful communications are known to have occurred, and define the last known point to which a conversation must be rolled back for missing or damaged elements to be replayed to recover from the effects of missing or damaged data • The Session layer’s primary job is to support communications between two networked parties, in which a sequence of messages or PDUs is typically exchanged

23. Layer 6: Presentation Layer • The Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the presentation service understands both and the mapping between them. • This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. • This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. • It is sometimes called the syntax layer.

24. Layer 7: Application Layer •  The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. • Application layer functions typically include: • identifying communication partners, • determining resource availability, • synchronizing communication. 

25. Layer 7: Application Layer • Identifying communication partners • Determines the identity and availability of communication partners for an application with data to transmit. • Determining resource availability • Decide whether sufficient network or the requested communication exist. • Synchronizing communication • All communication between applications requires cooperation that is managed by the application layer.

26. Layer 7: Application Layer •  Some examples of application layer implementations include • Hypertext Transfer Protocol (HTTP) • File Transfer Protocol (FTP) • Simple Mail Transfer Protocol (SMTP)

27. Sender Receiver Application Layer Application Layer HTTP Request HTTP Request Transport Layer Transport Layer TCP HTTP Request TCP HTTP Request Network Layer Network Layer IP TCP HTTP Request IP TCP HTTP Request Data Link Layer Data Link Layer Ethernet IP TCP HTTP Request Ethernet IP TCP HTTP Request Physical Layer Physical Layer

28. OSI Feature • Open system standards over the world • Rigorously defined structured, hierarchical network model • Complete description of the function • Provide standard test procedures • Still an excellent model for conceptualizing and understanding protocol architectures

29. Introduction TCP/IP • The Internet Protocol Suite (commonly known as TCP/IP) is the set of communications protocols used for the Internet and other similar networks. • It is named from two of the most important protocols in it: • the Transmission Control Protocol (TCP) and • the Internet Protocol (IP), which were the first two networking protocols defined in this standard.

30. TCP/IP Layers

31. TCP/IP Encapsulation

32. TCP/IP Some Protocol

33. De-jure vs. De-facto (OSI) • OSI • Standard legislated by official recognized body. (ISO) • The OSI reference model was devised before the protocols were invented. This ordering means that the model was not biased toward one particular set of protocols, which made it quite general. The down side of this ordering is that the designers did not have much experience with the subject and did not have a good idea of which functionality to put in which layer. • Being general,the protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes. • Not so widespread as compared with TCP/IP. (complex , costly) • More commonly used as teaching aids.

34. De-jure vs. De-facto (TCP/IP) • TCP/IP • Standards adopted due to widespread use. (Internet) • The protocols came first, and the model was really just a description of the existing protocols. There was no problem with the protocols fitting the model, but it is hardly possible to be use to describe other models. • “Get the job done" orientation. Over the years it has handled most challenges by growing to meet the needs. • More popular standard for internetworking for several reasons : • relatively simple and robust compared to alternatives such as OSI • available on virtually every hardware and operating system platform (often free) • the protocol suite on which the Internet depends.

35. Quote of the day • Almost everyone who has had an idea that's somewhat revolutionary or wildly successful was first told they're insane. LARRY PAGE