The OSI Model

1. The OSI Model

2. Topics • Why we need Protocols and Standards • What are Protocols ? • The OSI Reference Model • The Seven Layers of the Model • Responsibilities of Each Layer • The Application Layer • The Presentation Layer • The Session Layer • The Transport Layer • The Network Layer • The Data Link Layer • The Physical Layer

3. What are Protocols ? • Let’s look at what happen: Oh my God, what did he say ? Không biết thằng này đang nói cái gì nữa ? They do not understand each other because they are using different languages.

4. What are Protocols ? (cont) • Now, Let’s look at what happens next: They can understand each other because they are using the same language.

5. What are Protocols ? (cont) • So in order for two people to speak to each other, the first thing they should to agree on is the language, or protocols, to use. • Communication between hosts on the network is the same. When hosts begin communicating with each other, they first must agree on what protocols to use.

6. What are Protocols ? (cont)

7. What are Protocols ? (cont) • A group of protocol is called a protocol suite or a protocol stack. • When combined with other protocols, the protocol group that result is called a protocol suite. TCP/IP, for example, is a protocol suite.

8. Why We Need Protocols and Standards • Rules – or protocols and standards – are important to ensure compatibility between different kinds of things • However, before a protocol is accepted and widely implemented, it has to pass rigorous testing. So a standard framework is used to help design, compare, test, and evaluate protocols.

9. The OSI Reference Model • For network communications to take place, hundreds of questions must be answered by a set of protocol (how, when, what…). • Evaluating and working with these hundreds of questions would be unmanageable.

10. The OSI Reference Model (cont) • So, in 1977, the International Standards Organization (ISO) adopted the Open Standard Interconnection (OSI) model. • The OSI model breaks down the many tasks involved in moving data from one host to another. • The hundreds of questions are divided into seven smaller. The seven groups are called layers.

11. The OSI Reference Model (cont) • The OSI reference model is exactly that; it is only a model. • If we think of the model as a set of questions that have to be answered, then the protocols are the answers. • Any one protocol may answer only a few of the questions, or in other words, address specific layers in the model. By combining multiple protocols into a protocol suite, we can answer all the questions posed by the model.(Mô hình OSI ó thể trả lời những câu hỏi đặt ra)

12. The Seven Layers of the OSI Model • The goal of the OSI model is to break down the task of data communication into simple steps. These steps are called layers. • The OSI model is made up of seven distinct layers. Each layer has certain responsibilities.

13. The Seven Layers of the OSI Model

14. Responsibilities of Each Layer • The purpose of each layer in the OSI model is to provide services to the layer above it. • The higher layers do not need to what happened at the lower layers. • Following is the process of moving data from one host to another:

15. IP Header IP Header IP Header IP Header IP Header IP Header IP Header 1 1 1 1 1 1 1 Frame Header I IP Header 1 IP Header 1 I Frame Header Frame Header I

16. Web Server Web Browser The Application Layer The Website is delivered to you… • Let’s look at what is happening: You are surfing on the Internet… You type an address of a Website…

17. Web Server Web Browser The Application Layer • The Application layer is the top layer of the OSI model. • The purpose of the Application layer is to manage communications between applications.

18. The Application Layer • Example of the Application layer: • File transfer • Electronic mail • Terminal access • Word processing • Web Browser

19. The Presentation Layer • Let’s look at what is happening:

20. The Presentation Layer • The Presentation layer is the layer below the Application layer and above the Session layer. • The Presentation ensures that the information that the application layer of one system sends out is readable by the application layer of another system.

21. The Presentation Layer • Actions of the Presentation layer: • Format of data • Data structure • Data conversion • Data compression • Data encryption • Examples of the Presentation layer: • PICT – This is picture format used by Macintosh • MIDI – The Musical Instrument Digital Interface is used for digitized music. • MPEG – The Moving Picture Experts Group’s standard for the compression and coding of motion video for CDs. • RTF – Rich Text Format is a file format that lets you exchange text files between different word processors, even in different operating systems.

22. The Session Layer • Let’s look at what is happening:

23. The Session Layer • The Session is below the Presentation layer. • The Session layer establishes, manages, and terminates sessions between two communicating hosts.

24. The session layer • Actions of the Session layer: • Sessions • Dialog • Conversations • Data exchange • Examples of the Session layer: • Network File System (NFS) – Developed by Sun Microsystems and used with TCP/IP and Unix workstation to allow transparent access to remote resources. • AppleTalk Session Protocol (ASP) – client/server mechanism, which both establishes and maintains sessions between AppleTalk client and server machines.

25. The Transport Layer • The Transport layer is below the Session layer. • The Transport layer can guarantee that packets are received.

26. The Transport Layer • The Transport layers segments and reassembles data into a data stream

27. The Transport Layer • Connectionless transmission • Connection-oriented transmission • The three-way handshake • Flow control • Acknowledgement • Windowing

28. The Transport Layer • The primary function of the Transport layer is to ensure that the data packets it receives from the Session layer arrive reliably. • The Transport layer does this by using two types of transmissions: connectionless and connection oriented. • The Transport layer also has the job of managing the speed of communication between devices. This is known as flow control.

29. The Transport LayerConnectionless transmissions • Let’s look at what happens: • So he goes to the local Post Office and left the letter there • That’s OK. The letter is not important, it is just a friendly letter. • Harry want to send mail to Wilson • The letter then delivered from the local Post Office through the Post Office system arrive at the Destination Post Office then delivered to Wilson • But the letter may be lost on the way and never come to the destination.

30. The Transport LayerConnectionless transmissions(cont) • The first reason is that’s the data is not important • The second reason is that it’s OK if there is a good and reliable connection between the source and the destination (lease line) • Working in the same way, with connectionless transmission using connectionless protocol, when Host A want to send data to Host B, it only put the packet onto the network and hope that it will be arrive at the destination. • The advantage of connectionless transmission using connectionless protocol is that it is very fast. • The packets may be lost on the way (internet). But that’s OK.

31. The Transport LayerConnectionless transmissions(cont) • User Datagram Protocol (UDP) is an example of connectionless protocol • Applications that use UDP to send their data information is DNS, TFTP….

32. The Transport LayerConnection-oriented transmissions • Let’s look at what happens: • Harry want to send mail to Wilson again. • But this time, it is an important business letter. So he want Wilson phone him to acknowledge when Wilson receive the letter. • When Harry receive an acknowledge phone from Wilson, he is now stop worrying about the letter. • He then goes to the Local Post Office and send the letter in a certified form. It means that the Post Office guarantee the letter will arrive at the right destination. • The letter then sent and when Wilson receives the letter and see that it is an important business letter, so he then phone Harry to acknowledge (báo cho biết) that he had received the letter.

33. The Transport LayerConnection-oriented transmissions(cont) • Because Host A and Host B both use connection-oriented protocol to send and receive packets. So when Host B receive packet from A, it will send an acknowledge packet to specify that it has received. • The advantage of connection-oriented transmission using connection-oriented protocol is that it is reliable although it has higher latency than connectionless protocol. • But after waiting for a predefined time, Host A does not receive an acknowledgement packet from Host B, Host A will resend the packet and wait. The process happens until Host A receive an acknowledgement packet from Host B. • Working in the same way, with connection-oriented transmission using connection-oriented protocol, when Host A want to send data to Host B, it put the packet onto the network and wait for the destination to acknowledge of receiving packet. • When Host A receive an acknowledgement packet, it then stop worrying about the packet.

34. The Transport LayerConnectionless transmissions (cont) • Transmission Control Protocol (TCP) is an example of connection-oriented protocol • Applications that use TCP to send their data information is DNS, FTP, HTTP, SMTP,…

35. The Transport Layerthe three-way handshake • In connection-oriented transmission, both the sender and receiver have to join to the session. The receiver must know that the sender is sending packets in order to answer (acknowledgment) • So, the sender must first establish a connection-oriented session with the receiver, which is called a call setup, or three-way handshake

36. The Transport Layerthe three-way handshake (cont) • Let’s look at what happens: • In the three-way handshake process, when Host A (sender) want to establish a session with Host B (receiver), it first send to the receiver a synchronize segment. • Host A after receives the synchronize segment from Host B, it will send an acknowledgement segment to Host B to acknowledge to connection. • Host B (receiver) after examine the synchronize packet from Host A, if agree, it will send acknowledgement synchronize segment to Host A. Synchronize (Can I talk to you ?) Connection Established Ack (Yes), Synchronize ( Can I talk to you ?) Data Transfer (send segments) Ack (Yes)

37. The Transport layerFlow Control • Once data transfer is in progress, congestion can occur for two reasons.

38. The Transport layerFlow Control (cont) • First, the sending device might be able to generate traffic faster than the network can transfer it.

39. The Transport layerFlow Control (cont) • The second reason is that multiple devices need to send data to the same destination.

40. The Transport layerFlow Control (cont) • When datagram arrive too quickly for a device to process, it temporarily stores them in memory.

41. The Transport layerFlow Control (cont) • If the datagrams are part of a small burst, this buffering solves the problem.

42. The Transport layerFlow Control (cont) • However, if the traffic continues at this rate, the device eventually exhausts its memory and must discard additional datagrams that arrive.

43. The Transport layerFlow Control (cont) • Instead of losing the data, the transport function can issue a “not ready” indicator to the sender.

44. The Transport layerFlow Control (cont) • This acts like a stop sign and signal the sender to discontinue sending segment traffic to the receiver.

45. The Transport layerFlow Control (cont) • After the receiving device has processed sufficient segments to free space in its buffer, the receiver sends a “ready transport “ indicator – which is like a go signal.

46. The Transport layerFlow Control (cont) • When they receives this indicator, the senders can resume segment transmission.

47. The Transport LayerAcknowledgement • The transport layer provide a reliable service regardless of the quality of the underlying network

48. The Transport LayerAcknowledgement (cont) • One technique that is used to guarantee reliable delivery is called positive acknowledgement (Thông báo xác thực) with retransmission. • This requires the receiver to issue an acknowledgement message to the sender when it receivers data.

49. The Transport LayerAcknowledgement (cont) • The sender also start a timer when it sent a packet. • If the timer expires before an acknowledgement is received, it retransmits the packet.

50. The Transport LayerWindowing • Acknowledging every data segment, however, has its drawback (điều trở ngại). • If the sender has to wait for an acknowledgement of each data segment, the throughput will be very low.