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OSI Reference Model and TCP/IP (Lecture #3 ET3003 Sem1 2014/2015)


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OSI Reference Model and TCP/IP (Lecture #3 ET3003 Sem1 2014/2015)

  1. 1. The OSI Model & the TCP/IP Protocol Suite ET3003 Computer Networks Tutun Juhana Telecommunication Engineering School of Electrical Engineering & Informatics Institut Teknologi Bandung 3
  2. 2. When Communication is simple Example 1 • The communication is so simple that it can occur in only one layer Speak Spanish Speak English (sign language)
  3. 3. When Communication is not simple Example 2 • Ann has to move to another town because of her job • We need three layers • It is hierarchical (the tasks must be done in the order given in the hierarchy) • Each layer uses the services of the layer immediately below it • Each layer gives the services to the layer immediately above it
  4. 4. • Before 1990 everyone believed that the OSI model would become the ultimate standard for data communications - but this did not happen
  5. 5. • The TCP/IP protocol suite became the dominant commercial architecture because it was used and tested extensively in the Internet – the OSI model was never fully implemented
  7. 7. • Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards • An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model – It was first introduced in the late 1970s
  8. 8. • An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture • The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable • The OSI model was intended to be the basis for the creation of the protocols in the OSI stack
  9. 9. The OSI model The network support layers; they deal with the physical aspects of moving data from one device to another (such as electrical specifications, physical connections, physical addressing, and transport timing and reliability); They are a combination of hardware and software, except for the physical layer, which is mostly hardware The user support layers; they allow interoperability among unrelated software systems; They are almost always implemented in software the transport layer, links the two subgroups and ensures that what the lower layers have transmitted is in a form that the upper layers can use
  10. 10. Layered Architecture
  11. 11. An exchange using the OSI model Encapsulation Decapsulation
  12. 12. Physical Layer • Responsible for moving individual bits from one (node) to the next • It coordinates the functions required to carry a bit stream over a physical medium. • It deals with the mechanical and electrical specifications of the interface and transmission media • It defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur
  13. 13. The physical layer is also concerned with the following • Physical characteristics of interfaces and media • Representation of bits • Data rate • Synchronization of bits • Line configuration • Physical topology • Transmission mode
  14. 14. Data Link Layer • The data link layer is responsible for moving frames from one hop (node) to the next • The data link layer transforms the physical layer, a raw transmission facility, to a reliable link – It makes the physical layer appear error-free to the upper layer (network layer)
  15. 15. Other responsibilities of the data link layer include the following • Framing • Physical addressing • Flow control • Error control • Access control
  16. 16. Network Layer • The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links) • Whereas the data link layer oversees the delivery of the packet between two systems on the same network (link) • The network layer ensures that each packet gets from its point of origin to its final destination
  17. 17. • If two systems are connected to the same link, there is usually no need for a network layer • However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to- destination delivery
  18. 18. Other responsibilities of the network layer include the following • Logical addressing • Routing
  19. 19. Transport Layer • The transport layer is responsible for the delivery of a message from one process to another • A process is an application program running on the host
  20. 20. • Reliable process-to-process delivery of a message
  21. 21. • Differences with Network Layer • Network layer oversees source-to-destination delivery of individual packets – It does not recognize any relationship between those packets • The transport layer ensures that the whole message arrives intact and in order – Overseeing both error control and flow control at the source- to-destination level
  22. 22. • Other responsibilities of the transport layer – Service-point addressing (port address) – Segmentation and reassembly – Connection control • The transport layer can be either connectionless or connection oriented – Flow control – Error control
  23. 23. Session Layer • Session layer establishes, maintains, and synchronizes the interaction between communicating systems • Responsibilities – Dialog control – Synchronization
  24. 24. Presentation Layer • The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems – Translation – Encryption – Compression
  25. 25. Application Layer • The application layer enables the user, whether human or software, to access the network • It provides user interfaces and support for services (email, remote file access etc.) • Services provided : – Network virtual terminal – File transfer, access, and management (FTAM) – E-mail services – Directory services
  26. 26. Summary of OSI Layers
  28. 28. • The TCP/IP protocol suite was developed prior to the OSI model
  29. 29. TCP/IP versus OSI model
  30. 30. Layers in the TCP/IP Protocol Suite • To discuss the purpose of each layer in the TCP/IP protocol suite, we will study a small private internet
  31. 31. • A link is a network that allows a set of computers to communicate with each other • A link can be a LAN (local area network) serving a small area or a WAN (wide area network) serving a larger area • We assume that different links are connected together by devices called routers or switches that route the data to reach their final destinations
  32. 32. Physical Layer • TCP/IP does not define any specific protocol for the physical layer • It supports all of the standard and proprietary protocols • The communication is between two hops or nodes, either a computer or router • The unit of communication is a single bit
  33. 33. We assume that the most efficient way to communicate with each other is via routers R1, R3, and R4 if a node is connected to n links, it needs n physical-layer protocols, one for each link because links may use different physical-layer protocols
  34. 34. • The responsibility of the physical layer, in addition to delivery of bits, matches with what mentioned for the physical layer of the OSI model • It mostly depends on the underlying technologies that provide links
  35. 35. Data Link Layer • TCP/IP does not define any specific protocol for the data link layer either • It supports all of the standard and proprietary protocols • The communication is also between two hops or nodes • The unit of communication however, is a packet called a frame
  36. 36. • A frame is a packet that encapsulates the data received from the network layer with an added header and sometimes a trailer • The header, among other communication information, includes the source and destination of frame • The destination address is needed to define the right recipient of the frame because many nodes may have been connected to the link • The source address is needed for possible response or acknowledgment as may be required by some protocols
  37. 37. These frames may be different because link 1 and link 3 may be using different protocols and require frames of different formats
  38. 38. Network Layer • At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internet Protocol (IP) • IP is the transmission mechanism used by the TCP/IP protocols • IP transports data in packets called datagrams, each of which is transported separately • Datagrams can travel along different routes and can arrive out of sequence or be duplicated • IP does not keep track of the routes and has no facility for reordering datagrams once they arrive at their destination
  39. 39. Transport Layer • Only the two end computers need to have the transport layer • The transport layer is responsible for delivering the whole message, which is called a segment from A to B – A segment may consist of a few or tens of datagrams • Where as the network layer is responsible for sending individual datagrams from computer A to computer B
  40. 40. • The segments need to be broken into datagrams and each datagram has to be delivered to the network layer for transmission • Since the Internet defines a different route for each datagram, the datagrams may arrive out of order and may be lost • The transport layer at computer B needs to wait until all of these datagrams to arrive, assemble them and make a segment out of them
  41. 41. • The transport layer was represented in the TCP/IP suite by two protocols: – User Datagram Protocol (UDP) – Transmission Control Protocol (TCP) • A new protocol called Stream Control Transmission Protocol (SCTP) has been introduced in the last few years
  42. 42. Application Layer • The application layer in TCP/IP is equivalent to the combined session, presentation, and application layers in the OSI model • The application layer allows a user to access the services of our private internet or the global Internet – electronic mail, file transfer, accessing the World Wide Web, etc. • The unit of communication at the application layer is a message
  43. 43. Addressing • The address of a node as defined by its LAN or WAN • Have authority over the link Needed in which each host can be identified uniquely, regardless of the underlying physical network To enable communication between processes User-friendly addresses that are designed for specific application
  44. 44. Physical Addresses • Also known as the link address
  45. 45. • Destination physical addresses can be either – unicast  one single recipient – multicast  a group of recipients – broadcast  to be received by all systems in the network
  46. 46. Logical Addresses • Logical addresses are necessary for universal communications that are independent of underlying physical networks • Physical addresses are not adequate in an internetwork environment where different networks can have different address formats • A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical network • The logical addresses are designed for this purpose
  47. 47. • A logical address in the Internet is currently a 32-bit address (IPv4) and a 128-bit address (IPv6), that can uniquely define a host connected to the Internet • No two publicly addressed and visible hosts on the Internet can have the same IP address
  48. 48. datagram with dst. and src. logical address Frame with dst. and src. physical address The physical addresses will change from hop to hop, but the logical addresses remain the same
  49. 49. Port Addresses • Computers are devices that can run multiple processes at the same time • The end objective of Internet communication is a process communicating with another process • For example, computer A can communicate with computer C by using TELNET, at the same time, computer A communicates with computer B by using the File Transfer Protocol (FTP) • In the TCP/IP architecture, the label assigned to a process is called a port address • A port address in TCP/IP is 16 bits in length (represented by one decimal number)
  50. 50. The physical addresses change from hop to hop, but the logical and port addresses usually remain the same
  51. 51. Application-Specific Addresses • User-friendly addresses that are designed for specific application • Examples include the e-mail address (for example, and the • Universal Resource Locator (URL) (for example,