osi model - communication system for connecting end-systems
- 1. 1 Netprog: OSI Reference Model Networking
- 2. 2 Netprog: OSI Reference Model Network “ ... communication system for connecting end-systems” End-systems a.k.a. “hosts” PCs, workstations dedicated computers network components
- 3. 3 Netprog: OSI Reference Model Multiaccess vs. Point-to- point ● Multiaccess means shared medium. – many end-systems share the same physical communication resources (wire, frequency, ...) – There must be some arbitration mechanism. ● Point-to-point – only 2 systems involved – no doubt about where data came from !
- 4. 4 Netprog: OSI Reference Model Multiacces s Point-to- point
- 5. 5 Netprog: OSI Reference Model LAN - Local Area Network ● connects computers that are physically close together ( < 1 mile). – high speed – multi-access ● Technologies: – Ethernet 10 Mbps, 100Mbps – Token Ring 16 Mbps – FDDI 100 Mbps
- 6. 6 Netprog: OSI Reference Model WAN - Wide Area Network ● connects computers that are physically far apart. “long-haul network”. – typically slower than a LAN. – typically less reliable than a LAN. – point-to-point ● Technologies: – telephone lines – Satellite communications
- 7. 7 Netprog: OSI Reference Model MAN - Metropolitan Area Network ● Larger than a LAN and smaller than a WAN - example: campus-wide network - multi-access network ● Technologies: – coaxial cable – microwave
- 8. 8 Netprog: OSI Reference Model Internetwork ● Connection of 2 or more distinct (possibly dissimilar) networks. ● Requires some kind of network device to facilitate the connection. Net A Net B
- 9. 9 Netprog: OSI Reference Model OSI Reference Model ● Layered model: 7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical
- 10. 10 Netprog: OSI Reference Model The Physical Layer ● Responsibility: – transmission of raw bits over a communication channel. ● Issues: – mechanical and electrical interfaces – time per bit – distances
- 11. 11 Netprog: OSI Reference Model The Data Link Layer - Data Link Control ● Responsibility: – provide an error-free communication link ● Issues: – framing (dividing data into chunks) » header & trailer bits – addressing 10110110101 01100010011 10110000001
- 12. 12 Netprog: OSI Reference Model The Data Link Layer - The MAC sublayer ● Medium Access Control - needed by mutiaccess networks. ● MAC provides DLC with “virtual wires” on multiaccess networks.
- 13. 13 Netprog: OSI Reference Model The Network Layer ● Responsibilities: – path selection between end-systems (routing). – subnet flow control. – fragmentation & reassembly – translation between different network types. ● Issues: – packet headers – virtual circuits
- 14. 14 Netprog: OSI Reference Model The Transport Layer ● Responsibilities: – provides virtual end-to-end links between peer processes. – end-to-end flow control ● Issues: – headers – error detection – reliable communication
- 15. 15 Netprog: OSI Reference Model The Session Layer ● Responsibilities: – establishes, manages, and terminates sessions between applications. – service location lookup ● Many protocol suites do not include a session layer.
- 16. 16 Netprog: OSI Reference Model The Presentation Layer ● Responsibilities: – data encryption – data compression – data conversion ● Many protocol suites do not include a Presentation Layer.
- 17. 17 Netprog: OSI Reference Model The Application Layer ● Responsibilities: – anything not provided by any of the other layers ● Issues: – application level protocols – appropriate selection of “type of service”
- 18. 18 Netprog: OSI Reference Model Layering & Headers ● Each layer needs to add some control information to the data in order to do it’s job. ● This information is typically prepended to the data before being given to the lower layer. ● Once the lower layers deliver the the data and control information - the peer layer uses the control information.
- 19. 19 Netprog: OSI Reference Model Headers Process Transpor t Networ k Data Link Process Transpor t Networ k Data Link DATA DATA DATA DATA H H H H H H
- 20. 20 Netprog: OSI Reference Model What are the headers? Physical: no header - just a bunch of bits. Data Link: – address of the receiving endpoints – address of the sending endpoint – length of the data – checksum.
- 21. 21 Netprog: OSI Reference Model Network layer header - examples ● protocol suite version ● type of service ● length of the data ● packet identifier ● fragment number ● time to live ● protocol ● header checksum ● source network address ● destination network address
- 22. 22 Netprog: OSI Reference Model Important Summary ● Data-Link: communication between machines on the same network. ● Network: communication between machines on possibly different networks. ● Transport: communication between processes (running on machines on possibly different networks).
- 23. 23 Netprog: OSI Reference Model Connecting Networks ● Repeater: physical layer ● Bridge: data link layer ● Router: network layer ● Gateway: network layer and above.
- 24. 24 Netprog: OSI Reference Model Repeater ● Copies bits from one network to another ● Does not look at any bits ● Allows the extension of a network beyond physical length limitations REPEATER
- 25. 25 Netprog: OSI Reference Model Bridge ● Copies frames from one network to another ● Can operate selectively - does not copy all frames (must look at data-link headers). ● Extends the network beyond physical length limitations. BRIDGE
- 26. 26 Netprog: OSI Reference Model Router ● Copies packets from one network to another. ● Makes decisions about what route a packet should take (looks at network headers). ROUTER
- 27. 27 Netprog: OSI Reference Model Gateway ● Operates as a router ● Data conversions above the network layer. ● Conversions: encapsulation - use an intermediate network translation - connect different application protocols encrpyption - could be done by a gateway
- 28. 28 Netprog: OSI Reference Model Encapsulation Example Gatewa y Gatewa y ● Provides service connectivity even though intermediate network does not support protocols.
- 29. 29 Netprog: OSI Reference Model Translation ● Translate from green protocol to brown protocol Gatewa y
- 30. 30 Netprog: OSI Reference Model Encryption gateway Encryption/ Decryption Gateways Secure Networ k Secure Networ k GW GW ? ? ? Insecure Network
- 31. 31 Netprog: OSI Reference Model Hardware vs. Software ● Repeaters are typically hardware devices. ● Bridges can be implemented in hardware or software. ● Routers & Gateways are typically implemented in software so that they can be extended to handle new protocols. ● Many workstations can operate as routers or gateways.
- 32. 32 Netprog: OSI Reference Model Byte Ordering ● Different computer architectures use different byte ordering to represent multibyte values. ● 16 bit integer: Low Byte High Byte High Byte Low Byte Address A Address A+1
- 33. 33 Netprog: OSI Reference Model Byte Ordering Low Byte High Byte Addr A Addr A+1 High Byte Low Byte Addr A Addr A+1 Big-Endian IBM 370 Motorola 68000 Sun Little-Endian IBM 80x86 DEC VAX DEC PDP-11
- 34. 34 Netprog: OSI Reference Model Byte Order and Networking ● Suppose a Big Endian machine sends a 16 bit integer with the value 2: ● A Little Endian machine will think it got the number 512: 0000000000000010 0000001000000000
- 35. 35 Netprog: OSI Reference Model Network Byte Order ● Conversion of application-level data is left up to the presentation layer. ● But hold on !!! How do lower level layers communicate if they all represent values differently ? (data length fields in headers) ● A fixed byte order is used (called network byte order) for all control data.
- 36. 36 Netprog: OSI Reference Model Multiplexing ● “.. to combine many into one”. ● Many processes sharing a single network interface. ● A single process could use multiple protocols. ● More on this when we look at TCP/IP.
- 37. 37 Netprog: OSI Reference Model Modes of Service ● connection-oriented vs. connectionless ● sequencing ● error-control ● flow-control ● byte stream vs. message based ● full-duplex vs. half-duplex.
- 38. 38 Netprog: OSI Reference Model Connection-Oriented vs. Connectionless Service ● A connection-oriented service includes the establishment of a logical connection between 2 processes. – establish logical connection – transfer data – terminate connection. ● Connectionless services involve sending of independent messages.
- 39. 39 Netprog: OSI Reference Model Sequencing ● Sequencing provides support for an order to communications. ● A service that includes sequencing requires that messages (or bytes) are received in the same order they are sent.
- 40. 40 Netprog: OSI Reference Model Error Control ● Some services require error detection (it is important to know when a transmission error has occured). ● Checksums provide a simple error detection mechanism. ● Error control sometimes involves notification and retransmission.
- 41. 41 Netprog: OSI Reference Model Flow Control ● Flow control prevents the sending process from overwhelming the receiving process. ● Flow control can be handled a variety of ways - this is one of the major research issues in the development of the next generation of networks (ATM).
- 42. 42 Netprog: OSI Reference Model Byte Stream vs. Message ● Byte stream implies an ordered sequence of bytes with no message boundaries. ● Message oriented services provide communication service to chunks of data called datagrams.
- 43. 43 Netprog: OSI Reference Model Full- vs. Half-Duplex ● Full-Duplex services support the transfer of data in both directions. ● Half-Duplex services support the transfer of data in a single direction.
- 44. 44 Netprog: OSI Reference Model End-to-End vs. Hop-toHop ● Many service modes/features such as flow control and error control can be done either: between endpoints of the communication. -or- between every 2 nodes on the path between the endpoints.
- 45. 45 Netprog: OSI Reference Model End-to-End Process A Process B
- 46. 46 Netprog: OSI Reference Model Hop-by-Hop Process A Process B
- 47. 47 Netprog: OSI Reference Model Buffering ● Buffering can provide more efficient communications. ● Buffering is most useful for byte stream services. Process A Process B Send Buffer Recv. Buffer
- 48. 48 Netprog: OSI Reference Model Addresses ● Each communication endpoint must have an address. ● Consider 2 processes communicating over an internet: – the network must be specified – the host (end-system) must be specified – the process must be specified.
- 49. 49 Netprog: OSI Reference Model Addresses at Layers ● Physical Layer: no address necessary ● Data Link Layer - address must be able to select any host on the network. ● Network Layer - address must be able to provide information to enable routing. ● Transport Layer - address must
- 50. 50 Netprog: OSI Reference Model Broadcasts ● Many networks support the notion of sending a message from one host to all other hosts on the network. ● A special address called the “broadcast address” is often used. ● Some popular network services are based on broadcasting (YP/NIS, rup, rusers)