OSI model


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OSI model

  1. 1. (open systems interconnection) By: SAIF ULLAH KHAN
  2. 2.  The Open Systems Interconnection (OSI) model (ISO/IEC 7498-1) is a product of the Open Systems Interconnection effort at the International Organization for Standardization Similar communication functions are grouped into logical layers
  3. 3.  Work on a layered model of network architecture was started and the International Organization for Standardization (ISO) began to develop its OSI framework architecture The concept of a seven-layer model was provided by the work of Charles Bachman, Honeywell Information Services Each entity interacted directly only with the layer immediately beneath it, and provided facilities for use by the layer above it Protocols enabled an entity in one host to interact with a corresponding entity at the same layer in another host
  4. 4.  According to recommendation X.200, there are seven layers, labeled 1 to 7, with layer 1 at the bottom Each layer is generically known as an N layer At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU) A Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU) An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user
  5. 5.  The PDU at a layer N is the SDU of layer N-1 In effect the SDU is the payload of a given PDU That is, the process of changing an SDU to a PDU, consists of an encapsulation process, performed by the lower layer All the data contained in the SDU becomes encapsulated within the PDU The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1 The added headers or footers are part of the process used to make it possible to get data from a source to a destination
  6. 6.  The physical layer defines electrical and physical specifications for devices it defines the relationship between a device and a transmission medium, such as a copper or fiber optical cable This includes the layout of pins, voltages, line impedance, cable specifications, sig nal timing, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more Parallel SCSI buses operate in this layer The same applies to other local-area networks, such as token ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4
  7. 7. Network Layer Functions Protocols Component Repeater IEEE 802 Multiplexer Hubs ISO 2110 Transmits raw bit stream over physical cable ISDN Passive EIA/TIA-232 Active EIA/TIA-449 Defines cables, cards, and physical aspects ITU-T V-Series TDR I.430 Establishment and termination of a connection to I.431 Oscilloscope a communications medium PDHPhysical Defines techniques to transfer bit stream to SONET/SDHHardware; Amplifier cable PONRaw bit OTNStream Participation in the process whereby the DSL communication resources are effectively shared IEEE 1394 among multiple users. For ITU-T G.hn example, contention resolution and flow control PHY Modulation or conversion between the USB representation of digital data in user equipment Bluetooth and the corresponding signals transmitted over a RS-232 communications channel RS-449
  8. 8.  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 Data Link layer defines the format of data on the network In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used A network data frame, aka packet, includes checksum, source and destination address, and data
  9. 9.  The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU) Sliding window flow control and acknowledgment is used at the transport layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages The data link layer handles the physical and logical connections to the packets destination, using a network interface The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer which provides both error correction and flow control by means of a selective repeat Sliding Window Protocol Both WAN and LAN service arranges bits, from the physical layer, into logical sequences called frames
  10. 10.  A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address MAC addresses are usually represented as six colon- separated pairs of hex digits, e.g., 8:0:20:11:ac:85 This number is unique and is associated with a particular Ethernet device
  11. 11. Network Layer Function Protocol Component Logical Link Control: Turns packets into raw bits 100101 and at • Error correction and flow Bridge the receiving end turns bits into packets control Switch • Control and defines SAPs Handles data frames between the Network 802.2 Logical Link Control ISDN Router and Physical layers Media Access Control The receiving end packages raw data from • communicates with the Intelligent the Physical layer into data frames for adapter card HubData layer delivery to the Network layer • controls the type of mediaLink Data being used:frames to 802.3 CSMA/CD (Ethernet) bits Responsible for error-free transfer of frames 802.4 Token Bus (ARC net) NIC to other computer via the Physical Layer 802.5 Token Ring 802.12 Demand Priority This layer defines the methods used to transmit and receive data on the network. It ATM , SDLC, HDLC, CSLIP consists of the wiring, the devices use to SLIP , GFP, PLIP, IEEE802.2 Advanced connect the NIC to the wiring, the signaling LLC , L2TP, IEEE 802.3 Cable Tester involved to transmit / receive data and the Frame Relay, ITU-T G.hn ability to detect signaling errors on the DLL , PPP, X.25 network media
  12. 12.  The network layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network (in contrast to the data link layer which connects hosts within the same network), while maintaining the quality of service requested by the transport layer The network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors Routers operate at this layer, sending data throughout the extended network and making the Internet possible. IP is responsible for routing, directing datagrams from one network to another
  13. 13.  The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram The Internetwork Protocol identifies each host with a 32- bit IP address IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., The leading 1-3 bytes of the IP identify the network and the remaining bytes identifies the host on that network. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address A number of layer-management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the network layer
  14. 14. The network layer may be divided into three sub-layers: Sub-network access  that considers protocols that deal with the interface to networks, such as X.25 Sub-network (dependent convergence)  when it is necessary to bring the level of a transit network up to the level of networks on either side Sub-network (independent convergence)  handles transfer across multiple networks
  15. 15. Network Layer Function Protocols Component Translates logical network address and names to their Router Frame physical address (e.g. computer name ==> MAC address) IP ARP RARP Responsible for ICMP • Addressing RIP Network • Determining routes for sending OSFP Relay DeviceAddressing; • Managing network problems such as packet switching, IGMP Routing data congestion and routing IPX NWLink NetBEUI DDP ATM Switch If router can’t send data frame as large as the source DECnet computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, Advanced the network layer reassembles the data Cable Tester
  16. 16.  Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control 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/de-segmentation, and error control Some protocols are state and connection-oriented  This means that the transport layer can keep track of the segments and retransmit those that fail. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer
  17. 17.  Reliability and speed are the primary difference between these two protocols TCP:  TCP establishes connections between two hosts on the network through sockets which are determined by the IP address and port number  TCP keeps track of the packet delivery order and the packets that must be resent UDP:  UDP on the other hand provides a low overhead transmission service, but with less error checking. NFS (network file system) is built on top of UDP because of its speed and statelessness. Statelessness simplifies the crash recovery
  18. 18.  OSI defines five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet) Feature Name TP0 TP1 TP2 TP3 TP4 Connection oriented network YES YES YES YES YES Connectionless network NO NO NO NO YES Concatenation and separation NO YES YES YES YES Segmentation and reassembly YES YES YES YES YES Error Recovery NO YES YES YES YES Reinitiate connection (if an excessive number NO YES NO YES NO of PDUs are unacknowledged) Multiplexing and de-multiplexing over a single virtual NO NO YES YES YES circuit Explicit flow control NO NO YES YES YES Retransmission on timeout NO NO NO NO YES Reliable Transport Service NO YES NO YES YES
  19. 19. Network Layer Function Protocol Component Manages the flow control of data between parties TCP ARP Gateway across the network RARP Divides streams of data into chunks or packets; the SPX Brouter Transport transport layer of the receiving computer NWLink (Bridge Router)Packets; Flow reassembles the message from packets NetBIOS control & Provides error-checking to guarantee error-free data NetBEUI Advanced CableError-handling delivery, with on losses or duplications ATP Tester UDP Provides acknowledgment of successful transmissions; SCTP requests retransmission if some packets don’t arrive DCCP error-free SPX
  20. 20.  The session protocol defines the format of the data sent over the connections  The NFS uses the Remote Procedure Call (RPC) for its session protocol  RPC may be built on either TCP or UDP  Login sessions uses TCP whereas NFS and broadcast use UDP 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 check pointing, adjournment, termination, and restart procedures The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session check pointing and recovery, which is not usually used in the Internet Protocol Suite
  21. 21. Network Layer Function Protocol Component Establishes, maintains and ends sessions across the network NetBIOS Responsible for name recognition (identification) so only Named the designated parties can participate in the session Pipes Provides synchronization services by planning check points Mail in the data stream => if session fails, only data after the Slots Session most recent checkpoint need be transmitted RPCSyncs and Gateway SAP Sessions Manages who can transmit data at a certain time and for PPTP how long RTP Examples are interactive login and file transfer SOCKS connections, the session would connect and re-connect if SPDY there was an interruption; recognize names in sessions and TLS/SSL register names in history
  22. 22.  The presentation layer establishes context between application-layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them External Data Representation (XDR) sits at the presentation level  It converts local representation of data to its canonical form and vice versa The canonical uses a standard byte ordering and structure packing convention, independent of the host If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack
  23. 23.  This layer provides independence from data representation (e.g., encryption) by translating between application and network formats The presentation layer transforms data into the form that the application accepts This layer formats and encrypts data to be sent across a network It is sometimes called the syntax layer The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML
  24. 24. Network Layer Function Protocol Component Translates from application to network format and vice- versa All different formats from all sources are made into a common uniform format that the rest of the OSI model canPresentation understand MIME Gateway Translation Responsible for protocol conversion, character conversion, XDR Redirector data encryption / decryption, expanding graphics commands, data compression Sets standards for different systems to provide seamless communication from multiple protocol stacks
  25. 25.  Provide network services to the end-users  Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications 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 This layer interacts with software applications that implement a communicating component Application-layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit
  26. 26.  When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer Some examples of application-layer implementations also include:  On OSI stack:  FTAM File Transfer and Access Management Protocol  X.400 Mail  Common Management Information Protocol (CMIP)  On TCP/IP stack:  Hypertext Transfer Protocol (HTTP),  File Transfer Protocol (FTP),  Simple Mail Transfer Protocol (SMTP)  Simple Network Management Protocol (SNMP).
  27. 27. Network Layer Function Protocol Componen t Used for applications specifically written to run over DNS, FTP the network TFTP, BOOTP SNMP, RLOGIN Allows access to network services that support SMTP, MIME applications NFS, FINGERApplication Directly represents the services that directly support TELNET, NCP User user applications APPC, AFP Gateway Interface SMB, NNTP Handles network access, flow control and error SIP, SSI recovery Gopher, HTTP NTP, SMPP Example apps are file transfer, e-mail, NetBIOS- Netconf, based applications (more)
  28. 28.  There are some functions or services that are not tied to a given layer, but they can affect more than one layer Examples include the following:  Security service (telecommunication) as defined by ITU-T X.800 Recommendation  management functions:  functions that permit to configure, instantiate, monitor, terminate the communications of two or more entities: there is a specific application layer protocol, common management information protocol (CMIP) and its corresponding service, common management information service (CMIS), they need to interact with every layer in order to deal with their instances  ARP:  It is used to translate IPv4 addresses (OSI layer 3) into Ethernet MAC addresses (OSI layer 2)  Multiprotocol Label Switching (MPLS):  It operates at an OSI-model layer that is generally considered to lie between traditional definitions of layer 2 (data link layer) and layer 3 (network layer), and thus is often referred to as a "layer-2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames
  29. 29.  In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as in the OSI model However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and the scope of the direct links to other nodes on the local network Even though the concept is different from the OSI model, these layers are nevertheless often compared with the OSI layering scheme in the following way:  The Internet application layer includes the OSI application layer, presentation layer, and most of the session layer
  30. 30.  Its end-to-end transport layer includes the graceful close function of the OSI session layer as well as the OSI transport layer. The internetworking layer (Internet layer) is a subset of the OSI network layer, while the link layer includes the OSI data link and physical layers, as well as parts of OSIs network layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the network layer document
  31. 31. Thank you for your time