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Transcript
OSI Reference Model Hierarchical Software Layers 1 The ISO/OSI Reference Model Developed by the International Standards Organization (ISO) to facilitate the international standardization of communications protocols For U.S.: ANSI (American National Standard Institute) www.ansi.org OSI is ISO's Basic Reference Model for Open Systems Interconnect (hence: ISO/OSI) The Reference Model itself is not a Network Architecture (does not specify any protocols or services) ISO also developed Network Architecture standards 2 The ISO/OSI Reference Model The model describes computer communications protocols in a general sense to facilitate discussion No assumptions are made regarding: Programming language bindings Operating system bindings Applications programming interfaces Development of the model started in the mid-1970’s Biggest Problems Very long time to complete the model and protocol standards Very hard to understand the detailed standards Difficult (expensive) to get the standards documents 3 ISO/OSI 7-Layer Reference Model 7 Application Application 6 Presentation Presentation 5 Session Session 4 Transport Transport 3 Network Network Network 2 Data Link Data Link Data Link 1 Physical Physical Physical Physical Communications Media 4 ISO/OSI Reference Model - Why 7 Layers? One layer for each level of abstraction Each layer performs (ideally) a limited, well defined function Functions for each layer are selected with International Standardization as a goal Layer boundaries are chosen to minimize information crossing the interface Want to keep the model manageable (5 would have been nice) but not have to jumble together distinct functions 5 OSI Layer 1 - Physical layer Primary function is transmitting raw bits over a physical communications channel Primary design issues include: mechanical, electrical, coding, physical characteristics How many pins in the connector what voltage represents a “1” versus a “0” etc. By “raw bits” we mean there is no interpretation of the bits - stream of bits in and bits out 6 OSI Layer 2 - Data Link layer Primary function is to make Layer 1 into what appears to be a channel free of undetected errors Deals with data in chunks (typically 100s-1000s of bytes) generally called Frames This layer must create/recognize frame boundaries remember - Physical layer does not care often requires special bit patterns to signal boundaries may have to deal with possibility of pattern appearing in data 7 OSI Layer 2 - Data Link layer Among the key issues dealt with are: Error handling (e.g. corrupted frame) Flow control Providing various qualities of service For Broadcast networks, a key issue is controlling access to the channel: Use a sub-layer called the Media Access Control (MAC) sub-layer 8 OSI Layer 3 - Network layer Primary function is to control the operation of the layers below Among the key issues dealt with are: Routing packets from source to destination through the network (or multiple networks) using static or dynamic routing algorithms Controlling congestion in the network Accounting functions (for billing) Translating between protocols across heterogeneous networks Concerned with Addressing 9 OSI Layer 4 - Transport layer First end-to-end layer Uses the network to (most often) provide higher layers with a connection oriented, reliable, error free channel that delivers messages (or byte stream) in order May provide other types of services Often performs multiplexing of multiple transport connections over one or more network connections Generally requires Address (or naming) May also perform flow control 10 OSI Layer 5 - Session layer Sort of an unwanted layer, this layer is usually very thin and little more than a pass through for most protocols Manages dialog control (e.g. may manage who’s turn it is to talk in a high-level half-duplex protocol) Manages synchronization of transactions which may need to be able to roll back in case of a crash 11 OSI Layer 6 - Presentation layer Rather than being concerned with moving information, the Presentation layer is concerned with the interpretation of information representation Ensures that the syntax and meaning is the same for each participant in a communication Provides for standard representation and may provide capabilities for conversion of data 12 OSI Layer 7 - Application layer The layer where end-user applications live This is the highest level of abstraction and the level which is of primary importance (for most users) All the rest of the layers exist to support these applications Layering exists so we can move these around to different machines, and so they can communicate across any platforms - Open Systems Interconnect 13 Review - Functions of the OSI Layers Layer 1 (physical): Transmission of bits Layer 2 (data link): Transmission of frames on one given link Layer 3 (network): Routing of packets through the network Layer 4 (transport): End-to-end delivery of messages 14 Review - Functions of the OSI Layers Layer 5 (session): Setup and management of end-to-end conversation, synchronization Layer 6 (presentation): Formatting, encryption, and compression of data Layer 7 (application): user applications 15 TCP/IP Protocol Suite Advanced Research Project Agency (ARPA) of DoD sponsored the development of ARPANET in 1970s. TCP/IP has been adopted as the ARPANET protocol suite TCP/IP became popular by the inclusion of this protocol in BSD Unix system (a version of Unix developed by University of California @ Berkley) 16 TCP/IP (cont.) Transport Layer-TCP (Transmission Control Protocol) • Provides fully reliable, connection-oriented service • Byte-stream transmission Network Layer- IP (Internet Protocol) • IP provides datagram service (used in packet switching) • It is connectionless unreliable service • IP handles routing 17 TCP/IP suite and OSI 7 Layer Model Application Presentation Application Process Session Transport TCP Network IP Data Link Communication Network Most people define TCP/IP as a 5 layer protocol dividing the Communication layer into 2 separate layers (Network & Physical) Physical 18 Network Structure UDP: User Datagram Protocol: ICMP: Internet Control Message Protocol IP: Internet Protocol ARP: Address Resolution Protocol RARP: Reverse ARP ICMP user processor user processor TCP UDP IP ARP hardware interface OSI Layer 5-7 OSI Layer 4 RARP OSI Layer 3 OSI Layer 1-2 19 Port Number user user user user user user user proc.A proc.B proc.C proc.D proc.E proc.F proc.G UDP TCP/IP protocol suite TCP PEX IP Ethernet interface Ethernet cable 2 SPP IDP Ethernet interface Ethernet interface XNS protocol suite Ethernet cable 1 Ethernet interface 20 Hierarchical Addressing Scheme UDP = User Datagram Protocol Connection defines the communication link between two processes 16-bit UDP source port # 16-bit UDP dest. port # protocol = UDP internet 32-bit source addr internet 32-bit dest. addr frame type = IP Ethernet 48-bit source addr Ethernet 48-bit dest. addr Ethernet header data UDP header data IP header UDP header data IP header UDP header data Ethernet trailer Ethernet frame 21