1 n OSI Reference Model n Hierarchical Software Layers

2 The ISO/OSI Reference Model a Developed by the International Standards Organization (ISO) to facilitate the international standardization of communications protocols a For U.S.: ANSI (American National Standard Institute) - a OSI is ISO's Basic Reference Model for Open Systems Interconnect (hence: ISO/OSI) a The Reference Model itself is not a Network Architecture (does not specify any protocols or services) a ISO also developed Network Architecture standards

3 The ISO/OSI Reference Model a The model describes computer communications protocols in a general sense to facilitate discussion a No assumptions are made regarding: u Programming language bindings u Operating system bindings u Applications programming interfaces a Development of the model started in the mid-1970’s a Biggest Problems u Very long time to complete the model and protocol standards u Very hard to understand the detailed standards u Difficult (expensive) to get the standards documents

4 ISO/OSI 7-Layer Reference Model Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Application Network Data Link Physical Physical Communications Media

5 ISO/OSI Reference Model - Why 7 Layers? a One layer for each level of abstraction a Each layer performs (ideally) a limited, well defined function a Functions for each layer are selected with International Standardization as a goal a Layer boundaries are chosen to minimize information crossing the interface a Want to keep the model manageable (5 would have been nice) but not have to jumble together distinct functions

6 OSI Layer 1 - Physical layer a Primary function is transmitting raw bits over a physical communications channel a Primary design issues include: mechanical, electrical, coding, physical characteristics u How many pins in the connector u what voltage represents a “1” versus a “0” u etc. a By “raw bits” we mean there is no interpretation of the bits - stream of bits in and bits out

7 OSI Layer 2 - Data Link layer a Primary function is to make Layer 1 into what appears to be a channel free of undetected errors a Deals with data in chunks (typically 100s-1000s of bytes) generally called Frames a This layer must create/recognize frame boundaries u remember - Physical layer does not care u often requires special bit patterns to signal boundaries u may have to deal with possibility of pattern appearing in data

8 OSI Layer 2 - Data Link layer a Among the key issues dealt with are: u Error handling (e.g. corrupted frame) u Flow control u Providing various qualities of service a For Broadcast networks, a key issue is controlling access to the channel: u Use a sub-layer called the Media Access Control (MAC) sub-layer

9 OSI Layer 3 - Network layer a Primary function is to control the operation of the layers below a Among the key issues dealt with are: u Routing packets from source to destination through the network (or multiple networks) using static or dynamic routing algorithms u Controlling congestion in the network u Accounting functions (for billing) u Translating between protocols across heterogeneous networks u Concerned with Addressing

10 OSI Layer 4 - Transport layer a First end-to-end layer a 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 a May provide other types of services a Often performs multiplexing of multiple transport connections over one or more network connections a Generally requires Address (or naming) a May also perform flow control

11 OSI Layer 5 - Session layer a Sort of an unwanted layer, this layer is usually very thin and little more than a pass through for most protocols a Manages dialog control (e.g. may manage who’s turn it is to talk in a high-level half-duplex protocol) a Manages synchronization of transactions which may need to be able to roll back in case of a crash

12 OSI Layer 6 - Presentation layer a Rather than being concerned with moving information, the Presentation layer is concerned with the interpretation of information representation a Ensures that the syntax and meaning is the same for each participant in a communication a Provides for standard representation and may provide capabilities for conversion of data

13 OSI Layer 7 - Application layer a The layer where end-user applications live a This is the highest level of abstraction and the level which is of primary importance (for most users) a All the rest of the layers exist to support these applications a Layering exists so we can move these around to different machines, and so they can communicate across any platforms - Open Systems Interconnect

14 Review - Functions of the OSI Layers a Layer 1 (physical): Transmission of bits a Layer 2 (data link): Transmission of frames on one given link a Layer 3 (network): Routing of packets through the network a Layer 4 (transport): End-to-end delivery of messages

15 Review - Functions of the OSI Layers a Layer 5 (session): Setup and management of end-to-end conversation, synchronization a Layer 6 (presentation): Formatting, encryption, and compression of data a Layer 7 (application): user applications

16 TCP/IP Protocol Suite n Advanced Research Project Agency (ARPA) of DoD sponsored the development of ARPANET in 1970s. n TCP/IP has been adopted as the ARPANET protocol suite n TCP/IP became popular by the inclusion of this protocol in BSD Unix system (a version of Unix developed by University of Berkley)

17 TCP/IP (cont.) n Transport Layer-TCP (Transmission Control Protocol) Provides fully reliable, connection-oriented serviceProvides fully reliable, connection-oriented service Byte-stream transmissionByte-stream transmission n Network Layer- IP (Internet Protocol) IP provides datagram service (used in packet switching)IP provides datagram service (used in packet switching) It is connectionless unreliable serviceIt is connectionless unreliable service IP handles routingIP handles routing

18 TCP/IP suite and OSI 7 Layer Model Application Presentation Session Transport Network Data Link Physical Application Process TCP IP Communication Network Most people define TCP/IP as a 5 layer protocol dividing the Communication layer into 2 separate layers (Network & Physical)

19 hardware interface TCP IP UDP user processor user processor OSI Layer 5-7 OSI Layer 4 OSI Layer 3 OSI Layer 1-2 ARP ICMP RARP UDP: User Datagram Protocol: ICMP: Internet Control Message Protocol IP: Internet Protocol ARP: Address Resolution Protocol RARP: Reverse ARP Network Structure

20 Port Number user proc.A user proc.B user proc.C user proc.D user proc.E user proc.F user proc.G IDPIP SPPPEXTCPUDP Ethernet interface Ethernet interface Ethernet interface Ethernet interface TCP/IP protocol suite XNS protocol suite Ethernet cable 2 Ethernet cable 1

21 Hierarchical Addressing Scheme n Connection defines the communication link between two processes data UDP header data UDP header IP header UDP header data IP header Ethernet trailer Ethernet header 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 frame UDP = User Datagram Protocol