OSI Reference Model Hierarchical Software Layers

1. OSI Reference Model Hierarchical Software Layers

2. 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

3. 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

4. 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 ISO/OSI 7-Layer Reference Model

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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)

17. 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

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

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

21. 16-bit UDP source port # 16-bit UDP dest. port # data protocol = UDP internet 32-bit source addr internet 32-bit dest. addr UDP header data IP header UDP header data frame type = IP Ethernet 48-bit source addr Ethernet 48-bit dest. addr Ethernet header IP header UDP header Ethernet trailer data Ethernet frame Hierarchical Addressing Scheme • Connection defines the communication link between two processes UDP = User Datagram Protocol