CECS 474 Computer Network Interoperability CHAPTER 1 Network

CECS 474 Computer Network Interoperability CHAPTER 1 Network Protocols Tracy Bradley Maples, Ph. D. Computer Engineering & Computer Science Cal ifornia State University, Long Beach Notes for Douglas E. Comer, Computer Networks and Internets (5 th Edition)

Protocol Suites and Layering Models Defn: Protocols are agreements about how communication should take place. They specify: • Format of messages • Meaning of messages • Rules for exchanging messages • Procedures for handling problems Network hardware functions at a very low level. Hardware related problems can occur that need to be addressed by protocols: • Bits can be corrupted or destroyed • Entire packets can be lost • Packets can be duplicated • Packets can be delivered out of order 2

Protocols can also be used to distinguish among: • Multiple computers on a network • Multiple applications on a computer • Multiple copies of a single application on a computer Sets of Protocols • Sets of protocols are designed to work together. • Each protocol solves a small part of the communications problem • Sets of protocols are known as: -- Protocol Suites -- Protocol families • They are designed in layers. 3

Protocol Design • Protocols are divided into layers • Each layer is devoted to one sub-problem Example: The ISO (International Standards Organization) OSI (Open System Interconnection) 7 layer Reference Model The OSI 7 -Layer Model The 7 -Layer Model: • Was defined fairly early in the development of networks • Is now somewhat dated • Does not include the internet layer 4

The OSI 7 -Layers Layer 1: Physical • The underlying hardware Layer 2: Data Link (media access) • Hardware frame definitions Layer 3: Network • Packet forwarding Layer 5: Session • Login and passwords Layer 6: Presentation • Data representation Layer 7: Application • Individual application programs Layer 4: Transport • Reliability 5

The Layering Model Protocol software follows the layering model, with: • One software module per layer • Modules that work together • Incoming or outgoing data passing from one module to another The entire set of protocol layers (or modules) is known as a stack. 6

Layers and Packet Headers Each layer: • Prepends a header to the outgoing packet • Removes a header from the incoming packet This process is known as data encapsulation. 7

Layering Principle Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source. --Comer 8

TCP/IP Layering Notwithstanding the push by researchers to adopt the OSI model, it became clear that TCP/IP was technically more flexible and superior. TCP/IP is the primary protocol stack used today. Note: This TCP/IP layering is an actual implementation of protocols, unlike the ISO model. 9

TCP/IP Layers Layer 1: Physical • Basic network hardware • Similar to OSI Layer 1 Layer 2: Network Interface • MAC frame format • MAC addressing • Interface between computer and the network (i. e. , the NIC) • Similar to OSI Layer 2 Layer 3: Internet Layer 4: Transport • Specifies how to provide reliable transfer from one application on one computer to an application on another • Similar to OSI Layer 4 Layer 5: Application • Everything else (i. e. , how one application uses the Internet) • Similar to OSI Layer 6 and 7 • Format of packets • Mechanisms forwarding packets • Not in the OSI Model 10