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

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

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 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

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 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

5 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 4: Transport  Reliability Layer 5: Session  Login and passwords Layer 6: Presentation  Data representation Layer 7: Application  Individual application programs

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

10 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  Format of packets  Mechanisms for forwarding packets  Not in the OSI Model 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