Department of Computer Science Southern Illinois University Carbondale CS441 – Mobile & Wireless Computing Overview of Computer Networking Protocols and Layering Thanks to: Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2004 Data and Computer Communications 7th Edition. William Stallings Prentice-Hall PTR Computer Network 4th edition Andrew S. Tanenbaum Prentice-Hall PTR Mobile & Wireless Computing 1

What’s the Internet: “nuts and bolts” view Millions of connected computing devices: hosts = end systems Running network apps Communication links fiber, copper, radio, satellite transmission rate = bandwidth Routers: forward packets (chunks of data) Protocols control sending, receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP Internet: “network of networks” loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force local ISP company network regional ISP router workstation server mobile Mobile & Wireless Computing 2

The network edge End systems (hosts) Client/Server model Run application programs e.g., web, email at “edge of network” Client/Server model Client host requests, receives service from always-on server e.g., Web browser/server; email client/server Peer-Peer model Minimal (or no) use of dedicated servers e.g., Gnutella, KaZaA Mobile & Wireless Computing 3

What’s the Internet: a service view Communication infrastructure enables distributed applications Network edge: applications and hosts Network core: routers network of networks Physical media: communication links Applications: Web, email, games, e-commerce, file sharing Communication services provided to apps: Connectionless unreliable Connection-oriented reliable Various protocols are used for communication services Mobile & Wireless Computing 4

What’s a protocol? Human protocols: “What’s the time?” “I have a question” Introductions … specific msgs sent … specific actions taken when msgs received, or other events Network protocols: Machines rather than humans All communication activity in Internet governed by protocols Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt Mobile & Wireless Computing 5

What’s a protocol? A human protocol and a computer network protocol: Hi Got the time? 2:00 TCP connection req response Get http://www.awl.com/kurose-ross <file> time Q: Other human protocols? Key Elements of a Protocol: Syntax Data formats Signal levels Timing Speed matching Sequencing Semantics Control information Error handling Mobile & Wireless Computing 6

Protocol “Layers” Networks are complex! Many “pieces”: hosts routers links of various media applications protocols hardware, software Consider the following analogy: Organization of air travel ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing Question: Is there any hope of organizing structure of network? Or at least our discussion of networks? A series of steps Mobile & Wireless Computing 7

Layering of airline functionality ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival intermediate air-traffic control centers ticket (complain) baggage (claim gates (unload) runway (land) ticket baggage gate takeoff/landing Layers: Each layer implements a service via its own internal-layer actions relying on services provided by layer below Why Layering? Dealing with complex systems Explicit structure allows identification, relationship of complex system’s pieces Layered reference model for discussion Modularization eases maintenance, updating of system Change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system Mobile & Wireless Computing 8

Standardized Protocol Architectures Required for devices to communicate Vendors have more marketable products Customers can insist on standards based equipment Two standards: OSI Reference model Open Systems Interconnection Developed by the International Organization for Standardization (ISO) Seven layers Never lived up to early promises TCP/IP protocol suite Most widely used De facto standard Also: IBM Systems Network Architecture (SNA) Mobile & Wireless Computing 9

OSI - The Model A layer model Each layer performs a subset of the required communication functions Each layer relies on the next lower layer to perform more primitive functions Each layer provides services to the next higher layer Changes in one layer should not require changes in other layers Why OSI did not take over the world Bad timing Bad technology Bad implementations Bad politics Mobile & Wireless Computing 10

The OSI Environment Mobile & Wireless Computing 11

OSI as Framework for Standardization Mobile & Wireless Computing 12

Elements of Standardization Protocol specification Operates between the same layer on two systems May involve different operating system Protocol specification must be precise Format of data units Semantics of all fields Allowable sequence of PCUs Service definition Functional description of what is provided Addressing between applications Referenced by SAPs Services between adjacent layers expressed in terms of primitives and parameters Primitives specify function to be performed Parameters pass data and control info Mobile & Wireless Computing 13

OSI Layers Transport Physical Session Data Link Presentation Network Exchange of data between end systems Error free In sequence No losses No duplicates Quality of service Session Control of dialogues between applications Dialogue discipline Grouping Recovery Presentation Data formats and coding Data compression Encryption Application Means for applications to access OSI environment Physical Physical interface between devices Mechanical Electrical Functional Procedural Data Link Means of activating, maintaining and deactivating a reliable link Error detection and control Higher layers may assume error free transmission Network Transport of information Higher layers do not need to know about underlying technology Not needed on direct links Mobile & Wireless Computing 14

Use of a Relay Mobile & Wireless Computing 15

TCP/IP Protocol Architecture Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) Used by the global Internet No official model but a working one. Application layer Host to host or transport layer Internet layer Data link layer Physical layer Problems: Service, interface, and protocol not distinguished Not a general model No exact separate mention of physical and data link layers Sometimes called host-to-network layer Minor protocols deeply entrenched, hard to replace Mobile & Wireless Computing 16

Layer Descriptions Physical Layer Data Link Layer Internet (IP) Layer Physical interface between data transmission device (e.g. computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates Data Link Layer Exchange of data between neighboring network nodes Invoking services like priority Internet (IP) Layer Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers Transport Layer (TCP) Reliable delivery of data Ordering of delivery Application Layer Support for user applications: HTTP, SMTP Mobile & Wireless Computing 17

OSI vs TCP/IP Mobile & Wireless Computing 18

Protocols and networks in the TCP/IP model Mobile & Wireless Computing 19

How is a connection established? TCP/IP is already part of your computer’s OS Application layer initiates the process TCP adds the PORT number (known as SAP in OSI Model) to the packet For each application there is a different port number (e.g., SMTP= 25) Makes a connection request to the receiving side IP adds the IP address of the destination computer Link or MAC layer determines the destination MAC address and creates a frame by appending that address Passes it to physical layer for transmission Physical layer converts it to bits and transmits The reverse order is followed at the receiver site to receive the packet at the given PORT Mobile & Wireless Computing 20

PDUs in TCP/IP Mobile & Wireless Computing 21

Encapsulation source destination application transport network link message M application transport network link physical segment Ht M datagram Ht Hn M frame Ht Hn Hl M link physical Ht Hn Hl M Ht Hn Hl M switch destination network link physical Ht Hn M Ht Hn M M application transport network link physical Ht Hn Hl M Ht Hn Hl M Ht M Ht Hn M router Ht Hn Hl M Mobile & Wireless Computing 22

Some Protocols in TCP/IP Suite Mobile & Wireless Computing 23