MASSACHUSETTS INSTITUTE OF TECHNOLOGY SLOAN SCHOOL OF MANAGEMENT 15.565 Integrating Information Systems: Technology, Strategy, and Organizational Factors 15.578 Global Information Systems: Communications & Connectivity Among Information Systems Spring 2002 Lecture 9 NETWORK PROTOCOLSCOMPLEXITY OF COMMUNICATION NETWORKS Sources Receiver Web site Central File Server E-Mail Server Browser Your Files E-Mail Program T1 Line Satellite link The Intenet Your PC 2ROLE OF PROTOCOLS EXAMPLES FROM NORMAL TELEPHONE CONVERSATION • ASSUMING SENDER (S) AND RECEIVER (R) SPEAK ENGLISH – ITSELF A PROTOCOL ISSUE • ESTABLISHING CONNECTON: S: “IS JOHN THERE?” R: “YES, THIS IS JOHN.” • ERROR CHECKING: S: “DID YOU HEARD WHAT I JUST SAID?” R: “YES.” • FLOW CONTROL (E.G., DICTATING LETTER OVER PHONE): S: “ARE YOU READY FOR NEXT SENTENCE?” R: … pause … “YES.” BOTH SENDER AND RECEIVER MUST FOLLOW SAME RULES. • OTHER EXAMPLES ? 3LAYERING OF PROTOCOLS AND COMMUNICATIONS 4 Concepts Symbols on Paper Dot Pattern Electrical Signals Physical Wire User Paper Fax Machine Modem Telephone Line To: Joe To: Joe 617/253-3321617/253-3321 Issues: Example: Abstractions & Substitutions/Alternatives Sending a fax----ISO OSI REFERENCE MODEL • ISO = INTERNATIONAL STANDARDIZATION ORGANIZATION • OSI = OPEN SYSTEMS INTERCONNECTION • ISO OSI REFERENCE MODEL: IS A FRAMEWORK AND SET OF NOMENCLATURE IS NOT A PROTOCOL STANDARD • STANDARDS DEVELOPERS: CCITT, IFIP, ANSI, IEEE 5ISO REFERENCE MODEL FOR OSI 7. APPLICATION: APPLICATION DEPENDENT (E.G., USER PROGRAM) 6. PRESENTATION: COMPRESSION AND CONVERSIONS (E.G., LIBRARY) 5. SESSION: PROCESS-TO-PROCESS (E.G., OS SOFTWARE) 4. TRANSPORT: HOST-TO-HOST (E.G., OS SOFTWARE) 3. NETWORK: ROUTING (E.G., DEVICE DRIVER) 2. DATA: RELIABLE BIT STREAM (E.G., SPECIAL CHIP) 1. PHYSICAL: RAW BIT STREAM (E.G., HARDWARE) 7 6 5 4 3 2 1 7 6 5 4 3 2 1 APPLICATION LAYER PROTOCOL PRESENTATION LAYER PROTOCOL SESSION LAYER PROTOCOL TRANSPORT LAYER PROTOCOL NETWORK DATALINK PHYSICAL NETWORK DATALINK PHYSICAL HOST A NODE HOST B6 THE SEVEN-LAYER ISO REFERENCE MODELSIMPLE MAIL DELIVERY ANALOGY 7. APPLICATION: SPECIFIC HANDLING (e.g., PAY BILL, INQUIRY) 6. PRESENTATION: LANGUAGE TRANSLATION SERVICE 5. SESSION: GROUP IN COMPANY (e.g., A/P) or PERSON (e.g., E53-321) 4. TRANSPORT: SOURCE COMPANY TO RECEIVER COMPANY 3. NETWORK: ROUTING FROM POST OFFICE TO POST OFFICE 2. DATA: FLOW CONTROL AND TRAFFIC HANDLING ON HIGHWAY 1. PHYSICAL: TRUCKS AND PLANES USED • MUST USE SAME STANDARDS FROM …. TO … TO … FROM … 71. PHYSICAL LAYER • SUBNET TYPES – CIRCUIT SWITCHING (DEDICATED CHANNEL) – MESSAGE SWITCHING – PACKET SWITCHING (SHARED CHANNEL) • COMMUNICATION TECHNOLOGIES (SOME EXAMPLES) – TELEPHONE • T1 = 1.544M bps (USA & CANADA) OR 2.048M (ELSE) – SHARED CABLE (ETHERNET) • 10-100M bps (Typical) • CSMA/CD (CARRIER SENSE MULTIPLE ACCESS/COLLISION DETECT) – SATELLITE • 5-10 CHANNELS, EACH 50M bps • UP-LINK & DOWN-LINK = 270 MILLISECONDS • VSAT – FIBER-OPTIC • 100M - 10G bps (Typical) • INTERNET II (622M -> 2G) • PROJECT OXYGEN = 1.28T bps (before 2003) 82. DATA LINK LAYER FOCUS: RELIABLE TRANSMISSION: ERROR HANDLING & FLOW CONTROL ERROR HANDLING: DETECTION AND CORRECTION • CHECK SUM FOR ERROR DETECTION (AND OTHER ERROR DETECT/CORRECT CODES) BEGIN CODE END CODE 01111110 ADDRESS CONTROL INFORMATION CHECKSUM 01111110 8 88 ? 16 8 9FLOW CONTROL • TO HANDLE CONGESTION & SEQUENCING • RECEIVER INDICATES WILLINGNESS TO RECEIVE • LIKE RESERVATION FOR DINNER • POSITIVE AND NEGATIVE ACKNOWLEDGEMENTS TO RECEIPT • SEQUENCE NUMBERS TO HELP KEEP COORDINATED • RECYCLE SEQUENCE NUMBERS • MAY HAVE MULTIPLE OUTSTANDING MESSAGES • ESPECIALLY FOR SATELLITE -- 1/4 SEC ROUND TRIP • TYPES OF INFO USED: • SEQUENCE (THIS FRAME) • P/F (POLL OR FINAL) • NEXT (ACK RECEIVED UP TO) 10DYNAMIC CHANNEL SHARING • SATELLITE – MULTIPLEX (TIME OR FREQUENCY) – “SLOTTED” ALOHA (PRE-DATES ETHERNET) • EACH “SLOT”, EITHER SEND OR NO SEND • IF SEND, LISTEN FOR COLLISION (270 MS) • IF COLLIDE, TRY AGAIN LATER -- BUT WHEN? • “BEST ATTAINABLE UTILIZATION” ABOUT 37% • SIMILAR FOR SHARED LAN (E.G., ETHERNET) • IEEE 802 STANDARDS – MEDIA ACCESS: CSMA/CD AND TOKEN RING LA SF HAWAII TOKYO 113. NETWORK LAYER ROUTE DETERMINATION (TO BE DISCUSSED MORE LATER) • VIRTUAL CIRCUIT • vs DATAGRAM • E.G., X.25 NETWORK CCITT 3-LAYER PROTOCOL • -- VIRTUAL CIRCUIT ORIGINALLY • PROCEDURE 1. SET UP VIRTUAL CIRCUIT (CALL REQUEST) -- RECEIVER ACCEPTS OR REJECTS 2. IF ACCEPT, SEND DATA PACKETS (FULL-DUPLEX) 3. TERMINATE BY EITHER PARTY • EXTENSIONS -- DATAGRAM -- FAST SELECT (ONE PACKET MESSAGE) 12------4. TRANSPORT LAYER • PROVIDE “TRANSPARENT” USER-TO-USER (END-TO-END) • HANDLE RECOVERY, ETC. TRANSPARENTLY • EXAMPLE FUNCTIONS: CONNUM = CONNECT (LOCAL, REMOTE) CONNUM = LISTEN (LOCAL) STATUS = CLOSE (CONNUM) STATUS = SEND (CONNUM, BUFFER, BYTES) STATUS = RECEIVE (CONNUM, BUFFER, BYTES) • CCITT STANDARD X.25 ADDRESS = 14 DIGITS 3 = COUNTRY (MAYBE MULTIPLE CODES) 1 = COUNTRY NET 10 = NETWORK OPERATOR CHOICE (E.G., 5 = HOST #, 5 = USER #) 13------------SYNCHRONIZATION AND MULTIPLEXING ISSUES • SYNCHRONIZATION ISSUES: UNEXPECTED MESSAGE RECEIVED MULTIPLE PACKETS (DUE TO TIME-OUT & RETRANSMIT) CLOSING CONNECTIONS (E.G., TWO ARMY DIVISION PROBLEM: “YOU ATTACK WHEN YOU GET MY MESSAGE”) NEED TO HANDLE THESE CASES • CONNECTION MULTIPLEXING TO SHARE “VIRTUAL CIRCUIT” • FOR EFFICIENCY/COST SAVINGS (LIKE SOFTWARE MULTIPLEXING) TO USE MULTIPLE “VIRTUAL CIRCUITS” • FOR INCREASED TRANSMISSION CAPACITY 14------------5. SESSION LAYER • PROVIDE PROCESS-TO-PROCESS COMMUNICATION (E.G., WEB BROWSER VS. FILE TRANSFER VS. E-MAIL -- SIMULTANEOUS) 6. PRESENTATION LAYER • TYPICAL ACTIVITIES TEXT COMPRESSION & ENCRYPTION (OFTEN AT DATA LAYER) CONVERSION • “VIRTUAL” TERMINAL PROTOCOLS MANY TERMINAL DIFFERENCES TYPES: SCROLL, PAGE (CURSOR), FORM (E.G. ARPANET TELNET) E.G., X-WINDOWS • FILE TRANSFER PROTOCOLS BIT-FOR-BIT OR CONVERTED? (E.G. ASCII - > EBCDIC, FLOATING POINT #’s) 7. APPLICATION LAYER • ELECTRONIC MAIL, WEB BROWSER & OTHERS 15Layer ISO INTERNET SNA DECNET 7 6 5 4 3 2 1 Application Presentation Session Transport Network Data link Physical User Telnet, FTP, Web Transmission Control Internet Protocol