Introduction to Network Protocols PDF

What’s a protocol? human protocols: v v v “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: v v machines rather than humans all communication activity in Internet governed by protocols protoc...

What’s a protocol? human protocols: v v v “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: v v machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt Intro 11 A closer look at network structure: v network edge: § § v v mobile network hosts: clients and servers servers often in data centers access networks, physical media: wired, wireless communication links global ISP home network regional ISP network core: § interconnected routers § network of networks institutional network Intro 14 Access net: digital subscriber line (DSL) central office DSL splitter modem voice, data transmitted at different frequencies over dedicated line to central office v v v telephone network DSLAM ISP DSL access multiplexer use existing telephone line to central office DSLAM § data over DSL phone line goes to Internet § voice over DSL phone line goes to telephone net < 2.5 Mbps upstream transmission rate (typically < 1 Mbps) < 24 Mbps downstream transmission rate (typically < 10 Mbps) Intro 16 Access net: cable network cable headend … cable splitter modem V I D E O V I D E O V I D E O V I D E O V I D E O V I D E O D A T A D A T A C O N T R O L 1 2 3 4 5 6 7 8 9 Channels frequency division multiplexing: different channels transmitted in different frequency bands Intro 17 Access net: cable network cable headend … cable splitter modem data, TV transmitted at different frequencies over shared cable distribution network v v CMTS cable modem termination system ISP HFC: hybrid fiber coax § asymmetric: up to 30Mbps downstream transmission rate, 2 Mbps upstream transmission rate network of cable, fiber attaches homes to ISP router § homes share access network to cable headend § unlike DSL, which has dedicated access to central office Intro 18 Access net: home network wireless devices to/from headend or central office often combined in single box cable or DSL modem wireless access point (54 Mbps) router, firewall, NAT wired Ethernet (100 Mbps) Intro 19 Intro 20 Enterprise access networks (Ethernet) institutional link to ISP (Internet) institutional router Ethernet switch v v v institutional mail, web servers typically used in companies, universities, etc 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates today, end systems typically connect into Ethernet switch Intro 21 Two key network-core functions routing: determines sourcedestination route taken by packets § routing algorithms forwarding: move packets from router’s input to appropriate router output routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 1 3 2 1 011 dest address in arriving packet’s header Packet switching versus circuit switching packet switching allows more users to use network! • 100 kb/s when “active” • active 10% of time N users ….. example: § 1 Mb/s link § each user: 1 Mbps link v circuit-switching: § 10 users v packet switching: § with 35 users, probability > 10 active at same time is less than .0004 * Q: how did we get value 0.0004? Q: what happens if > 35 users ? * Check out the online interactive exercises for more examples Intro 37 Packet switching versus circuit switching is packet switching a “slam dunk winner?” v v v great for bursty data § resource sharing § simpler, no call setup excessive congestion possible: packet delay and loss § protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? § bandwidth guarantees needed for audio/video apps § still an unsolved problem Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? Intro 38 Internet structure: network of networks v v v v End systems connect to Internet via access ISPs (Internet Service Providers) § Residential, company and university ISPs Access ISPs in turn must be interconnected. v So that any two hosts can send packets to each other Resulting network of networks is very complex v Evolution was driven by economics and national policies Let’s take a stepwise approach to describe current Internet structure Intro 39 Internet structure: network of networks Question: given millions of access ISPs, how to connect them together? access net … access net access net … google access net access net access net … … access net access net access net access net access net access net access net access net … … ouhk access net access net Intro 40 Internet structure: network of networks Option: connect each access ISP to every other access ISP? access net access net … access net access net … access net … … access net access net … … connecting each access ISP to each other directly doesn’t scale: O(N2) connections. … … access net access net access net access net access net access net access net … … access net access net Intro 41 … Caravan analogy (more) 100 km ten-car caravan v v v toll booth 100 km toll booth suppose cars now “propagate” at 1000 km/hr and suppose toll booth now takes one min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at first booth? § A: Yes! after 7 min (1+6), 1st car arrives at second booth; three cars still at 1st booth. Intro 56 v v v v v v R: link bandwidth (bps) L: packet length (bits) a: average packet arrival rate average queueing delay Queueing delay (revisited) traffic intensity = La/R La/R ~ 0: avg. queueing delay small La/R -> 1: avg. queueing delay large La/R > 1: more “work” arriving than can be serviced, average delay infinite! La/R ~ 0 La/R -> 1 Intro 57 Throughput: Internet scenario per-connection endend throughput: min(Rc,Rs,R/10) v in practice: Rc or Rs is often bottleneck v Rs Rs Rs R Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec Intro 63 Outline 1.1 what is the Internet? 1.2 network edge § end systems, access networks, links 1.3 network core § packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Intro 64 Protocol “layers” Networks are complex, with many “pieces”: § hosts § routers § links of various media § applications § protocols § hardware, software Question: is there any hope of organizing structure of network? …. or at least our discussion of networks? Intro 65 Organization of air travel ticket (purchase) ticket (complain) baggage (check) baggage (claim) gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing airplane routing v a series of steps Intro 66 Why layering? dealing with complex systems: v explicit structure allows identification, relationship of complex system’s pieces § layered reference model for discussion v 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 v v v layering considered harmful? one layer may duplicate lower-layer functionality (e.g., error) functionality at one layer may need information (e.g., timestamp value) that is present only in another layer Intro 68 Internet protocol stack （5-layered) v application: supporting network applications § FTP, SMTP, HTTP v transport: process-process data transfer § TCP, UDP v network: routing of datagrams from source to destination § IP, routing protocols v link: data transfer between neighboring network elements application transport network link physical § Ethernet, 802.111 (WiFi), PPP v physical: bits “on the wire” Intro 69 ISO/OSI reference model ISO stands for International organization of Standardization The Open Systems Interconnection model (OSI model) application v presentation: allow applications to presentation interpret meaning of data, e.g., session encryption, compression, machinespecific conventions transport v session: synchronization, network checkpointing, recovery of data link exchange v Internet stack “missing” these layers! physical v § these services, if needed, must be implemented in application § needed? Intro 70 e 1.23 Application Presentation Application Session Transport Transport Network Network Link Link Physical Physical a. Five-layer Internet protocol stack ! b. Seven-layer ISO OSI reference model The Internet protocol stack (a) and OSI reference model (b) Intro 71 Encapsulation source message segment M Ht M datagram Hn Ht M frame M Hl Hn Ht application transport network link physical link physical switch M Ht M Hn Ht M Hl Hn Ht M destination Hn Ht M application transport network link physical Hl Hn Ht M network link physical Hn Ht M router Intro 72 Outline 1.1 what is the Internet? 1.2 network edge § end systems, access networks, links 1.3 network core § packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Intro 73 Network security v field of network security: § how bad guys can attack computer networks § how we can defend networks against attacks § how to design architectures that are immune to attacks v Internet not originally designed with (much) security in mind § original vision: “a group of mutually trusting users attached to a transparent network” J § Internet protocol designers playing “catch-up” § security considerations in all layers! Intro 74 Bad guys: put malware into hosts via Internet v malware can get in host from: § virus: self-replicating infection by receiving/executing object (e.g., e-mail attachment) § worm: self-replicating infection by passively receiving object that gets itself executed v spyware malware can record keystrokes, web sites visited, upload info to collection site v infected host can be enrolled in botnet, used for spam. DDoS attacks Intro 75 Outline 1.1 what is the Internet? 1.2 network edge § end systems, access networks, links 1.3 network core § packet switching, circuit switching, network structure 1.4 delay, loss, throughput in networks 1.5 protocol layers, service models 1.6 networks under attack: security 1.7 history Intro 79 Internet history 1961-1972: Early packet-switching principles v v v v 1961: Kleinrock queueing theory shows effectiveness of packetswitching 1964: Baran - packetswitching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational v 1972: § ARPAnet public demo § NCP (Network Control Protocol) first host-host protocol § first e-mail program § ARPAnet has 15 nodes Intro 80 Internet history 1972-1980: Internetworking, new and proprietary nets v v v v v v 1970: ALOHAnet satellite network in Hawaii 1974: Cerf and Kahn architecture for interconnecting networks 1976: Ethernet at Xerox PARC late70’s: proprietary architectures: DECnet, SNA, XNA late 70’s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Cerf and Kahn’s internetworking principles: § minimalism, autonomy - no internal changes required to interconnect networks § best effort service model § stateless routers § decentralized control define today’s Internet architecture Intro 81 Review v v v v v v v v v The Internet Access technologies Types of Physical media Packet vs circuit switching Concept of network of networks Sources of packet delay Throughput and bottleneck TCP/IP layers Encapsulation