Network Functionality Network functionality is only useful if it benefits users also applies to any computer system but easier to forget in networking e g Is it worth it to upgrade a 56Kb s modem link in your network to 100Mb s Ethernet Only if it significantly improves user experience EE 122 Network Applications Kevin Lai Aug 28 2002 Generalization of Amdahl s Law from computer architecture Speedupoverall execution timeold 1 fractionenhanced execution timenew 1 fraction enhanced speedupenhanced Also applies to metrics other than time laik cs berkeley edu Network Applications Different Network Applications how networks affect users buy stock in networking companies work for networking companies use network applications telephony making a phone call sending email web browsing buying something reading news need to understand application requirements to build useful networks to understand why networks are built the way they are to understand why some ideas failed laik cs berkeley edu 2 3 sending instant messages file sharing playing a distributed game watching listening to streaming media laik cs berkeley edu 4 1 Web Browsing Network Requirements Requirements of the Network throughput latency cost Transfer 1KB 100KB files Depends on size of content at sites visited News oriented sizes e g www nytimes com e g HTML file JPEG image compatibility with existing equipment usually lowers cost Average throughput 32Kb s 1Mb s Average latency 100 250ms Average loss 10 qualifications distribution mean worst case in small bursts all the time ease of programmability routing Flat rate 20 50 month unicast multicast broadcast laik cs berkeley edu 5 laik cs berkeley edu Telephony Network Requirements File Sharing Requirements Stream data at 9 6Kb s 128Kb s Transfer 3MB 600MB file Inbound throughput 128Kb s 10Mb s Outbound throughput 128Kb s Mb s Flat rate 20 50 month an encoded audio signal MP3 audio file MPEG video file Wired 6 Worst case throughput 64Kb s 128Kb s Worst case latency 100ms 125ms Loss 1 01 10 minute Mobile Worst case throughput 9 6Kb s 128Kb s Worst case latency 100ms 250ms Loss 10 10 1 minute laik cs berkeley edu 7 laik cs berkeley edu 8 2 Distributed Game Requirements Comparing Applications e g WarCraft III Quake III EverQuest unlimited number of players stream data about player s state e g location in the world appearance items carried units controlled etc Throughput 128Kb s Mb s Latency 0ms 250ms Flat rate 20 50 month laik cs berkeley edu actually email actually WWW actually instant messaging actually file sharing maybe distributed gaming Otherwise little uniformity in application requirements laik cs berkeley edu 10 Network Service Model Given different application requirements how to design network that can run different applications well Specifies what the network does for an application Examples leaves the rest to the application Establishes a circuit that guarantees 1Mb s from one node to another for 10 s regardless of other traffic Deliver a packet from one node to another with a delay of at most 100ms for 10 Mb Deliver a packet with unknown delay for 10 Mb but less delay than people paying 05 Mb Deliver a packet with unknown delay maybe define network service model implement service model on physical technology satisfy service model when multiple applications and users are sharing network replace obsolete service model laik cs berkeley edu predicted telnet predicted ftp predicted IP telephony predicted streaming predicted streaming 9 Designing for Requirements Research community consistently failed to predict next popular application 11 Which model satisfies which application What model do existing networks provide laik cs berkeley edu 12 3 Service Model Philosophies More Functionality Service Model Philosophies Less Functionality Provide functionality for existing popular application Integrated Services Asynchronous Transfer Mode ATM e g telephone network less work for application developers some functionality can only be provided by the network more expensive network per flow state not all applications may use functionality target applications may be replaced with new applications More Functionality Put only the most basic functionality in e g internet invert above properties laik cs berkeley edu 13 Best effort delivery Ethernet Unicast Simpler has predominated so far Reduced Instruction Set Computing RISC analogy 14 Sharing How to implement service model on physical medium Many users of same application using network Users of different applications using network How to do resource allocation Not all packets are created equal e g Category 5 cabling optical fiber radio waves analog digital digital analog encoding how to separate packets framing physical medium is subject to errors e g fading multipath microwave ovens sun spots etc service model may specify higher reliability than physical medium provides reliability error detection laik cs berkeley edu Differentiated Services Multicast Explicit Congestion Notification ECN laik cs berkeley edu Implementing service model Less Functionality resources bandwidth memory CPU cycles IP telephony packet must be delivered with low delay File sharing packet can be delayed Not all users created equal some users pay more some users follow rules some do not for personal gain selfish users some do not just to be mean malicious users 15 laik cs berkeley edu 16 4 Replacing Network Model Every service model developed so far has eventually become obsolete service model optimizes for a particular application mix application mix changes Network is much harder to upgrade than other systems bad news 100 s M nodes full upgrade takes decades good news use old network to bootstrap new network e g first Internet nodes used phone lines How to implement new network service model on top of existing network solution overlay networks laik cs berkeley edu 17 5
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