Transcript ppt
EE 122: Network Applications Kevin Lai Aug 28, 2002 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 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 [email protected] 2 Network Applications how networks affect users - buy stock in networking companies, work for networking companies - use network applications 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 [email protected] 3 Different Network Applications telephony (making a phone call) sending email web browsing - buying something - reading news sending instant messages file sharing playing a distributed game watching/listening to streaming media [email protected] 4 Requirements of the Network throughput latency cost - compatibility with existing equipment usually lowers cost qualifications - distribution: mean, worst-case - in small bursts / all the time ease of programmability routing - unicast, multicast, broadcast [email protected] 5 Web Browsing Network Requirements Transfer 1KB-100KB files - e.g., HTML file, JPEG image Depends on size of content at sites visited News oriented sizes (e.g., www.nytimes.com) - Average throughput 32Kb/s – 1Mb/s - Average latency 100 – 250ms - Average loss < 10% Flat rate $20-$50/month [email protected] 6 Telephony Network Requirements Stream data at 9.6Kb/s – 128Kb/s - an encoded audio signal Wired - 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 [email protected] 7 File Sharing Requirements Transfer 3MB – 600MB file - MP3 audio file, MPEG video file Inbound throughput 128Kb/s – 10Mb/s Outbound throughput 128Kb/s – ? Mb/s Flat rate $20-$50/month [email protected] 8 Distributed Game Requirements 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 [email protected] 9 Comparing Applications Research community consistently failed to predict next popular application - predicted: telnet predicted: ftp predicted: IP telephony predicted: streaming predicted: streaming actually: email actually: WWW actually: instant messaging actually: file sharing maybe: distributed gaming Otherwise, little uniformity in application requirements [email protected] 10 Designing for Requirements Given different application requirements, how to design network that can run different applications well? - 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 [email protected] 11 Network Service Model Specifies what the network does for an application - leaves the rest to the application Examples - 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 Which model satisfies which application? What model do existing networks provide? [email protected] 12 Service Model Philosophies More Functionality Less Functionality Provide functionality for existing popular application - 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 Put only the most basic functionality in - e.g., internet - invert above properties [email protected] 13 Service Model Philosophies More Functionality -Integrated Services -Asynchronous Transfer Mode (ATM) Less Functionality -Differentiated Services -Multicast -Explicit Congestion Notification (ECN) -Best effort delivery -Ethernet -Unicast Simpler has predominated (so far) Reduced Instruction Set Computing (RISC) analogy [email protected] 14 Implementing service model How to implement service model on physical medium? - 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 [email protected] 15 Sharing Many users of same application using network Users of different applications using network How to do resource allocation? - resources: bandwidth, memory, CPU cycles Not all packets are created equal - 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) [email protected] 16 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 [email protected] 17