Introduction To Computer Networks - Department Of Computer Science
Introduction to Computer Networks Andreas Terzis terzis@cs.jhu.edu JHU Computer Science Dept 9/12/04 Intro to Computer Networks 1 Outline Show how the World Wide Web works Present some of the key ideas behind the Internet architecture 9/12/04 Intro to Computer Networks 2 1
World Wide Web We use it every day Millions of websites http://www.google.com/index.html Webpages contain objects – HTML, Text, images, etc – Base HTML file contains reference to other objects – Each object addressable by a URL 9/12/04 Intro to Computer Networks 3 Questions How to find what we are looking for How to transmit bits How to direct bits towards the right destination How to protect against errors How to express what we are looking for 9/12/04 Intro to Computer Networks 4 2
Solution Divide and Conquer – Break the problem in smaller parts – Solve each part separately – Put the pieces back together Layering Protocol Stack 9/12/04 application application transport transport network network link link physical physical Intro to Computer Networks 5 Physical Layer Send bits between directly connected machines Physical Media – Twisted Pair – Coaxial Cable – Fiber Optics – Radio Link Bandwidth Dedicated vs. shared link 9/12/04 Intro to Computer Networks 6 3
Data Link Layer Data Link Layer functions router base station – Framing – Error Detection & Correction mobile hosts – Link sharing Examples – Dialup, ADSL – Cable Modems – Wireless (WiFi) 9/12/04 Intro to Computer Networks 7 Internet: Network of Networks local ISP Tier 3 ISP Tier-2 ISP local ISP local local ISP ISP Tier-2 ISP ? Tier 1 ISP Tier 1 ISP local ISP Tier-2 ISP local ISP 9/12/04 NAP Tier 1 ISP Tier-2 ISP local ISP Intro to Computer Networks Tier-2 ISP local ISP 8 4
ISP Topology 9/12/04 Intro to Computer Networks 9 Network Layer Networking layer functions – Transport packets from sending to receiving hosts – Packet Switching vs. Circuit Switching application transport network data link physical application transport network 2. Receive data data link physical 1. Send data 9/12/04 Intro to Computer Networks 10 5
Forwarding vs. Routing Forwarding – How to send packet to the next hop towards the destination packet Routing F – Distance Vector (RIP) 5 3 B 2 A 2 1 – Link State (OSPF) 9/12/04 D Incoming Link – How to compute the next hops Routing algorithms Outgoing Links Forwarding Table dest nhop C F 1 3 D 5 E 1 2 Intro to Computer Networks 11 Transport Layer application transport network data link physical Packets get lost – Physical errors network data link physical gi ca le – Congestion lo – Equipment failures nd -e nd Transport layer functions network data link physical network data link tr physical an sp t – share resources among all network users or – Deliver packets reliably network data link physical application transport network data link physical 9/12/04 Intro to Computer Networks 12 6
Reliable Transfer Sender How to recover from losses Receiver 1 – Introduce ACKs A:1 – Timers – Stop-and-Wait 2 Sender sends packet set timer Receiver sends ACK Timer Expires 2 If no ACK received when timer expires sender retx 9/12/04 Intro to Computer Networks 13 Reliable Transfer (2) Stop-and-Wait has low Utilization Pipelining Sender Receiver 1 2 3 – Senders allows multiple yet-to-be-acked packets (window) A:1 A:2 A:3 Two variants – Go-Back-N – Selective repeat 9/12/04 Intro to Computer Networks 14 7
Congestion Control Congestion – Too many sources sending too much data too fast for network to handle Results bottleneck router capacity R – Lost packets – Long delays Solutions – End-to-End – Network assisted 9/12/04 Intro to Computer Networks 15 Application Layer-HTTP So far: data reliably transferred end-toend HT selene HTTP: hypertext transfer protocol client/server model – client: browser that requests, receives, “displays” Web objects HT TP TP HT T req res pon eq Pr HT TP ues ue se st s re t po nse Server running Apache Web server – server: Web server sends objects in response to requests 9/12/04 Intro to Computer Networks 16 8
Request/Response Timeline We enter URL www.google.com/index.html 1a. HTTP client initiates TCP connection to HTTP server (process) at www.google.com 2. HTTP client sends HTTP time request message (containing URL). Message indicates that client wants index.html 9/12/04 (contains text, reference to 1 jpeg image) 1b. HTTP server at host www.google.com waiting for TCP connection. “accepts” connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends response Intro to Computer Networks 17 Request/Response Timeline (cont.) 5. HTTP client receives time response message containing html file, displays html. Parsing html file, finds 1 referenced jpeg object 6. Steps 1-5 repeated for jpeg object 9/12/04 Intro to Computer Networks 18 9
HTTP Request Message HTTP request message: – ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /index.html HTTP/1.1 Host: www.google.com User-agent: Mozilla/4.0 header Connection: close lines Accept-language:fr Carriage return, line feed indicates end of message 9/12/04 (extra carriage return, line feed) Intro to Computer Networks 19 HTTP Response Message status line (protocol status code status phrase) header lines data, e.g., requested HTML file 9/12/04 HTTP/1.1 200 OK Connection close Date: Tue, 04 Mar 2003 07:28:29 GMT Server: GWS/2.0 Content-Length: 2824 Content-Type: text/html data data data data data . Intro to Computer Networks 20 10
Application Layer-DNS Translate from www.google.com to 216.239.51.101 Domain Name System: – distributed database implemented in hierarchy of many name servers – application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) 9/12/04 Intro to Computer Networks 21 Name Servers No server has all name-to-IP address mappings Local name servers: – each ISP, company has local (default) name server – host DNS query first goes to local name server Authoritative name server: – for a host: stores that host’s IP address, name – can perform name/address translation for that host’s name 9/12/04 Intro to Computer Networks 22 11
DNS Example root name server Root name server: 2 may not know authoritative name server may know intermediate name server: who to contact to find authoritative name server Results are cached 6 3 7 4 local name server 5 128.220.13.50 1 8 authoritative name server ns1.google.com requesting host selene.cs.jhu.edu 9/12/04 Intro to Computer Networks www.google.com 23 Alternate applications Email Instant Messaging P2P (Gnutella, Kazaa, etc) Online Games (Counter Strike) 9/12/04 Intro to Computer Networks 24 12
Alternate architectures Wireless and Mobile Networks – Cellular networks – Ad-hoc wireless networks Sensor networks 9/12/04 Intro to Computer Networks 25 Summary Explained how the WWW works – From physical to application layer Key problems – Resource sharing – Resource discovery – Distributed control and algorithms Key concepts – Packet switching – Layering 9/12/04 Intro to Computer Networks 26 13
Impact Tim Berners-Lee invented the Web Shawn Fanning was the (co-)creator of Napster Both tried to solve a pressing problem Both are not computer networking researchers! 9/12/04 Intro to Computer Networks 27 14
9/12/04 Intro to Computer Networks 22 Name Servers No server has all name-to-IP address mappings Local name servers: -each ISP, company has local (default) name server -host DNS query first goes to local name server Authoritative name server: -for a host: stores that host's IP address, name
DCAP406 COMPUTER NETWORKS Sr. No. Description 1. Introduction to Computer Networks: uses of computer networks, 2. Network hardware, network software, Reference models, Example networks 3. Physical Layer : Theoretical Basis for Data Communication, Guided Transmission Media, Wireless Transmission, Communication Satellites 4.
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Computer Networks Vs. Distributed Systems Computer Networks: – A computer network is an interconnected collection of autonomous computers able to exchange information. – A computer network usually require users to explicitly login onto one machine, explicitly submit jobs remotely, explicitly move files/data around the network .
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