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CSE 561 – Introduction and Layering David Wetherall Spring 2000.

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1 CSE 561 – Introduction and Layering David Wetherall djw@cs.washington.edu Spring 2000

2 djw // CS 561, Spring 2000 L1.2 This Lecture 1.Administrative Stuff 2.Introduction to Networking 3.Protocols and Layering 4.Medium Access

3 djw // CS 561, Spring 2000 L1.3 People David Wetherall –djw@cs.washington.edudjw@cs.washington.edu –Sieg 210, Th 11  12 Eric Hoffman –hoffman@cs.washington.eduhoffman@cs.washington.edu

4 djw // CS 561, Spring 2000 L1.4 Materials Course Web page –www.cs.washington.edu/education/course/561/00spwww.cs.washington.edu/education/course/561/00sp Mailing list –Important – make sure you join! Readings –Select papers distributed in class Textbook –Peterson and David, Computer Networks: A Systems Approach –2 nd edition came out late 1999 and is great.

5 djw // CS 561, Spring 2000 L1.5 Work and Grading Paper reviews and class participation (30%) –Reviews due by noon on day of class Written assignment (20%) –Mini-study of development of an area or technique Project (30%) –Groups, research-oriented, simulation or implementation Exam (20%) –In-class quiz in the latter part of the quarter

6 djw // CS 561, Spring 2000 L1.6 Mini-Reviews Half a page or so –Post on web site by noon before class Identify contributions/significance of paper –Summarize thrust of paper –Identify strengths and weaknesses Tell me what you think –What you learned, what you found interesting, what you want to know

7 djw // CS 561, Spring 2000 L1.7 Projects Done in groups of two or three Research-oriented –Evaluate proposed or new solutions/techniques –Simulation, measurement, or implementation –Topic is up to you Proposal –Two pages only stating purpose, prior work, plan of attack, and measure for success Presentations –In class, midterm and final (with paper)

8 djw // CS 561, Spring 2000 L1.8 Timeline Projects –Proposal end of week 2 (two pages only) –Midterm status report due end of week 5 –Final presentation/paper in class end of week 10 Assignment –Due end of week 6 Exam –In class, start of week 8

9 djw // CS 561, Spring 2000 L1.9 Goal of this Course For you to understand the design of large-scale computer networks. Fundamental problems in building networks –That are fast, efficient, secure and robust Design principles of proven value –Networking is young and there are few! Common implementation technologies –These will change of course …

10 djw // CS 561, Spring 2000 L1.10 Lecture Topics Multi-access (Ethernet) Routing and forwarding (IP routers) Network and Protocol Design (E2E etc.) Reliable transport (TCP) Congestion control (TCP) Multicast (Mbone) Scheduling and QOS (Intserv, DiffServ) Naming (DNS)

11 djw // CS 561, Spring 2000 L1.11 Elements of a Network Links carry information (bits) –Wire, wireless, fiber optic, smoke signals … Switches move bits between links –Routers, gateways, bridges, CATV headend, PABXs, … Hosts are the communication endpoints –PC, PDA, cell phone, tank, toaster, …

12 djw // CS 561, Spring 2000 L1.12 Example – Local Area Network Your home network –Ethernet is a broadcast-capable multi-access LAN Cable Modem PC Laptop Printer Ethernet Hub

13 djw // CS 561, Spring 2000 L1.13 Example – An Internetwork Internetwork is a network of networks The Internet is a global internetwork in which all participants speak a common language, IP. Local Net 2 Local Net 1 ISP 2 ISP 1

14 djw // CS 561, Spring 2000 L1.14 Our Focus We are interested in networks that are: –Distributed –Large scale –Multi-purpose

15 djw // CS 561, Spring 2000 L1.15 The meaning of “Distributed” There are distributed and parallel networks: –Cash machines versus a parallel computer What is the essential difference? –Tolerance of failed components –Decentralized operation –Heterogeneity Hard to get it right –“A distributed system is a system in which I can’t do my work because some computer has failed that I’ve never even heard of.” – Lamport

16 djw // CS 561, Spring 2000 L1.16 The meaning of “Large-scale”

17 djw // CS 561, Spring 2000 L1.17 The meaning of “Multi-purpose” Telephone network –Designed for telephone calls Internet –Web, email, Quake, e-commerce, audio/video, … –But evolution was at work: Web/email a “surprise” Computer networks –Carry digital information and support a rich variety of distributed applications

18 djw // CS 561, Spring 2000 L1.18 Why Build Networks? Communication at a distance –Want performance sufficient to given task Video conference, etc. Cost-effective resource sharing –Networks are shared among users Statistical multiplexing

19 djw // CS 561, Spring 2000 L1.19 Statistical Multiplexing Static partitioning schemes work well for a fixed number of users that always have data to send Not suited to data communications: peak>>average If we share on demand we can support more users –Based on the statistics of their transmissions –Occasionally we might be oversubscribed Statistical multiplexing is heavily used in data networks

20 djw // CS 561, Spring 2000 L1.20 Example One user sends at 1 Mbps and is idle 90% of the time. –10 Mbps channel; 10 users if statically allocated What are the likely loads if we share on demand? 0 1 2 Mbps Prob 0 1 … 10 Mbps Prob 2 users 10 users

21 djw // CS 561, Spring 2000 L1.21 You do the Math For 10 users, Prob(need 10 Mbps) = 10 -10 So keep adding users … For 35 users, Prob(>10 active users) = 0.17% We can support three times as many users!

22 djw // CS 561, Spring 2000 L1.22 Protocols and Layering Need abstractions to handle complexity –Protocols and layering Protocol –Agreement dictating the form and function of data exchanged between parties to effect communication –Two parts: Syntax: where the bits go Semantics: what they mean, what to do with them –Examples: IP, the Internet protocol TCP and HTTP, for the Web

23 djw // CS 561, Spring 2000 L1.23 Protocol Standards Different functions require different protocols Thus there are many protocol standards –E.g., IP, TCP, UDP, HTTP, DNS, FTP, SMTP, NNTP, ARP, Ethernet/802.3, 802.11, RIP, OPSF, 802.1D, NFS, ICMP, IGMP, DVMRP, IPSEC, PIM-SM, BGP, … Organizations: IETF, IEEE, ITU IETF specifies Internet-related protocols –RFCs (Requests for Comments) –“We reject kings, presidents and voting. We believe in rough consensus and running code.” – Dave Clark.

24 djw // CS 561, Spring 2000 L1.24 Protocol Layering Layering –Higher level protocols build on services provided by lower levels –Peer layers communicate with each other Layer N+1 e.g., HTTP Layer N e.g., TCP Home PC Netscape

25 djw // CS 561, Spring 2000 L1.25 Example – Layering at work We can connect different systems TCP IP Ethernet TCP IP CATV IP EthernetCATV host router

26 djw // CS 561, Spring 2000 L1.26 Layering Mechanics Encapsulation and decapsulation Hdr Data + + Layer N+1 PDU becomes Layer N ADU Messages passed between layers

27 djw // CS 561, Spring 2000 L1.27 More Layering Mechanics Multiplexing and demultiplexing in a protocol graph UDPTCP ARPIP Ethernet SMTPHTTP 802.2 identifier IP protocol field TCP port number

28 djw // CS 561, Spring 2000 L1.28 A Packet on the Wire Starts looking like an onion! This isn’t entirely accurate –ignores segmentation and reassembly, Ethernet trailers, etc. But you can see that layering adds overhead IP HdrPayload (Web object)TCP HdrHTTP Hdr Ethernet Hdr Start of packetEnd of packet

29 djw // CS 561, Spring 2000 L1.29 Internet Protocol Stacks Application Transport Network Link Many (HTTP, SMTP) TCP / UDP IP Many (Ethernet, …) ModelProtocols

30 djw // CS 561, Spring 2000 L1.30 OSI Reference Model Seven Layers Their functions: Your call Encode/decode messages Manage connections Reliability, congestion control Routing Framing, multiple access Symbol coding, modulation Application Presentation Session Transport Network Link Physical

31 djw // CS 561, Spring 2000 L1.31 The Problem of Multiple Access Multiple nodes share a broadcast channel –wired LAN, wireless LAN, cell phones, packet radio, satellites –How do they coordinate their transmissions? Ideal solution for N nodes sharing bandwidth B bps: –high goodput (B), low delay (0), fair (B/N), decentralized, stable, and simple! nodes broadcast media

32 djw // CS 561, Spring 2000 L1.32 Key Concepts Networks are used to share distributed resources Protocol layers are used to handle complexity The Internet/OSI models give us a roadmap

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