CSSE 460 Computer Networks by Dr Yingwu Zhu

CSSE 460 Computer Networks by Dr. Yingwu Zhu 1

Course Overview 2

Class Materials r Will be posted on my website m Lecture slides m Projects m Additional materials m Others r What counts in this class? m Performance (Pop quizzes, Assignments, Projects, Exams) m Active participation in discussion r 2 sessions per class, 10 minutes break 3

What I Expect You to Do … r Reading is very important m Pre- and post-class reading textbook and supplemental materials m Lectures do not cover full stories r Active participation in class discussion r Accomplish projects 4

Networking is everywhere! r Internet, ad-hoc wireless networks, sensor networks r Networking devices: m Computers, PDAs, i-pods, sensor nodes, others r Networking services m Web, emails, P 2 P file sharing, Vo. IP, VOD, multimedia streaming … r Changing our lives in many ways! 5

Expectations ü This class IS about. . . ü Principles and Concepts in Computer Networks ü General-Purpose Computer Networks ü Internet Perspective ü Network Software and Programming ü Understanding Network Design Principles ü Application, Transport, Network Layers 6

r This class IS NOT about. . . m Survey of protocol standards m Special Purpose Networks m OSI - TCP/IP Battle m Network Hardware Components m Queuing Theory m Link and physical layers 7

Project machines r CS 1. seattleu. edu r SEATTLE U account and password 8

Introduction Goal: r get context, overview, “feel” of networking r more depth, detail later in course r approach: m descriptive m use Internet as example Overview: r what’s the Internet r what’s a protocol? r network edge r network core r access net, physical media r performance: loss, delay r protocol layers, service models r history 9

What’s the Internet: “nuts and bolts” view r millions of connected computing devices: hosts, end-systems m m pc’s workstations, servers PDA’s phones, toasters router server mobile local ISP running network apps r communication links m workstation regional ISP fiber, copper, radio, satellite r routers: forward packets (chunks) of data thru network company network 10

What’s the Internet: “nuts and bolts” view r protocols: control sending, receiving of msgs m e. g. , TCP, IP, HTTP, FTP, PPP r Internet: “network of router server workstation mobile local ISP networks” m m loosely hierarchical public Internet versus private intranet r Internet standards m RFC: Request for comments m IETF: Internet Engineering Task Force regional ISP company network 11

What’s the Internet: a service view r communication infrastructure enables distributed applications: m m WWW, email, games, ecommerce, database. , voting, more? r communication services provided: m m connectionless connection-oriented 12

Perspective r Network users: Does the network support the users’ applications m Reliability Error free service m Speed of data transfer m r Network designers: Cost efficient network design m Good utilization of network resources m Cost of building the network m Types of services to be supported 13

Perspective r Network providers: Network administration and customer service m Maximize Revenue m Minimize Operations Expenses m Survivability and Resiliency (Why) 14

A closer look at network structure: r network edge: applications and hosts r network core: m m routers network of networks r access networks, physical media: communication links 15

Connectivity r We may want a set of hosts (or devices) to be directly connected! WHY ? r We may not always strive for global connectivity! WHY? r Building Blocks to connect at the physical level: m links: coax cable, optical fiber. . . m nodes: general-purpose workstations… (though sometimes very specialized) 16

Basic Connectivity r Two types of direct connectivity: m point-to-point m multiple access 17

Discussions r If all computers had to be directly connected, networks would be either very limited or expensive and unmanageable! r Question: If n computers were to be completely and directly connected by pointto-point links of cost C, what would be the total cost of the net? r Question: Would it be less expensive to use a multiple-access network? What are the drawbacks and limitations? 18

Building Blocks r Switches, Routers, Gateways m Special network components responsible for “moving” packets across the network from source to destination. r Network hosts, workstations, etc. m they generally represent the source and sink (destination) of data traffic (packets) r We can recursively build large networks by interconnecting networks via gateways and routers. 19

An Interconnection Network 20

What’s a protocol? human protocols: r “what’s the time? ” r “I have a question” r introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: r machines rather than humans r all communication activity in Internet governed by protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt 21

What’s a protocol? a human protocol and a computer network protocol: Hi TCP connection req. Hi TCP connection reply. Got the time? Get http: //gaia. cs. umass. edu/index. htm 2: 00 <file> time Q: Other human protocol? 22

Protocols r Building blocks of a network architecture r Each protocol object has two different interfaces m service interface: defines operations on this protocol m peer-to-peer interface: defines messages exchanged with peer r Term “protocol” is overloaded m specification of peer-to-peer interface m module that implements this interface 23

The network edge: r end systems (hosts): m m m run application programs e. g. , WWW, email at “edge of network” r client/server model m m client host requests, receives service from server e. g. , WWW client (browser)/ server; email client/server r peer-peer model: m m host interaction symmetric e. g. : teleconferencing, Gnutella, Kazza 24

Network edge: connection-oriented service Goal: data transfer between end sys. r handshaking: setup (prepare for) data transfer ahead of time m m Hello, hello back human protocol set up “state” in two communicating hosts r TCP - Transmission Control Protocol m Internet’s connectionoriented service TCP service [RFC 793] r reliable, in-order byte- stream data transfer m loss: acknowledgements and retransmissions r flow control: m sender won’t overwhelm receiver r congestion control: m senders “slow down sending rate” when network congested 25

Network edge: connectionless service Goal: data transfer between end systems m same as before! r UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service m unreliable data transfer m no flow control m no congestion control App’s using TCP: r HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email) App’s using UDP: r streaming media, teleconferencing, Internet telephony 26

The Network Core r mesh of interconnected routers r the fundamental question: how is data transferred through net? m circuit switching: dedicated circuit per call: telephone net m packet-switching: data sent thru net in discrete “chunks” 27

Different Types of Switching r Different Types of Switching: m Circuit Switching (telephone network) • dedicated circuit, sending and receiving bit streams m Message Switching m Packet Switching • store and forward, sending and receiving packets m Virtual Circuit Switching m Cell Switching (ATM) r What are Packets? m Data to be transmitted is divided into discrete blocks 28

Network Core: Circuit Switching End-end resources reserved for “call” r link bandwidth, switch capacity r dedicated resources: no sharing r circuit-like (guaranteed) performance r call setup required 29

Cost-Effective Resource Sharing r Must share (multiplex) network resources among multiple users. r Common Multiplexing Strategies m Time-Division Multiplexing (TDM) m Frequency-Division Multiplexing (FDM): Frequency bandwidth r Multiplexing multiple logical flows over a single physical link. 30

Network Core: Circuit Switching network resources (e. g. , bandwidth) divided into “pieces” r pieces allocated to calls r resource piece idle if not used by owning call (no sharing) r dividing link bandwidth into “pieces” m frequency division m time division 31

Network Core: Packet Switching each end-end data stream divided into packets r user A, B packets share network resources r each packet uses full link bandwidth r resources used as needed, Bandwidth division into “pieces” Dedicated allocation Resource reservation resource contention: r aggregate resource demand can exceed amount available r congestion: packets queue, wait for link use r store and forward: packets move one hop at a time m transmit over link m wait turn at next link 32

Network Core: Packet Switching 10 Mbs Ethernet A B On-demand sharing statistical multiplexing C 1. 5 Mbs queue of packets waiting for output link D 45 Mbs E 33

Network Core: Packet Switching Packet-switching: store and forward behavior 34

Packet switching versus circuit switching Packet switching allows more users to use network! r 1 Mbit link r each user: m 100 Kbps when “active” m active 10% of time r circuit-switching: m 10 users N users 1 Mbps link r packet switching: m with 35 users, probability > 10 active less than. 004 35

Packet switching versus circuit switching Is packet switching a “slam dunk winner? ” r Great for bursty data m resource sharing m no call setup r Excessive congestion: packet delay and loss m protocols needed for reliable data transfer, congestion control r Q: How to provide circuit-like behavior? m bandwidth guarantees needed for audio/video apps still an unsolved problem! 36

Packet-switched networks: routing r Goal: move packets among routers from source to destination m we’ll study several path selection algorithms r datagram network: m destination address determines next hop m routes may change during session m analogy: driving, asking directions r virtual circuit network: m each packet carries tag (virtual circuit ID), tag determines next hop m fixed path determined at call setup time, remains fixed thru call m routers maintain per-call state m ATM 37

Access networks and physical media Q: How to connect end systems to edge router? r residential access nets r institutional access networks (school, company) r mobile access networks Keep in mind: r bandwidth (bits per second) of access network? r shared or dedicated? 38

Residential access: point to point access r Dialup via modem m up to 56 Kbps direct access to router (conceptually) r ISDN: intergrated services digital network: 128 Kbps alldigital connect to router r ADSL: asymmetric digital subscriber line m up to 1 Mbps home-to-router m up to 8 Mbps router-to-home 39

Residential access: cable modems r HFC: hybrid fiber coax m asymmetric: up to 10 Mbps upstream, 1 Mbps downstream r network of cable and fiber attaches homes to ISP router m m shared access to router among home issues: congestion, dimensioning r deployment: available via cable companies, e. g. , Media. One, Comcast 40

Institutional access: local area networks r company/univ local area network (LAN) connects end system to edge router r Ethernet: m shared or dedicated cable connects end system and router m 10 Mbs, 100 Mbps, Gigabit Ethernet r deployment: institutions, home LANs soon 41

Wireless access networks r shared wireless access network connects end system to router r wireless LANs: m m radio spectrum replaces wire e. g. , Lucent Wavelan 10 Mbps router base station r wider-area wireless access m CDPD: wireless access to ISP router via cellular network (base stations) mobile hosts 42

Physical Media r physical link: transmitted data bit propagates across link r guided media: m signals propagate in solid media: copper, fiber r unguided media: m signals propagate freely, e. g. , radio Twisted Pair (TP) r two insulated copper wires m m Category 3: traditional phone wires, 10 Mbps ethernet Category 5 TP: 100 Mbps ethernet 43

Physical Media: coax, fiber Coaxial cable: r wire (signal carrier) within a wire (shield) m m baseband: single channel on cable broadband: multiple channel on cable r bidirectional r common use in 10 Mbs Fiber optic cable: r glass fiber carrying light pulses r high-speed operation: m m 100 Mbps Ethernet high-speed point-to-point transmission (e. g. , 5 Gps) r low error rate Ethernet 44

Physical media: radio r signal carried in electromagnetic spectrum r no physical “wire” r bidirectional r propagation environment effects: m m m reflection obstruction by objects interference Radio link types: r microwave m e. g. up to 45 Mbps channels r LAN (e. g. , wave. LAN) m 2 Mbps, 11 Mbps r wide-area (e. g. , cellular) m e. g. CDPD, 10’s Kbps r satellite m up to 50 Mbps channel (or multiple smaller channels) m 270 Msec end-end delay m geosynchronous versus LEOS 45

Different Types of Links Sometimes you install your own! 46

Bigger Pipes! Sometimes leased from the phone company STS: Synchronous Transport Signal 47

Delay in packet-switched networks packets experience delay on end-to-end path r four sources of delay at each hop transmission A r nodal processing: m check bit errors m determine output link r queueing m time waiting at output link for transmission m depends on congestion level of router propagation B nodal processing queueing 48

Delay in packet-switched networks Transmission delay: r R=link bandwidth (bps) r L=packet length (bits) r time to send bits into link = L/R transmission A Propagation delay: r d = length of physical link r s = propagation speed in medium (~2 x 108 m/sec) r propagation delay = d/s Note: s and R are very different quantitites! propagation B nodal processing queueing 49

Performance Bandwidth (throughput) r Amount of data that can be transmitted per time unit r Example: 10 Mbps r link versus end-to-end r Notation m KB = 210 bytes m Mbps = 106 bits per second 50

Performance r Bandwidth related to “bit width” a) 1 second b) 1 second 51

Latency (delay) r Time it takes to send message from point A to point B r Example: 24 milliseconds (ms) r Sometimes interested in in round-trip time (RTT) r Components of latency Latency = Propagation + Transmit + Queue + Proc. Propagation = Distance / Speed. Of. Light Transmit = Size / Bandwidth 52

Transmission and Propagation Delays r Propagation delay m The propagation delay over a link is the time it takes a bit to travel from on end of the link to the other m = d/s r Transmission delay m It is the amount of time it takes to push the packet onto the link m =L/B r Total latency over the link m = transmission delay + propagation delay 53

Bandwidth v. s. Latency r Consider a standard 6250 bpi magnetic tape that holds 180 Megabytes. A station wagon can easily transport 200 tapes. Suppose the source and destination are an hour’s drive apart. r Calculate the effective throughput: m 288000 megabits in 3600 seconds m or 80 Mbps r What is the moral of this story ? 54

Bandwidth v. s. Latency r Never underestimate the bandwidth of a station wagon full of tapes pacing down the highway. r But … What happens to the latency? 55

The “hard” limit r Speed of light m 3. 0 x 108 meters/second in a vacuum m 2. 3 x 108 meters/second in a cable m 2. 0 x 108 meters/second in a fiber r Notes m no queuing delays in direct link m bandwidth not relevant if Size = 1 bit m process-to-process latency includes software overhead m software overhead can dominate when Distance is small 56

r Relative importance of bandwidth and latency m small message (e. g. , 1 byte): 1 ms vs. 100 ms dominates 1 Mbps vs. 100 Mbps m large message (e. g. , 25 MB): 1 Mbps vs. 100 Mbps dominates 1 ms vs. 100 ms r Consider two channels of 1 Mbps and 100 Mbps respectively. For a 1 byte message, the available bandwidth is relatively insignificant given a RTT of 1 ms. The transmit delay for each channel is 8 s and 0. 08 s, respectively. 57

r Delay x Bandwidth Product Delay Bandwidth e. g. , 100 ms RTT and 45 Mbps Bandwidth = 560 KB of data r We have to view the network as a buffer. This may have interesting consequences: m How much data did the sender transmit before a response can be received? 58

r Application Needs m bandwidth requirements: burst versus peak rate m jitter: variance in latency (inter-packet gap) r Average Bandwidth Requirement is Not enough: m consider a source with an avg. BW-requirement of 2 Mbps. If the application generates 1 Mbit in one second interval and 3 Mbit in a second, a channel that can support 2 Mbps max. will have a tough time. r Other Quality of Service (QOS) Parameters: m max. and min. delay m max. and min. bandwidth demand m rates for dynamic increase of demands m Cell-Loss Rate 59

Queueing delay (revisited) r R=link bandwidth (bps) r L=packet length (bits) r a=average packet arrival rate traffic intensity = La/R r La/R ~ 0: average queueing delay small r La/R -> 1: delays become large r La/R > 1: more “work” arriving than can be serviced, average delay infinite! 60

What Goes Wrong in the Network? r Different types of Error: m Bit-level errors (electrical interference) • 1 in 106 -107 in copper - 1 in 1012 - 1014 in fiber m Packet-level errors (congestion) m Link and node failures r How should we deal with these types of error? r What are the consequences of errors? 61

Other types of problems in the Network? r Messages are delayed r Messages are deliver out-of-order r Third parties eavesdrop The key problem is to fill in the gap between what applications expect and what the underlying technology provides. 62