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Transcript
Internet Technology Fundamentals 1 Why Study Networks? • Integral part of society • Work, entertainment, community • Pervasive • Home, car, office, school, mall … • Understand what they do, how they work, and limitations – Any jobs left? – What happened to IT? – Future of the IT industry. 2 Impact of the Net on People • Anytime access to remote information – HW assignments from my server • Person-to-person and group communication – email, blogs, chat, meeting • Form and strengthen communities – chat rooms, MUDs, newsgroups 3 Impact of the Net on Society • Huge impact! – Continuation of technologies that reduce problems of time & space • (e.g. railroads,phone,autos,TV) • Good, bad and ugly – mirror of society • Changes still on the horizon – Commerce, services, entertainment, socializing 4 Internet Roles • Users • Everyone (mom and pop, kids) • work, leisure, serious, frivolous • Designers • protocol design and implementation • performance, cost and scale • Service Providers • Administrators and ISPs • Management, revenue, deployment 5 What is Internet Technology? • What is an internet? – Network of networks • What is the Internet? – A global internet based on the IP protocol • To what does “Internet technology” refer? – Architecture, protocols and services 6 Sample Internet Applications • • • • • • • • • Electronic mail Remote terminal File transfer Newsgroups File sharing Resource distribution World Wide Web Video conferencing Games 7 What is a Network? • Carrier of information between 2 or more entities • Interconnection may be any media capable of communicating information: – – – – copper wire lasers microwave satellite link 8 Some Definitions – – – – – – – Network: Collection of interconnected machines Host: Machine running user application Channel: Logical line of communication Media: Physical process used Protocol: Rules of communication Router: decide were to send data next Topology: How network is interconnected 9 How Do Computers Communicate? • With 1’s and 0’s – Computers only deal with 1’s and 0’s – So do networks – Must build all further structures from this basic representation • How do we transmit 1’s and 0’s in a network? 10 Physical Transmission A physical quantity (e.g. voltage), varying over time represents a digital 0 or 1 11 Concepts for this week • Layering and encapsulation – IP Hourglass • • • • Core and Edge of the Internet Circuit, message and packet switching Single link transmission delay Multi-link transmission delay – – – – Circuit switching Message switching Packet switching Computing general pipelining delay 12 Layering and Encapsulation 13 Why Layering? • Network communication is very complex • Separation of concerns – Different vendors and organizations responsible for different layers – Testing and maintenance is simplified – Easy to replace a single layer with a different version 14 Protocol Hierarchy • Use layers to hide complexity – Each layer implements a service • Layer N uses service provided by layer N-1 • layer N-1 provides a service to layer N – Protocols • Each layer communicates with its peer by a set of rules • Interface – A layers interface specifies the operations 15 Protocol Hierarchy (cont’d) Host A Layer 7 Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 Host B Layer 7 Protocol Layer 6 Protocol Layer 5 Protocol Layer 4 Protocol Layer 3 Protocol Layer 2 Protocol Layer 1 Protocol Physical Medium Layer 7 Layer 6 Layer 5 Layer 4 Layer 3 Layer 2 Layer 1 16 Different Layering Architectures • ISO OSI 7-Layer Architecture • TCP/IP 4-Layer Architecture – + application layer = 5 layers in Kurose • Novell NetWare IPX/SPX 4-Layer Architecture 17 Standards Making Organizations ISO = International Standards Organization ITU = International Teletraffic Union (formerly CCITT) ANSI = American National Standards Institute IEEE = Institute of Electrical and Electronic Engineers IETF = Internet Engineering Task Force ATM Forum = ATM standards-making body ...and many more 18 Why So Many Standards Organizations? • Multiple technologies • Different areas of emphasis and history – Telecommunications/telephones • ITU,ISO,ATM – Local area networking/computers • IETF, IEEE – System area networks/storage • ANSI 19 ISO OSI Layering Architecture Host A Host B Application Layer Application Protocol Presentation Layer Presentation Protocol Session Layer Session Protocol Transport Layer Transport Protocol Application Layer Presentation Layer Session Layer Transport Layer Network Layer Network Layer Network Layer Network Layer Data Link Layer Data Link Layer Data Link Layer Data Link Layer Physical Layer Physical Layer Physical Layer Physical Layer Router Router 20 ISO’s Design Principles • A layer should be created where a different level of abstraction is needed • Each layer should perform a well-defined function • The layer boundaries should be chosen to minimize information flow across the interfaces • The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy 21 Layer 1: Physical Layer • Functions: – Transmission of a raw bit stream – Forms the physical interface between devices • Issues: – – – – – – Which modulation technique (bits to pulse)? How long will a bit last? Bit-serial or parallel transmission? Half- or Full-duplex transmission? How many pins does the network connector have? How is a connection set up or torn down? 22 Layer 2: Data Link Layer • Functions: – Provides reliable transfer of information between two adjacent nodes – Creates frames, or packets, from bits and vice versa – Provides frame-level error control – Provides flow control • In summary, the data link layer provides the network layer with what appears to be an error-free link for packets 23 Layer 3: Network Layer • Functions: – Responsible for routing decisions • Dynamic routing • Fixed routing – Performs congestion control 24 Layer 4: Transport Layer • Functions: – Hide the details of the network from the session layer • Example: If we want replace a point-to-point link with a satellite link, this change should not affect the behavior of the upper layers – Provides reliable end-to-end communication 25 Transport Layer (cont’d) Host A first end-to-end layer Host B Application Layer Application Protocol Presentation Layer Presentation Protocol Session Layer Session Protocol Transport Layer Transport Protocol Application Layer Presentation Layer Session Layer Transport Layer Network Layer Network Layer Network Layer Network Layer Data Link Layer Data Link Layer Data Link Layer Data Link Layer Physical Layer Physical Layer Physical Layer Physical Layer Router Router 26 Transport Layer (cont’d) • Functions (cont’d): – Perform end-to-end flow control – Perform packet retransmission when packets are lost by the network 27 Layer 5: Session Layer • May perform synchronization between several communicating applications or logical transmissions • Groups several user-level connections into a single “session” • Examples: – Banking session – Network meetings 28 Layer 6: Presentation Layer • Performs specific functions that are requested regularly by applications • Examples: – encryption – ASCII to Unicode, Unicode to ASCII – LSB-first representations to MSB-first representations 29 Layer 7: Application Layer • Application layer protocols are applicationdependent • Implements communication between two applications of the same type • Examples: – FTP – Quake – SMTP (email) 30 Encapsulation Treat the neighboring layer’s information as a “black box”, can’t look inside or break message • Sending: add information needed by the current layer “around” the higher layers’ data – headers in front – trailers in back • Receiving: Strip off headers and trailers before handing up the stack 31 Encapsulation Data Application Layer AH Presentation Layer PH Session Layer SH Transport Layer TH Network Layer NH Data Link Layer DH Physical Layer PH Data Data • Headers Data Data • Trailer Data Data Data DT 32 Internet “Hourglass” Architecture • Defined by Internet Engineering Task Force (IETF) • “Hourglass” Design FTP HTTP RTP TFTP UDP TCP IP Ethernet CAT-5 802.11 Single-Mode Fiber … PPP RS-232 33 Internet Design Principles – Scale • Protocols should work in networks of all sizes and distances – Incremental deployment • New protocols need to be deployed gradually – Heterogeneity • Different technologies, autonomous organizations – End-to-end argument • Some functions can only be correctly implemented at the end hosts; the network should not provided these. 34 TCP/IP Layering Architecture Application Transport Internet/Network Host-to-Net • A simplified model • The network layer – Hosts drop packets into this layer, layer routes towards destination- only promise- try my best • The transport layer – reliable byte-oriented stream 35 TCP/IP Layering Architecture (cont’d) Host A Host B Application Protocol Application Layer Transport Protocol (TCP) Transport Layer Network Layer Host-toNet Layer Application Layer IP Network Layer Host-toNet Layer IP Network Layer Host-toNet Layer Transport Layer IP Network Layer Host-toNet Layer 36 Internet Topology • Current structure divides network into the “core” and “edge” networks • Core ISP’s “tiers” – Tier 1: Biggest ISPs • E.g. MCI, Sprint, AT&T – Tier 2 and 3: Regional and very small. • Edges: – Companies, organizations with a “default route” • E.g. Rutgers 37 Edge Networks Company A Internet Service Provider 1 ISP 2 Company B Edge router 38 Core Networks Internet Service Provider 1 Company A ISP 2 Company B 39 Single link Network Performance A Brief Introduction 40 Why Study Network Performance • Networks cost $ – OC-3 line ~= $10,000/month – Cable modem: $40/month – Are you getting your $/worth? • Why is the network “slow”? • Approach: – Build abstract models of network performance – Observe where real networks deviate from model – Simple Models: Tells us average/best/worse cases->useful, practical – Complex Models: Hard to understand -> useless 41 Digression: Units • Bits are the units used to describe an amount of data in a network – 1 kilobit (Kbit) – 1 megabit (Mbit) – 1 gigabit (Gbit) • Seconds are the units used to measure time – 1 millisecond (msec) – 1 microsecond (msec) – 1 nanosecond (nsec) • = 1 x 103 bits = 1,000 bits = 1 x 106 bits = 1,000,000 bits = 1 x 109 bits = 1,000,000,000 bits = 1 x 10-3 seconds = 0.001 seconds = 1 x 10-6 seconds = 0.000001 seconds = 1 x 10-9 seconds = 0.000000001 seconds Bits per second are the units used to measure channel capacity/bandwidth and throughput – bit per second (bps) – kilobits per second (Kbps) – megabits per second (Mbps) 42 Types of Delay • Processing – Time to execute protocol code • Queuing – Time waiting in queue to be processed • Transmission – Time to “get bits on wires” • Propagation – Time for bits to “move across wires” 43 Some Definitions • Packet length: size of a packet (units = bits or bytes) • Channel speed: How fast the channel can transmit bits (units = bits/second) • Packet transmission time: amount of time to transmit an entire packet (units = seconds) • Propagation delay: Delay imposed by the properties of the link. Depends on the link’s distance (units = seconds) 44 Transmission vs. Prop. delay A single transmission link as a water pipe 1. 2. The thicker the pipe, the more water it can carry from one end to the other Water is carried from one end of the pipe to the other at constant speed, no matter how thick the pipe is Water = Data bits Thickness of the pipe = Channel capacity Speed of water through the pipe = Propagation delay 45 Transmission vs. Prop. Delay (cont) pipe 1. 2. Propagation delay is how long takes to cross the pipe, irrespective of volume Transmission (bandwidth delay) is related to how much water can be pushed in through the openning per unit time Assumption that inserting water and propagation overlap 46 Transmission Time How long does it take A to transmit an entire packet onto the link? Relevant information: packet length = 1500 bytes channel capacity = 100 Mbps Another way to ask this question: If the link can transmit 10 million bits in a second, how many seconds does it take to transmit 1500 bytes (8x1500 bits)? 100 Mbits 1 sec = 1500 x 8 bits t Solving for t… t = 0.00012 sec (or 120 msec) 47 Propagation Delay How long does it take a single bit to travel on the link from A to B? Relevant information: link distance = 500 m prop. delay factor = 5 msec/km Another way to ask this question: If it takes a signal 5 msec to travel 1 kilometer, then how long does it take a signal to travel 500 meters? 5 msec 1000 m = t 500 m Solving for t… t = 2.5 msec 48 Processing Delay Stylized format required to send data Analogy: adding and removing envelopes to letters Host Host Application Layer Application Layer Transport Layer Transport Layer Router Network Layer Network Layer Network Layer Host-toNet Layer Host-toNet Layer Host-toNet Layer How long does it take to execute all these layers? Why is this time important? 49 Example A B 500 m Protocol Processing Time = 40 msec packet length = 1500 bytes channel capacity = 100 Mbps propagation delay factor = 5 msec/km 1. 2. 3. How long to format the data? How long does it take a single bit to travel on the link from A to B? How long does it take A to transmit an entire packet onto the link? 50 Timeline Method Host A 40 1st bit Host B Protocol Delay 2.5 Propagation delay 120 Transmission time Time last bit 40 Protocol Delay Total time: 40+120+2.5+40 = 202.5 msec 51 Queuing Delay Router Network Layer Host-toNet Layer Packets waiting processing at input ports 2 0 3 1 0 0 Router 0 0 1 2 2 3 Packets waiting transmission at output ports Packets arriving faster than processing or transmission delay => queuing (I.e. waiting in line) 52 Switching Schemes How much “state” about the connection between two hosts does each node/router along a path through the network maintain? 53 Switching Schemes (1) Circuit Switching (2) Message Switching (Store-and-Forward) (3) Packet Switching (Store-and-Forward) 54 Circuit Switching • Provides service by setting up the total path of connected lines hop-by-hop from the origin to the destination • Example: Telephone network 55 Circuit Switching (cont’d) 1. Control message sets up a path from origin to destination 2. Return signal informs source that data transmission may proceed 3. Data transmission begins 4. Entire path remains allocated to the transmission (whether used or not) 5. When transmission is complete, source releases the circuit 56 Circuit Switching (cont’d) Call request signal Time Propagation Delay Transmission Delay Call accept signal Data Transmission Time Data A B C D Routers/Switches 57 Message Switching • Each message is addressed to a destination • When the entire message is received at a router, the next step in its journey is selected; if this selected channel is busy, the message waits in a queue until the channel becomes free • Thus, the message “hops” from node to node through a network while allocating only one channel at a time • Analogy: Postal service 58 Message Switching (cont’d) Header Time Msg Transmission Delay Msg Queueing Delay Msg A B C D Routers/switches 59 Packet Switching • Messages are split into smaller pieces called packets • These packets are numbered and addressed and sent through the network one at a time • Allows Pipelining – Overlap sending and receiving of packets on multiple links 60 Packet Switching (cont’d) Pkt 1 Time Header Pkt 2 Pkt 1 Pkt 3 Transmission Delay Pkt 2 Pkt 1 Pkt 3 Pkt 2 Pkt 3 A B C D IMPs 61 Comparisons (1) Header Overhead Circuit < Message < Packet (2) Transmission Delay Short Bursty Messages: Packet < Message < Circuit Long Continuous Messages: Circuit < Message < Packet 62 Analytic Comparison • Given choice of 2 switching schemes, how would you compare their performance? – What would you need to know? – What are the independent variables? – What is the dependent variable? • Could you come up with a closed form expression based on your choices? 63 Example: Circuit Switching vs. Packet Switching • Goal: Determine which is faster – Formal definition: Least time to move a fixed amount of data • Approach: – Compute time where circuit switching and packet switching are equal based on all possible factors – A factor moving in one direction or the other will tip the balance in favor of one or the other – We’ll ignore wire-line propagation delay in this example 64 Factors: • • • • • • Number of bytes in the message: N Time to set up circuit: c Per-link bandwidth: B Size of the packet: p Size of the header: h Number of switches: s 65 Circuit Switching Time • Time to send N bytes using circuit switching • = Set-up cost + bandwidth delay N C B 66 Pipelining “Parallelogram” for packet switching Host A Switch 1 Switch 2 Host B Packet 1 Packet 2 Time Packet 3 Bandwidth Delay Packet 4 Propagation Delay 67 Note on Pipelining • The above analysis is very general: – Packets in a computer network • Messages/packets are the unit of work. – Instructions in a processor • Instructions are the unit of work. – Jobs through a batch Q in an operating system. • Processes are the unit of work. • Pipelining speeds up work over time. – How? 68 Packet Switching Time Delay = Transmission + “Propagation” delays Transmission delay (also bandwidth delay): Time to push all the packets into the network + “Propagation” delay: Time for the last packet to cross - not really prop. delay in the traditional sense N ( p h) ( p h) S 1 P B B 69 Packet Switching Time Transmission delay Number of packets “Propagation” delay Number of links/hops N ( p h) ( p h) S 1 P B B Time for each packet 70 Equilibrium Point N p h N C S 1 B B P Assuming all other factors equal, solve for C Q: Can you add link propagation delay to this example? 71 Homework Questions • If we use message switching, how does the time increase as we scale s? • How does packet switching reduce the impact of increasing s? • Show, using an equation, how reducing the packet size and packet switching reduces the impact of increasing s. • Where does the approach of reducing packet size fail to give any benefit? 72