Lec_1.pdf

Data Communications Network Chapter 1 ▪ Module 01: Computer Networks and the Internet ▪ Module 02: Application Layer ▪ Module 03: Transport Layer ▪ Module 04: The Network Layer ▪ Module 05: The Link Layer: Links, Access Networks, and LANs ▪ Module 06: The physical layer Computer Networking: A Top-Down Approach 8th edition Jim Kurose, Keith Ross Pearson, 2020 Introduction: 1-2 Chapter 1: introduction Chapter goal: Overview/roadmap: ▪ Get “feel,” “big picture,” ▪ What is the Internet? What is a introduction to terminology protocol? more depth, detail later in ▪ Network edge: hosts, access network, physical media course ▪ Network core: packet/circuit switching, internet structure ▪ Performance: loss, delay, throughput ▪ Protocol layers, service models ▪ Security ▪ History Introduction: 1-3 The Internet: a “nuts and bolts” view Billions of connected mobile network computing devices: national or global ISP ▪ hosts = end systems ▪ running network apps at Internet’s “edge” Packet switches: forward local or packets (chunks of data) Internet regional ISP ▪ routers, switches home network content Communication links provider network datacenter ▪ fiber, copper, radio, satellite network ▪ transmission rate: bandwidth Networks enterprise ▪ collection of devices, routers, network links: managed by an organization Introduction: 1-4 “Fun” Internet-connected devices Tweet-a-watt: monitor energy use bikes Pacemaker & Monitor Amazon Echo Web-enabled toaster + IP picture frame weather forecaster Internet refrigerator Slingbox: remote cars control cable TV Security Camera AR devices sensorized, scooters bed mattress Fitbit Gaming devices Others? Internet phones diapers Introduction: 1-5 The Internet: a “nuts and bolts” view mobile network 4G ▪ Internet: “network of networks” national or global ISP Interconnected ISPs ▪ protocols are everywhere Skype IP Streaming video control sending, receiving of messages local or regional ISP e.g., HTTP (Web), streaming video, Skype, TCP, IP, WiFi, 4/5G, Ethernet home network content provider HTTP network datacenter ▪ Internet standards network Ethernet RFC: Request for Comments IETF: Internet Engineering Task enterprise TCP Force network WiFi Introduction: 1-6 The Internet: a “services” view ▪ Infrastructure that provides mobile network services to applications: national or global ISP Web, streaming video, multimedia teleconferencing, email, games, e- Streaming commerce, social media, inter- Skype video connected appliances, … local or regional ISP ▪ provides programming interface to distributed applications: home network content provider “hooks” allowing sending/receiving HTTP network datacenter network apps to “connect” to, use Internet transport service provides service options, analogous enterprise to postal service network Introduction: 1-7 What’s a protocol? Human protocols: Network protocols: ▪ “what’s the time?” ▪ computers (devices) rather than humans ▪ “I have a question” ▪ all communication activity in Internet ▪ introductions governed by protocols Rules for: Protocols define the format, order of … specific messages sent messages sent and received among … specific actions taken network entities, and actions taken when message received, or other events on message transmission, receipt Introduction: 1-8 What’s a protocol? A human protocol and a computer network protocol: Hi TCP connection request Hi TCP connection response Got the time? GET http://gaia.cs.umass.edu/kurose_ross 2:00 time Q: other human protocols? Introduction: 1-9 Chapter 1: roadmap ▪ What is the Internet? ▪ What is a protocol? ▪ Network edge: hosts, access network, physical media ▪ Network core: packet/circuit switching, internet structure ▪ Performance: loss, delay, throughput ▪ Security ▪ Protocol layers, service models ▪ History Introduction: 1-10 A closer look at Internet structure mobile network Network edge: national or global ISP ▪ hosts: clients and servers ▪ servers often in data centers local or regional ISP home network content provider network datacenter network enterprise network Introduction: 1-11 A closer look at Internet structure mobile network Network edge: national or global ISP ▪ hosts: clients and servers ▪ servers often in data centers local or Access networks, physical media: regional ISP ▪wired, wireless communication links home network content provider network datacenter network enterprise network Introduction: 1-12 A closer look at Internet structure mobile network Network edge: national or global ISP ▪ hosts: clients and servers ▪ servers often in data centers local or Access networks, physical media: regional ISP ▪wired, wireless communication links home network content provider network datacenter Network core: network ▪ interconnected routers ▪ network of networks enterprise network Introduction: 1-13 Access networks and physical media Q: How to connect end systems mobile network national or global ISP to edge router? ▪ residential access nets ▪ institutional access networks (school, company) local or ▪ mobile access networks (WiFi, 4G/5G) regional ISP home network content provider network datacenter network enterprise network Introduction: 1-14 Access networks: cable-based access cable headend … cable splitter modem C O V V V V V V N I I I I I I D D T D D D D D D A A R E E E E E E T T O O O O O O O A A L 1 2 3 4 5 6 7 8 9 Channels frequency division multiplexing (FDM): different channels transmitted in different frequency bands Introduction: 1-15 Access networks: digital subscriber line (DSL) central office telephone network DSL splitter modem DSLAM voice, data transmitted ISP at different frequencies over DSL access dedicated line to central office multiplexer ▪ use existing telephone line to central office DSLAM data over DSL phone line goes to Internet voice over DSL phone line goes to telephone net ▪ 24-52 Mbps dedicated downstream transmission rate ▪ 3.5-16 Mbps dedicated upstream transmission rate Introduction: 1-16 Access networks: home networks Wireless and wired devices to/from headend or central office often combined in single box cable or DSL modem WiFi wireless access router, firewall, NAT point (54, 450 Mbps) wired Ethernet (1 Gbps) Introduction: 1-17 Wireless access networks Shared wireless access network connects end system to router ▪ via base station aka “access point” Wireless local area networks Wide-area cellular access networks (WLANs) ▪ provided by mobile, cellular network ▪ typically within or around operator (10’s km) building (~100 ft) ▪ 10’s Mbps ▪ 802.11b/g/n (WiFi): 11, 54, 450 ▪ 4G/5G cellular networks Mbps transmission rate to Internet to Internet Introduction: 1-18 Access networks: enterprise networks Enterprise link to ISP (Internet) institutional router Ethernet institutional mail, switch web servers ▪ companies, universities, etc. ▪ mix of wired, wireless link technologies, connecting a mix of switches and routers (we’ll cover differences shortly) ▪ Ethernet: wired access at 100Mbps, 1Gbps, 10Gbps ▪ WiFi: wireless access points at 11, 54, 450 Mbps Introduction: 1-19 Access networks: data center networks mobile network ▪ high-bandwidth links (10s to 100s national or global ISP Gbps) connect hundreds to thousands of servers together, and to Internet local or regional ISP home network content provider network datacenter network Courtesy: Massachusetts Green High Performance Computing enterprise Center (mghpcc.org) network Introduction: 1-20 Host: sends packets of data host sending function: ▪ takes application message ▪ breaks into smaller chunks, two packets, known as packets, of length L bits L bits each ▪ transmits packet into access 2 1 network at transmission rate R link transmission rate, aka link host capacity, aka link bandwidth R: link transmission rate packet time needed to L (bits) transmission = transmit L-bit = delay packet into link R (bits/sec) Introduction: 1-21 Links: physical media ▪ bit: propagates between Twisted pair (TP) transmitter/receiver pairs ▪ two insulated copper wires ▪ physical link: what lies Category 5: 100 Mbps, 1 Gbps Ethernet between transmitter & Category 6: 10Gbps Ethernet receiver ▪ guided media: signals propagate in solid media: copper, fiber, coax ▪ unguided media: signals propagate freely, e.g., radio Introduction: 1-22 Links: physical media Coaxial cable: Fiber optic cable: ▪ two concentric copper conductors ▪ glass fiber carrying light pulses, each pulse a bit ▪ bidirectional ▪ high-speed operation: ▪ broadband: high-speed point-to-point multiple frequency channels on cable transmission (10’s-100’s Gbps) 100’s Mbps per channel ▪ low error rate: repeaters spaced far apart immune to electromagnetic noise Introduction: 1-23 Links: physical media Wireless radio Radio link types: ▪ signal carried in various ▪ Wireless LAN (WiFi) “bands” in electromagnetic 10-100’s Mbps; 10’s of meters spectrum ▪ wide-area (e.g., 4G/5G cellular) ▪ no physical “wire” 10’s Mbps (4G) over ~10 Km ▪ broadcast, “half-duplex” ▪ Bluetooth: cable replacement (sender to receiver) short distances, limited rates ▪ propagation environment effects: ▪ terrestrial microwave reflection point-to-point; 45 Mbps channels obstruction by objects ▪ satellite Interference/noise up to < 100 Mbps (Starlink) downlink 270 msec end-end delay (geostationary) Introduction: 1-24 Chapter 1: roadmap ▪ What is the Internet? ▪ What is a protocol? ▪ Network edge: hosts, access network, physical media ▪ Network core: packet/circuit switching, internet structure ▪ Performance: loss, delay, throughput ▪ Security ▪ Protocol layers, service models ▪ History Introduction: 1-25 The network core ▪ mesh of interconnected routers mobile network national or global ISP ▪ packet-switching: hosts break application-layer messages into packets network forwards packets from one local or regional ISP router to the next, across links on path from source to destination home network content provider network datacenter network enterprise network Introduction: 1-26 Two key network-core functions routing algorithm Routing: Forwarding: local local forwarding forwarding table table ▪ global action: header value output link determine source- ▪ aka “switching” 0100 3 destination paths ▪ local action: 0101 2 move arriving 0111 1001 2 1 taken by packets packets from ▪ routing algorithms router’s input link 1 to appropriate router output link 3 2 destination address in arriving packet’s header Introduction: 1-27 routing Introduction: 1-28 forwarding forwarding Introduction: 1-29 Packet-switching: store-and-forward L bits per packet 3 2 1 source destination R bps R bps ▪ packet transmission delay: takes L/R seconds to One-hop numerical example: transmit (push out) L-bit packet into link at R bps ▪ L = 10 Kbits ▪ store and forward: entire packet must arrive at ▪ R = 100 Mbps router before it can be transmitted on next link ▪ one-hop transmission delay = 0.1 msec Introduction: 1-30 Packet-switching: queueing R = 100 Mb/s A C D B R = 1.5 Mb/s E queue of packets waiting for transmission over output link Queueing occurs when work arrives faster than it can be serviced: Introduction: 1-31 Packet-switching: queueing R = 100 Mb/s A C D B R = 1.5 Mb/s E queue of packets waiting for transmission over output link Packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps) of link for some period of time: ▪ packets will queue, waiting to be transmitted on output link ▪ packets can be dropped (lost) if memory (buffer) in router fills up Introduction: 1-32 Alternative to packet switching: circuit switching end-end resources allocated to, reserved for “call” between source and destination ▪ in diagram, each link has four circuits. call gets 2nd circuit in top link and 1st circuit in right link. ▪ dedicated resources: no sharing circuit-like (guaranteed) performance ▪ circuit segment idle if not used by call (no sharing) ▪ commonly used in traditional telephone networks Introduction: 1-33 Circuit switching: FDM and TDM Frequency Division Multiplexing (FDM) 4 users frequency ▪ optical, electromagnetic frequencies divided into (narrow) frequency bands ▪ each call allocated its own band, can transmit at max rate of that narrow time band Time Division Multiplexing (TDM) frequency ▪ time divided into slots ▪ each call allocated periodic slot(s), can transmit at maximum rate of (wider) time frequency band (only) during its time slot(s) Introduction: 1-34 Packet switching versus circuit switching example: ▪ 1 Gb/s link N ▪ each user: users 1 Gbps link 100 Mb/s when “active” active 10% of time Q: how many users can use this network under circuit-switching and packet switching? ▪ circuit-switching: 10 users ▪ packet switching: with 35 users, Q: how did we get value 0.0004? probability > 10 active at same time is less than.0004 * A: HW problem (for those with course in probability only) Introduction: 1-35 Packet switching versus circuit switching Is packet switching a “slam dunk winner”? ▪ great for “bursty” data – sometimes has data to send, but at other times not resource sharing simpler, no call setup ▪ excessive congestion possible: packet delay and loss due to buffer overflow protocols needed for reliable data transfer, congestion control ▪ Q: How to provide circuit-like behavior with packet-switching? “It’s complicated.” We’ll study various techniques that try to make packet switching as “circuit-like” as possible. Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet switching)? Introduction: 1-36 Internet structure: a “network of networks” mobile network ▪ hosts connect to Internet via access national or global ISP Internet Service Providers (ISPs) ▪ access ISPs in turn must be interconnected so that any two hosts (anywhere!) local or regional ISP can send packets to each other ▪ resulting network of networks is home network content provider very complex network datacenter network evolution driven by economics, enterprise national policies network Let’s take a stepwise approach to describe current Internet structure Internet structure: a “network of networks” Question: given millions of access ISPs, how to connect them together? access access net net access net access access net net access access net net access access net net access net access net access net access net access access net access net net Introduction: 1-38 Internet structure: a “network of networks” Question: given millions of access ISPs, how to connect them together? access access net net access net access access net net access access net net connecting each access ISP to each other directly doesn’t scale: access access net O(N2) connections. net access net access net access net access net access access net access net net Introduction: 1-39 Internet structure: a “network of networks” Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement. access access net net access net access access net net access access net net global access net ISP access net access net access net access net access net access access net access net net Introduction: 1-40 Internet structure: a “network of networks” But if one global ISP is viable business, there will be competitors …. access access net net access net access access net net access access net net ISP A access net ISP B access net access ISP C net access net access net access net access access net access net net Introduction: 1-41 Internet structure: a “network of networks” But if one global ISP is viable business, there will be competitors …. who will want to be connected Internet exchange point access access net net access net access access net net IXP access access net net ISP A access net IXP ISP B access net access ISP C net access net access net peering link access net access access net access net net Introduction: 1-42 Internet structure: a “network of networks” … and regional networks may arise to connect access nets to ISPs access access net net access net access access net net IXP access access net net ISP A access net IXP ISP B access net access ISP C net access net access net regional ISP access net access access net access net net Introduction: 1-43 Internet structure: a “network of networks” … and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users access access net net access net access access net net IXP access access net net ISP A Content provider network access net IXP ISP B access net access ISP C net access net access net regional ISP access net access access net access net net Introduction: 1-44 Internet structure: a “network of networks” Tier 1 ISP Tier 1 ISP Google IXP IXP IXP Regional ISP Regional ISP access access access access access access access access ISP ISP ISP ISP ISP ISP ISP ISP At “center”: small # of well-connected large networks ▪ “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage ▪ content provider networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs Introduction: 1-45 Chapter 1: roadmap ▪ What is the Internet? ▪ What is a protocol? ▪ Network edge: hosts, access network, physical media ▪ Network core: packet/circuit switching, internet structure ▪ Performance: loss, delay, throughput ▪ Security ▪ Protocol layers, service models ▪ History Introduction: 1-46 How do packet delay and loss occur? ▪ packets queue in router buffers, waiting for turn for transmission ▪ queue length grows when arrival rate to link (temporarily) exceeds output link capacity ▪ packet loss occurs when memory to hold queued packets fills up packet being transmitted (transmission delay) A B packets in buffers (queueing delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Introduction: 1-47 Packet delay: four sources transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dproc: nodal processing dqueue: queueing delay ▪ check bit errors ▪ time waiting at output link for ▪ determine output link transmission ▪ typically < microsecs ▪ depends on congestion level of router Introduction: 1-48 Packet delay: four sources transmission A propagation B nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop dtrans: transmission delay: dprop: propagation delay: ▪ L: packet length (bits) ▪ d: length of physical link ▪ R: link transmission rate (bps) ▪ s: propagation speed (~2x108 m/sec) ▪ dtrans = L/R ▪ dprop = d/s dtrans and dprop very different Introduction: 1-49 “Real” Internet delays and routes ▪ what do “real” Internet delay & loss look like? ▪ traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path towards destination (with time-to-live field value of i) router i will return packets to sender sender measures time interval between transmission and reply 3 probes 3 probes 3 probes Introduction: 1-50 Real Internet delays and routes tracert : gaia.cs.umass.edu to www.eurecom.fr 3 delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu 1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 3 delay measurements 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms to border1-rt-fa5-1-0.gw.umass.edu 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms trans-oceanic link 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms looks like delays 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms decrease! Why? 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * 18 * * * * means no response (probe lost, router not replying) 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms * Do some traceroutes from exotic countries at www.traceroute.org Introduction: 1-51 Packet loss ▪ queue (aka buffer) preceding link in buffer has finite capacity ▪ packet arriving to full queue dropped (aka lost) ▪ lost packet may be retransmitted by previous node, by source end system, or not at all buffer (waiting area) packet being transmitted A B packet arriving to full buffer is lost Introduction: 1-52 Throughput ▪ throughput: rate (bits/time unit) at which bits are being sent from sender to receiver instantaneous: rate at given point in time average: rate over longer period of time link capacity pipe that can carry linkthat pipe capacity can carry Rsfluid bits/sec at rate Rfluid c bits/sec at rate serverserver, sends with bits (fluid) into pipe (Rs bits/sec) (Rc bits/sec) file of F bits to send to client Introduction: 1-53 Throughput Rs < Rc What is average end-end throughput? Rs bits/sec Rc bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec bottleneck link link on end-end path that constrains end-end throughput Introduction: 1-54 Throughput: network scenario ▪ per-connection end- Rs end throughput: Rs Rs min(Rc,Rs,R/10) ▪ in practice: Rc or Rs is R often bottleneck Rc Rc Rc * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/ 10 connections (fairly) share backbone bottleneck link R bits/sec Introduction: 1-55 Chapter 1: roadmap ▪ What is the Internet? ▪ What is a protocol? ▪ Network edge: hosts, access network, physical media ▪ Network core: packet/circuit switching, internet structure ▪ Performance: loss, delay, throughput ▪ Security ▪ Protocol layers, service models ▪ History Introduction: 1-56 Network security ▪ Internet not originally designed with (much) security in mind original vision: “a group of mutually trusting users attached to a transparent network” ☺ Internet protocol designers playing “catch-up” security considerations in all layers! ▪ We now need to think about: how bad guys can attack computer networks how we can defend networks against attacks how to design architectures that are immune to attacks Introduction: 1-57 Bad guys: packet interception packet “sniffing”: ▪ broadcast media (shared Ethernet, wireless) ▪ promiscuous network interface reads/records all packets (e.g., including passwords!) passing by A C src:B dest:A payload B Wireshark software used for our end-of-chapter labs is a (free) packet-sniffer Introduction: 1-58 Bad guys: fake identity IP spoofing: injection of packet with false source address A C src:B dest:A payload B Introduction: 1-60 Bad guys: denial of service Denial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic 1. select target 2. break into hosts around the network (see botnet) 3. send packets to target target from compromised hosts Introduction: 1-61 Lines of defense: ▪ authentication: proving you are who you say you are cellular networks provides hardware identity via SIM card; no such hardware assist in traditional Internet ▪ confidentiality: via encryption ▪ integrity checks: digital signatures prevent/detect tampering ▪ access restrictions: password-protected VPNs ▪ firewalls: specialized “middleboxes” in access and core networks: ▪ off-by-default: filter incoming packets to restrict senders, receivers, applications ▪ detecting/reacting to DOS attacks Introduction: 1-62 Chapter 1: roadmap ▪ What is the Internet? ▪ What is a protocol? ▪ Network edge: hosts, access network, physical media ▪ Network core: packet/circuit switching, internet structure ▪ Performance: loss, delay, throughput ▪ Security ▪ Protocol layers, service models ▪ History Introduction: 1-63

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