Networking Primer - The Internet and the Link Layer ECE 256 Romit Roy Choudhury Dept.
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Networking Primer - The Internet and the Link Layer ECE 256 Romit Roy Choudhury Dept. of ECE and CS 1 Slides are from ECE 156 Designed to help you recall undergrad material Please be patient if you remember most of this … 2 On the Shoulders of Giants 1961: Leonard Kleinrock published a work on packet switching 1962: J. Licklider described a worldwide network of computers called Galactic Network 1965: Larry Roberts designed the ARPANET that communicated over long distance links 1971: Ray Tomilson invents email at BBN 1972: Bob Kahn and Vint Cerf invented TCP for reliable packet transport 3 On the Shoulders of Giants … 1973: David Clark, Bob Metcalfe implemented TCP and designed ethernet at Xerox PARC 1975: Paul Mockapetris developed DNS system for host lookup 1980: Radia Perlman invented spanning tree algorithm for bridging separate networks Things snowballed from there on … 4 What we have today is beyond any of the inventors’ imagination … 5 And YOU are here 6 And by “YOU” I mean … 7 “Cool” internet appliances Web-enabled toaster + weather forecaster IP picture frame http://www.ceiva.com/ World’s smallest web server http://www-ccs.cs.umass.edu/~shri/iPic.html Internet phones 8 And Of Course real people … 9 InterNetwork Millions of end points (you, me, and toasters) connected across a mesh of links Many end points can be addressed by numbers Many others lie behind a virtual end point Many networks form a bigger network The overall strcture called the Internet With a capital I Defined as a network of networks 10 Internet structure: network of networks roughly hierarchical at center: “tier-1” ISPs (e.g., MCI, Sprint, AT&T, Cable and Wireless), national/international coverage treat each other as equals Tier-1 providers interconnect (peer) privately Tier 1 ISP Tier 1 ISP NAP Tier-1 providers also interconnect at public network access points (NAPs) Tier 1 ISP 11 Tier-1 ISP: e.g., Sprint Sprint US backbone network Seattle Tacoma DS3 (45 Mbps) OC3 (155 Mbps) OC12 (622 Mbps) OC48 (2.4 Gbps) POP: point-of-presence to/from backbone Stockton … … Kansas City . … Anaheim peering … … San Jose Cheyenne New York Pennsauken Relay Wash. DC Chicago Roachdale Atlanta to/from customers Fort Worth Orlando 12 Internet structure: network of networks “Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs France telecome, Tiscali, etc. buys from Sprint Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet Tier-2 ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP NAP Tier 1 ISP Tier-2 ISPs also peer privately with each other, interconnect at NAP Tier-2 ISP Tier-2 ISP 13 Internet structure: network of networks “Tier-3” ISPs and local ISPs (Time Warner, Earthlink, etc.) last hop (“access”) network (closest to end systems) local ISP Local and tier3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP NAP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP 14 Internet structure: network of networks a packet passes through many networks! Local ISP (taxi) -> T1 (bus) -> T2 (domestic) -> T3 (international) local ISP Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP NAP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP 15 Organizing the giant structure Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software Question: Is there any hope of organizing structure of network? 16 Turn to analogies in air travel ticket (purchase) ticket (complain) baggage (check) baggage (claim) gates (load) gates (unload) runway takeoff runway landing airplane routing airplane routing airplane routing a series of steps 17 Layering of airline functionality ticket (purchase) ticket (complain) ticket baggage (check) baggage (claim baggage gates (load) gates (unload) gate runway (takeoff) runway (land) takeoff/landing airplane routing airplane routing airplane routing departure airport airplane routing airplane routing intermediate air-traffic control centers arrival airport Layers: each layer implements a service layers communicate with peer layers rely on services provided by layer below 18 Why layering? explicit structure allows identification, relationship of complex system’s pieces modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in aircraft runway does not affect boarding gate layering considered harmful? 19 Protocol “Layers” Service of each layer encapsulated Universally agreed services called PROTOCOLS A large part of this course will focus on designing protocols for networking systems 20 Internet protocol stack application: supporting network applications FTP, SMTP, HTTP transport: host-host data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet, WiFi, Bluetooth application transport network link physical physical: bits “on the wire” 21 source message segment Ht datagram Hn Ht frame Hl Hn Ht M M M M Encapsulation application transport network link physical Hl Hn Ht M link physical Hl Hn Ht M switch destination M Ht M Hn Ht Hl Hn Ht M M application transport network link physical Hn Ht Hl Hn Ht M M network link physical Hn Ht Hl Hn Ht M M router 22 PHY and Link Layer 23 PHY and Link Layer The Layers that make the connections Sends signals on physical media Schedules who gets to transmit Detects transmission errors and collisions Etc. 24 Physical Link / Media Guided media Twisted pair Coaxial cable Fiber optics Unguided media terrestrial microwave up to 45 Mbps WiFi LAN 11Mbps, 54 Mbps Cellular Wide-area 3G: hundreds of kbps Satellite Kbps to 45Mbps 25 Laying out Access networks Q: How to connect end systems to edge router? Mobile users Residential access nets Institutions, schools Backbones 26 Residential access: point to point access Dialup via modem up to 56Kbps direct access to router (often less) Can’t surf and phone at same time: can’t be “always on” ADSL: asymmetric digital subscriber line up to 1 Mbps upstream (today typically < 256 kbps) up to 8 Mbps downstream (today typically < 1 Mbps) FDM: 50 kHz - 1 MHz for downstream 4 kHz - 50 kHz for upstream 0 kHz - 4 kHz for ordinary telephone 27 Residential access: Networked Cable modems HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream, 2 Mbps upstream Network of cable/fiber attach homes to ISP router • Homes share access to router Deployment: available via cable TV companies 28 Residential access: cable modems Diagram: http://www.cabledatacomnews.com/cmic/diagram.html 29 Cable Network Architecture: Overview Typically 500 to 5,000 homes cable headend cable distribution network (simplified) home 30 Cable Network Architecture: Overview server(s) cable headend cable distribution network home 31 Cable Network Architecture: Overview cable headend cable distribution network (simplified) home 32 Cable Network Architecture: Overview FDM: V I D E O V I D E O V I D E O V I D E O V I D E O V I D E O D A T A D A T A C O N T R O L 1 2 3 4 5 6 7 8 9 Channels cable headend cable distribution network home 33 ADSL DSL is point to point Thus data rate does not reduce when neighbor uses DSL But, DSL uses twisted pair Transmission technology cannot support more than ~10Mbps Vs Cable Cable modems share pipe to the cable headend Data rate reduces when neighbor surfing However, fiber optic lines offer significantly higher data rate (fat pipe) Even with neighbors, your data rate can be higher 34 Wireless Access Networks shared wireless access network connects end system to router via base station aka “access point” wireless LANs: 802.11b/g (WiFi): 11 or 54 Mbps router base station wider-area wireless access provided by telco operator 4G, WiMax, LTE • Will it happen?? mobile hosts 35 Communication on Links (Delay, Queuing, and Packet Loss) 36 How do loss and delay occur? packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) A B packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers 37 Four sources of packet delay 1. nodal processing: 2. queueing check bit errors determine output link time waiting at output link for transmission depends on congestion level of router transmission A propagation B nodal processing queueing 38 Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R transmission A 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s Note: s and R are very different quantities! propagation B nodal processing queueing 39 Nodal delay dnodal dproc dqueue dtrans dprop dproc = processing delay typically a few microsecs or less dqueue = queuing delay depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs 40 Queueing delay (revisited) R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite! 41 “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 router i will return packets to sender sender times interval between transmission and reply. 3 probes 3 probes 3 probes 42 “Real” Internet delays and routes traceroute: gaia.cs.umass.edu to www.eurecom.fr Three 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 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 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 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms link 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 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 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 * * * * means no response (probe lost, router not replying) 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms 43 Medium Access Control In Computer Networks 44 Random Access Protocols Trivial Solution: When node has packet to send transmit at full channel data rate R. no a priori coordination Two or more transmitting nodes ➜ “collision” Collision detected by comparing signal with channel content Random access MAC protocol specifies: how to schedule communications how to recover from collisions Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA 45 Pure (unslotted) ALOHA unslotted Aloha: simple, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t0 collides with other frames sent in [t0-1,t0+1] 46 Pure Aloha efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [p0-1,p0] . P(no other node transmits in [p0-1,p0] = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1) … choosing optimum p and then letting n -> infty ... = 1/(2e) = .18 Very Poor ! 47 Slotted ALOHA Assumptions all frames same size time is divided into equal size slots nodes start to transmit frames only at beginning of slots nodes are synchronized Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success 48 Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes must be able to detect collision in less than time to transmit packet clock synchronization Why? 49 Slotted Aloha efficiency Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1p)N-1 prob that any node has a success = Np(1-p)N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1 For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37 At best: channel used for useful transmissions 37% of time! 50 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others! 51 Don’t transmit Carrier Sensing A B C Listen before you talk Carrier sense multiple access (CSMA) Defer transmission when signal on channel Can collisions still occur? 52 Deterministic MAC protocols Polling: master node “invites” slave nodes to transmit in turn concerns: polling overhead latency single point of failure (master) Token passing: control token passed from one node to next sequentially. token message concerns: token overhead latency single point of failure (token) 53 Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code • Time Division, Frequency Division Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet CSMA/CA used in 802.11 • • Taking Turns • polling from a central site, token passing 54 Questions ? 55 Backup Slides 56 Wired Vs Wireless Media Access Both are on shared media. Then, what’s really the problem ? 57 Wired A B C Collision Detection Tx can transmit and listen at the same time • If (Transmitted_Signal != Sensed_Signal) Collision Channel Condition ~ identical at Tx and Rx 58 Wireless Collision Avoidance H/W can either transmit or receive While transmitting, cannot detect a collision Detection is based on SINR • Thus must take educated decision when to transmit Channel Condition Tx unaware of signal quality at receiver Channel dispersion large – high uncertainty 59 Thoughts Please attend seminars Equivalent to reading 3 papers in 1 hour Priceless Check out the Comp. Eng Seminar series • Lot of great speakers are scheduled over the semester People have begun selecting slots Please start looking 60 CSMA collisions spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability 61 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting human analogy: the polite conversationalist 62 CSMA/CD collision detection 63 Physical Media Bit: propagates between transmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fiber, coax Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps Ethernet unguided media: signals propagate freely, e.g., radio 64 Physical Media: coax, fiber Coaxial cable: two concentric copper conductors bidirectional baseband: single channel on cable legacy Ethernet broadband: multiple channels on cable HFC Fiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 10’s-100’s Gps) low error rate: repeaters spaced far apart ; immune to electromagnetic noise 65 Physical media: radio signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference Radio link types: terrestrial microwave e.g. up to 45 Mbps channels LAN (e.g., Wifi) 11Mbps, 54 Mbps wide-area (e.g., cellular) e.g. 3G: hundreds of kbps satellite Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude 66 Applying the concepts later Several protocol designs will require solid understanding of delay Bandwidth estimation TCP congestion control TCP flow control TCP loss discrimination MAC protocols for wireless networks 67 Link Layer 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS 68