Introduction to Computer Networks Miguel A. Labrador Department of Computer Science & Engineering labrador@csee.usf.edu http://www.csee.usf.edu/~labrador 1 Dr. Miguel A. Labrador Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Protocol layers, service models 2 Intro 2 1
What s the Internet: nuts and bolts view Millions of connected computing devices: hosts = end systems Running network protocols and applications Communication links Fiber, copper, radio, satellite Transmission rate = bandwidth Routers: forward packets (chunks of data) router server local ISP workstation mobile regional ISP 3 company network Intro 3 What s the Internet: nuts and bolts view Protocols control sending, receiving of messages e.g., TCP, IP, HTTP, FTP, PPP Internet: network of networks Loosely hierarchical Public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force router local ISP company network server workstation mobile regional ISP 4 Intro 4 2
What s the Internet: a service view Communication infrastructure enables distributed applications: Web, email, games, e- commerce, file sharing Communication services provided to applications: Connectionless unreliable Connection-oriented reliable 5 Intro 5 What s a protocol? Human protocols: What s the time? I have a question Introductions Network protocols: Machines rather than humans All communication activity in Internet governed by protocols specific msgs sent specific actions taken when msgs received, or other events Protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt 6 Intro 6 3
What s a protocol? A human protocol and a computer network protocol: Hi Hi Got the time? 2:00 time TCP connection req TCP connection response Get http://www.csee.usf.edu/~labrador <file> 7 Intro 7 Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Protocol layers, service models 8 Intro 8 4
A closer look at network structure Network edge: applications and hosts Network core: Routers Network of networks Access networks, physical media: communication links 9 Intro 9 The network edge End systems (hosts) Run application programs e.g. Web, email At edge of network Client/server model Client host requests, receives service from always-on server e.g. Web browser/server; email client/server Peer-peer model Minimal (or no) use of dedicated servers e.g. Skype, BitTorrent Two types of services Connection-oriented Connectionless 10 Intro 10 5
Network edge: connection-oriented service Goal: data transfer between end systems Handshaking: setup (prepare for) data transfer ahead of time Hello, hello back human protocol Set up state in two communicating hosts TCP - Transmission Control Protocol Internet s connection-oriented service TCP service [RFC 793] Reliable, in-order byte-stream data transfer Loss: acknowledgements and retransmissions Flow control: Sender won t overwhelm receiver Congestion control: Senders slow down sending rate when network congested 11 Intro 11 Network edge: connectionless service Goal: data transfer between end systems Same as before! UDP - User Datagram Protocol [RFC 768]: Connectionless Unreliable data transfer No flow control No congestion control App s using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP (email) App s using UDP: Streaming media, teleconferencing, DNS, Internet telephony 12 Intro 12 6
Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Protocol layers, service models 13 Intro 13 The network core Mesh of interconnected routers The fundamental question: how is data transferred through the network? Circuit switching: dedicated circuit per call: telephone network Packet-switching: data sent through the network in discrete chunks As a result, we have: Circuit-switched networks Packet-switched networks 14 Intro 14 7
Delay, loss and throughput in packet switched networks A parenthesis to look at three important metrics in networks Analyze the performance of switching technologies 15 Intro 15 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 16 Intro 16 8
Four sources of packet delay 1. Nodal processing: Check bit errors Determine output link 2. Queueing Time waiting at output link for transmission Depends on congestion level of router A transmission propagation B nodal processing queueing 17 Intro 17 Delay in packet-switched networks 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) Time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) Propagation delay = d/s A transmission propagation B nodal processing queueing 18 Intro 18 9
nodal proc Nodal delay d = d + d + d + d queue trans prop d proc = processing delay Typically a few microsecs or less d queue = queuing delay Depends on congestion d trans = transmission delay = L/R, significant for low-speed links d prop = propagation delay A few microsecs to hundreds of msecs 19 Intro 19 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! 20 Intro 20 10
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 21 Intro 21 Real Internet delays and routes traceroute: gaia.cs.umass.edu to www.eurecom.fr Three delay measements 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 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 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 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms trans-oceanic link * means no reponse (probe lost, router not replying) 22 Intro 22 11
Packet loss Queue (aka buffer) preceding link has finite capacity When packet arrives to full queue, packet is dropped (aka lost) Lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all A buffer (waiting area) packet being transmitted B packet arriving to full buffer is lost 23 Intro 23 Throughput Throughput: rate (bits/time unit) at which bits are transferred between sender/receiver Instantaneous: rate at given point in time Average: rate over longer period of time server, with pipe link that capacity can carry file of F bits fluid R s bits/sec at rate to send to client R s bits/sec) pipe link that capacity can carry Rfluid c bits/sec at rate R c bits/sec) 24 Intro 24 12
Throughput (more) R s < R c What is average end-end throughput? R s bits/sec R c bits/sec R s > R c What is average end-end throughput? R s bits/sec R c bits/sec bottleneck link link on end-end path that constrains end-end throughput 25 Intro 25 Network Core: Circuit Switching End-to-end resources reserved for call Link bandwidth, switch capacity Dedicated resources: no sharing Circuit-like (guaranteed) performance Call setup (signaling mechanism) required Three phases Circuit connection phase Transmission phase Circuit termination phase Telephone network is a typical example 26 Intro 26 13
Network Core: Circuit Switching Network resources (e.g., bandwidth) divided into pieces Pieces allocated to calls Resource piece idle if not used by owning call (no sharing) Dividing link bandwidth into pieces Frequency division Time division 27 Intro 27 Circuit Switching: FDM and TDM FDM Example: 4 users Frequency TDM time Frequency 28 time Intro 28 14
Circuit Switching Example 29 Intro 29 Circuit Switching Performance Analysis Total Time (T), given by: S: Set up time M: Message Size B: Bit rate K: Number of links D: Propagation delay per link M T = S + + KD B S N1 N2 N3 L1 L2 Total Time Data N4 L3 Call request signal D Queueing delay Call accept signal Propagation delay Transmission delay 30 Time Link A Link B Link C Intro 30 15
Circuit Switching Performance Advantages Constant delay for all frames belonging to the same message Incoming data are switched to the appropriate outgoing channel without delay Efficient for transmitting large amounts of data Elimination of the per packet header overhead No queueing delay at intermediate nodes Good if traffic is constant Disadvantages Circuit set up overhead Inefficient use of the available bandwidth Not good if station is idle most of the time 31 Intro 31 Network Core: Packet Switching Each end-end data stream divided into packets User A, B packets share network resources Each packet uses full link bandwidth Resources used as needed Bandwidth division into pieces Dedicated allocation Resource reservation Resource contention: Aggregate resource demand can exceed amount available Congestion: packets queue, wait for link use Store and forward: packets move one hop at a time Node receives complete packet before forwarding 32 Intro 32 16
Packet Switching: Statistical Multiplexing A 10 Mb/s Ethernet statistical multiplexing C B queue of packets waiting for output link 1.5 Mb/s D E Sequence of A & B packets does not have fixed pattern statistical multiplexing In TDM each host gets same slot in revolving TDM frame 33 Intro 33 Network Core: Packet Switching Packet switching technology can be further subdivided Connection oriented A circuit is established but link can still be shared Virtual circuit All packets follow the same route Delay at intermediate node is possible Each packet carries tag (virtual circuit ID), tag determines next hop Fixed path determined at call setup time, remains fixed through the call Routers maintain per-call state Connectionless No connection is made a priori Datagrams Packets can take any route; Routes may change during session Delay at intermediate node is possible Destination address in packet determines next hop Analogy: driving, asking directions 34 Intro 34 17
Packet Switching Connection-Oriented or Virtual Circuit Service Multiple packets of a single message take the same path, although no dedicated physical path is set up The virtual circuit has to be set up though Each virtual circuit is identified by a Virtual Circuit Identifier (VCI) Delays are more variable than with a dedicated circuit Guaranteed, reliable, in-sequence delivery, with suppression of duplicates Best example: ATM networks VC 1 2 3 B A 1 6 C 35 VC 2 4 5 Intro 35 Packet Switching - VC Service 36 Combination of Circuit Switching and Datagrams Total Time (T), given by: M: Message Size P:Packet size (contains overhead) S: Set up time B: Bit rate K: Number of links D: Propagation delay per link Q: Queueing delay N: Number of nodes * P M T = S + ( K 1) + NQ+ + KD B B * In packets S N1 N2 N3 L1 L2 Total Time Time P P P Link A P P P Link B N4 D L3 Call request signal P P P Link C Call accept signal Queueing delay Propagation delay Intro 36 18
Packet Switching Performance Virtual Circuit Service Advantages Can provide QoS guarantees to different types of applications Efficient use of the available bandwidth, since data from other sources may use the same link Flow control can be exercised by each intermediate node or end-to-end No packets out of order Disadvantages Complex Signaling, resource reservations Mechanism to offer bounded delay and loss Access control and policing 37 Intro 37 Packet Switching Connectionless or Datagram Service Multiple packets of a single message are treated individually, and may take different routes This causes different delays for each packet Packets may be lost or duplicated, or arrive out of order Best Effort Service It is the user application that is responsible for enhancing the quality of the basic service Best example: IP networks (TCP/IP), the Internet 2 3 B A 1 6 C 38 4 5 Intro 38 19
Packet Switching - Datagram Service 39 Total Time (T), given by: M: Message Size P:Packet size (contains overhead) B: Bit rate K: Number of links D: Propagation delay per link Q: Queueing delay N: Number of nodes * P M T = ( K 1) + NQ+ + KD B B * In packets S N1 N2 N3 N4 L1 L2 L3 Total Time P P P Time Link A P P P Link B P P P Link C D Queueing delay Propagation delay Intro 39 Packet Switching Performance Datagram Service Advantages Very simple Efficient use of the available bandwidth, since data from other sources may use the same link Low delay for interactive data Flow control can be exercised by each intermediate node or end-to-end Disadvantages Two packets from the same message may take different routes, and experience different delays No reservation of resources Packets are dropped Packets can reach the destination out of order 40 Intro 40 20
Circuit Switching vs. Packet Switching Interesting questions can be asked: What applications are better off with which service? Short and long lived applications Under what circumstances one is better than the other? Which one offers shorter T? What is the optimal packet size that maximizes the pipeline effect? Which one is better for bursty traffic or constant bit rate traffic? Which one offers better and easier QoS guarantees? Now, you can answer some of these questions; the others will be answered during the course 41 Intro 41 Packet switching versus circuit switching Packet switching allows more users to use network! 1 Mb/s link Each user: 100 kb/s when active Active 10% of time Circuit-switching: 10 users Packet switching: with 35 users, probability > 10 active less than.0004 N users 1 Mbps link 42 Intro 42 21
Network Taxonomy Telecommunication networks Circuit-switched networks Packet-switched networks FDM TDM Networks with VCs Datagram Networks 43 Datagram network is not either connection-oriented or connectionless Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps Intro 43 Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Protocol layers, service models 44 Intro 44 22
Access networks and physical media Q: How to connect end systems to edge router? Residential access networks Institutional access networks (school, company) Mobile access networks Keep in mind Bandwidth (bits per second) of access network? Shared or dedicated? 45 Intro 45 Residential access: Dial-up Modem Dialup via modem Uses existing telephony infrastructure Home is connected to central office Up to 56Kbps direct access to router (often less) Can t surf and phone at same time: can t be always on central office telephone network Internet home PC home dial-up modem ISP modem (e.g., AOL) 46 Intro 46 23
Asymmetric Digital Subscriber Line (DSL) ADSL: Asymmetric Digital Subscriber Line Also uses telephone infrastructure Dedicated physical line to telephone central office Up to 1 Mbps upstream (today typically < 256 kbps) Up to 8 Mbps downstream (today typically < 1 Mbps) home phone Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz- 1MHz downstream data Internet DSLAM home PC DSL modem splitter central office telephone network 47 Intro 47 Residential access: cable modems Does not use telephone infrastructure Instead uses cable TV infrastructure HFC: Hybrid Fiber Coax Asymmetric: up to 30Mbps downstream, 2 Mbps upstream Network of cable and fiber attaches homes to ISP router Homes share access to router Unlike DSL, which has dedicated access 48 Intro 48 24
Residential access: cable modems 49 Diagram: http://www.cabledatacomnews.com/cmic/diagram.html Intro 49 Cable Network Architecture: Overview Typically 500 to 5,000 homes cable headend cable distribution network (simplified) home 50 Intro 50 25
Cable Network Architecture: Overview 51 cable headend cable distribution network (simplified) home Intro 51 Cable Network Architecture: Overview server(s) cable headend 52 cable distribution network home Intro 52 26
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 Channels 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 cable headend cable distribution network home 53 Intro 53 Fiber to the Home Optical links from central office to the home Two competing optical technologies Passive Optical Network (PON) Active Optical Networks (PAN) Much higher Internet rates; fiber also carries television and phone services ONT Internet optical fibers OLT optical fiber ONT central office optical splitter 54 ONT Intro 54 27
Ethernet Internet access Typically used in companies, universities, etc. Company/univ local area network (LAN) connects end system to edge router Ethernet: 10 Mbs, 100Mbps, 1 Gbps, 10 Gbps Ethernet Today, end systems typically connect into Ethernet switch 100 Mbps 100 Mbps Ethernet switch Institutional router To Institution s ISP 100 Mbps 1 Gbps 55 server Intro 55 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 Wider-area wireless access Provided by telco operator ~1Mbps over cellular system (EVDO, HSDPA) Next up (?): WiMAX (10 s Mbps) over wide area router base station mobile hosts 56 Intro 56 28
Home networks Typical home network components: ADSL or cable modem Router/firewall/NAT Ethernet Wireless access point to/from cable headend cable modem router/ firewall Ethernet wireless access point wireless laptops 57 Intro 57 Physical Media Bit: propagates between transmitter/receiver pairs Physical link: what lies between transmitter & receiver Guided media: Signals propagate in solid media: copper, fiber, coax Unguided media: Signals propagate freely, e.g., radio Twisted Pair (TP) Two insulated copper wires Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps Ethernet 58 Intro 58 29
Physical Media: coax, fiber Coaxial cable: Two concentric copper conductors Bidirectional Baseband: Single channel on cable Legacy Ethernet Broadband: Multiple channel 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 Gbps) Low error rate: repeaters spaced far apart ; immune to electromagnetic noise 59 Intro 59 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) 2Mbps, 11Mbps Wide-area (e.g., cellular) e.g. 3G: hundreds of kbps Satellite Up to 50Mbps channel (or multiple smaller channels) 270 msec end-end delay Geosynchronous versus low altitude 60 Intro 60 30
Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Protocol layers, service models 61 Intro 61 Internet structure: network of networks Roughly hierarchical At center: tier-1 ISPs (e.g., Verizon, Sprint, AT&T), national/international coverage Treat each other as equals Tier-1 providers interconnect (peer) privately Tier 1 ISP NAP Tier 1 ISP Tier 1 ISP Tier-1 providers also interconnect at public network access points (NAPs) 62 Intro 62 31
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 Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider Tier-2 ISP Tier-2 ISP Tier 1 ISP NAP Tier 1 ISP Tier 1 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISPs also peer privately with each other, interconnect at NAP Tier-2 ISP 63 Intro 63 Internet structure: network of networks Tier-3 ISPs and local ISPs Last hop ( access ) network (closest to end systems) Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet local ISP local ISP Tier 3 ISP Tier 1 ISP Tier-2 ISP Tier-2 ISP local ISP local ISP Tier 1 ISP local ISP Tier-2 ISP NAP Tier 1 ISP Tier-2 ISP local ISP local ISP Tier-2 ISP local ISP 64 Intro 64 32
Internet structure: network of networks A packet passes through many networks! local ISP Tier 3 ISP Tier-2 ISP local ISP Tier 1 ISP local ISP Tier-2 ISP NAP local ISP local ISP Tier 1 ISP Tier-2 ISP local ISP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP 65 Intro 65 Roadmap 1.1 What is the Internet? 1.2 Network edge 1.3 Network core 1.4 Network access and physical media 1.5 Internet structure and ISPs 1.6 Protocol layers, service models 66 Intro 66 33
Protocol Layers 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? Or at least our discussion of networks? 67 Intro 67 Network Design To reduce design complexity, task of communication broken up into modules Most networks are organized into layers Each layer is like a module implementing some functions or services Each layer enhances the service of the layer below Added value service Two layer n entities in different computers use a peer-to-peer layer n protocol to communicate with each other Two adjacent layers within the same computer communicate through an interface The interface defines the primitive operations and services the lower layer provides to the upper layer Modularization eases maintenance, updating of system Change of implementation of layer s service transparent to rest of system 68 Intro 68 34
Network Architecture Layer N Layer N<->N-1 Interface Layer N-1 Layer N-1<->N-2 Interface Layer N Protocol Layer N-1 Protocol Layer N Layer N-1 Layer 2<->1 Interface Layer 2 Layer 1 Layer 2 Protocol Layer 1 Protocol Layer 2 Layer 1 Physical Medium 69 Intro 69 Information Flow Data Layer N Protocol Data H Data Layer N-1 Protocol H Data H H Data Layer 2 Protocol H H Data H H H Data Layer 1 Protocol H H H Data Physical Medium 70 Intro 70 35
Interfaces and Services Layer N entity implements a service used by Layer N+1 Layer N is the service provider Layer N+1 is the service user At each layer, protocols are used to communicate Control information is added to user data at each layer Services are available through Service Access Points (SAPs) SAPs are uniquely identified by their addresses Example: Telephone network A SAP is the wall socket The SAP address is the phone number 71 Intro 71 ISO/OSI Communication Model 72 Intro 72 36
Internet protocol stack Application: supporting network applications FTP, SMTP, STTP Transport: End-to-End data transfer (segments) TCP (CO), reliable, flow and congestion control UDP (CL), unreliable, no flow and congestion control Socket interface Network: routing of datagrams from source to destination (packets) IP, routing protocols Link: data transfer between neighboring network elements (frames) Point-to-Point, reliable service PPP, Ethernet Physical: bits on the wire application transport network link physical 73 Intro 73 message segment H t datagram H n H t frame H l H n H t M M M M source application transport network link physical Encapsulation H l H n H t M link H l H n H t physical M switch H t H n H t H l H n H t M M M M destination application transport network link physical H n H t H l H n H t M M network link physical H n H t H l H n H t M M router 74 Intro 74 37
LAN Interconnection and OSI Application Presentation Session Transport Network Link Physical LLC MAC Repeater Bridge Router Application Presentation Session Transport Network Link Physical 75 Focus of the course Intro 75 Some Protocols in TCP/IP Suite 76 Intro 76 38