CMPE 150/L : Introduction to Computer Networks. Chen Qian Computer Engineering UCSC Baskin Engineering Lecture 3

Transcription

1 CMPE 150/L : Introduction to Computer Networks Chen Qian Computer Engineering UCSC Baskin Engineering Lecture 3 1

2 No Class 1/23 (next Tuesday) Dr. Qian absent to serve a duty for US Department of Energy in DC 1-2

3 Chapter 1: roadmap 1.1 what is the Inter? 1.2 work edge end systems, works, links 1.3 work core packet switching, circuit switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 3

4 The work core packet-switching: hosts break application-layer messages into packets forward packets from one router to the next, across links on path from source to destination each packet transmitted at full link capacity 3

5 Packet Switching: queueing delay, loss A R = 100 Mb/s C B queue of packets waiting for output link R = 1.5 Mb/s D E queuing and loss: If arrival rate (in bits) to link exceeds transmission rate of link for a period of time: packets will queue, wait to be transmitted on link packets can be dropped (lost) if memory (buffer) fills up 5

6 Alternative core: circuit switching end-end resources allocated to, reserved for call between source & dest: dedicated resources: no sharing circuit-like (guaranteed) performance circuit segment idle if not used by call (no sharing) Commonly used in traditional telephone works 6

7 Packet switching versus circuit switching packet switching allows more users to use work! example: 1 Mb/s link each user: 100 kb/s when active active 10% of time N users 1 Mbps link circuit-switching: 10 users packet switching: with 35 users, probability > 10 active at same time is less than.0004 * Q: how did we get value ? Q: what happens if > 35 users? 7

8 Packet switching versus circuit switching is packet switching a slam dunk winner? great for bursty data resource sharing simpler, no call setup excessive congestion possible: packet delay and loss protocols needed for reliable data transfer, congestion control Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? 8

9 Inter structure: work of works End systems connect to Inter via ISPs (Inter Service Providers) Residential, company and university ISPs Access ISPs in turn must be interconnected. So that any two hosts can send packets to each other Resulting work of works is very complex Evolution was driven by economics and national policies

10 Inter structure: work of works Question: given millions of ISPs, how to connect them together?

11 Inter structure: work of works Option: connect each ISP to every other ISP? connecting each ISP to each other directly doesn t scale: O(N 2 ) connections.

12 Inter structure: work of works Option: connect each ISP to a global transit ISP? Customer and provider ISPs have economic agreement. global ISP

13 Inter structure: work of works But if one global ISP is viable business, there will be competitors. ISP A ISP B ISP C

14 Inter structure: work of works But if one global ISP is viable business, there will be competitors. which must be interconnected Inter exchange point ISP A IXP IXP ISP B ISP C peering link

15 Inter structure: work of works and regional works may arise to connect s to ISPS ISP A IXP IXP ISP B ISP C regional

16 Inter structure: work of works and content provider works (e.g., Google, Microsoft, Akamai ) may run their own work, to bring services, content close to end users ISP A ISP B ISP B IXP Content provider work IXP regional

17 Inter structure: work of works Tier 1 ISP Tier 1 ISP Google IXP Regional ISP IXP Regional ISP IXP ISP ISP ISP ISP ISP ISP ISP at center: small # of well-connected large works tier-1 commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage content provider work (e.g, Google): private work that connects it data centers to Inter, often bypassing tier-1, regional ISPs ISP 17

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19 Leonard Kleinrock talks about packet switch vs. circuit switching. Time: 2:55 7:00 19

20 Chapter 1: roadmap 1.1 what is the Inter? 1.2 work edge end systems, works, links 1.3 work core packet switching, circuit switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 1.7 history 20

21 How do loss and delay occur? packets queue in router buffers packet arrival rate to link (temporarily) 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 21

22 Four sources of packet delay A transmission propagation B nodal processing queueing d nodal = d proc + d queue + d trans + d prop d proc : nodal processing check bit errors determine output link typically < msec d queue : queueing delay time waiting at output link for transmission depends on congestion level of router 22

23 Four sources of packet delay A transmission propagation B nodal processing queueing d nodal = d proc + d queue + d trans + d prop d trans : transmission delay: L: packet length (bits) R: link bandwidth (bps) d trans = L/R d prop : propagation delay: d: length of physical link s: propagation speed in medium (~2x10 8 m/sec) d prop = d/s 23

24 Analogy using Mailing work Transmission delay: Time to write a mail Propagation delay Post truck carrying the mail moves to the next post office Processing delay Post officers distribute the mail to the right box for next station Queuing delay Mails waiting in the post office for processing 24

25 Packet loss queue (aka 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 A buffer (waiting area) packet being transmitted B packet arriving to full buffer is lost 25

26 Animation of queuing 26

27 Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time server server, sends withbits (fluid) file of into F bits pipe to send to client link pipe capacity that can carry R s bits/sec fluid at rate R s bits/sec) link pipe capacity that can carry R c bits/sec fluid at rate R c bits/sec) 27

28 Throughput (more) R s < R c What is average end-end throughput? No higher than R s! R s bits/sec R c bits/sec R s > R c What is average end-end throughput? No higher than R c! R s bits/sec R c bits/sec bottleneck link link on end-end path that constrains end-end throughput 28

29 Throughput: Inter scenario per-connection endend throughput: min(r c,r s,r/10) in practice: R c or R s is often bottleneck R s R s R s R R c R c R c 10 connections (fairly) share backbone bottleneck link R bits/sec 29

30 Chapter 1: roadmap 1.1 what is the Inter? 1.2 work edge end systems, works, links 1.3 work core packet switching, circuit switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 1.7 history 30

31 Protocol layers Networks are complex, with many pieces : hosts routers links of various media applications protocols hardware, software Question: is there any hope of organizing structure of work?. or at least our discussion of works? 31

32 Organization of air travel ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing a series of steps 32

33 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 airplane routing airplane routing departure airport intermediate air-traffic control centers arrival airport layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below 33

34 Why layering? dealing with complex systems: explicit structure allows identification, relationship of complex system s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer s service transparent to rest of system e.g., change in gate procedure doesn t affect rest of system layering considered harmful? 34

35 Inter protocol stack application: supporting work applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP work: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring work elements Ether, (WiFi), PPP physical: bits on the wire application transport work link physical 35

36 ISO/OSI reference model presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions session: synchronization, checkpointing, recovery of data exchange Inter stack missing these layers! these services, if needed, must be implemented in application needed? application presentation session transport work link physical 36

37 segment datagram frame message H l H t H n H t H n H t M M M M source application transport work link physical Encapsulation link physical switch H l H n H n H t H t H t M M M M destination application transport work link physical H l H n H n H t H t M M work link physical H n H t M router 37

38 Chapter 1: roadmap 1.1 what is the Inter? 1.2 work edge end systems, works, links 1.3 work core packet switching, circuit switching, work structure 1.4 delay, loss, throughput in works 1.5 protocol layers, service models 1.6 works under attack: security 38

39 Network security field of work security: how bad guys can attack computer works how we can defend works against attacks how to design architectures that are immune to attacks Inter not originally designed with (much) security in mind original vision: a group of mutually trusting users attached to a transparent work Inter protocol designers playing catch-up security considerations in all layers! Introduction 1-39

40 Bad guys: put malware into hosts via Inter malware can get in host from: virus: self-replicating infection by receiving/executing object (e.g., attachment) worm: self-replicating infection by passively receiving object that gets itself executed spyware malware can record keystrokes, web sites visited, upload info to collection site infected host can be enrolled in bot, used for spam. DDoS attacks Introduction 1-40

41 Robert Tappan Morris Son of Robert Morris Sr., chief scientist at National Security Agency (NSA) Introduction 1-41

42 Robert Tappan Morris Developed the first worm on the Inter in1988, while he was a graduate student at Cornell University. He said it was designed to gauge the size of the Inter. released the worm from MIT, rather than Cornell. Caused $10M - $100M loss Sentenced to three years of probation, 400 hours of community service A Computer Science professor at MIT since Not because of his worm!! Introduction 1-42

43 Bad guys: attack server, work infrastructure 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 work (see bot) 3. send packets to target from compromised hosts target Introduction 1-43

44 Bad guys can sniff packets packet sniffing : broadcast media (shared ether, wireless) promiscuous work interface reads/records all packets (e.g., including passwords!) passing by A C src:b dest:a payload B wireshark software used for labs is a (free) packetsniffer Introduction 1-44

45 Bad guys can use fake addresses IP spoofing: send packet with false source address A C src:b dest:a payload B lots more on security (throughout, Chapter 8) Introduction 1-45

46 Next class Please read Chapter of your textbook BEFORE Class 46