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CS211: Protocol and Systems Design for Wireless and Mobile Networks

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1 CS211: Protocol and Systems Design for Wireless and Mobile Networks
Instructor: Songwu Lu Office: 4531D BH Lectures: 2:00-3:50am M&W office hours: 4:00-5:00pm M&W

2 What this course is about...
Introduce Internet design philosophy Wireless networking protocols Mobile computing system software design Trendy topics System programming skills How to start research

3 A picture of the course coverage
Networking fundamentals: Internet philosophy and principles Wireless Protocols MAC protocol Standard - Scheduling - Mobility management, ad-hoc routing - wireless TCP Mobile Computing - middleware, OS, file sys. services, applications Topical Studies -Wireless security Sensor networks -QoS and Energy-efficient design -Mesh Networks -MIMO Systems

4 Emerging Wireless Networks
Internet Backbone Base Station Fixed Host Wireless Cell Mobile Host

5 Growth of Wireless Users

6 The Wi-Fi Space It is one of the fastest growing industry sectors
100,000 public hotspots by 2005 Most notebooks will have embedded wi-fi card Go and check the local hotspots online

7 Protocol Stack Application Layer Middleware and OS Transport Layer
Wireless Web, Location Services, etc. Content adaptation, Consistency, File system Wireless TCP Mobility, Routing QoS Scheduling MAC Application Layer Middleware and OS Transport Layer Network Layer Link Layer & Below

8 The Course Description
No required textbook for this course, only a set of papers Read and discuss your class participation counts practice what you have learned get your hand dirty: do a term project make your contributions Heavy workload expected You are expected to be prepared for each lecture by reading the paper BEFORE coming to the lecture

9 Prerequisites basic knowledge of packet switched networks & familiarity with TCP/IP protocol suite adequate programming experience familiar with C/C++/UNIX useful reference books: “Internetworking with TCP/IP, Vol’s I, II, III” by Doug Comer “TCP/IP Illustrated, Vol’s 1 & 2” by Stevens

10 Course Workload One midterm, no final exam reading assignment:
Midterm: November 10th, in class. reading assignment: 1~2-page summary for the assigned reading of each lecture 3 strong points, 3 weak points, suggestions Similar to the paper review process you are going to do for your field in the future all assignments due 12:00pm(noon) before lecture on the due date to with subject “cs211: homework #”

11 Course Project A few big projects
Several topics within each big project to be distributed this Wednesday 2-3 persons on each topic Pick a topic and a team by next Monday Proposal + Checkpoint + Presentation + Final Report

12 Why such projects? Interact closely within your topic team
Discuss every three weeks within your big project to have the big picture in mind Stimulate discussions across teams Most topics are well defined, and you have a good starting point

13 Grading Policy Grading breakdown: in-class presentation: 10%
5~10 min each person Will get an assigned paper (expanding the topic scope of the paper discussed in class) from me midterm exam: 30% homework assignments: 20% There would be 19 assignments, you are expected to turn in at least 15 The 15 critiques with highest scores to be counted term project: 40% proposal 5%, checkpoint 10%, final report 15%, presentation & demo 10%

14 Course policies Homeworks, project proposals & reports all due 12:00pm on the due date No late turn-in accepted for credit!!! No makeup exam!!! Course homepage:

15 Tips on Doing Research in Graduate School
How to do productive research in graduate school What are the bad practices you should avoid Your feedback?

16 The content of this presentation
We take slides and points from many outstanding researchers: Dave Patterson, Richard Hamming, Craig Patridge, Nitin Vaidya, and the many references and sources cited there. They deserve all the credits I also share some of my own experiences We need your input and feedback too

17 Caveats Only opinions from some people. Others may not agree, including your advisors. Use advice at your own risk I do not necessarily follow the advice all the time This presentation may not follow some rules it talks about

18 What is Research, Anyway?
Research is not really about coming up with a nice solution to a hard (possibly new) problem, to show how smart you are. It is a process: identifying a research problem Coming up with a nice/new result (including simulating, implementing, testing your solution) Writing your results well Presenting your results Marketing your work Engineering is not science, it is about different tradeoff (whether u can do things easier, efficient, more convenient, … at acceptable cost/complexity), precisely true/false is not the main concern

19 A Few EQ Rules Motivation: “you are indeed interested in PhD research”
Think carefully about your career goal when you start your PhD NOT: “My family asks me to get a PhD…”, “It is hard to find a job with a MS degree now…”, “I want to hang around in school a little longer…” We can get you interested in something for some time, but not all the time Good start: “well begun, half done” Work harder during the first two years to settle down in research Have a taste of what is good research; not poisoned by the bad taste Believe yourself: your mindset has not be “framed” by conventional approaches yet; you can be innovative since you do not know much You have more energy and can have less distraction at this time Take the initiative: “you do care about what you are working on” Do not be afraid to talk to your advisor or others, and let people know the negative results/setbacks etc. “If u do not talk to these folks, who can u talk to???” disconnected communication causes more confusion among people Be honest to research and yourself; do not hide the nasty findings. If you do not understand something, ask; then you will know it. The reality of “capture effect”: Each advisor has more students than (s)he can handle; whoever is more aggressive gets more feedback more output Push for the project schedule from your side: call for meetings, set deadlines for internal drafts, look for places where to publish, etc.

20 EQ Rules (Cont’d) Regular life: “manage your time and life properly”
Shift from “deadline scheduling” to “priority scheduling” Evaluate your progress periodically. No one else will tell you that you are not efficient Have a “to-do” list on a daily/weekly/monthly basis Keep your most productive time-slot during a day to yourself No interruption even by your advisor, full concentration Even when the deadline comes

21 How to put yourself into the best position?
Keep yourself informed and networked: “know what is going on and talk to people” Know the literature on the topic you are working on; not let us tell you what to read. A quick rule “10+10” for breadth and depth: ten top systems/network conferences and ten leading groups People networking: the best way to be a missionary for your work Conference is a best place to talk to people. “Do not spend most time to polish your slides/talk there!!” When people contact you for your work, be responsive. “If you do not care about your work, who should care?” Attend seminars: people present the “meat” and “dark side” of their work in a talk Balance between quality and quantity: “make your record without controversy” Target a top conference each year: show your work quality Try at least a couple of small conferences: show your productivity Good way to practice writing, independent research, presentation,… A nice way to go to scenery places for sightseeing, vacations…

22 Selecting a Problem Solve a real problem that sb. cares about
Follow the industry technology trend and try to stay ahead of it a little Bad move: even if technology appears to leave you behind, stand by your problem Bad move: avoid payoffs of less than 20 years Working on a new problem is always easier People have worked on some problems, e.g., congestion control, for years. It is debatably harder for you to jump in and make major contributions Select a topic that you are interested for some extended period of time, not just for a month Interdisciplinary topics are always better, they can be very fruitful Running real experiments to discover new problems For systems topic, start from yourself: what do you need the systems to do for you?

23 Coming up with a solution
Do not rush for a solution simply based on the literature or what others tell you Understand the problem better, the solution naturally follows Use common sense Do not try to simply combine several existing solutions Explore new approaches: the alternative/opposite first Ask questions based on your intuition Keep things simple unless a very good reason not to – Pick innovation points carefully Best results are obvious in retrospect “Anyone could have thought of that” Complexity cost is in longer design, construction, test, and debug Fast changing field + delays => less impressive results Bad move: best compliment: it is so complicated, I cannot understand the ideas Best solutions are a combination of simplicity and depth Keep the solution core simple Depth is on second-level issues and fixes A relevant issue: How do I know mine is different from others READING PAPERS

24 How to read a paper? Know why you want to read the paper
To know what’s going on title, authors, abstract Track a few leading groups/researchers in your area, typically less than 10 is enough Only a few conferences (and journals): sigcomm, mobicom, infocom, sosp, sigmetrics, mobisys, … Papers in your broad research area introduction, motivation, solution description, summary, conclusions sometimes reading more details useful, but not always Papers that are directly relevant to your work read entire paper carefully, and several times

25 What to note Authors and research group Theme of the solution
Need to know where to look for a paper on particular topic Theme of the solution Should be able to go back to the paper if you need more info Approach to performance evaluation Note any shortcomings Be critical. It is easy to say nice words about a work, it is harder to identify limitations/flaws

26 In the process of a research project
Get Periodic Reviews/Feedbacks with Others Talk to people and ask what they think Give a seminar within your group periodically to collect feedback Explain the results to your friends, see whether they can grasp your problem and your solution For both technical people and non-technical people Exchange s, publish technical reports

27 Evaluate Quantitatively
If you can’t be proven wrong, then you can’t prove you’re right Report in sufficient detail for others to reproduce results can’t convince others if they can’t get same results For better or for worse, benchmarks shape a field Good ones accelerate progress good target for development Bad benchmarks hurt progress help real users v. help sales? Before you run real experiments, do an intuitive analysis Math does not need to be fancy, as long as it proves the point; in fact, best theory starts from scratch, not from some complex theorem you never heard about

28 Marketing Publishing papers is not equivalent to marketing
Missionary work: “Sermons” first, then they read papers Selecting problem is key: “Real stuff” Ideally, more interest as time passes Change minds with believable results Dave Patterson’s experience: industry is reluctant to embrace change Howard Aiken, circa 1950: “The problem in this business isn’t to keep people from stealing your ideas; its making them steal your ideas!” Need 1 bold company (often not no. 1) to take chance and be successful RISC with Sun, RAID with (Compaq, EMC, …) Then rest of industry must follow Publicize software: when people use your tools, they know your results think about how ns-2 and its wireless extension evolve

29 Do unto others as you would have them do unto you
How to write a paper Do unto others as you would have them do unto you When you have truly exceptional results P == NP Probably doesn’t matter how you write, people will read it anyway Most papers are not that exceptional Good writing makes significant difference Better to say little clearly, than saying too much unclearly

30 Readability a must If the paper is not readable, author has not given writing sufficient thought Two kinds of referees If I cannot understand the paper, it is the writer’s fault If I cannot understand the paper, I cannot reject it Don’t take chances. Write the paper well. Badly written papers typically do not get read

31 Do not irritate the reader
Define notation before use No one is impressed anymore by Greek symbols If you use much notation, make it easy to find summarize most notation in one place Avoid Using Too Many Acronyms AUTMA ?! You may know the acronyms well. Do not assume that the reader does (or cares to)

32 Writing a draft First read Strunk and White, then follow these steps;
1. 1-page paper outline, with tentative page budget/section 2. Paragraph map 1 topic phrase/sentence per paragraph, handdrawn figures w. captions 3. (Re)Write draft Long captions/figure can contain details ~ Scientific American Uses Tables to contain facts that make dreary prose 4. Read aloud, spell check & grammar check (MS Word; Under Tools, select Grammar, select Options, select “technical” for writing style vs. “standard”; select Settings and select) 5. Get feedback from friends and critics on draft; go to 3.

33 How to write a systems paper
Provide sufficient information to allow people to reproduce your results people may want to reproduce exciting results do not assume this won’t happen to your paper besides, referees expect the information Do not provide wrong information Sometimes hard to provide all details in available space may be forced to omit some information judge what is most essential to the experiments cite a tech report for more information

34 Discuss related work Explain how your work relates to state of the art
Discuss relevant past work by other people too Remember, they may be reviewing your paper. Avoid: The scheme presented by FOO performs terribly Prefer: The scheme by FOO does not perform as well in scenario X as it does in scenario Y Avoid offending people, unless you must

35 Tell them your shortcomings
If your ideas do not work well in some interesting scenarios, tell the reader People appreciate a balanced presentation

36 How to write weak results
If results are not that great, come up with better ones Do not hide weak results behind bad writing Be sure to explain why results are weaker than you expected If you must publish: write well, but may have to go to second-best conference Only a few conferences in any area are worth publishing in Too many papers in poor conferences bad for your reputation Just because a conference is “IEEE” or “ACM” or “International” does not mean it is any good If results not good enough for a decent conference, rethink your problem/solution

37 Miscellaneous Read some well-written papers Avoid long sentences
award-winning papers from conferences Avoid long sentences If you have nothing to say, say nothing don’t feel obliged to fill up space with useless text if you must fill all available space, use more line spacing, greater margins, bigger font, bigger figures, anything but drivel

38 How to present a poster Answer Five Heilmeier Questions
1. What is the problem you are tackling? 2. What is the current state-of-the-art? 3. What is your key make-a-difference concept or technology? 4. What have you already accomplished? 5. What is your plan for success? Do opposite of Bad Poster commandments Poster tries to catch the eye of person walking by 9 page poster might look like Problem Statement State-of- the-Art Key Concept Accomplish -ment # 1 Title and Visual logo Accomplish -ment # 2 Accomplish -ment # 3 Plan for Success Summary & Conclusion

39 How to present a paper (at a conference)
Objectives, in decreasing order of importance Keep people awake and attentive everything has been tried: play fiddle, cartoons, jokes in most cases, extreme measures should not be needed humor can help Get the problem definition across people in audience may not be working on your problem Explain your general approach most productive use of your time Dirty details most people in the audience probably do not care a typical conference includes 30+ paper presentations, yours could be the N-th

40 How many slides? Depends on personal style Rules of thumb
1 slides for 1-2 minutes Know your pace I tend to make more slides than I might need, and skip the not-so-important ones dynamically Anticipate technical questions, and prepare explanatory slides

41 Accent may not be easy to understand Talk slowly Easier said than done
How to present a paper Practice makes perfect (or tolerable) May need several trials to fit your talk to available time particularly if you are not an experienced speaker English issue Accent may not be easy to understand Talk slowly Easier said than done I have a tough time slowing down myself

42 The rest of the notes Overview/Review: Internet protocol stack
IP protocol TCP protocol If you forget these materials, go back and review what you learned in CS118 ASAP

43 Packet Switched Networks
Hosts send data in packets network supports all data communication services by delivering packets Web, , multimedia Host Host video Application Host Web Host Host

44 One network application example

45 What is happening inside ?
msg Transport protocol Network protocol Physical net physical net

46 Layered Network Architecture
network consists of geographically distributed hosts and switches (nodes) Nodes communicate with each other by standard protocols B A C physical connectivity Protocol layers host switch A B C D network topology

47 a picture of protocol layers
Application (data) header data Transport segment header DATA network packet header DATA tail Ethernet frame B physical connectivity What’s in the header: info needed for the protocol’s function

48 TCP/IP Protocol Suite IP Protocol: Inter-networking protocol
RFC791 TCP Protocol: reliable transport protocol RFC793

49 Why IP a number of different network technologies developed in early 70’s: ARPAnet, Ethernet, Satnet, PRnet different trans. media: copper, radio, satellite different protocol designs, e.g. ARPAnet: reliable message delivery Ethernet: unreliable packet delivery under different administrative control

50 Fundamental Goal of IP developing an effective technique for multiplexed utilization of all existing networks no centralized control no changes to individual subnets To read next time “The Design Philosophy of Internet Protocols” by Dave Clark, SIGCOMM'88

51 The picture of the world according to IP
application protocols TCP UDP transport (end-to-end) transport layer protocols universal datagram delivery IP inter-network layer subnets hardware-specific network technologies ethernet token-ring FDDI dialup ATM

52 IP Packet Header Format
vers IHL Type-of-Service total length identifier fragment offset time-to-live protocol checksum source address destination address options (variable length) padding data DF MF

53 IP: two basic functions
a globally unique address for each reachable interface datagram delivery from any host to any other host(s) two supporting protocols ICMP (Internet Control Message Protocol) ARP (Address Resolution Protocol)

54 Fundamental challenge: How to scale better
Original design: two levels of hierarchy, network, host Observed problems: class-based address assignment infeasible too many networks visible at the top level two approaches: subnetting & (CIDR) supernetting

55 Longer-Term Scaling issues
We've run out of all IPv4 unicast space far before theoretical limit of 4 billion hosts, due to inevitable inefficiency of address block allocation Short term patch: NAT boxes One long term solution: IP version 6 expanded addressing capability: 16 bytes cleanup of IPv4 design after 15 years of running experience improved support for options/extensions

56 The IPv6 Header Version Priority Flow Label Payload Length Next Header
Hop Limit Source Address Destination Address  32 bits 

57 The IPv4 Header shaded fields are absent from IPv6 header Version
Hdr Len Prec TOS Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source Address Destination Address Options Padding  32 bits  shaded fields are absent from IPv6 header

58 TCP: Transmission Control Protocol
a transport protocol IP delivers packets “from door to door” TCP provides full-duplex, reliable byte-stream delivery between two application processes Application process Application process More terminology: TCP segment Max. segment size (MSS) W rite Read bytes bytes TCP TCP Send buffer Receive buffer segment segment

59 TCP: major functionalities
Header format Connection Management Open, close State management Reliability management Flow and Congestion control Flow control: Do not flood the receiver’s buffer Congestion control: Do not stress the network by sending too much too fast

60 TCP header format IP header data source port destination port
16 31 IP header source port destination port Data sequence number acknowledgment number u a p r s f r c s s y i g k h t n n Hlen unused window size checksum urgent pointer Options (viable length) data

61 opening a connection: three-way hand-shake
client open request(x) server Passive open ack(x+1) + request(y) ack(y+1) (now in estab. state) enter estab. state

62 Closing a TCP Connection
I-finished(M) ACK (M+1) I-finished(N) ack(N+1) wait for 2MSL before deleting the conn state Done, delete conn. state

63 Mechanisms for Reliability Management
Sequence number Acknowledgment number Error detection at the receiver side Retransmission timeout

64 TCP Flow and Congestion Control
Window-based protocol Flow control is easy: set the sender’s window no larger than the advertised window by the receiver 4 algorithms in TCP congestion control Control congestion window variable: cwnd slow start, congestion avoidance, fast retransmit and fast recovery, retransmission upon timeout Sender_window=min(adv_win, cwnd)

65 Slow Start & Congestion Avoidance
start conservatively: cwnd <= min(2*SMSS bytes, 2 segments) when cwnd <ssthresh, use slow start: increase cwnd exponentially to quickly fill up the pipe: upon receiving each ACK, cwnd+=SMSS; when cwnd > ssthresh, use congestion avoidance cwnd += SMSS*SMSS/cwnd; continue until loss is detected

66 Fast Retransmit When the 3rd DUP_ACK is received, ssthresh=max(FlightSize/2, 2*SMSS) ReXmit the lost segment, set cwnd=ssthresh+3*SMSS Design questions: why FlightSize, not cwnd ? FlightSize: data sent but not yet acked Why add 3 SMSS to cwnd ?

67 Fast Recovery For each additional DUP_ACK: When a NEW ACK arrives,
cwnd+=SMSS; (why ?) transmit a new segment if min(cwnd, rwnd) permits When a NEW ACK arrives, cwnd=ssthresh; (why ?)

68 Retransmission Timeout
Initial design: RTT=*old_RTT+ (1-)*New_RTT_sample RTO= *RTT;  = 2 for original spec variation in RTT: ~1/(1-L); factor 4, for L=50%; factor 10, for L=80%; load <= 30% for =2. RTO improvement in addition to mean, also estimate the deviation of RTT Diff=New_RTT_sample - old-RTT; Smoothed_RTT=old_RTT+1/8*Diff Dev=old_RTT+1/4*(|Diff|-Old_Dev) RTO = Smoothed_RTT+4*Dev

69 Karn’s Algorithm how to measure RTT in retransmission cases?
take the delay between the first (last) transmission and final ack? do not update SRTT in case of retransmission? Karn’s algorithm: do not take RTT samples in case of retransmission double the retransmission timer for next packet, till one can get a RTT sample without retransmission

70 Putting all together: RFC2581
how TCP congestion control works Start with slow start for bootstrapping phase: quickly open up the window At ssthresh, switch to congestion avoidance When 3rd duplicate ACK is received (indicating a packet loss), use fast retransmit; if more than 3 duplicate ACKs, use fast recovery Upon retransmission timeout (indicating a packet loss too): cwnd=1, binary exponential backoff

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