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CS211/Fall 2003 9/29
CS211: Protocol and Systems Design for Wireless
and Mobile Networks
Instructor: Songwu Lu [email protected]: 4531D BH
Lectures: 2:00-3:50am M&Woffice hours: 4:00-5:00pm M&W
CS211/Fall 2003 9/29
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
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Networking fundamentals: Internet philosophy and principles
Wireless Protocols-MAC protocol-802.11 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
A picture of the course coverage
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Emerging Wireless Networks
Base Station
Fixed Host
Wireless Cell
InternetBackbone
Mobile Host
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Growth of Wireless Users
010203040506070
1991 1993 1995 1997
Wireless Phone Subscribers (in millions)
Source: cellular telecom. Indus. Assn.
0
2
4
6
8
10
12
1997 1998 1999 2000 2001 2002
Wireless Data Subscriber (in millions)
Source: Strategis Market Res.
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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– www.ezgoal.com/hotspots/
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Protocol Stack• Wireless Web, Location
Services, etc.
Content adaptation, Consistency, File system
Wireless TCP
Mobility, Routing QoS
o Schedulingo MAC
Application Layer
Middleware and OS
Transport Layer
Network Layer
Link Layer & Below
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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
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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
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Course Workload• One midterm, no final exam
– 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– email to [email protected] with subject “cs211:
homework #”
CS211/Fall 2003 9/29
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
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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
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Grading PolicyGrading 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%
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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:http://www.cs.ucla.edu/classes/fall03/cs211/
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Tips on Doing Research in Graduate School
1. How to do productive research in graduate school
2. What are the bad practices you should avoid
3. Your feedback?
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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
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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
CS211/Fall 2003 9/29
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
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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.
CS211/Fall 2003 9/29
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
CS211/Fall 2003 9/29
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…
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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?
CS211/Fall 2003 9/29
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
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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
CS211/Fall 2003 9/29
What to note
• Authors and research group– 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
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•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 emails, publish technical
reports
In the process of a research project
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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
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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
CS211/Fall 2003 9/29
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
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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
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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)
CS211/Fall 2003 9/29
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.
• www.cs.berkeley.edu/~pattrsn/talks/writingtips.html
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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
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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
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Tell them your shortcomings
• If your ideas do not work well in some interesting scenarios, tell the reader
• People appreciate a balanced presentation
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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
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Miscellaneous
• Read some well-written papers– 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
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How to present a poster
• Answer Five Heilmeier Questions1. 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
ProblemStatement
State-of-the-Art
KeyConcept
Accomplish-ment # 1
Title and Visual logo
Accomplish-ment # 2
Accomplish-ment # 3
Plan for Success
Summary &Conclusion
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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
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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
CS211/Fall 2003 9/29
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
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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
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Packet Switched Networks
• Hosts send data in packets• network supports all data
communication services by delivering packets– Web, email, multimedia
Host Host
Application
Host
Web
Host Host
video
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What is happening inside ?
[email protected] [email protected]
msg
Physical net physical netPhysical net
Networkprotocol
Networkprotocol
Networkprotocol
Networkprotocol
Transportprotocol
Transportprotocol
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A B C
network topology
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
D
host switch
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Ethernet frame
network packet
Transport segment
header tail
header
header
DATA
DATA
data
What’s in the header: info needed for the protocol’s function
Application (data)
B
A
physical connectivity
a picture of protocol layers
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TCP/IP Protocol Suite
• IP Protocol: Inter-networking protocol– RFC791
• TCP Protocol: reliable transport protocol– RFC793
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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
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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
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transport(end-to-end)
subnets
ethernet token-ring FDDI dialup ATM
IP
TCP UDP
inter-network layer
application protocols
transport layer protocols
universal datagram delivery
hardware-specific network technologies
The picture of the world according to IP
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IP Packet Header Formatvers. 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
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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)
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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
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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
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The IPv6 Header
Destination Address
Version Priority Flow Label
Payload Length Next Header Hop Limit
Source Address
32 bits
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The IPv4 Header
shaded fields are absent from IPv6 header
Version Hdr Len Total Length
Identification Fragment Offset
Prec TOS
Time to Live Protocol Header Checksum
Flags
Source Address
Destination Address
PaddingOptions
32 bits
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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
Writebytes
TCP
Send buffer
Application process
Readbytes
TCP
Receive buffer
segment segment
More terminology:• TCP segment• Max. segment size (MSS)
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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
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u a p r s fr c s s y ig k h t n n
source port destination port
Data sequence number
acknowledgment number
Hlen unused window size
checksum urgent pointer
Options (viable length)
0 16 31
TCP header format
data
IP header
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client
serveropen request(x)
Passive open
ack(x+1) + request(y)
ack(y+1)(now in estab. state)
enter estab. state
opening a connection:three-way hand-shake
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Done, delete conn. state
Closing a TCP Connection
I-finished(M)
I-finished(N)
ACK (M+1)
ack(N+1)wait for 2MSLbefore deletingthe conn state
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Mechanisms for Reliability Management
• Sequence number• Acknowledgment number• Error detection at the receiver side• Retransmission timeout
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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)
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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
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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 ?
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Fast Recovery
• For each additional DUP_ACK:– cwnd+=SMSS; (why ?)– transmit a new segment if min(cwnd,
rwnd) permits
• When a NEW ACK arrives, – cwnd=ssthresh; (why ?)
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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
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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
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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
