Networked Systems and Services, Fall 2018 Chapter 1. Jussi Kangasharju Markku Kojo Lea Kutvonen

Transcription

1 Networked Systems and Services, Fall 2018 Chapter 1 Jussi Kangasharju Markku Kojo Lea Kutvonen

2 Course topic and goals Networked systems and services In particular, their reliability from various points of view After the course, you: Can explain how reliability can be achieved at various levels of the protocol stack Can justify why perfectly reliable systems cannot be built and can justify the tradeoffs in their design Can explain how the applications and services covered in the key themes work and can discuss their design tradeoffs Have designed one of the key themes and can justify its design tradeoffs

3 Course Organization Course sessions: Mon, Thu in D123 and Fri in D122 Lectures present new material Discussion sessions about articles Q&A sessions for exercises Lectures mostly in the beginning of the course First 4 weeks of the course Discussion and Q&A sessions mostly towards the end of the course Lecturers: Main lecturer: Jussi Kangasharju Other lectures: Markku Kojo, Lea Kutvonen Assistants: AleksandrZavodovski, Pengyuan Zhou

4 Passing the Course No course exam, only exercises 1 system design and implementation exercise (weight 1/2) 1 system design exercise (weight 1/6) 1 programming exercise (weight 1/6) 3 essays about scientific articles (weight 1/6) Half of maximum points/passing all exercises needed for passing the course All assignments are mandatory: No return à Failed course

5 Course Timeline Week 1: Lecture on Monday and Thursday; Friday network programming tutorials, article essay Week 2: Lectures, start of programming exercise, deadline Week 3: Lectures, start of design/implementation exercise Week 4: Lectures, deadline for first design Week 5: Start of second design exercise, Q&A sessions Week 6: Q&A sessions for exercises Week 7: Q&A sessions, Return implementation exercise, deadline , return second design exercise, deadline

6 Practical Details See course page for info Announcements on Twitter #UnivHelsinkiCS_NSS18 Slack team: See sign-up info on course page All returns via Moodle, see link on course page Office hours/appointments: Via or ask in person after lectures

7 Next week timeline Thursday 6.9.: Lecture Friday 7.9.: Python programming tutorials See course website for more info Sunday 9.9.: Deadline for returning first assignment (see Moodle) Monday 10.9.: Discussion on first assignment, lecture Start of programming exercise, deadline Thursday 13.9., Friday 14.9.: Lecture and Q&A for programming exercise

8 Questions?

9 Introduction

10 What Is Reliability?

11 What is reliability? Dependability, or reliability, describes the ability of a system or component to function under stated conditions for a specified period of time IEEE Standard Computer Dictionary: A Compilation of IEEE Standard Computer Glossaries, Open questions: Which system or component? What conditions? For how long? Where to implement? How to implement? What is the cost?

12 End-to-End Argument Seminal paper: J. H. Saltzer, D. P. Reed, and D. D. Clark (1981) "End-to- End Arguments in System Design". Discusses where to implement reliability in a networked system See assignment on website/moodle Deadline to return Sunday September 9 by 23:55 Class discussion about paper on Monday

13 Remember the Protocol Stack? User What is the right place to implement reliability? User Application Transport How many places are there? Application Transport Network Network Network Link Link Link Physical Physical Physical

14 Just turn all the knobs to 11?!? Well No Reliability solutions on different levels interact Interactions can lead to inefficiency What does inefficiency mean? Possible issues: Reduced throughput Increased latency What else could go wrong?

15 Remember the Application! Everything starts (and ends) with the application Application = Whatever is the thing of interest to you Application, service, combination of these, What level of reliability does the application require? We will return to this many times How much X can the application tolerate? How much X does the application absolutely require?

16 Combination of (some) X s Reliability Optimize for one or more of these properties Throughput Latency Do all possible combinations actually exist?

17 What about application requirements? Downloading these slides? Overnight backup? Video conference? Watching a video? Recording a video stream? Instant message? Bank transfer? Travel reservation? Class enrollment? On-line exam? Reliability Throughput Latency

18 Painful truth Tradeoff between reliability, throughput, and latency Often: High reliability à Low performance Not always true, but often How much reliability is really needed for the application? How much performance can we sacrifice?

19 How about non-technical requirements? Privacy? Security? Timing? Business requirements? How do these relate to reliability? How to measure these?

20 Mechanisms for reliability Detecting lost messages Timeouts, sequence numbers, Detecting corrupted messages Correcting corrupted messages Retransmissions Heartbeats Measurements

21 Roadmap for the rest of the course We will make our way up the protocol stack Look at reliability mechanisms on each level Lower level mechanisms as building blocks? Exercises also look at bigger picture Forces YOU to think about interactions and where to implement what Always remember the end-to-end argument User Application Transport Network Link Physical

22 System Model

23 What does our world look like? Endpoint Channel Endpoint 1-to-1 Communication

24 What does our world look like? Endpoint Endpoint Channel Endpoint 1-to-N Communication

25 What does our world look like? Endpoint Endpoint Channel Endpoint Endpoint N-to-M Communication

26 Endpoints Mostly 1-to-1 communications, some 1-to-N Endpoint is a loaded term! Traditional networking definition: Endpoint = endhost For us: End = Any entity of interest that needs reliability Endhost Application Service Any higher level entity

27 Communication channel Channel between endpoints One hop, multiple hops Hop-by-hop vs. end-to-end Wired, wireless, or a combination Different properties

28 Channel properties Channel may lose data Channel may corrupt data Perfect ideal channel does NOT exist J Our focus more on channel properties/problems rather than hosts

29 How to build a reliable enough system over an unreliable network?