1. 1 An Introduction to Computer Networks Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, and Sam Madden Prof. Dina Katabi Chapter 7

2. 2 Chapter Outline  Introduction (slides and 7.A)  Layered Architecture (slides and 7.B & 7.D)  Routing (slides and 7.D)  Reliable Transmission & Flow Control (slides and read 7.E)  Congestion Control (slides and read 7.F)

3. 3 This Lecture  What is a network?  Sharing the infrastructure  Circuit switching  Packet switching  Best Effort Service  Analogy: the mail system  Internet’s Best Effort Service

4. 4 Networks  Why they are interesting?  Overcome geographic limits  Access remote data  Separate clients and server  Goal: Universal Communication (any to any)  Design the cloud Network

5. 5 Connectivity Link  DSL, T1, T3,...  Characterized by  Capacity or bit-rate (1.5 Mb/s, 100Mb/s, …)  Propagation delay (10us, 10ms, 100ms,..)  Transfer time on a link = #bit/bit-rate + propagation delay

6. 6 Connectivity  A mesh requires N 2 links  too costly

7. 7  We Have to Share the Infrastructure  Intermediate nodes called switches or routers allow the hosts to share the infrastructure

8. 8 This Lecture  What is a network?  Sharing the infrastructure  Circuit switching  Packet switching  Best Effort Service  Analogy: the mail system  Internet’s Best Effort Service

9. 9 Two ways to share  Circuit switching (isochronous)  Packet switching (asynchronous)

10. 10 Circuit Switching DATA CallerCallee Boston Switch LA Switch propagation delay between caller and and Boston switch processing delay at switch  It’s the method used by the telephone network  A call has three phases: 1. Establish circuit from end-to-end (“dialing”), 2. Communicate, 3. Close circuit (“tear down”).  If circuit not available: “busy signal” (1) (2) (3)

11. 11 Switch Circuit Switching: Multiplexing/Demultiplexing  Time divided into frames and frames divided into slots  Relative slot position inside a frame determines which conversation the data belongs to  E.g., slot 0 belongs to the red conversation  Need synchronization between sender and receiver Frames 012 3 45012 3 45 Slots = One way for sharing a circuit is TDM: Lecture notes use the word “frame” for slot

12. 12 Circuit Switching  Assume link capacity is C bits/sec  Each communication requires R bits/sec  #slots = C/R  Maximum number of concurrent communications is C/R  What happens if we have more than C/R communications?  What happens if the a communication sends less/more than R bits/sec?  Design is unsuitable for computer networks where transfers have variable rate (bursty)

13. 13 Internet Traffic Is Bursty Daily traffic at an MIT-CSAIL router Max In:12.2Mb/sAvg. In: 2.5Mb/s Max Out: 12.8Mb/sAvg. Out: 3.4 Mb/s

14. 14 Packet Switching  Used in the Internet  Data is sent in Packets (header contains control info, e.g., source and destination addresses)  Per-packet routing  At each node the entire packet is received, stored, and then forwarded (store-and- forward networks)  No capacity is allocated HeaderData Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 processing delay of Packet 1 at Node 2 propagation delay between Host 1 & Node 2 transmission time of Packet 1 at Host 1 Host 1 Host 2 Node 1Node 2

15. 15 Router Packet Switching: Multiplexing/Demultiplexing  Multiplex using a queue  Routers need memory/buffer  Demultiplex using information in packet header  Header has destination  Router has a routing table that contains information about which link to use to reach a destination Queue

16. 16 Queues introduce  Variable Delay  Delay = Queuing delay + propagation delay + transmission delay + processing delay  Losses  When packets arrive to a full queue/buffer they are dropped

17. 17 Packet switching also show reordering Host A Host B Host E Host D Host C Node 1 Node 2 Node 3 Node 4 Node 5 Node 6 Node 7 Packets in a flow may not follow the same path (depends on routing as we will see later)  packets may be reordered

18. 18 This Lecture  What is a network?  Sharing the Infrastructure  Circuit switching  Packet switching  Best Effort Service  Analogy: the mail system  Internet’s Best Effort Service

19. 19 The mail system DinaNick MIT Stanford Admin

20. 20 Characteristics of the mail system  Each envelope is individually routed  No time guarantee for delivery  No guarantee of delivery in sequence  No guarantee of delivery at all!  Things get lost  How can we acknowledge delivery?  Retransmission  How to determine when to retransmit? Timeout?  If message is re-sent too soon  duplicates

21. 21 The mail system DinaNick MIT Stanford Admin

22. 22 The Internet DinaNick Nms.csail.mit.edu Leland.Stanford.edu O.S. HeaderDataHeaderData Packet

23. 23 Characteristics of the Internet  Each packet is individually routed  No time guarantee for delivery  No guarantee of delivery in sequence  No guarantee of delivery at all!  Things get lost  Acknowledgements  Retransmission  How to determine when to retransmit? Timeout?  If packet is re-transmitted too soon  duplicate

24. 24 Best Effort No Guarantees:  Variable Delay (jitter)  Variable rate  Packet loss  Duplicates  Reordering  (notes also state maximum packet length)

25. 25 Differences Between Circuit & Packet Switching Circuit-switchingPacket-Switching Guaranteed capacityNo guarantees (best effort) Capacity is wasted if data is bursty More efficient Before sending data establishes a path Send data immediately All data in a single flow follow one path Different packets might follow different paths No reordering; constant delay; no pkt drops Packets may be reordered, delayed, or dropped

26. 26 This Lecture  We learned how to share the network infrastructure between many connections/flows  We also learned about the implications of the sharing scheme (circuit or packet switching) on the service that the traffic receives