Chapter 7 An Introduction to Computer Networks Prof. Dina Katabi

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

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) 2

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 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 8

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

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

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

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 18

The mail system MIT Stanford Dina Nick Admin Admin 19

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 20

The mail system MIT Stanford Dina Nick Admin Admin 21

The Internet Leland.Stanford.edu Nms.csail.mit.edu Dina O.S. Packet Packet Nick Data Header Data Header O.S. 22

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 23

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

Differences Between Circuit & Packet Switching Circuit-switching Packet-Switching Guaranteed capacity No 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 Packets may be reordered, delayed, or drops dropped 25

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 26