An Introduction to Com puter Networks

Transcription

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

2 Chapter Outline Int roduct ion (slides and 7.A) Layered Architecture (slides and 7.B & 7.D) Rout ing (slides and 7.D) Reliable Transm ission & Flow Control (slides and read 7.E) Congestion Control (slides and read 7.F) 2

3 This Lecture What is a network? Sharing the infrastructure Circuit switching Packet swit ching Best Effort Service Analogy: the m ail system Internet s Best Effort Service 3

4 Networks Why they are interesting? Overcom e geographic lim its Access rem ote data Separate clients and server Goal: Universal Com m unication (any to any) Design the cloud Net work 4

5 Connectivity Link DSL, T1, T3,... Charact erized by Capacity or bit-rate (1.5 Mb/s, 100Mb/s, ) Propagation delay (10us, 10m s, 100m s,..) Transfer tim e on a link = # bit/bit-rate + propagation delay 5

6 Connectivity A m esh requires N 2 links too costly 6

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

8 This Lecture What is a network? Sharing the infrastructure Circuit switching Packet swit ching Best Effort Service Analogy: the m ail system Internet s Best Effort Service 8

9 Two ways to share Circuit swit ching (isochronous) Packet swit ching (asynchronous) 9

10 Circuit Switching It s the m ethod used by the telephone net work A call has three phases: 1. Establish circuit from end-to-end ( dialing ), 2. Com m unicate, 3. Close circuit ( tear down ). If circuit not available: busy signal Caller (1 ) (2 ) (3 ) Boston Switch DATA LA Switch processing delay at switch Callee propagatio n delay between caller and and Boston switch 10

11 Circuit Switching: Mult iplexing/dem ult iplexing Switch Frames Slots = One way for sharing a circuit is TDM: 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 synchronizat ion bet ween sender and receiver Lecture notes use the word fram e for slot 11

12 Circuit Switching Assume link capacity is C bits/sec Each communication requires R bits/sec # slots = C/R Maxim um 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 com puter networks where transfers have variable rate (bursty) 12

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

14 Packet Switching Used in the Internet Data is sent in Packet s (header contains control info, e.g., source and dest inat ion addresses) Host 1 Host 2 Node 1 Node 2 Header Data Per-packet rout ing At each node the entire packet is received, stored, and then forwarded (st ore-andforw ard net w orks) No capacity is allocated transmission time of Packet 1 at Host 1 Packet 1 Packet 2 Packet 3 propagatio n delay between Host 1 & Node 2 Packet 1 Packet 2 Packet 3 processin g delay of Packet 1 at Node 2 Packet 1 Packet 2 Packet 3 14

15 Packet Switching: Multiplexing/Demultiplexing Router Queue Multiplex using a queue Routers need m em ory/buffer Dem ultiplex using inform ation in packet header Header has destination Router has a routing table that contains inform ation about which link to use to reach a destination 15

16 Queues int roduce Variable Delay Delay = Queuing delay + propagation delay + t ransm ission delay + processing delay Losses When packets arrive to a full queue/buffer they are dropped 16

17 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 A Host D Node 1 Node 2 Node 3 Node 5 Host B Node 4 Node 6 Node 7 Host E 17

18 This Lecture What is a network? Sharing the Infrastructure Circuit switching Packet swit ching Best Effort Service Analogy: the m ail system Internet s Best Effort Service 18

19 The m ail system MIT Dina St an ford Nick Adm in Adm in 19

20 Characteristics of the m ail system Each envelope is individually rout ed No tim e guarantee for delivery No guarant ee of delivery in sequence No guarantee of delivery at all! Things get lost How can we acknowledge delivery? Ret ransm ission How to determine when to retransmit? Timeout? If m essage is re-sent too soon duplicat es 20

21 The m ail system MIT Dina St an ford Nick Adm in Adm in 21

22 The Internet Nm s.csail.m it.ed u Dina Leland.Stanford.ed u Nick O.S. Packet Data Header Data Heade r O.S. 22

23 Characteristics of the Internet Each packet is individually rout ed No tim e guarantee for delivery No guarant ee of delivery in sequence No guarantee of delivery at all! Things get lost Acknowledgem ent s Ret ransm ission How to determine when to retransmit? Timeout? If packet is re-transm itted too soon duplicat e 23

24 Best Effort No Guarant ees: Variable Delay (jit t er) Variable rat e Packet loss Duplicat es Reordering (notes also state m axim um packet lengt h) 24

25 Differences Between Circuit & Packet Swit ching Circuit -sw it ching Packet -Sw it ching Guarant eed capacit y Capacity is wasted if data is bursty Before sending data establishes a path All data in a single flow follow one path No reordering; constant delay; no pkt drops No guarantees (best effort) More efficient Send data immediately Different packets might follow different pat hs Packets may be reordered, delayed, or dropped 25

26 This Lecture We learned how to share the network infrastructure between m any connect ions/flows We also learned about the im plications of the sharing schem e (circuit or packet switching) on the service that the traffic receives 26