6.02 Fall 2014!Lecture #19

• Packet switching vs. circuit switching • Queues and Little’s Law

Communications Model

last week: multiple nodes sharing a single channel

first two parts of 6.02: point-to-point communications

Networks as Graphs

today: many interconnected points (networks)

model networks as graphs: nodes are vertices, edges are links; a link between two nodes means they’re directly connected

end point

switch

link

MIT Network

bs2

e3/0: w5 (18.249.0.1/23)e3/2: w11 (18.11.0.1/24)e3/5: w59 (18.156.0.1/23)e4/1: w31 (18.174.0.1/23)e4/2: n57 (18.230.0.1/23)e5/0: dke (18.233.0.1/23)e5/1: dp (18.234.0.1/23)e5/2: ato (18.232.0.1/23)e5/3: pbe (18.236.0.1/23)e5/4: ks (18.235.0.1/23)e5/5: tc (18.237.0.1/23)e8/5: nw61 (18.243.0.1/21)e9/1: nw62 (18.41.0.1/24)e9/5: nw12srv (18.9.0.1/24) e10/1: w8 (18.136.0.1/24)e10/2: w53 (18.137.0.1/24)e10/3: zp (18.228.0.1/23)e10/4: adp (18.202.0.1/23)e10/5: wilg (18.226.0.1/23)

fa3/2: n42 (18.152.0.1/23)fa3/3: w16 (18.175.0.1/23)fa3/4: w20srv158 (18.158.0.1/24)fa3/5: sipb (18.181.0.1/24)fa3/6: w7 (18.245.0.1/23)fa3/7: nw30 (18.139.0.1/23)fa3/8: nw17 (18.120.0.1/23)fa3/9: w20 (18.187.0.1/23)fa3/10: w91srv92 (18.92.0.1/23)fa3/11: w89 (18.169.0.1/21) fa3/12: n52 (18.150.0.1/21)fa3/13: w61 (18.239.0.1/21)fa3/14: fbml (18.165.0.1/24)fa3/15: nw16 (18.166.0.1/21)fa3/16: nw21 (18.167.0.1/23)fa3/17: w1 (18.250.0.1/21)fa3/18: nw86 (18.95.0.1/21)fa3/19: w51 (18.247.0.1/21)fa3/20: w79 (18.96.0.1/21)fa3/21: w70 (18.241.0.1/21)fa3/22: w71 (18.242.0.1/21)fa3/24: w4 (18.240.0.1/21)fa4/1: w84 (18.251.0.1/21)fa4/2: w13 (18.246.0.1/21)fa4/3: w85 (18.98.0.1/21)fa4/4: nw10 (18.252.0.1/21)gig7/1: w35 (18.183.0.1/21)gig7/3: nw14 (18.140.0.1/21)gig7/5: w34(18.182.0.1/21)

fa3/2: amt (18.85.0.1/18)fa3/3: e52 (18.170.0.1/21)fa3/4: 68 (18.79.0.1/21)fa3/5: e25 (18.42.0.1/21)fa3/6: 16 (18.55.0.1/21)fa3/7: 54 (18.83.0.1/21)fa3/8: 38 (18.48.0.1/21)fa3/9: 56 (18.56.0.1/21)fa3/10: 66 (18.63.0.1/21)fa3/11: e51 (18.186.0.1/21)fa3/12: e53 (18.171.0.1/21)fa3/14: e40srv91 (18.91.0.1/24)fa3/16: e40 (18.172.0.1/21)fa3/17: e48 (18.178.0.1/21)fa3/18: 62 (18.248.0.1/21)fa3/19: e1 (18.147.0.1/23)fa3/20: 64 (18.238.0.1/21)fa3/21: 32 (18.50.0.1/21)fa3/22: 57 (18.119.0.1/21)gig7/1: bluebox (18.162.0.1/16)gig7/3: e19 (18.142.0.1/21)gig7/5: e18 (18.114.0.1/21)gig7/7: e2 (18.244.0.1/21)gig7/9: e15 (18.134.0.1/21)

e8/5: e55 (18.97.0.1/21)e8/4: medical (18.64.0.1/21)e8/1: e23 (18.143.0.1/21)e5/0: 50 (18.160.0.1/21)e4/2: e34 (18.28.0.1/21)e4/1: e32 (18.22.0.1/23)e3/5: e39 (18.173.0.1/23)e3/1: e56 (18.154.0.1/21)

fa3/1: 48 (18.84.0.1/21)fa3/2: 6 (18.74.0.1/21)fa3/3: n4 (18.40.0.1/24)fa3/4: 41t (18.112.0.1/23)fa3/5: nmis (18.39.0.1/24)fa3/6: 1 (18.58.0.1/21)fa3/7: 5 (18.38.0.1/21)fa3/8: wibr (18.157.0.1/16)fa3/9: 14 (18.51.0.1/21)fa3/10: 7 (18.113.0.1/21)fa3/11: math (18.87.0.1/21)fa3/12: 13 (18.82.0.1/21)fa3/13: 17 (18.67.0.1/23)fa3/14: rle (18.62.0.1/22)fa3/15: lms (18.60.0.1/21)fa3/16: 10 (18.90.0.1/21)fa3/17: n9 (18.65.0.1/21)gig7/1: 24srv7 (18.7.7.1/24)gig7/3: 3 (18.80.0.1/21)gig7/5: 26 (18.109.0.1/21)gig7/7: 35 (18.78.0.1/21)

e9/2: 39 (18.109.0.1/23)e9/0: n10 (18.20.0.1/23)e8/5: 37srv145 (18.145.0.1/24)e8/3: 45 (18.32.0.1/23)e8/0: 31 (18.47.0.1/21)fa5/0/0: lcs (18.201.1.1/31)e4/3: 42 (18.163.0.1/23)e3/5: csr (18.75.0.1/22)e3/1: lns (18.77.0.1/22)e1/0: 44 (18.44.0.1/24)

gig3/15: w91 (18.94.0.1/21)gig3/13: w92 (18.18.0.1/21)gig3/7: w92srv21 (18.7.21.1/24)gig3/6: w92srv17 (18.7.16.1/24)gig3/5: w92srv16 (18.7.16.1/24)gig3/4: w92srv15 (18.7.15.1/24)gig3/3: w92srv14 (18.7.14.1/24)gig3/1: w92srv10 (18.7.10.7/24)

(18.168.0.2/24)

alpha phi

ww15

oc1

bhf

ne25

zeta beta tau

epsilon theta

e70

pika

delta tau delta

fenway house

pi lambda pi

nu delta

sigma phi epsilon

delta upsilon

theta chi

phi kappa sigma

sigma chi

phi kappa theta

tau epsilon pi

alpha chi omega

phi sigma kappa

sigma kappa

sigma nu

chi phi

theta xi

phi delta theta

lambda chi alpha

student house

beta theta pi

alpha epsilon pi

MIT Campus Network

extr2

w92r1gig0/0/0 (20)

e19r2gig1/1 (22)

e19r1fa1/0/0: (15)

e0: (18.229.0.1/16)

e0: (18.99.0.1/16)

e0: (18.141.0.1/24)

e0: (18.36.0.1/16)

e0: (18.227.0.1/23)

e0: (18.208.0.1/23)

e0: (18.193.0.1/24)

e0: (18.214.0.1/23)

e0: (18.153.0.1/23)

e0: (18.209.0.1/23)

e0: (18.206.0.1/23)

e0: (18.218.0.1/23)

e0: (18.223.0.1/23)

e0: (18.207.0.1/23)

e0: (18.222.0.1/23)

e0: (18.211.0.1/23)

e0: (18.215.0.1/23)

e0: (18.220.0.1/23)

e0: (18.216.0.1/23)

e0: (18.224.0.1/23)

e0: (18.231.0.1/23)

e0: (18.217.0.1/23)

e0: (18.195.0.1/23)

e0: (18.194.0.1/23)

e0: (18.205.0.1/23)

e0: (18.225.0.1/23)

e0: (18.212.0.1/23)

e0: (18.210.0.1/23)

e0: (18.221.0.1/23)

e0: (18.204.0.1/23)

e0: (18.203.0.1/23)

fa0/0 (16)

nw12r1

24r2

gig1/1 (23)

nw12r2gig1/1 (21)

ds3

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

ds1

d s1

ds1

ds1

ds1

s10/0

s10/1

s10/2

s10/4

s11/0/0

gig3/1: nox (19 2.5.89.90/ 30)

g ig3/ 3: cogent (18.3.4. 1/ 24)

Topology Overviewne20

e0: (18.88.0.1/24)

ds1

s10/3

13 May 2003tom@mit.edu

Verizon

server networkresidential networknon-IS operated network56kbs interface1.544Mbs (T1) interface44.376Mbs (T3) interface155Mbs interface10Mbs Ethernet interface100Mbs FDDI interface100Mbs Ethernet interface1Gbs Ethernet interfaceATM interfaceSerial interface

ds0ds1ds3oc3eX

fX/YfaX/Y[/Z]gigX/Y[/Z]

atX/YsX[/Y]

Legend

24r1fa12/1/0 (14)

24r3extr1

gig1/1 (27)

gig1/1 (26)

to Abilene, RCN, Woods Hole, Lincoln Labs and other New England universities

to the other greater Internet areaGenuity

Comcast

Cogent

gig0/0/0 (18)

NOX

to the broadband network(formerly AT&T formerly MediaOne)

p os4/1: gen uity (4.24 .88.50/30)

1000LX

1000L X

OC12

1 00 0LX

gig3/1: 12 (18.19.0.1/21)gig3/2: 44t (18.32.0.1/21)gig3/3: 34 (18.105.0.1/21)gig3/4: ne47 (18.125.0.1/21)gig3/5: 41 (18.116.0.1/21)gig3/7: 4 (18.53.0.1/21)gig3/9: 11 (18.81.0.1/21)gig3/11: 18 (18.148.0.1/21)gig3/13: ne49 (18.124.0.1/21)gig3/15: ne43 (18.127.0.1/21)

gig7/9: 33 (18.34.0.1/21)gig7/11: 38 (18.107.0.1/21)gig7/13: 37 (18.33.0.1/21)gig7/15: 8 (18.115.0.1/21)gig8/1: 9 (18.89.0.1/21)gig8/3: 24 (18.54.0.1/21)gig8/5: 36 (18.108.0.1/21)

to the greater Internet area

http://mit.edu/maps/networks/mit/mit3topology.pdf

Circuit Switchingendpoint endpointswitch switch

DATA

setup

communicate

tear down

time

Paul Baran’s Proposal

http://www.rand.org/pubs/research_memoranda/RM3420.html

Packet Switchingendpoint endpointswitch switch

time

packetA1

packetA1

packetA1

packetA2

packetA2

packetA2

packetA3

packetA3

packetA3

1

2

4

8

16

32

64

128

256

number.of.senders

time

numberAofAbytes

bursts become smoother as number of senders increases

Statistical Multiplexing(of randomly generated traffic)

Statistical Multiplexing(of real traffic)

(in 2010)

queues: we need queues to absorb bursts of traffic, but they also add delay into the network. we need a way to analyze how large our queues will be, and thus how much delay they will add

Types of DelayendpointA1 endpointA2switchA1 switchA2

packetA1

packetA1

packetA1

packetA2

packetA2

packetA2

packetA3

packetA3

packetA3

queueing delay would also show up in the vertical dimension, along with

processing delay

transmission delay of packet 1 at

endpoint 1

propagation delay!of packet 1 between

endpoint 1 and switch 1

processing delay of

packet 1 at switch 1

Little’s Lawn(

t) nu

mbe

r of p

acke

ts

in q

ueue

at t

ime

t

T time

N = λ * D

averageArateAatAwhichApacketsAarriveAatAtheAqueue

meanAdelayAofApacketsAinAqueue

meanAnumberAofApacketsAinAqueue

• Circuit Switching Explicit setup/teardown phases, guarantees a particular rate. Simpler than packet switching, but can waste capacity.

• Packet Switching No guarantees, requires more effort to make delivery reliable, but is (generally) more efficient than circuit switching

• Queues Needed to absorb bursts of traffic (which often occur in packet-switched networks), but increase delay. Little’s Law gives us a relationship between the size of the queue and delay