ccn2001 quiz1 solutions

5/17/2018 Ccn2001 Quiz1 Solutions

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Shivkumar KalyanaramanRensselaer Polytechnic Institute

1

ECSE-4670: Computer

Communications Netwoks

Exam 1: SOLUTONS

Time: 7! min "strict#$ en%orce&'

Points:!0

(OU) N*+E: ,,,,,,,

Be brief, but DO NOT omit necessary detail{Note: Simply copying text directly from the slides or notes will not

earn (partial) credit. Brief, clear and consistent explanation will.


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2

Below, yo! are gi"en a tr!e or false statement and as#ed a follow !p

$!estion.

%. .4 pts/False statement:&onnecti"ity is e$!i"alent to a point'to'point

physical lin#.

n: xplain how the notion of *connecti"ity+ differs from an e$!i"alent of

a *physical lin#.+

&onnecti"ity refers to two hosts eing ale to comm!nicate. S!chcomm!nication co!ld e estalished "ia a reliale transport protocol

which can !se !nderlying layers to reach remote hosts (those that are

not directly connected y a physical lin#). Physical lin# has well'

defined performance, !t with connecti"ity performance is defined on

a pac#et y pac#et asis.

-efinition of connecti"ity and physical lin# ' /pts

0ny one alone ' 1pts

-ifference is clearly artic!lated ' 2pts


3/10


3

1 "6 pts'False statement:&irc!it'switching exploits statistical m!ltiplexing and henceneeds to tac#le staility iss!es.

n: xplain:

a) 3hy circ!it switching -4S N4T exploit statistical m!ltiplexing, and

) 3hy do statistically m!ltiplexed systems (li#e pac#et'switched systems) need to tac#le

staility iss!es !nli#e circ!it'switched systems.

a) &irc!it switching in"ol"es reser"ing some fixed andwidth when the circ!it isset!p. B!t statistical m!ltiplexing means sharing a lin# among "ario!sconnections depending on the demands posed y the connections. Sincecirc!it switching does not care if indi"id!al connections offer any demand and

reser"es andwidth, it is clear that it does not !se statistical m!ltiplexing.5/pts6

) The statistically m!ltiplexed systems pro"ide gains y trading off delays and$!e!ing of pac#ets. 7f the system does not estimate the load of theconstit!ent connections properly, there is a possiility that an excess trafficmight e admitted. S!ch a sit!ation can ca!se !no!nded delay and $!e!ing

leading to system instaility.5/pts6


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4

/. "6 pts'True statement:8TTP !ses T&P. -NS !ses 9-P

n: a) -isc!ss why 8TTP -NS need T&P and 9-P respecti"ely.

) 3hat co!ld happen in -NS if its 9-P'ased re$!est or response

pac#et is lost;

a) -NS is in"o#ed prior to estalishing T&P connections andhence response time is important. 7t cannot tolerate largelatencies d!e to T&P connection set!p. 0lso 8TTPtypically transfers larger amo!nt of data, and needs areliale mechanism. -NS re$!ests on the other hand, are

short pac#ets which gi"e the 7P address for a name andproaly some a!thoritati"e name ser"er addresses. Sothe reliale protocol semantics of T&P wo!ld e costly inthe case of -NS. 5/pts6 partial is 1pts

) 7f -NS re$!est pac#ets are lost the resol"er wo!ld intimatethat there is a temporary fail!re in name resol!tion or itco!ld stall witho!t any reply. 7f a retransmit mechanism haseen !ilt into it, it co!ld proaly retry after a timeo!t.5/pts6 partial is 1pts


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5

2. (< pts) False statement:The chec#s!m (as 9-P wo!ld comp!te) of

the three n!mers:

=%%%=%%==%%%=%%=, ==%=%%%=%=%=%%%=, %====%%%====%=%% is

==%%=%=%==%%=%=%

n: 3hat is the correct chec#s!m ; Show how yo! otained yo!r

answer. &orrect answer >


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6

@. "6 pts'False statement:&!m!lati"e 0&As, Selecti"e 0&As and N0As are alle$!i"alent.

n: -isc!ss 1 pros and 1 cons 0&8 of &!m!lati"e 0&As, Selecti"e ac#s andN0As.

a) &!m!lati"e 0&As, pros:

a) ewer 0&As can e sent for the same n!mer of pac#ets, so there can e a sa"ing in re"erse channelandwidth.

) Sender window can mo"e faster since 7t does not ha"e to wait for all 0&As

) &!m!lati"e 0&As cons:

a) -oes not help in red!cing retransmissions. 0ltho!gh the recei"er might ha"e later pac#ets, it wo!ld 0&Aan earler one if an intermediate pac#et is lost.

) &an ca!se !rsty transmission, since the window can increase s!ddenly d!e to a c!m!lati"e 0&A.

c) Selecti"e 0&As pros:

a) 7ndicates the pac#ets that need to e retransmitted explicitly and a"oids !nnecessary retransmission.) ewer 0&As sent

d) Selecti"e 0&As cons:

a) Se$!ence n!mer prolems > if se$!ence n!mers wrap there is no way to find o!t if the 0&A is for anolder session.

) 8igher !ffer re$!irements since o!t'of'order pac#ets are not discarded.

e) N0A pros:

a) 7ndicates only the pac#ets that were not recei"ed or corr!pted and hence pre"ents !nnecessary

retransmissions.) ewer *0&A+s, no pac#ets sent to confirm a proper receipt, hence andwidth sa"ed.

f) N0A cons:

a) 7f a N0A for a pac#et is lost, that pac#et might ne"er e reco"ered.

) 8igher !ffer re$!irements since o!t'of'order pac#ets need to e processed.

C pt for each point in *pro+ or *con+


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7

.10 pts/&onsider a % Dps 30N channel with %= msec propagationdelay. -ata pac#ets are %=== its long while 0&AEN0A pac#ets arenegligile in length. 3indow siFe (N) G @ pac#ets, and there are %=pac#ets to e transmitted. 0ss!me a Selecti"e ?epeat 0? protocol

with a new addition: N0As are sent for pac#ets detected lost at therecei"er.

a) 0ss!me no errors and no lost pac#ets or 0&AsEN0As. 8ow m!chtime is re$!ired to complete the transfer of the %= p#ts and recei"ethe final ac#.

) Now ass!me e"ery Hthpac#et which crosses the forward channel is

lost. 0&AsEN0As are not lost or corr!pted. 8ow m!ch time is re$!iredto complete the transfer of the %= p#ts and recei"e the final ac#.

a) irst pac#et of first window recei"es an 0&A at 1%ms. So first pac#etof 1ndwindow recei"es an 0&A at 21ms and the last pac#et recei"es

an 0&A at 21I2 G 2 so 2


8/10


8

Tota# 46 "Timeout '

T2e timeout s2ou#& 3e reater t2an 51ms


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9

.7 pts/T&P ?TT estimation and Timeo!t Setting

0ss!me a T&P flow has samples of ?TT: (%, /, 1) expressed in !nits of seconds.3hat is the 0"erage ?TT, -e"iation and "al!e of Timeo!t calc!lated yT&P when it recei"es all these samples; 0ss!me that the initial "al!e of

0"erage ?TT is /s, initial "al!e of -e"iation is =. 3D0 parameter is =.%.-iff G ?TT ' S?TT

S?TT5n6 G S?TT5n'%6 I %E%= K diff

-e"5n6 G -e"5n'%6 I %E%= K (LdiffL > de"5n'%6)

0ll right ' Jpts, correct proced!re ' @pts, reasonale try ' /pts

7teration ?TT -iff S?TT -e"

= ' ' / =

% % '1 1.M =.1

1 / =.1 1.M1 =.1

/ 1 '=.M1 1.J/M =.1


10/10

Shivkumar KalyanaramanRensselaer Polytechnic Institute1

7 .! pts/a) 3hy is 9-P necessary when 7P already pro"ides a

connectionless deli"ery ser"ice ;

) Oi"e an example !sing 9-P soc#ets etween the same pair of

client and ser"er machines. 7n yo!r example three of the fo!r fields(7P address%, port%, 7P address1, port1) sho!ld e the same, !t

one field sho!ld e different. xplain how m!ltiplexing de'

m!ltiplexing is correctly done in this case.

a) 7P pro"ides protocol m!ltiplexing, while 9-P pro"ides port(application) m!ltiplexing. 7f the application were to recei"e all 7P

pac#ets it wo!ld ha"e to sort o!t pac#ets which elong to it which

can e oth !ndesirale and time cons!ming.

) &onsider (client, port%, ser"er, port1) and (client, port/, ser"er,

port1). There are 1 clients exchanging 9-P pac#ets with a ser"erport. The fact that the client side port n!mers are different is

s!fficient for dem!ltiplexing the pac#ets. 1 pts for a) and / pts for

).

ccn2001 quiz1 solutions

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