Assignment 2 -Solution

Advanced computer networks
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  ECS 152A Computer NetworksAssignment IIDue: 10/26/07 by pm! Q 1)a)Why do we need an unreliable UDP service when we have a reliable TCP service?[2] b)What advantae does the !elective e#eat #rotocol have over the $o %ac& ' #rotocol? [2]c)(#lain the conce#t o* *ast retrans+it in the TCP #rotocol, [-] a)  .* we incorrectly esti+ate the ti+e out interval/ it can cause #roble+s in our #ac&ettrans+ission, .* the ti+e out interval 0 T) is too s+all/ then ti+e outs will occur very*reuently and the sender will end u# retrans+ittin a lot o* #ac&ets unnecessarily, Thiswill add to the conestion in the networ&, [1].* the T is lare/ then in the event o* #ac&et loss/ the sender will wait *or a lon ti+e be*ore retrans+ittin/ thus slowin down the e**ective rate, [1]  b) .* in a #articular connection/ the window si3e and the bandwidth4delay #roduct are both lare/ +any #ac&ets can be in the #i#eline, 5 sinle #ac&et error can thus cause $%'to retrans+it a lare nu+ber o* #ac&ets/ +any unnecessarily, 5s the #robability o* channel errors increases/ the #i#eline can beco+e *illed with these unnecessaryretrans+issions, !elective re#eat avoids unnecessary retrans+issions by havin thesender retrans+it only those #ac&ets that were received in error at the receiver, [2] c) .* the ti+e out interval is too lon/ then the sender will wait too lon be*oreretrans+ittin a #ac&et causin larer delays, !ince a sender sends a lare nu+ber o* se+ents bac& to bac& and one se+ent ets lost/ there will be +any bac& to bac& du#licate 5C6s, .* the sender receives three du#licate 5C6s *or the sa+e data #ac&et be*ore its ti+er e#ires/ it will send the #ac&ets aain be*ore the ti+e out, This is *astretrans+it,Q 2)a)Draw the 7!8 *or the receiver side o* #rotocol rdt -,9, [-]  a) Same as FSM for rdt 2.2 receiver in the text book.  b)$ive a trace o* the o#eration o* #rotocol rdt -,9 when data #ac&ets andac&nowlede+ent #ac&ets are arbled, 0!i+ilar to *i, -,1: in the tet boo&) [-]c) ;999 6+ 1999 6+ .n the iven setu#/ 5 sends +essaes to % and % *orwards the+ to C/ both usin sto#and wait #rotocol, Data rate between 5 and % is 199 6b#s, Pac&et lenth is 1999 bits,The #ro#aation delay is < +icroseconds = 6+, The 5C6 #ac&ets are neliible inlenth, There are no errors and losses, What would be the data rate between % and C such that % is not overloaded 0arrivalrate at % is less than rate at which % can send the #ac&ets? [;] B receives a packet every 30 ms. So, B must send a packet in ess than 30 ms to not be overoaded. !F is the transmission time, then # $ % 30  % 2$ &' (000* % 2$ &' * ' +0 bps-+/herefore, the rate from B to  shoud at east be +0 bps. Q -) !tate whether the *ollowin are true or *alse, $ive reason i* *alse, [-]a)TCP uses cu+ulative ac&nowlede+ent, b)>ost 5 sends >ost % a *ile over a TCP connection, >ost % has no data to send to>ost 5, >ost % will not send ac&nowlede+ents to >ost 5 because >ost % cannot #iybac& the ac&nowlede+ents on data,c).n selective re#eat/ the receiver will not ac&nowlede a correctly received #ac&eti* it is out o* order, a) /rueb) Fase. 5%C  c) Fase.


Jul 23, 2017
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