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1 Exam #1 for Compuer Neworks (CIS 6930) Fall 2003 >>> SOLUTIONS <<< Welcome o Exam #1 in Compuer Neworks (CIS 6930). You have 75 minues. Read each oblem carefully. There are eigh required oblems (each worh 12.5 poins) and one exra credi oblem worh 5 poins. You may have wih you a calculaor, pencils, eraser, blank paper, and one 8.5 x 11 inch formula shee. On his formula shee you may have anyhing you wan (definiions, formulas, ec.) handwrien by you. You may use boh sides. Compuer generaed ex, phoocopies, and scans are no allowed on his shee. Please submi your formula shee wih your exam. No sharing of calculaors. Sar each numbered oblem on a new shee of paper and do no wrie on he back of he shees. Submi everyhing in oblem order. Good luck and be sure o show your work. Problem #1 a) Cenral o he sudy of neworks is he layered model. In his class we have a leas wo layered models ha we work wih. They are he OSI model and he Kurose/Ross model ha srucures our ex book. Skech hese wo models. For he OSI model, describe he funcion of he layers wih one senence for each layer. OSI is Applicaions, Presenaion, Session, Transpor, Nework, Daa Link, and Phyiscal (op o boom). Kurose/Ross is Applicaion, Transpor, Nework, Daa Link, and Physical (basically, he TCP/IP sack). For he OSI model: Applicaion ovides access o user applicaions Presenaion ovides daa independence Session manages end-o-end connecions Transpor ovides reliable end-o-end daa ranspor Nework mainains poin-o-poin connecions Daa Link ovide reliable poin-o-poin daa ranspor Physical ransmission of bi sream b) Wha are he four causes, or componens, of delay in a packe swiched nework? Wha can be done o reduce each of hese componens? The delay componens are Processing, queueing, ransmission, and opagaion. Processing delay can be reduced by faser CPUs, queueing and ransmission delay by faser links, and opagaion delay only by moving conen closer o is user. Problem #2 a) Describe he operaion of a Web server wih high-level pseudocode. I is sufficien o show operaion for only he case of HTTP GET. This sudy resource was Main ocess Lisen for a connecion on por 80 Accep a connecion and spin-off a hread or ocess o handle he connecion Handle GET ocess Parse he HTTP header for he GET objec name Open he file for he objec If file no found, send an HTTP 404 message If file found, send an HTTP 200 message followed by he file Close he connecion Exi he hread or ocess

2 b) Aached is a sockes ogram. The ogram compiles successfully, bu here are several errors in i. Idenify hese errors. Use he line numbers o help you describe where he errors are. The ogram is missing hon*() conversions in lines 31 and 32. A bind() is missing following line 32. Also missing afer line 32 is a lisen(). In line 40 hese should be a +1 o he sring lengh (o include he end of sring characer needed in he receiver). Finally, a he end of he ogram a close of he connec_s socke is missing. Problem #3 Consider a channel ha connecs a sender and a receiver. Assume ha he channel never loses packes or ges hem ou of order, bu i may corrup hem. Assume ha he sender always sends unique packes and ha he receiver does no care if i receives duplicae packes (e.g., i may have some way o deermine if a packe is a duplicae). Design a reliable daa ransfer oocol for his sysem. You may use FSMs o describe your oocol. FSM for he sender Wai for call om above Wai for ACK or NACK (a) > (c) (b) < < For ransiion (a) he even is rd_send(daa). The acion is sendpk=make(daa, checksum), ud_send(sendpk). For ransiion (b) he even is rd_rcv(recvpk) && (iscorreup(rcvpk) isnack(rcvpk)). The acion is ud_send(sendpk). For ransiion (c) he even is rd_rcv(recvpk) && isack(rcvpk). The acionis nohing (null). FSM for he receiver Wai for call om above (a) < ) (b) < This sudy resource was For ransiion (a) he even is rd_rcv(rcvpk) && corrup(rcvpk). The acion is ud_send(nack). For ransiion (b) he even is rd_rcv(rcvpk) && nocorrup(rcvpk). The exrac(rcvpk, daa), deliver_daa(daa), ud_send(ack). For ransiion (b) he even is rd_rcv(recvpk) && (iscorreup(rcvpk) isnack(rcvpk)). The acion is ud_send(sendpk).

3 Problem #4 Consider a link connecing a sender and receiver. The sender always has packes o send of lengh L byes. GBN ARQ flow and error conrol is used. The link has a bi error rae of p. Wrie he exession for U in erms of,, N, and p for he case of window size is greaer han pipe size (i.e., N muliplied by is greaer han wo imes plus ). ( 1 p) U = for p = obabiliy of packe error. If p = obabiliy of bi error, we haveu + 2 p which simplifies o: U p) 8 L = 8 L (1 p). ( ) ( 8 L 1 p ) ) ( p) ( L) ( ) = 8 + Problem #5 Carefully explain how TCP compues is rery ime-ou value. TCP uses he following algorihm (wih parameers g, h, and f) o esimae RTT and he possible deviaion o RTT. The deviaion is added o he esimaed RTT. Exponenial smoohing is used here. SRTT(k+1) = (1 g)*srtt(k) + g * RTT(k+1) SERR(k+1) = RTT(k+1) SRTT(k) SDEV(k+1) = (1 h)*sdev(k)+h*abs(serr(k+1)) RTO(k+1) = SRTT(k+1) + f*sdev(k+1) On a reransmi use exponenial backoff of RTO (i.e., do no execue he above). Problem #6 Consider five nodes (call hem x 1, x 2,, x 5 ) ha reques bandwidh on a common channel. The nodes reques 2, 5, 5, 7, and 10 unis of bandwidh, respecively. a) Give he max-min fair bandwidh allocaion for a channel wih 25 unis of bandwidh. All nodes can ge wha hey reques (i.e., he allocaions are 2, 5, 7, and 10). b) Repea (a) for a channel wih 15 unis of bandwidh. In he firs round we allocae 15/5 = 3 unis of bandwidh and ge allocaions 2, 3, 3, 3, and 3. There is 1 uni lef o allocae o 4 nodes. This sudy resource was In he second round we allocae 1/4 = 0.25 unis of bandwidh and ge allocaions of 2, 2.25, 2.25, 2.25, and There is nohing lefover, so his is he final allocaion c) Repea (a) for a channel wih 10 unis of bandwidh. In he firs round we allocae 10/5 = 2 unis of bandwidh and ge allocaions 2, 2, 2, 2, and 2. There is nohing lefover, so his is also he final allocaion.

4 Problem #7 a) Wha is he key difference beween disance-vecor and link-sae rouing oocols in erms of how oocol messages are sen? In disance-vecor oocols message exchanges are beween neighbors. In link-sae oocols message exchanges are gobal (i.e., broadcas) o all nodes. b) Give he names of a disance-vecor algorihm and oocol. Give he names of a link-sae algorihm and oocol. RIP and BGP are disance-vecor oocols. OSPF is a link-sae oocol. c) Give a simple example of he coun-o-infiniy oblem. Consider hree nodes wih A conneced o B a cos 1 and B conneced o C a cos 1. The roue om A o C is hrough B a cos 2. If he link om B o C fails (now cos om B o C is infiniy) he following will happen. A he nex vecor exchange A will learn ha cos o B is infiniy and ener his in is vecor. However, B will learn ha he cos o C is 2 via A and will ener 3 as is cos o C (going hrough A as he case may be). In he nex exchange A will learn om B ha he cos o C is 3 and now ener 4 as is cos o C. In his same exchange, B will learn om A ha he cos o C is infiniy, so i will ener infiniy as is cos. And so on, unil he cos o C is infiniy for boh A and B. Problem #8 a) Wha is he purpose of hierarchical address in he Inerne (i.e., why are IP addresses hierarchical)? Explain how he hierarchical address is used in IP rouing. By having an IP address hierarchical as nework-hos, we can reduce he size of rouing ables by having rouing ables only handle forwarding om nework o nework. Only a he desinaion nework is he hos address porion used for delivering he packe o he desinaion hos. b) Assume ha hoss A and B have a TCP connecion esablished. Assume ha he wo hoss are separae by one rouer (i.e., hey are one hop apar). Why does hos A no direcly use he MAC (LAN) address of hos B when consrucing is packes o send o hos B? Hos A has no way of knowing he MAC address of hos B on he oher side of a rouer. A rouer does no pass broadcasr ARP ames. In any case, he MAC address of he rouer por connecing hos A s LAN o hos B s LAN mus be used o forward an A-o-B packe hrough he rouer. If some oher MAC address (e.g., ha of hos B) were used, rouing would no ake place. Exra Credi This sudy resource was Give he names of up o five neworking conferences. Each correc name will earn you one poin of exra credi. Trade shows and local (e.g., om your universiy only) conferences do no coun. ICC, INFOCOM, GLOBECOM, SIGCOMM, and LCN are all neworking conferences. Oher possible answers include IPCCC, IC3N, Mobicom, and even VTC is accepable. All have been discussed or menioned in some conex in he class. In any case, each of hese conferences are likely o appear in he references in any paper you pick-up.

5 Powered by TCPDF ( Program for oblem #2 1 //============================================================ file = s.c ===== 2 //= A message "server" ogram = 3 //============================================================================= 4 #include <sdio.h> // Needed for inf() 5 #include <sring.h> // Needed for memcpy() and srcpy() 6 #include <windows.h> // Needed for all Winsock suff 7 8 #define PORT_NUM 1050 // Arbirary por number for he server 9 10 void main(void) 11 { 12 WORD wversionrequesed = MAKEWORD(1,1); // Suff for WSA funcions 13 WSADATA wsadaa; // Suff for WSA funcions 14 unsigned in server_s; // Server socke descripor 15 sruc sockaddr_in server_addr; // Server Inerne address 16 unsigned in connec_s; // Connecion socke descripor 17 sruc sockaddr_in clien_addr; // Clien Inerne address 18 sruc in_addr clien_ip_addr; // Clien IP address 19 in addr_len; // Inerne address lengh 20 char ou_buf[100]; // 100-bye oupu buffer for daa 21 char in_buf[100]; // 100-bye inpu buffer for daa // Iniializes winsock 24 WSASarup(wVersionRequesed, &wsadaa); // Creae a socke 27 server_s = socke(af_inet, SOCK_STREAM, 0); // Fill-in my socke's address informaion 30 server_addr.sin_family = AF_INET; // Address family o use 31 server_addr.sin_por = PORT_NUM; // Por number o use 32 server_addr.sin_addr.s_addr = INADDR_ANY; // Lisen on any IP address // Accep a connecion. 35 addr_len = sizeof(clien_addr); 36 connec_s = accep(server_s, (sruc sockaddr *)&clien_addr, &addr_len); // Send o he clien 39 srcpy(ou_buf, "Tes message om server o clien"); 40 send(connec_s, ou_buf, srlen(ou_buf), 0); // Receive om he clien 43 recv(connec_s, in_buf, sizeof(in_buf), 0); 44 inf("received om clien... daa = '%s' \n", in_buf); // Close and clean-up 47 closesocke(server_s); 48 WSACleanup(); 49 } This sudy resource was