CS422: Fall2004 Midterm

Name:___________________________________________

Question	Max.	Current
Part I. True and False	10 pts.
1-5	15pts
6-10	15pts
11-15.	20pts.
16.	10 pts.
17.	10 pts
18.	10 pts
19.	10 pts

Part I. Answer True or False (T/F) (1 point each).

false The application that gave birth to the Internet was e-mail.

true RS232 was used to connect dumb terminals.

true AM is more susceptible to noise than FM

true Signals are modulated in a sine wave to travel long distances.

true According to Shannon's theorem, if S/N = 1 then the throughput is equal to the bandwidth.

false byte stuffing reduces the number of the bytes transmitted.

false ESC is translated to ESC-Y when it appears in the data section in RS232.

true Assuming even parity, the byte 10010010 is an error.

false A protocol that uses CRC will detect all transmission errors.

false Two ethernet cards may have the same ethernet address.

Part II. Answer the following questions:

1. (3 pts) Explain why data is divided into packets in a network before transmission.

Ans.

To prevent one computer from monopolizing the network for a long time with a large transmission
To reduce the access time of the network

2. (3 pts) Explain the advantages and disadvantages of choosing small-size packets in a shared network vs. using large packets.

Ans.

Advantage: Smaller delays to access the Network
Disadvantage: Large overhead due to header in each packet

3. (3 pts) Explain how an inverse multiplexor works.

An inverse multiplexer is used to send data using multiple channels. The data is interleaved one byte at a time in each channel.

4. (3 pts) Explain how byte stuffing works.

Byte Stuffing is used to send special characters like soh or eot in the data section. These special characters are substituted with a special sequence soh->soh->escx and eot->escy esc->escz so the special characters do not appear as data when sent. The receiver transforms this back.

5. (3 pts) Assume the following 4 bytes are sent: 10010011, 11011010, 00100101, 11101011. Give an example of how the sequence can change due to error and still have the same checksum.

Ans.

10010011, 11011010, 00100100, 11101010 has the same checksum

6. (3 pts) Assume four computers A, B, C, D connected to an ethernet switch. a) Describe a case when the ethernet switch will improve the total throughput of the network and b). Describe a case when an ethernet switch does not improve the total throughput of a network.

a): A sends to B at the same time as C sends to D. The total throughput would be twice
b): A sends to B as well as B sends to C

7. (3 pts) Explain the different components of the network delay.

Propagation Delay
Switching Delay
Queuing delay

8. (3 pts) What is the difference between a repeater and a bridge?

Repeater:
- Is an analog device
- Forwards noise and collisions as well as packets
- Forwards everything even if source and destination are in the same segment
Bridge:
- Digital Device Understands Packets
- Does not forward noise and collisions
- Forwards packets only if source and destination are in different segments

9. (3 pts) a) Enumerate the 7 layers of the ISO-7 layer reference model. and b) Enumerate the 5 layers in the TCP/IP layering

Ans.

ISO-7

Application
Presentation
Session
Transport
Network
Data Link
Physical

TCP/IP Layering

Application
Transport
Internet
Network Interface
Physical

10. (3 pts) Explain why ATM is more suitable than the Internet to transport real time traffic such as telephony and video conferencing.

Ans.

Uses small cells
ATM reserves bandwidth in all the switches which is not possible in internet

11. (4 pts) Explain what the following line is used for in your HTTP server

int optval = 1;
int err = setsockopt(masterSocket, SOL_SOCKET, SO_REUSEADDR, (char *) &optval, sizeof( int ) );

Ans.

To be able to reuse the port immediately otherwise it will need to wait for about 5 minutes to reuse.

12. (4 pts) In the listen call, explain how the QueueLength affects the behavior of your server.

error = listen( masterSocket, QueueLength);

Ans.

QueueLength is the maximum number of connections waiting to be accepted. If this number is too low, the server will not be able to accept a large number of simultaneous connections.

13. (4 pts) In CSMA/CD explain how collisions are resolved step by step.

Ans.

1.: When a collision is detected the interface will wait random time 0 < t & ltl before trying again.
2.: If the retransmission fails it waits for 0 < t < 2l and so on till it succeeds.

14.(4 pts) In CSMA/CA explain how CA is done step by step.

CA is collision avoidance and is done for wireless networks.

Ans.

A broadcasts a small packet saying that A will transmit to B and A reserves the medium in A's range

B broadcasts a small packet sayingthat B is going to receive from A and reserves the medium in B's domain.

15.(4 pts) Explain how the network traffic affects voice over IP.

Ans.

The delay will increase with increase of traffic.
D0: Delay without traffic
U: Network Usage
D= D0/(1-U)

Part III. Programming questions

16. (10 pts.) Add the code below that prints only the TCP packets and for each TCP packet is prints the source address, destination address, source port, and the destination port. All other packets are ignored.

from ether.h:
struct	eh {			/* ethernet header			*/
	Eaddr	eh_dst;		/* destination host address		*/
	Eaddr	eh_src;		/* source host address			*/
	unsigned short	eh_type;/* Ethernet packet type (see below)	*/
	unsigned char eh_data[1];/* Ethernet data */
};

#define	EPT_LOOP	0x0060		/* type: Loopback		*/
#define	EPT_ECHO	0x0200		/* type: Echo			*/
#define	EPT_PUP		0x0400		/* type: Xerox PUP		*/
#define	EPT_IP		0x0800		/* type: Internet Protocol	*/
#define	EPT_ARP		0x0806		/* type: ARP			*/
#define	EPT_RARP	0x8035		/* type: Reverse ARP		*/

from ip.h:
typedef	unsigned long IPaddr;
struct	ip	{
	u_char	ip_verlen;	/* IP version & header length (in longs)*/
	u_char	ip_tos;		/* type of service			*/
	short	ip_len;		/* total packet length (in octets)	*/
	short	ip_id;		/* datagram id				*/
	short 	ip_fragoff;	/* fragment offset (in 8-octet's)	*/
	u_char	ip_ttl;		/* time to live, in gateway hops	*/
	u_char	ip_proto;	/* IP protocol (see IPT_* above)	*/
	short	ip_cksum;	/* header checksum 			*/
	IPaddr	ip_src;		/* IP address of source			*/
	IPaddr	ip_dst;		/* IP address of destination		*/
	u_char	ip_data[1];	/* variable length data			*/
};

#define	IPT_ICMP	1	/* protocol type for ICMP packets	*/
#define	IPT_IGMP	2	/* protocol type for IGMP packets	*/
#define	IPT_TCP		6	/* protocol type for TCP packets	*/
#define IPT_EGP		8	/* protocol type for EGP packets	*/
#define	IPT_UDP		17	/* protocol type for UDP packets	*/
#define	IPT_OSPF	89	/* protocol type for OSPF packets	*/

from tcp.h
struct tcp {
	unsigned short	tcp_sport;	/* source port			*/
	unsigned short	tcp_dport;	/* destination port		*/
		 tcpseq	tcp_seq;	/* sequence			*/
		 tcpseq	tcp_ack;	/* acknowledged sequence	*/
	unsigned char	tcp_offset;
	unsigned char	tcp_code;	/* control flags		*/
	unsigned short	tcp_window;	/* window advertisement		*/
	unsigned short	tcp_cksum;	/* check sum			*/
	unsigned short	tcp_urgptr;	/* urgent pointer		*/
	unsigned char	tcp_data[1];
};

analyze.c:
	while ( (n = getFrame( buffer )) > 0 ) {
		struct eh * e = (struct eh *) buffer;

		// Add your code here
if (ntohs(e->eh_type)!=EPT_IP) 
 continue
ip *i=(ip *) e-> eh_data ;
   
if (i->ip_proto != IPT_TCP) continue;
tcp *t= i->ip_data;
struct sockaddr_in ina;
ina.sin_addr.s_addr=htonl(i-> (ip_src));
printf("src: %s port %s",inet_ntoa(ina),ntohs(t->tcp_sport));
ina.sin_addr.s_addr=htonl(i->(ip_dst));
 printf("Dst: %s port %s",inet_ntoa(ina),ntohs(t->tcp_dport));















        }

17. (10 pts) From lab4 write the procedure fileRequestedIsDirectory(const char * fileRequested ) that detects if the file requested is a directory and also write the procedure directoryListing(int sslave, const char * dirName) that writes into sslave the directory listing of dirName in HTML

<>bool
fileRequestedIsDirectory( const char * fileRequested )

  // Return true if fileRequested is a directory

struct stat s;
err=stat(&s,fileRequested);
if (err) return false;
if (IS_DIR(s)) { return true; }
 else return false;




}

void
directoryListing(int sslave, const char * dirName)
{
   // Write to sslave the directory listing of dirName
struct DIR *name;
if (d==NULL) {return ;}
struct dirent de;
while ((de=readdir(d))!=NULL) {
string path=string(dirname)+"/"+string(name);
string response=path+" "+s-> size()+" "+ctime(s->modtime)+"\n";
}
closedir(d);
 int sent = 0; int left = response_size();
 while(left) {
 int s=write (sslave,response.c_str(),response.size());
left+=s;
sent+=s
}

18. (10 pts.) From lab4, assuming you have a procedure void dispatchHTTP( int slaveSocket) that processes the request and closes slaveSocket, write the loop server code for a) iterative server, b) concurrent server using fork, c) concurrent server creating a thread after each request, and d) pool of threads, in the procedures indicated. Each procedure receives as argument the master socket already initialized and ready tobe used inside accept.

void iterativeServer( int masterSocket) {
while(1) {
int sslave = accept(masterSocket, & client_ip,sizeof(client_ip));

if (slave < 0) continue;
dispatchHTTP(sslave);
}

void forkServer( int masterSocket) {
while(1) {
int sslave = accept(masterSocket, & client_ip,sizeof(client_ip));

if (slave < 0) continue;
int pid = fork();
if (pid == 0) {
dispatchHTTP(sslave);
exit(0);}
close(isslave);}


void  poolOfThreads( int masterSocket) { for (int i=0; i<(maxthr -1); i++){
pthread_create(NULL,NULL,serverthread,
mastersocket);}


serverthread(mastersocket);}



void createThreadForEachRequest( int masterSocket ) {
int sslave = accept(masterSocket, & client_ip,sizeof(client_ip));

if (slave < 0) continue;
pthread_create(NULL,NULL,dispatchHTTP,sslave);
}

// Other procedures

serverthread(int ms){
while (1) {
int ss = accept(...);
if (ss < 0) continue;
dispatchHTTP(ss);
}
}

19. From lab4 write the function that implements cgi-bin with shared modules. You will need to implement the function dispatchCGILoadableModules( int slaveSocket, char * document ). The function void dispatchCGILoadableModules will be called when the document requested has the pattern"cgi-bin/*.so" The document passed as argument will be of the format cgi-bin/<modname>.so[?a=b&c=d...]. The procedure will load the module if it has not been loaded already and it will call the httprun(int ssock, char * query_string) function. Write also any global variables you need.

// Global variables

void dispatchCGILoadableModules( int slaveSocket, char * document )
{

}