CS 779/879
Design of Network Protocols
Spring 2011
Second Exam
Time 2 & 1/2 hours
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All questions are of equal weights.

Question 1:

A. Sometimes we may use read/write with a UDP socket. Give an example.

B. Sometimes we have to use send/recv with a TCP socket. Give an example.

Question 2:

Suppose we want to implement a general chat server (GCS) that allows the user to use the following types of chat clients:

TCP, UDP (Unicast or Multicast) and SCTP (Stream or Sequence Packets) each can use either IPV4 or IPV6.

What is the maximum possible number of different types of chat clients can be used?

What is the minimum possible number of server sockets needed to implement the GCS?

Explain briefly how the server is going to switch the chat messages among all participating clients.

Question 3:

The following are several possible implementations of a chat client: f, t, s, n, b1 and b2.

Assuming all chatters will use the same implementation; describe which of these implementations will:

1. Perfectly perform its function. Do not explain..

2. Poorly perform its function. Explain why?

3. Fail to perform its function. Explain why?

GeneralChatClient.c
int sockfd;
int main(int argc, char **argv)
{
int pid;
struct sockaddr_in servaddr;
struct hostent *hp, *gethostbyname();
sockfd = socket(AF_INET, SOCK_STREAM, 0);
bzero(&servaddr, sizeof(servaddr));
hp = gethostbyname(argv[1]);
bcopy(hp->h_addr, &(servaddr.sin_addr.s_addr), hp->h_length);
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(atoi(argv[2]));
if (connect(sockfd,(SA*)&servaddr,sizeof(servaddr))<0) exit(-1);
if (strcmp(argv[3], "f") == 0) Chat_fork();
else if (strcmp(argv[3], "t") == 0) Chat_thread();
else if (strcmp(argv[3], "s") == 0) Chat_select();
else if (strcmp(argv[3], "n") == 0) Chat_nonblock();
else if (strcmp(argv[3], "b1") == 0) Chat_block1();
else if (strcmp(argv[3], "b2") == 0) Chat_block2();
}
int Chat_fork()
{
int pid = fork();
if (pid == 0) {
HandleSocket();
kill(getppid(), 9);
} else {
HandleStdin();
kill(pid, 9);
}
}
int Chat_thread()
{
pthread_t tid1;
pthread_t tid2;
pthread_create(&tid1, NULL, &HandleSocket, NULL);
pthread_create(&tid1, NULL, &HandleStdin, NULL);
pthread_join(tid1, NULL);
pthread_join(tid2, NULL);
}

int Chat_select()
{
fd_set readset;
int nread;
char buffer[512];
for (;;) {
FD_SET(sockfd, &readset);
FD_SET(0, &readset);
select(4, &readset, NULL, NULL, NULL);
if (FD_ISSET(sockfd, &readset)) {
nread = read(sockfd, buffer, 512);
if (nread <= 0) return (0);
write(1, buffer, nread);
}
if (FD_ISSET(0, &readset)) {
nread = read(0, buffer, 512);
if (nread <= 0) return (0);
write(sockfd, buffer, nread);
}
}
}
int Chat_nonblock()
{
int nread, valu;

char buffer[512];
val = fcntl(sockfd, F_GETFL, 0);
fcntl(sockfd, F_SETFL, val | O_NONBLOCK);
val = fcntl(0, F_GETFL, 0);
fcntl(0, F_SETFL, val | O_NONBLOCK);
for (;;) {
nread = read(sockfd, buffer, 512);
if (nread == 0) return (0);
else if (nread > 0) write(1, buffer, nread);
nread = read(0, buffer, 512);
if (nread == 0) return (0);
else if (nread > 0) write(sockfd, buffer, nread);
usleep(100000);
}
}
int Chat_block1()
{
int nread;
char buffer[512];
for (;;) {
nread = read(0, buffer, 512);
if (nread == 0) return (0);
else if (nread > 0) write(sockfd, buffer, nread);
nread = read(sockfd, buffer, 512);
if (nread == 0) return (0);
else if (nread > 0) write(1, buffer, nread);
}
}

int Chat_block2()
{
int nread;
char buffer[512];
for (;;) {
nread = read(sockfd, buffer, 512);
if (nread == 0) return (0);
else if (nread > 0) write(1, buffer, nread);
nread = read(0, buffer, 512);
if (nread == 0) return (0);
else if (nread > 0) write(sockfd, buffer, nread);
}
}
int HandleSocket()
{
int nread;
char buffer[512];
for (;;) {
nread = read(sockfd, buffer, 512);
if (nread == 0) return (0);
write(1, buffer, nread);
}
}
int HandleStdin()
{
int nread;
char buffer[512];
for (;;) {
nread = read(0, buffer, 512);
if (nread == 0) return (0);
write(sockfd, buffer, nread);
}
}

Question 4:

The following is a ChatClient code.

Modify this code to send and receive Out-of-Band data as described below:

§ Any message M typed by the user that starts with the specail character ‘^’ is sent to the server as Out-of-Band data.

§ Any message M received from the server as Out-of-Band data is displayed to the user preceeded with the special character ‘!’

The client may use this mechanism to send and receive non-chat information with the Chat Server.

For example, the client may use it to request the Server to send back the list of current chat participants.

ChatClient.c

int sockfd;

int
main(int argc, char **argv)
{

struct sockaddr_in servaddr;
struct hostent *hp, *gethostbyname();
sockfd = socket(AF_INET, SOCK_STREAM, 0);
bzero(&servaddr, sizeof(servaddr));
hp = gethostbyname(argv[1]);
bcopy(hp->h_addr, &(servaddr.sin_addr.s_addr), hp->h_length);
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(atoi(argv[2]));
if(connect(sockfd,(SA*)&servaddr,sizeof(servaddr))<0)exit(-1);



Chat_thread();
}

int Chat_thread()
{
pthread_t tid1;
pthread_t tid2;
pthread_create(&tid1, NULL, &HandleSocket, NULL);
pthread_create(&tid1, NULL, &HandleStdin, NULL);
pthread_join(tid1, NULL);
pthread_join(tid2, NULL);
}
int HandleStdin()
{

int nread;
char buffer[512];
for (;;) {
nread = read(0, buffer, 512);
if (nread == 0) return(0);

write(sockfd, buffer, nread);

}
}
int HandleSocket()
{

int nread;
char buffer[512];
for (;;) {
nread = read(sockfd, buffer, 512);
if (nread == 0) return(0);

write(1, buffer, nread);
}
}

Question 5:

The following is the code for a pre-threaded echo server.

Modify this code so that when all the pre-threads are busy, the program will create temporary on-demand threads.

PreEchoServer.c

int len, listenfd;
struct sockaddr_in servaddr, cliaddr;
int NThreads;
int Available;
int PortNumber;

int
main(int argc, char *argv[])
{

NThreads = atoi(argv[1]);
PortNumber = atoi(argv[2]);

listenfd = socket(AF_INET, SOCK_STREAM, 0);

bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(PortNumber);

if (bind(listenfd,(SA*)&servaddr,sizeof(servaddr))<0)exit(-1);

listen(listenfd, 0);

Available = NThreads;
for (i = 1; i <= NThreads; i++) {
pthread_create(NULL, NULL, &EchoToClient, NULL);
}

pause();
}

void *EchoToClient()
{

int nread;
char buffer[512];
int connfd;

for (;;) {
len = sizeof(cliaddr);
connfd = accept(listenfd, (SA *) & cliaddr, &len);
--Available;
for (;;) {
nread = read(connfd, buffer, sizeof(buffer));
if (nread == 0) {
++Available;
break;
}
write(connfd, buffer, nread);
}
}
}