CS 779/879
Design of Network Protocols
Spring 2012
Second Exam
Time 2 & 1/2 hours
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All questions are of equal weights.

Question 1:

A. Under what condition we can use read/write with a UDP socket. .

B. Under what condition we have to use send/recv with a TCP socket.

Question 2:

A. In assignment 3, the server was implemented using: fork, thread and select.

The following is an implementation using Alarm. Modify this code to implement the server using non-blocking IO.

_{Show only necessary changes.}

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

_{

_{strcpy(GroupIP, argv[1]);}

_{strcpy(GroupPort, argv[2]);}

_{TCPServer.sin_family = AF_INET;}

_{TCPServer.sin_addr.s_addr = htonl(INADDR_ANY);}

_{TCPServer.sin_port = htons(atoi(GroupPort));}

_{GroupServer.sin_family = AF_INET;}

_{GroupServer.sin_addr.s_addr = htonl(inet_addr(GroupIP));}

_{GroupServer.sin_port = htons(atoi(GroupPort));}

_{UDPSocket = socket(AF_INET, SOCK_DGRAM, 0);}

_{bind(UDPSocket, (SA *) & GroupServer, sizeof(GroupServer));}

_{joinGroup(UDPSocket, (char *) GroupIP);}

_{setLoopback(UDPSocket, 1);}

_{for (i = 0; i <= MAXTCP; i++) connfd[i] = -1;}

_{TCPSocket = socket(AF_INET, SOCK_STREAM, 0);}

_{bind(TCPSocket, (SA *) & TCPServer, sizeof(TCPServer));}

_{listen(TCPSocket, 4);}

_{signal(SIGALRM, handle_ALARM);}

_{ualarm(1000);}

_{for (;;) {}

_handleUDP();

_{for (i = 0; i <= MAXTCP; i++) {}

_{if (connfd[i] == -1) {}

_{connfd[i] = accept(TCPSocket, 0, 0);}

_break;

_}

_}

_{for (i = 0; i <= MAXTCP; i++) {}

_{if ((connfd[i]) == -1) continue;}

_{handleTCP(i);}

_}

_}

_}

_{void handleUDP()}

_{

_{recvfrom(UDPSocket, buffer, sizeof(buffer), 0, (SA *)&from, &len));}

_{for (i = 0; i <= MAXTCP; i++)}

_{if (connfd[i] != -1) write(connfd[i], buffer, nread);}

_}

_{void handleTCP(int index)}

_{

_{int myIndex = index;}

_{nread = read(connfd[myIndex], buffer, sizeof(buffer));}

_{if (nread == 0) {}

_{connfd[myIndex] = -1;}

_return;

_}

_{sendto(UDPSocket,buffer,nread,0,(SA *)&GroupServer,sizeof(GroupServer));}

_}

_{void handle_ALARM(int signo)}

_{

_{signal(SIGALRM, handle_ALARM);}

_{ualarm(1000);}

_}

B.Compare the behavior of Chat1 and Chat2 as possible implementations of a chat client.

_ChatClient.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);
Chat1();// OR Chat2();}

_}

_{int nread;
char buffer[512];
int Chat1()
{
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 Chat2()
{
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);
}
}}

Question 3:

_{The following is a pre-threaded echo server that creates on-demand threads if allthe pre-threads are busy. Modify the code to use fork instead of threads. Make sure the server cleans up the <defunct> processes. Show only the needed changes.}

_{PreOnDemandEchoServerThread.c}

_{int len, listenfd, i, NTH, PortNumber;}

_{struct sockaddr_in servaddr, cliaddr;}

_{pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;}

_{int Available;}

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

_{

_{fd_set rset, allset;}

_{int connfd;}

_{if (argc != 3) {}

_{printf("Usage: %s <Number of Threads> <Port Number>\n", argv[0]);}

_exit(-1);

_}

_{NTH = 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);}

_{bind(listenfd,(SA*)&servaddr,sizeof(servaddr));}

_{listen(listenfd, 0);}

_{Available = NTH;}

_{for (i = 1; i <= NTH; i++)}

_{pthread_create(NULL, NULL, &EchoToClient, NULL);}

_{FD_ZERO(&allset);}

_{FD_SET(listenfd, &allset);}

_{for (;;) {}

_{rset = allset;}

_{select(4, &rset, NULL, NULL, NULL);}

_{pthread_mutex_lock(&mutex1);}

_{if (FD_ISSET(listenfd, &rset) && Available == 0) {}

_{len = sizeof(cliaddr);}

_{connfd = accept(listenfd, (SA *) & cliaddr, &len);}

_{pthread_create(NULL,NULL,&TempEchoToClient,(void*)connfd);}

_}

_{pthread_mutex_unlock(&mutex1);}

_}

_}

_{void *EchoToClient()}

_{

_{int nread;}

_{char buffer[512];}

_{int connfd;}

_{for (;;) {}

_{len = sizeof(cliaddr);}

_{pthread_mutex_lock(&mutex1);}

_{connfd = accept(listenfd, (SA *) & cliaddr, &len);}

_--Available;

_{pthread_mutex_unlock(&mutex1);}

_{doit(connfd);}

_{pthread_mutex_lock(&mutex1);}

_++Available;

_{pthread_mutex_unlock(&mutex1);}

_}

_}

_{void *TempEchoToClient(void *arg)}

_{

_{int connfd = (int) arg;}

_{doit(connfd);}

_{pthread_exit(0);}

_}

_{doit(int connfd)}

_{

_{int nread;}

_{char buffer[512];}

_{for (;;) {}

_{nread = read(connfd, buffer, sizeof(buffer));}

_{if (nread == 0) break;}

_{write(connfd, buffer, nread);}

_}

_}

Question 4:

_{A. In assignment #4, the YouChatC can handle the following set of sockets:}

_{{t4, t6, s4, s6, q4, q6, m4, m6, ux}}

_{Suppose we like to expand this set to include u4 and u6, where u stands for unicast udp.}

_{Briefly explain the necessary modifications to both YouChatC & YouChatS to achieve this goal.}

B. How did IPv6 manage to quadruple the IP address length while only doubling the overall header length?