1. Chapter 8 Elementary UDP Socket

2. Contents u recvfrom and sendto Function u UDP Echo Server( main, de_echo Function) u UDP Echo Client( main, de_cli Function) u Lost datagrams u Verifying Received Response u Sever not Running u Connect Function with UDP u Lack of Flow Control with UDP u Determining Outgoing Interface with UDP u TCP and UDP Echo Server Using select

3. 8.1 Introduction u connectionless u unreliable u datagram protocol u popular using DNS(the Domain Name System) NFS(the Network File System) SNMP(Simple Network Management Protocol)

4. UDP Server socket( ) bind( ) recvfrom( ) sendto( ) socket( ) sendto( ) recvfrom( ) close( ) Process request block until datagram received from a client UDP Client data(request) data(reply) Socket functions for UDP client-server

5. 8.2 recvfrom and sendto Functions #include ssize_t recvfrom(int sockfd, void *buff, size_t nbyte, int flag, struct sockaddr *from, socklen_t *addrlen); ssize_t sendto(int sockfd, const void *buff, size_t nbyte, int flag, const struct sockaddr *to, socklen_t addrlen); Both return: number of bytes read or written if OK,-1 on error

6. 8.3 UDP Echo Server: main Function #include “unp.h” int main(int argc, char **argv) { int sockfd; struck sockaddr_in servaddr,cliaddr; sockfd=Socket(AF_INET,SOCK_DGRAM,0); bzero(&servaddr,sizeof(servaddr)); servaddr.sin_fammily=AF_INET; servaddr.sin_addr.s_addr=htonl(INADDR_ANY); servaddr.sin_port=htons(SERV_PORT); bind(sockfd, (SA *) &servaddr,sizeof(servaddr)); dg_echp(sockfd, (SA *) &cliaddr,sizeof(cliaddr)); } sendto UDP client recvfrom sendto UDP server fgets fputs stdin stdout Simple echo client-server using UDP UDP echo server

7. 8.4 UDP Echo Server:dg_echo Function #include “unp.h” void dg_echo(int sockfd, SA *pcliaddr, socklen_t clilen) { int n; socklen_t len; char mesg[MAXLINE]; for( ; ; ) { len=clilen; n=Recvfrom(sockfd, mseg, MAXLINE, 0, pcliaddr, &len); sendto(sockfd, mesg, n, 0, pcliaddr, len); } dg_echo funtion: echo lines on a datagram socket.

8. 1.This function never terminates. 2.This function provides an iterative, not a concurrent server as we had with TCP connection fock connection client TCP server child server child listening server Summary of TCP client-server with two clients.

9. Socket receive buffer client server UDP datagram Summary of UDP client-server with two clients.

10. 8.5 UDP Echo client: main Function #include “unp.h” int main(int argc, char **argv) { int sockfd; struct sockaddr_in servaddr; if (argc != 2) err_quit( “usage : udpcli ”); bzero(&servaddr, sizeof(servaddr); servaddr.sin_family = AF_INET; servaddr.sin_port = htons(SERV_PORT); Inet_pton(AF_INET, argv[1], &servaddr.sin_addr); sockfd = Socket(AF_INET, SOCK_DGRAM, 0); dg_cli(stdin, sockfd, (SA *) &servaddr, sizeof(servaddr); exit(0); }

11. 8.6 UDP Echo Client: dg_cli Function #include “unp.h” void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, soklen_t servlen) { int n; char sendline[MAXLINE], recvline[MAXLINE+1]; while(Fgets(sendline, MAXLINE, fp) != NULL) { sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen); n = Recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL); recvline[n] = 0; /* null terminate */ Fputs(recvline,stdout); } dg_cli function: client processing loop

12. 8.7 Lost Datagrams If the client datagram arrives at the server but the server’s reply is lost, the client will again block forever in its call to recvfrom. The only way to prevent this is to place a timeout on the recvfrom.

13. 8.8 Verify Received Response #include “unp.h” void dg_cli(FILE *fp, int sock, const SA *pseraddr, socklen_t servlen) { int n; char sendline[MAXLINE], recvline[MAXLINE]; socklen_t len; struct sockaddr *preply_addr; preply_addr = Malloc(servlen); while(Fget(sendline, MAXLINE, fp) ! = NULL) { Sendto(sockfd,sendline, strlen(sendline), 0, pservaddr, servlen); len = servlen; n = Recvfrom(sockfdm, recvline, MAXLINE, 0, preply_addr,&len) continue

14. If(len != servlen || memcmp(pservaddr, preply_addr, len) != 0) { printf(“reply from %s (ignore)\n”, Sock_ntop(preply_addr, len); continue; } recvline[n] = 0; /*NULL terminate */ Fputs(recvline, stdout); } Version of dg_cli that verifies returned socket address. Solaris % udpcli02 206.62.226.66 hello reply from 206.62.226.35.7 (ignore) goodbye reply from 206.62.226.35.7 (ignore) The server has not bound an IP address to its socket, the kernel choose the source address for the IP datagram. It is chosen to be the primary IP address of the outgoing interface. Bsdi.kohala.com has address 206.62.226.35 Bsdi.kohala.com has address 206.62.226.66

15. 8.9 Server Not Running  Client blocks forever in the call to recvfrom. u ICMP error is asynchronous error. u The basic rule is that asynchronous errors are not returned for UDP sockets unless the socket has been connected.

16. 8.11 connect Function with UDP This does not result in anything like a TCP connection: there is no three-way handshake. Instead, the kernel just records the IP address and port number of the peer. With a connect UDP socket three change: 1. We can no long specify the destination IP address and port for an output operation. That is, we do not use sendto but use write or send instead. 2. We do not use recvfrom but read or recv instead.

17. 3. Asynchronous errors are returned to the process for a connected UDP socket. } Stores peer IP address and port#from connect UDP UDP datagram ??? application peer UDP datagram from some other IP address and/or port#

18. DNS client DNS server DNS client DNS client connect OK cannot connect (configured to use one server) (configured to use two server) Datagrams arriving from any other IP address or port are not passed to the connected socket because either the source IP address or source UDP port does not match the protocol address to which the socket is connected. ?

19. 8.12 dg_cli Function (Revisited) #include “unp.h” void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, soklen_t servlen) { int n; char sendline[MAXLINE], recvline[MAXLINE+1]; Connect(sockfd, (SA *) pservaddr, servlen); while(Fgets(sendline, MAXLINE, fp) != NULL) { sendto(sockfd, sendline, strlen(sendline), 0, pservaddr, servlen); n = Recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL); recvline[n] = 0; /* null terminate */ Fputs(recvline,stdout); }

20. 8.13 Lack of Flow Control with UDP #include “unp.h” #define NDG 2000 #define DGLEN 1400 void dg_cli(FILE *fp, int sockfd, const SA *pservaddr, socklen_t, servlen) { int i; char sendline[MAXLINE]; for(I = 0; I< NDG ; I++) { Sendto(sockfd, sendline, DGLEN, 0, pservaddr, servlen); } dg_cli function that writes a fixed number of datagram to server

21. #include “unp.h” static void recvfrom_int(int); static int count; void dg_echo(int sockfd, SA *pcliaddr, socklen_t clilen) { socklen_t len; char mesg[MAXLINE]; Signal(SIGHT, recvfrom_int); for( ; ; ) { len=clilen; Recvfrom(sockfd, mesg, MAXLINE, 0, pcliaddr, &len); count++; } static void recvfrom_int(int signo) { printf(“\nreceived %d datagram\n”, count); exit(0); } dg_echo function that counts received datagram

22. uThe interface’s buffers were full or they could have been discarded by the sending host. uThe counter “dropped due to full socket buffers” indicates how many datagram were received by UDP but were discarded because the receiving socket’s receive queue was full uThe number of datagrams received b the server in this example is nondeterministic. It depends on many factors, such as the network load, the processing load on the client host, and the processing load in the server host. u Solution fast server, slow client. Increase the size of socket receive buffer.

23. 8.14 Determining Outgoing Interface with UDP This local address is chosen by searching the routing table for the destination IP address, and then using the primary IP address for the resulting interface. N= 240 * 1024; Setsockopt(sockfd, sol_SOCKET, so_RECVBUF, &n, sizeof(n)); Increases the size of the socket receive queue

24. Connect(sockfd,(SA *) &servaddr, sizeof(servaddr)); len = sizeof(cliaddr); Getsockname(sockfd, (SA *) &cliaddr, &len); printf(“local address &s\n”,Soxk_ntop((SA *) &cliaddr, len); exit(0); Use connect to determine outgoing interface. This local IP address is chosen by searching the routing table for the destination IP address, and then using the primary IP address for the resulting

25. 8.15 TCP and UDP Echo Server Using select #include “unp.h” int main(int argc, char **argv) { int listenfd, connfd, udpfd, nready, maxfd1; char mesg[MAXLINE]; pid_t childpid; fd_set rset; ssize_t n; socklen_t len; const int on = 1; struct sockaddr_in cliaddr, servaddr; void sig_chld(int);

26. /* Create listening TCP socket */ listenfd = Socket(AF_INET,SOCK_STREAM, 0); bzero(&seraddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htol(INADDR_ANY); servaddr.sin_port = htos(SERV_PORT); Setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)); Bind(listenfd, (SA *)&servaddr, sizeof(servaddr)); Listenfd, LISTENQ); /* Create UDP socket */ udpfd = Socket(AF_INET, SOCK_DGRAM, 0); bzero(&seraddr, sizeof(servaddr)); servaddr.sin_family = AF_INET; servaddr.sin_addr.s_addr = htol(INADDR_ANY); servaddr.sin_port = htos(SERV_PORT); Bind(udpfd, (SA *) &servaddr, sizeof(servaddr));

27. Signal(SIGCHLD, sig_chld); /* must call waitpd( )*/ FD_ZERO(&rset); maxfdp1=max(listenfd, udpfd)+1; for( ; ; ) { FD_SET(listenfd, &rset); FD_SET(udpfd, &rset); if((nready = selext[,axfdp1, &rset, NULL, NULL,NULL) < 0) { if(errno == EINTR) continue; else err_sys(“select error”); } if(FD_ISSET(listenfd,&rset)) { len = sizeof(cliaddr); connfd = Accept(listenfd, (SA *) &cliaddr, &len); if((childpid = fork( )) == 0) { /* child process */ Close(listenfd); /* Close listening socket */ str_echo(connfd); /* process the request */ exit(0); } Close(connfd); }

28. if(FD_ISSET(udpfd, &rset)) { len = sizeof(cliaddr); n = Recvfro,(udp, mesg, MAXLINE, 0, (SA *) &cliaddr, &len); Sendto(udpfd,,esg, n, 0, (SA *) &cliaddr, len); } } /* for */ } /* main */