1. 240-322 Cli/Serv.: sockets 3/91 Client/Server Distributed Systems v Objectives –describe iterative clients and servers using the UDP protocol 240-322, Semester 1, 2005-2006 9. Sockets (3)

2. 240-322 Cli/Serv.: sockets 3/92 Overview 1. Why use UDP? 2. The UDP Protocol 3. A Connectionless Example 4. A Client for the UDP daytime Service

3. 240-322 Cli/Serv.: sockets 3/93 1. Why use UDP? v The connectionless protocol, UDP, has two big drawbacks: –datagrams may arrive in any order –datagrams may be lost v TCP does not have these problems, so why use UDP?

4. 240-322 Cli/Serv.: sockets 3/94 v UDP is reliable over a LAN –it is very unlikely that datagrams will be lost or reordered unless there is a hardware problem v UDP is less resource-intensive than TCP v Use UDP if the client-server communication is simple, such as: –get finger information from a server –get time/day information –messages must be small and self-contained

5. 240-322 Cli/Serv.: sockets 3/95 2. The UDP Protocol socket() bind() recvfrom() blocks until data received from a client process request sendto() socket() bind() recvfrom() data (request) data (reply) sendto() Server Client

6. 240-322 Cli/Serv.: sockets 3/96 Notes v The system calls for reading and writing are different from TCP.  The client does not need to connect with the server using connect() (although it is possible).  The server does not need to call accept().

7. 240-322 Cli/Serv.: sockets 3/97 Communication Diagram server client

8. 240-322 Cli/Serv.: sockets 3/98 sendto()  Send a message from a socket to an address:  Send a message from a socket to an address: #include #include int sendto(int sd, char *msg, int len, int flags, struct sockaddr *addr, int addrlen); v Return the length of the sent message if ok, -1 on error.

9. 240-322 Cli/Serv.: sockets 3/99 sendto() Diagram sendto(sd, msg, len, flags, addr, addrlen); socket descriptor len data to be sent special delivery options; usually 0 addrlen address of destination

10. 240-322 Cli/Serv.: sockets 3/910 recvfrom()  Receive a message at a socket from an address:  Receive a message at a socket from an address: #include #include int recvfrom(int sd, char *msg, int len, int flags, struct sockaddr *addr, int *addrlen); v Return the length of the received message if ok, -1 on error.

11. 240-322 Cli/Serv.: sockets 3/911 recvfrom() Diagram n = recvfrom(sd, msg, len, flags, addr, &addrlen); socket descriptor len received data placed here special options; usually 0 addrlen address of sender n

12. 240-322 Cli/Serv.: sockets 3/912 Other send/receive Functions #include #include int send(int sd, char *msg, int len, int flags); int recv(int sd, char *msg, int len, int flags); int sendmsg(int sd, struct msghdr msg[], int flags); int recvmsg(int sd, struct msghdr msg[], int flags);

13. 240-322 Cli/Serv.: sockets 3/913  send() and recv() are used when the client has used connect() to specify its server –may not be supported in the future  sendmsg() supports gather-write –collect data from different places in memory, and send as a single datagram  recvmsg() supports scatter-read –split received datagram into separate places in memory

14. 240-322 Cli/Serv.: sockets 3/914 3. A Connectionless Example 3.1. A UDP echo Server 3.2. A UDP echo Client 3.3. Usage

15. 240-322 Cli/Serv.: sockets 3/915 3.1. A UDP echo Server  Receive a string (ending in ‘\n’ ) from a client, convert it to uppercase, and send it back. v A simple communication protocol. v Messages are short and self-contained.

16. 240-322 Cli/Serv.: sockets 3/916 echo_userv.c #include : /* usual includes */ #include /* for toupper() */ #define PORT 3667 #define BUFSIZE 128 void touppers(char buf[]); int udp_serv_sock(int port); :

17. 240-322 Cli/Serv.: sockets 3/917 int main() { int sd, client_len, n; struct sockaddr_in client; char msg[BUFSIZE+1]; sd = udp_serv_sock(PORT); while(1) { client_len = sizeof(client); n = recvfrom(sd, msg, BUFSIZE, 0, (struct sockaddr *)&client, &client_len); :

18. 240-322 Cli/Serv.: sockets 3/918 if (n < 0) fprintf(stderr, "recvfrom failed\n"); else { /* msg[n] = '\0'; printf("n: %d, msg: \"%s\"\n", n, msg); */ touppers(msg); if (sendto(sd, msg, n, 0, (struct sockaddr *)&client, client_len) != n) fprintf(stderr, "sendto error\n"); } } return 0; }

19. 240-322 Cli/Serv.: sockets 3/919 int udp_serv_sock(int port) /* Bind a socket with the server's address using the UDP protocol. */ { int sockfd; struct sockaddr_in serv_addr; /* create a UDP socket */ if ((sockfd = socket(AF_INET,SOCK_DGRAM,0)) < 0) { fprintf(stderr, "Could not open a socket"); exit(1); } :

20. 240-322 Cli/Serv.: sockets 3/920 /* initialise socket address */ memset((char *)&serv_addr, 0,sizeof(serv_addr)); serv_addr.sin_family = AF_INET; serv_addr.sin_addr.s_addr = htonl(INADDR_ANY); serv_addr.sin_port = htons(port); /* bind socket to address */ if (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) { fprintf(stderr, "Could not bind socket to address\n"); exit(1); } return sockfd; }

21. 240-322 Cli/Serv.: sockets 3/921 Notes  main() reads the client information from the recvfrom() call. Then it knows where to send the reply with sendto().  udp_serv_sock() is the same as tcp_serv_sock() (part 7, slide 10) except for the socket() type, and no listen() call.  We cannot test this server with telnet because telnet uses the TCP protocol.

22. 240-322 Cli/Serv.: sockets 3/922 3.2. A UDP echo Client v Read server details from the command line: $./echo_ucli takasila 3667  main() reads a line from stdin, sends it to the UDP echo server, and then prints the reply to stdout.  The program continues until the user types EOF ( control-D ).

23. 240-322 Cli/Serv.: sockets 3/923 echo_ucli.c #include : /* usual includes */ #include #define PORT 3667 #define BUFSIZE 128 struct sockaddr_in get_addr(int argc, char *argv[]); int open_udp_sock(void); :

24. 240-322 Cli/Serv.: sockets 3/924 int main(int argc, char *argv[]) { struct sockaddr_in serv_addr; int sd, serv_len, n; char buf[BUFSIZE+1]; serv_addr = get_addr(argc, argv); serv_len = sizeof(serv_addr); sd = open_udp_sock(); :

25. 240-322 Cli/Serv.: sockets 3/925 while (fgets(buf, BUFSIZE, stdin) != NULL) { n = strlen(buf); if (sendto(sd, buf, n, 0, (struct sockaddr *)&serv_addr, serv_len) != n) fprintf(stderr, "sendto failed\n"); else { if ((n = recvfrom(sd, buf, BUFSIZE, 0, (struct sockaddr *)0, (int *)0)) < 0) fprintf(stderr, "recvfrom failed\n"); else { buf[n] = '\0'; fputs(buf, stdout); } } } close(sd); return 0; }

26. 240-322 Cli/Serv.: sockets 3/926 int open_udp_sock(void) /* Bind to any local address */ { int sd; struct sockaddr_in addr; if ((sd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { fprintf(stderr, "socket() error\n"); exit(1); } :

27. 240-322 Cli/Serv.: sockets 3/927 memset(&addr, 0, sizeof(addr)); addr.sin_family = AF_INET; addr.sin_addr.s_addr = htonl(INADDR_ANY); addr.sin_port = htons(0); if (bind(sd, (struct sockaddr *)&addr, sizeof(addr)) < 0) { fprintf(stderr, "Can not bind local address\n"); exit(1); } return sd; }

28. 240-322 Cli/Serv.: sockets 3/928 Notes  get_addr() is taken from part 7, slide 77.  open_udp_sock() attaches any free port (between 1024-5000) to the socket by using 0 as the port value. –this port is used by the server when it replies

29. 240-322 Cli/Serv.: sockets 3/929 3.3. Usage v v $./echo_userv &/* on takasila */ [1] 1745 $ netstat -a | grep 3667 udp 0 0 *:3667 *:* v v $./echo_ucli takasila /*on fivedots*/ hello HELLO good bye GOOD BYE $ control-D typed FT

30. 240-322 Cli/Serv.: sockets 3/930 4. A Client for the UDP daytime Service  The daytime service can be contacted by using TCP or UDP. Shown in /etc/services : : daytime 13/tcp daytime 13/udp  It's also necessary to check /etc/inet.conf to see if the service is switched on –usually most UDP services are disabled due to hacker attack weaknesses continued

31. 240-322 Cli/Serv.: sockets 3/931 v The availability of the service via TCP can be checked with telnet: –telnet 13  Execute the daytime UDP client (called day_cus ) with the server’s hostname as a command line argument: $./day_cus calvin

32. 240-322 Cli/Serv.: sockets 3/932 day_cus.c #include : /* usual includes */ #include #define MSG ”\n" /* dummy 'wake-up' msg */ #define BUFSIZE 128 struct sockaddr_in get_udp_serv_addr(char *host, char *service); int open_udp_sock(void); :

33. 240-322 Cli/Serv.: sockets 3/933 int main(int argc, char *argv[]) { struct sockaddr_in serv_addr; int sd, serv_len, n; char buf[BUFSIZE+1]; serv_addr = get_udp_serv_addr(argv[1], "daytime"); serv_len = sizeof(serv_addr); sd = open_udp_sock(); n = strlen(MSG); :

34. 240-322 Cli/Serv.: sockets 3/934 if (sendto(sd, MSG, n, 0, (struct sockaddr *)&serv_addr, serv_len) != n) fprintf(stderr, "sendto failed\n"); else { if ((n = recvfrom(sd, buf, BUFSIZE, 0, (struct sockaddr *)0, (int *)0)) < 0) fprintf(stderr, "recvfrom failed\n"); else { buf[n] = '\0'; fputs(buf, stdout); } } close(sd); return 0; }

35. 240-322 Cli/Serv.: sockets 3/935 struct sockaddr_in get_udp_serv_addr(char *host, char *service) /* Build a socket address struct for a system service using the host name (given in dotted-decimal or dotted-name format) and the service name. */ { struct sockaddr_in addr; unsigned long lgaddr; struct hostent *hp; struct servent *sp; if ((sp = getservbyname(service, "udp")) == NULL) { fprintf(stderr, "Unknown service\n"); exit(1); } :

36. 240-322 Cli/Serv.: sockets 3/936 memset(&addr, 0, sizeof(addr)); addr.sin_family = AF_INET; addr.sin_port = sp->s_port; if ((lgaddr = inet_addr(host)) != -1) addr.sin_addr.s_addr = lgaddr; else if ((hp = gethostbyname(host)) != NULL) memcpy(&addr.sin_addr.s_addr, hp->h_addr, hp->h_length); else { fprintf(stderr, "Unknown address\n"); exit(1); } return addr; }

37. 240-322 Cli/Serv.: sockets 3/937 Notes  The client must ‘wake up’ the server by sending it a dummy message ( MSG ).  The server uses the client address information included in the sendto() call to direct its answer. continued

38. 240-322 Cli/Serv.: sockets 3/938  get_udp_serv_addr() is the same as get_tcp_serv_addr() (part 7, slide 100) except that the service type in getservbyname() is “udp”.

39. 240-322 Cli/Serv.: sockets 3/939 Usage $./day_cus calvin /*on fivedots*/ Wed May 4 09:42:33 2005 $./day_cus fivedots Wed May 4 09:42:39 2005 The usual response is to get nothing back; day_cus hangs, waiting forever

40. 240-322 Cli/Serv.: sockets 3/940 Timeouts in UDP v Have the code wait for a message for a certain amount of time, then timeout. v Three main approaches: –use the timeout feature of select(); –set an alarm which interrupts the waiting process after a certain amount of time; –configure the socket to timeout using (non- standard) socket options. continued

41. 240-322 Cli/Serv.: sockets 3/941 v Details in: –UNIX Network Programming (Vol. 1: Networking APIs: Sockets and XTI) W. Richard Stevens