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CMU CS 15213 - Recitation

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15-213 Recitation 10 Greg ReshkoOutlineInternet connectionBerkeley Sockets InterfaceWhat is a socket?Key data structuresKey data structuresOverview of the Sockets InterfaceEcho clientEcho client: open_clientfd()Echo client: open_clientfd() (socket)Echo client: open_clientfd() (gethostbyname)Echo client: open_clientfd() (connect)Echo serverEcho server: open_listenfd()Slide 16Echo server: open_listenfd() (socket)Echo server: open_listenfd() (setsockopt)Echo server: open_listenfd()Echo server: open_listenfd() (bind)Echo server: open_listenfd (listen)Echo server: main loopEcho server: accept()accept()Echo server: identifying the clientEcho server: echo()Q&A15-213 Recitation 10Greg ReshkoOffice Hours: Wed 2:00-3:00PMApril 28th, 2003OutlineSockets InterfaceEcho Client/ServerFCEsInternet connectionClients and se rvers com munic ate by sendin g stream s o f bytes over c onnec tions :point -to-point, full-du plex, and reliabl eA socke t is an endpoi nt of a con necti onSock et add ress is an IPaddress:port pairA port is a 16 -bit i nteger that identif ies a proce ss:ephemeral po rt: ass igne d autom atical ly on c li ent when clien t makes a con nectio n reque stwel l-know n po rt: ass ocia ted wi th som e servic e provide d by a se rver (e.g., port 8 0 is as so ciate d with Web servers)A connec tion is uniq uely id entifie d by the s ocket addres ses of its endpoin ts (s ocke t pair)(cliaddr:cliport, servaddr:servport)Berkeley Sockets InterfaceCreated in the early 80’s as part of the original Berkeley distribution of Unix that contained an early version of the Internet protocols.Provides a user-level interface to the network.Underlying basis for all Internet applications.Based on client/server programming model.What is a socket?A socket is a descriptor that lets an application read/write from/to the networkKey idea: Unix uses the same abstraction for both file I/O and network I/OClients and servers communicate with each by reading from and writing to socket descriptorsUsing regular Unix read and write I/O functionsThe main difference between file I/O and socket I/O is how the application “opens” the socket descriptorsKey data structuresDefined in /usr/include/netinet/in.hInternet-style sockets are characterized by a 32-bit IP address and a port/* Internet address */struct in_addr { unsigned int s_addr; /* 32-bit IP address */ };/* Internet style socket address */struct sockaddr_in { unsigned short int sin_family; /* Address family (AF_INET) */ unsigned short int sin_port; /* Port number */ struct in_addr sin_addr; /* IP address */ unsigned char sin_zero[...]; /* Pad to sizeof “struct sockaddr” */};Key data structures Defined in /usr/include/netdb.hhostent is a DNS host entry that associates a domain name (e.g., cmu.edu) with an IP addr (128.2.35.186)Can be accessed from user programs gethostbyname() [domain name key]gethostbyaddr() [IP address key]Can also be accessed from the shell using nslookup or dig/* Domain Name Service (DNS) host entry */struct hostent { char *h_name; /* official name of host */ char **h_aliases; /* alias list */ int h_addrtype; /* host address type */ int h_length; /* length of address */ char **h_addr_list; /* list of addresses */}Overview of the Sockets InterfaceClientServersocket socketbindlistenacceptreadlinereadlinewritenclosereadlineconnectwritencloseconnectionrequestEOFAwait connectionrequest fromnext clientopen_listenfdopen_clientfdEcho clientint main(int argc, char **argv){ int clientfd, port; char *host, buf[MAXLINE]; if (argc != 3) { fprintf(stderr, "usage: %s <host> <port>\n", argv[0]); exit(0); } host = argv[1]; port = atoi(argv[2]); clientfd = open_clientfd(host, port); while (Fgets(buf, MAXLINE, stdin) != NULL) { Writen(clientfd, buf, strlen(buf)); Readline(clientfd, buf, MAXLINE); Fputs(buf, stdout); } Close(clientfd);}Echo client: open_clientfd()int open_clientfd(char *hostname, int port){ int clientfd; struct hostent *hp; struct sockaddr_in serveraddr; clientfd = Socket(AF_INET, SOCK_STREAM, 0); /* fill in the server's IP address and port */ hp = Gethostbyname(hostname); bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; bcopy((char *)hp->h_addr, (char *)&serveraddr.sin_addr.s_addr, hp->h_length); serveraddr.sin_port = htons(port); /* establish a connection with the server */ Connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr)); return clientfd;}Echo client: open_clientfd() (socket)The client creates a socket that will serve as the endpoint of an Internet (AF_INET) connection (SOCK_STREAM).socket() returns an integer socket descriptor.int clientfd; /* socket descriptor */clientfd = Socket(AF_INET, SOCK_STREAM, 0);Echo client: open_clientfd() (gethostbyname)The client builds the server’s Internet address. int clientfd; /* socket descriptor */ struct hostent *hp; /* DNS host entry */ struct sockaddr_in serveraddr; /* server’s IP address */ typedef struct sockaddr SA; /* generic sockaddr */... /* fill in the server's IP address and port */ hp = Gethostbyname(hostname); bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; bcopy((char *)hp->h_addr, (char *)&serveraddr.sin_addr.s_addr, hp->h_length); serveraddr.sin_port = htons(port);Echo client: open_clientfd() (connect)Then the client creates a connection with the serverThe client process suspends (blocks) until the connection is created with the server.At this point the client is ready to begin exchanging messages with the server via Unix I/O calls on the descriptor sockfd. int clientfd; /* socket descriptor */ struct sockaddr_in serveraddr; /* server address */... /* establish a connection with the server */ Connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr));Echo serverint main(int argc, char **argv) { int listenfd, connfd, port, clientlen; struct sockaddr_in clientaddr; struct hostent *hp; char *haddrp; port = atoi(argv[1]); /* the server listens on a port passed on the command line */ listenfd = open_listenfd(port); while (1) { clientlen = sizeof(clientaddr);


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