Internetworking I - Carnegie Mellon School of Computer Science

Internetworking I - Carnegie Mellon School of Computer Science

15-213 The course that gives CMU its Zip! Network programming Nov 27, 2001 Topics Client-server model Sockets interface Echo client and server class26.ppt Client-server model Every network application is based on the client-server model: Application is a server process and one or more client processes Server manages some resource, and provides service by manipulating resource for clients. Client makes a request for a service request may involve a conversation according to some server protocol Server provides service by manipulating the resource on behalf of client and then returning a response 1. client sends request client

process 4. client handles response class26.ppt server process 3. server sends response 2 resource 2. server handles request CS 213 F01 Clients Examples of client programs Web browsers, ftp, telnet, ssh How does the client find the server? The address of the server process has two parts: IPaddress:port The IP address is a unique 32-bit positive integer that identifies

the host (adapter). dotted decimal form: 0x8002C2F2 = 128.2.194.242 The port is positive integer associated with a service (and thus a server process) on that machine. port 7: echo server port 23: telnet server port 25: mail server port 80: web server class26.ppt 3 CS 213 F01 Using ports to identify services server machine 128.2 194.242 client machine client service request for 128.2.194.242:80 (i.e., the Web server)

Web server (port 80) kernel Echo server (port 7) client service request for 128.2.194.242:7 (i.e., the echo server) Web server (port 80) kernel Echo server (port 7) class26.ppt 4 CS 213 F01 Servers

Servers are long-running processes (daemons). Created at boot-time (typically) by the init process (process 1) Run continuously until the machine is turned off. Each server waits for requests to arrive on a wellknown port associated with a particular service. port 7: echo server port 25: mail server port 80: http server A machine that runs a server process is also often referred to as a server. class26.ppt 5 CS 213 F01 Server examples Web server (port 80) resource: files/compute cycles (CGI programs) service: retrieves files and runs CGI programs on behalf of the client FTP server (20, 21) resource: files

service: stores and retrieve files Telnet server (23) resource: terminal service: proxies a terminal on the server machine Mail server (25) resource: email spool file service: stores mail messages in spool file See /etc/services for a comprehensive list of the services available on a Linux machine. class26.ppt 6 CS 213 F01 The two basic ways that clients and servers communicate Connections (TCP):

reliable two-way byte-stream. looks like a file. akin to placing a phone call. slower but more robust. ... , Bk, Bk-1, ... , B1, B0 client connection server B0, B1, ..., Bk-1, Bk, ... Datagrams (UDP): data transferred in unreliable chunks. can be lost or arrive out of order. akin to using surface mail. faster but less robust. We will only discuss

connections. class26.ppt dgram dgram client server dgram 7 dgram CS 213 F01 Internet connections (review) Clients and servers communicate by sending str eams of bytes over connections: point-to-point, full-duplex, and reliable. A socket is an endpoint of a connection Socket address is an IPaddress:port pair

A port is a 16-bit integer that identifies a process: ephemeral port: assigned automatically on client when client makes a connection request well-known port: associated with some service provided by a server (e.g., port 80 is associated with Web servers) A connection is uniquely identified by the socket addresses of its endpoints (socket pair) (cliaddr:cliport, servaddr:servport) class26.ppt 8 CS 213 F01 Anatomy of an Internet connection (review) client socket address 128.2.194.242:51213 client server socket address 208.216.181.15:80 connection socket pair

(128.2.194.242 :51213, 208.216.181.15:80) client host address 128.2.194.242 class26.ppt server (port 80) server host address 208.216.181.15 9 CS 213 F01 Berkeley Sockets Interface Created in the early 80s 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. class26.ppt

10 CS 213 F01 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 descriptors. class26.ppt 11 CS 213 F01 Key data

structures Defined in /usr/include/netinet/in.h /* 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; unsigned short int sin_port; struct in_addr sin_addr; unsigned char sin_zero[...]; }; */ /* /* /* /* Address family (AF_INET) */ Port number */ IP address */ Pad to sizeof struct sockaddr */

Internet-style sockets are characterized by a 32-bit IP address and a port. class26.ppt 12 CS 213 F01 Key data structures Defined in /usr/include/netdb.h /* Domain Name Service (DNS) struct hostent { char *h_name; /* char **h_aliases; /* int h_addrtype; /* int h_length; /* char

**h_addr_list; /* } host entry */ official name of host */ alias list */ host address type */ length of address */ list of addresses */ 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. class26.ppt 13 CS 213 F01 Overview of the Sockets Client Server

Interface socket socket bind open_listenfd open_clientfd listen connect connection request writen readline readline close accept writen

EOF Await connection request from next client readline close class26.ppt 14 CS 213 F01 Echo client int main(int argc, char **argv) { int clientfd, port; char *host, buf[MAXLINE]; if (argc != 3) { fprintf(stderr, "usage: %s \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); } class26.ppt 15 CS 213 F01 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; } class26.ppt 16 CS 213 F01 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); class26.ppt 17 CS 213 F01 Echo client: open_clientfd() (gethostbyname) The client builds the servers Internet address. int clientfd; /* socket descriptor */ struct hostent *hp; /* DNS host entry */ struct sockaddr_in serveraddr; /* servers 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); class26.ppt 18 CS 213 F01 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; struct sockaddr_in serveraddr; /* socket descriptor */ /* server address */ ... /* establish a connection with the server */

Connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr)); class26.ppt 19 CS 213 F01 Echo server int 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); connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); hp = Gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr, sizeof(clientaddr.sin_addr.s_addr), AF_INET);

haddrp = inet_ntoa(clientaddr.sin_addr); printf("server connected to %s (%s)\n", hp->h_name, haddrp); echo(connfd); Close(connfd); } } class26.ppt 20 CS 213 F01 Echo server: open_listenfd() int open_listenfd(int port) { int listenfd; int optval; struct sockaddr_in serveraddr; /* create a socket descriptor */ listenfd = Socket(AF_INET, SOCK_STREAM, 0); /* eliminates "Address already in use" error from bind. */ optval = 1; Setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof(int));

... (more) class26.ppt 21 CS 213 F01 Echo server: open_listenfd() (cont) ... /* listenfd will be an endpoint for all requests to port on any IP address for this host */ bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; serveraddr.sin_addr.s_addr = htonl(INADDR_ANY); serveraddr.sin_port = htons((unsigned short)port); Bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)); /* make it a listening socket ready to accept connection requests */ Listen(listenfd, LISTENQ); return listenfd; } class26.ppt

22 CS 213 F01 Echo server: open_listenfd() (socket) socket() creates a socket descriptor. AF_INET: indicates that the socket is associated with Internet protocols. SOCK_STREAM: selects a reliable byte stream connection. int listenfd; /* listening socket descriptor */ listenfd = Socket(AF_INET, SOCK_STREAM, 0); class26.ppt 23 CS 213 F01 Echo server: open_listenfd() (setsockopt) The socket can be given some attributes. /* eliminates "Address already in use" error from bind. */ optval = 1;

Setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof(int)); Handy trick that allows us to rerun the server immediately after we kill it. Otherwise we would have to wait about 15 secs. Eliminates Address already in use error from bind(). Strongly suggest you do this for all your servers to simplify debugging. class26.ppt 24 CS 213 F01 Echo server: open_listenfd() (initialize socket address) Next, we initialize the socket with the servers Internet address (IP address and port) struct sockaddr_in serveraddr; /* server's socket addr */ /* listenfd will be an endpoint for all requests to port on any IP address for this host */ bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);

serveraddr.sin_port = htons((unsigned short)port); IP addr and port stored in network (big-endian) byte order htonl() converts longs from host byte order to network byte order. htons() convers shorts from host byte order to network byte order. class26.ppt 25 CS 213 F01 Echo server: open_listenfd() (bind) bind() associates the socket with the socket address we just created. int listenfd; /* listening socket */ struct sockaddr_in serveraddr; /* servers socket addr */ /* listenfd will be an endpoint for all requests to port on any IP address for this host */ Bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)); class26.ppt 26

CS 213 F01 Echo server: open_listenfd (listen) listen() indicates that this socket will accept connection (connect) requests from clients. int listenfd; /* listening socket */ /* make listenf it a server-side listening socket ready to accept connection requests from clients */ Listen(listenfd, LISTENQ); Were finally ready to enter the main server loop that accepts and processes client connection requests. class26.ppt 27 CS 213 F01 Echo server: main loop

The server loops endlessly, waiting for connection requests, then reading input from the client, and echoing the input back to the client. main() { /* create and configure the listening socket */ while(1) { /* Accept(): wait for a connection request */ /* echo(): read and echo input line from client */ /* Close(): close the connection */ } } class26.ppt 28 CS 213 F01 Echo server: accept() accept() blocks waiting for a connection request. int listenfd; /* listening descriptor */ int connfd; /* connected descriptor */ struct sockaddr_in clientaddr; int clientlen;

clientlen = sizeof(clientaddr); connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); accept() returns a connected socket descriptor (connfd) with the same properties as the listening descriptor (listenfd) Returns when connection between client and server is complete. All I/O with the client will be done via the connected socket. accept()also fills in clients address. class26.ppt 29 CS 213 F01 accept() illustrated listenfd(3) server client clientfd connection request client

1. Server blocks in accept, waiting for connection request on listening descriptor listenfd. listenfd(3) server 2. Client makes connection request by calling and blocking in connect. clientfd listenfd(3) client clientfd class26.ppt server connfd(4) 30 3. Server returns connfd from accept.

Client returns from connect. Connection is now established between clientfd and connfd. CS 213 F01 Echo server: identifying the client The server can determine the domain name and IP address of the client. struct hostent *hp; char *haddrp; /* pointer to DNS host entry */ /* pointer to dotted decimal string */ hp = Gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr, sizeof(clientaddr.sin_addr.s_addr), AF_INET); haddrp = inet_ntoa(clientaddr.sin_addr); printf("server connected to %s (%s)\n", hp->h_name, haddrp); class26.ppt 31 CS 213 F01

Echo server: echo() The server uses Unix I/O to read and echo text lines until EOF (end-of-file) is encountered. EOF notification caused by client calling close(clientfd). NOTE: EOF is a condition, not a data byte. void echo(int connfd) { size_t n; char buf[MAXLINE]; while((n = Readline(connfd, buf, MAXLINE)) != 0) { printf("server received %d bytes\n", n); Writen(connfd, buf, n); } } class26.ppt 32 CS 213 F01 Testing servers using telnet The telnet program is invaluable for testing servers

that transmit ASCII strings over Internet conections our simple echo server Web servers mail servers Usage: unix> telnet creates a connection with a server running on and listening on port . class26.ppt 33 CS 213 F01 Testing the echo server with telnet bass> echoserver 5000 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 5 bytes: 123 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 8 bytes: 456789 kittyhawk> telnet bass 5000 Trying 128.2.222.85... Connected to BASS.CMCL.CS.CMU.EDU.

Escape character is '^]'. 123 123 Connection closed by foreign host. kittyhawk> telnet bass 5000 Trying 128.2.222.85... Connected to BASS.CMCL.CS.CMU.EDU. Escape character is '^]'. 456789 456789 Connection closed by foreign host. kittyhawk> class26.ppt 34 CS 213 F01 Running the echo client and server bass> echoserver 5000 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 4 bytes: 123 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 7 bytes: 456789 ...

kittyhawk> echoclient bass 5000 Please enter msg: 123 Echo from server: 123 kittyhawk> echoclient bass 5000 Please enter msg: 456789 Echo from server: 456789 kittyhawk> class26.ppt 35 CS 213 F01 For detailed info W. Richard Stevens, Unix Network Programming: Networking APIs: Sockets and XTI, Volume 1, Second Edition, Prentice Hall, 1998. This is the network programming bible. Complete versions of the echo client and server are developed in the text. You should compile and run them for yourselves to see how they work. Feel free to borrow any of this code.

class26.ppt 36 CS 213 F01

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