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 4 client handles response client process class26 ppt 1 client sends request 3 server sends response 2 server process resource 2 server handles request CS 213 F 01 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 F 01 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 F 01 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 F 01 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 F 01 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 F 01 Internet connections review Clients and servers communicate by sending streams of bytes over connections point to po int 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 assign ed au tomatically on client when client makes a conn ection 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 F 01 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 F 01 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 class26 ppt 10 CS 213 F 01 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 F 01 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 F 01 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 F 01 Overview of the Sockets Interface Client Server socket socket bind open listenfd open clientfd listen connect connection request accept writen readline readline writen close EOF Await connection request from next client readline close class26 ppt 14 CS 213 F 01 Echo client int 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 class26 ppt 15 CS 213 F 01 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
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