Carnegie Mellon Network Programming 15 213 Introduc0on to Computer Systems 20th Lecture Nov 2 2010 Instructors Randy Bryant and Dave O Hallaron 1 Carnegie Mellon Last Time Client Server Transac on 4 Client handles response Client process 1 Client sends request 3 Server sends response Server process Resource 2 Server handles request Note clients and servers are processes running on hosts can be the same or di erent hosts 2 Carnegie Mellon Last Time Logical Structure of an internet host router host router router router router router 3 Carnegie Mellon Internet Connec ons Clients and servers communicate by sending streams of bytes over connecAons Point to point full duplex 2 way communica0on and reliable A socket is an endpoint of a connec on Socket address is an IPaddress port pair A port is a 16 bit integer that iden es a process Ephemeral port Assigned automa0cally on client when client makes a connec0on request Well known port Associated with some service provided by a server e g port 80 is associated with Web servers A connec on is uniquely iden ed by the socket addresses of its endpoints socket pair cliaddr cliport servaddr servport 4 Carnegie Mellon PuJng it all Together Anatomy of an Internet Connec on Client socket address 128 2 194 242 51213 Client Server socket address 208 216 181 15 80 Connec on socket pair 128 2 194 242 51213 208 216 181 15 80 Client host address 128 2 194 242 51213 is an ephemeral port allocated by the kernel Server port 80 Server host address 208 216 181 15 80 is a well known port associated with Web servers 5 Carnegie Mellon Clients Examples of client programs Web browsers ftp telnet ssh How does a client nd the server The IP address in the server socket address iden0 es the host more precisely an adapter on the host The well known port in the server socket address iden0 es the service and thus implicitly iden0 es the server process that performs that service Examples of well know ports Port 7 Echo server Port 23 Telnet server Port 25 Mail server Port 80 Web server 6 Carnegie Mellon Using Ports to Iden fy Services Server host 128 2 194 242 Client host 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 7 Carnegie Mellon Servers Servers are long running processes daemons Created at boot 0me typically by the init process process 1 Run con0nuously un0l the machine is turned o Each server waits for requests to arrive on a well known port associated with a par cular service Port 7 echo server Port 23 telnet server Port 25 mail server Port 80 HTTP server A machine that runs a server process is also o en referred to as a server 8 Carnegie Mellon Server Examples Web server port 80 Resource les compute cycles CGI programs Service retrieves les and runs CGI programs on behalf of the client FTP server 20 21 Resource les Service stores and retrieve les See etc services for a comprehensive list of the port mappings on a Linux machine Telnet server 23 Resource terminal Service proxies a terminal on the server machine Mail server 25 Resource email spool le Service stores mail messages in spool le 9 Carnegie Mellon Sockets Interface Created in the early 80 s as part of the original Berkeley distribu on of Unix that contained an early version of the Internet protocols Provides a user level interface to the network Underlying basis for all Internet applica ons Based on client server programming model 10 Carnegie Mellon Sockets What is a socket To the kernel a socket is an endpoint of communica0on To an applica0on a socket is a le descriptor that lets the applica0on read write from to the network Remember All Unix I O devices including networks are modeled as les Clients and servers communicate with each other by reading from and wri ng to socket descriptors Client clientfd Server serverfd The main dis nc on between regular le I O and socket I O is how the applica on opens the socket descriptors 11 Carnegie Mellon Example Echo Client and Server On Client On Server greatwhite echoserveri 15213 linux echoclient greatwhite ics cs cmu edu 15213 server connected to BRYANT TP4 VLSI CS CMU EDU 128 2 213 29 port 64690 type hello there server received 12 bytes echo HELLO THERE type D Connection closed 12 Carnegie Mellon Watching Echo Client Server 13 Carnegie Mellon Ethical Issues Packet Sni er Program that records network tra c visible at node Promiscuous mode Record tra c that does not have this host as source or des0na0on University Policy Network Traffic Network traffic should be considered private Because of this any packet sniffing or other deliberate attempts to read network information which is not intended for your use will be grounds for loss of network privileges for a period of not less than one full semester In some cases the loss of privileges may be permanent Note that it is permissable to run a packet sniffer explicitely configured in non promiscuous mode you may sniff packets going to or from your machine This allows users to explore aspects of networking while protecting the privacy of others 14 Carnegie Mellon Overview of the Sockets Interface Client Server socket socket bind open listenfd open clientfd listen Connec on request Client Server Session connect accept rio writen rio readlineb rio readlineb rio writen close EOF Await connec on request from next client rio readlineb close 15 Carnegie Mellon Socket Address Structures Generic socket address For address arguments to connect bind and accept Necessary only because C did not have generic void pointers when the sockets interface was designed struct sockaddr unsigned short sa family char sa data 14 protocol family address data sa family Family Speci c 16 Carnegie Mellon Socket Address Structures Internet speci c socket address Must cast sockaddr in to sockaddr for connect bind and accept struct sockaddr in unsigned short sin family unsigned short sin port struct in addr sin addr unsigned char sin zero 8 sin port AF INET address family always AF INET port num in network byte order IP addr in network byte order pad to sizeof struct sockaddr sin addr 0 0 0 0 0 0 0 0 sa family sin family Family Speci c 17 Carnegie Mellon Echo Client Main Rou ne include csapp h Send line to server Receive line from server usage echoclient host port int main int argc char argv int clientfd port char host buf MAXLINE rio t rio host argv 1 port atoi argv 2 clientfd
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