Simplex Talk Project

by: burt rosenberg
at: january 2016

Our first trip on the information highway!

Simplex Talk

The major software abstraction that we use as programmers is the socket, more properly the Berkeley Socket. This abstraction summarizes all of the machinery into a file handle, very similar to reading and writing a simple file, except slightly different concepts associated with opening a socket, rather than file, such as setting networking addresses.

This project introduces the use of sockets, as well as informing you on subversion, ssh and the lab, and makefiles.

Project

1. Create folder proj1 inside your userfolder in the respository, e.g. [repo]/burt/proj1,
2. Add the files simplex-talk-s.c, simplex-talk.c, and Makefile, as below. (Note, these files are also available as [repo]/class/examples/textbook-1.)
3. Build and run.
4. Setup ssh so that you can log into two different machines in the lab.
5. Modify the makefile so as to demonstrate that your code works when client and server are run on different machines.
6. Commit for grading by the due date

Server code

/*
 * simplex-talk-s.c
 *
 * network programming example from
 * "computer networks a systems approach" peterson and davie
 * slightly modified to obtain a clean compile
 *
 * last update by: burt
 * last update: 23 jan 2017
 */


#include<stdio.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>

#include<strings.h>
#include<stdlib.h>
#include<unistd.h>

#define SERVER_PORT 5432
#define MAX_PENDING 5 
#define MAX_LINE 256

int main(int argc, char * argv[]) {
        struct sockaddr_in sin ;
        char buf[MAX_LINE] ;
        int len ;
        int s, new_s ;

        bzero((char *)&sin,sizeof(sin)) ;
        sin.sin_family = AF_INET ;
        sin.sin_addr.s_addr = INADDR_ANY ;
        sin.sin_port = htons( SERVER_PORT) ; 
        
        if ( (s = socket(PF_INET,SOCK_STREAM,0)) < 0) {
                perror("simplex-talk: socket") ;
                exit(1) ;
        }
        if   (bind( s, (struct sockaddr *) &sin, sizeof(sin) ) < 0 ) {
                perror("simplex-talk: bind") ; 
                exit(1) ;
        }
        listen( s, MAX_PENDING ) ;

        while(1) {
                if ( (new_s = accept(s,(struct sockaddr *)&sin,
                        (socklen_t *) &len) ) < 0 ) {
                        perror("simplex-talk: accept") ; 
                        exit(1) ;
                }
                while ( (len = recv(new_s, buf, sizeof(buf), 0 )) )
                        fputs(buf, stdout) ;
                close(new_s) ;
        }
}


 

Client code

/*
 * simplex-talk.c
 *
 * network programming example from
 * "computer networks a systems approach" peterson and davie
 * slightly modified to obtain a clean compile
 *
 * last update by: burt
 * last update: 23 jan 2017
 */
 

#include<stdio.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>

#include<strings.h>
#include<stdlib.h>

#define SERVER_PORT 5432
#define MAX_LINE 256



int main(int argc, char * argv[]) {
        FILE *fp ;
        struct hostent *hp ;
        struct sockaddr_in sin ;
        char * host ;
        char buf[MAX_LINE] ;
        int s ;
        int len ;

        if ( argc==2 ) {
                host = argv[1] ;
        } else {
                fprintf(stderr, "usage: simplex-talk host\n") ;
                exit(1) ;
        }
        
        hp = gethostbyname(host) ;
        if (!hp) {
                fprintf(stderr, "simplex-talk: unknown host: %s\n", host ) ;
                exit(1); 
        }
        bzero((char *)&sin,sizeof(sin)) ;
        sin.sin_family = AF_INET ;
        bcopy( hp->h_addr, (char *) &sin.sin_addr, hp->h_length) ;
        sin.sin_port = htons( SERVER_PORT) ;
        
        if ( (s = socket(PF_INET,SOCK_STREAM,0)) < 0) {
                perror("simplex-talk: socket") ;
                exit(1) ;
        }
        if ( connect(s, (struct sockaddr *) &sin, sizeof(sin) ) < 0 ) {
                perror("simplex-talk: connect") ; 
                exit(1) ;
        }

        while ( fgets(buf, sizeof(buf), stdin) )  {
                buf[MAX_LINE-1] = '\0' ; 
                len = strlen(buf) + 1 ;
                send(s, buf, len, 0 ) ; 
        }
}


 

Makefile

#
# example networking program from
# "computer networks a systems apprroach" by Peterson and Davie 
#
# makefile author: bjr
# created: 23 jan 2017
# last-update: 23 jan 2017
#

HOST= localhost
MESSAGE= "hello world" 

all: simplex-talk simplex-talk-s
        ./simplex-talk-s &
        echo ${MESSAGE} | ./simplex-talk ${HOST}
        killall simplex-talk-s
        make clean

run-s: simplex-talk-s
        ./simplex-talk-s 

run: simplex-talk-s simplex-talk
        echo ${MESSAGE} | ./simplex-talk ${HOST}

simplex-talk: simplex-talk.c
        cc -o $@ $<

simplex-talk-s: simplex-talk-s.c
        cc -o $@ $<

clean:
        -rm simplex-talk simplex-talk-s

Internet crash course

In order to accomplish this project, you will need sneak previews on four networking technologies:

• Client-Server: Not so much a technology as a discipline, in which one endpoint of a communication channel is assigned the responsibility of client and the other of server. Communication proceeds with the client taking the initiative to contact to server, and the server responding in reply. Naturally, this assigns to the endpoint which a resource the role of server. Naturally, those softwares desiring the resource will take the initiative to contact the server to obtain that resource.
• IP Addresses: IP provides packet communication from network endpoint to network endpoint. A network endpoint is identified by an IP address, which in IP version 4 (IPv4) is a 32 bit integer. The IP address accomplishes both the naming of an endpoint, and provides routing hints for reaching the endpoint. It is usually rewritten in dotted-octet notation. E.g. the lab machine antietam has IP address 172.19.1.9. This is the 32 bit integer 0xac130109, however mostly the IP address is referred to in dotted-octet notation.
  • Private-local addresses: These days, most IP addresses for client machines are private-local addresses. They cannot be found on internet backbones, and are not routable. They are reused by location, and are only good within the location.
  • Public addresses: These addresses are routable from where ever on the Internet. They specify for the world at large a network endpoint. For instance, a publicly available web server will have a public IP address.
• Ports: The level 4 protocol add to the level 3 IP procotol the concept of ports. A port is a a 16-bit number the identifies in the context of the IP-endpoint which application-endpoint the data comes from or goes to.
  • Well-known ports: As the initiator of the communication, the client must know the IP address and port number where listens the server's network endpoint. Typically, this requires some outside mechanism to inform the client of these values. For the port, often well known services, such as a web server, are associated with well known port numbers — port 80 for a web server.
  • Ephemeral ports: As the respondent to a request from a client, the server will learn the port number of the client endpoint as part of the datapacket that initiates communication. That port number is typically assigned for the client automatically by the socket software so that it is unique for the network endpoint, and are retired for a sufficiently long period of time after the socket closes so that there will be no confusion about routing communication.
• DNS: IP addresses are the thing, from the point of view of the internet. However, machines are given names, and there is a system in place, called the Domain Naming Service (DNS), to map those names to IP addresses. The DNS system is an Internet-wide database with query mechanism with which will answer queries for names, and provide various bits of information, for instance the A record for the name, which is an IP address associated with that name.

This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

author: burton rosenberg (except Peterson/Davie code)
created: 30 jan 2017
update: 30 jan 2017