1.3 Protocol Independence

Our program in Figure 1.5 is protocol-dependent on IPv4. We allocate and initialize a sockaddr_in structure, we set the family of this structure to AF_INET, and we specify the first argument to socket as AF_INET.

To modify the program to work under IPv6, we must change the code. Figure 1.6 shows a version that works under IPv6, with the changes highlighted in bold.

Figure 1.6 Version of Figure 1.5 for IPv6.

intro/daytimetcpcliv6.c

 1 #include     "unp.h"

 2 int
 3 main(int argc, char **argv)
 4 {
 5     int     sockfd, n;
 6     char    recvline[MAXLINE + 1];
 7     struct sockaddr_in6 servaddr;

 8     if (argc != 2)
 9         err_quit("usage: a.out <IPaddress>");

10     if ( (sockfd = socket(AF_INET6, SOCK_STREAM, 0)) < 0)
11         err_sys("socket error");

12     bzero(&servaddr, sizeof(servaddr));
13     servaddr.sin6_family = AF_INET6;
14     servaddr.sin6_port = htons(13);    /* daytime server */
15     if (inet_pton(AF_INET6, argv[1], &servaddr.sin6_addr) <= 0)
16         err_quit("inet_pton error for %s", argv[1]);

17     if (connect(sockfd, (SA *) &servaddr, sizeof(servaddr)) < 0)
18         err_sys("connect error");

19     while ( (n = read(sockfd, recvline, MAXLINE)) > 0) {
20         recvline[n] = 0;    /* null terminate */
21         if (fputs(recvline, stdout) == EOF)
22             err_sys("fputs error");
23     }
24     if (n < 0)
25         err_sys("read error");

26     exit(0);
27 }

Only five lines are changed, but what we now have is another protocol-dependent program; this time, it is dependent on IPv6. It is better to make a program protocol-independent. Figure 11.11 will show a version of this client that is protocol-independent by using the getaddrinfo function (which is called by tcp_connect).

Another deficiency in our programs is that the user must enter the server's IP address as a dotted-decimal number (e.g., 206.168.112.219 for the IPv4 version). Humans work better with names instead of numbers (e.g., www.unpbook.com). In Chapter 11, we will discuss the functions that convert between hostnames and IP addresses, and between service names and ports. We purposely put off the discussion of these functions and continue using IP addresses and port numbers so we know exactly what goes into the socket address structures that we must fill in and examine. This also avoids complicating our discussion of network programming with the details of yet another set of functions.