1.4 Software¶

Application Programming Interface (Sockets)¶

The place to start when implementing a network application is the interface exported by the network. Since most network protocols are in software (especially those high in the protocol stack), and nearly all computer systems implement their network protocols as part of the operating system, when we refer to the interface “exported by the network,” we are generally referring to the interface that the OS provides to its networking subsystem. This interface is often called the network application programming interface (API).

Although each operating system is free to define its own network API (and most have), over time certain of these APIs have become widely supported; that is, they have been ported to operating systems other than their native system. This is what has happened with the socket interface originally provided by the Berkeley distribution of Unix, which is now supported in virtually all popular operating systems, and is the foundation of language-specific interfaces, such as the Java or Python socket library. We use Linux and C for all code examples in this book, Linux because it is open source and C because it remains the language of choice for network internals. (C also has the advantage of exposing all the low-level details, which is helpful in understanding the underlying ideas.)

Before describing the socket interface, it is important to keep two concerns separate in your mind. Each protocol provides a certain set of services, and the API provides a syntax by which those services can be invoked on a particular computer system. The implementation is then responsible for mapping the tangible set of operations and objects defined by the API onto the abstract set of services defined by the protocol. If you have done a good job of defining the interface, then it will be possible to use the syntax of the interface to invoke the services of many different protocols. Such generality was certainly a goal of the socket interface, although it’s far from perfect.

The main abstraction of the socket interface, not surprisingly, is the socket. A good way to think of a socket is as the point where a local application process attaches to the network. The interface defines operations for creating a socket, attaching the socket to the network, sending/receiving messages through the socket, and closing the socket. To simplify the discussion, we will limit ourselves to showing how sockets are used with TCP.

The first step is to create a socket, which is done with the following operation:

int socket(int domain, int type, int protocol);

The reason that this operation takes three arguments is that the socket interface was designed to be general enough to support any underlying protocol suite. Specifically, the domain argument specifies the protocol family that is going to be used: PF_INET denotes the Internet family, PF_UNIX denotes the Unix pipe facility, and PF_PACKET denotes direct access to the network interface (i.e., it bypasses the TCP/IP protocol stack). The type argument indicates the semantics of the communication. SOCK_STREAM is used to denote a byte stream. SOCK_DGRAM is an alternative that denotes a message-oriented service, such as that provided by UDP. The protocol argument identifies the specific protocol that is going to be used. In our case, this argument is UNSPEC because the combination of PF_INET and SOCK_STREAM implies TCP. Finally, the return value from socket is a handle for the newly created socket—that is, an identifier by which we can refer to the socket in the future. It is given as an argument to subsequent operations on this socket.

The next step depends on whether you are a client or a server. On a server machine, the application process performs a passive open—the server says that it is prepared to accept connections, but it does not actually establish a connection. The server does this by invoking the following three operations:

int bind(int socket, struct sockaddr *address, int addr_len);
int listen(int socket, int backlog);
int accept(int socket, struct sockaddr *address, int *addr_len);

The bind operation, as its name suggests, binds the newly created socket to the specified address. This is the network address of the local participant—the server. Note that, when used with the Internet protocols, address is a data structure that includes both the IP address of the server and a TCP port number. Ports are used to indirectly identify processes. They are a form of demux keys. The port number is usually some well-known number specific to the service being offered; for example, web servers commonly accept connections on port 80.

The listen operation then defines how many connections can be pending on the specified socket. Finally, the accept operation carries out the passive open. It is a blocking operation that does not return until a remote participant has established a connection, and when it does complete it returns a new socket that corresponds to this just-established connection, and the address argument contains the remote participant’s address. Note that when accept returns, the original socket that was given as an argument still exists and still corresponds to the passive open; it is used in future invocations of accept.

On the client machine, the application process performs an active open; that is, it says who it wants to communicate with by invoking the following single operation:

int connect(int socket, struct sockaddr *address, int addr_len);

This operation does not return until TCP has successfully established a connection, at which time the application is free to begin sending data. In this case, address contains the remote participant’s address. In practice, the client usually specifies only the remote participant’s address and lets the system fill in the local information. Whereas a server usually listens for messages on a well-known port, a client typically does not care which port it uses for itself; the OS simply selects an unused one.

Once a connection is established, the application processes invoke the following two operations to send and receive data:

int send(int socket, char *message, int msg_len, int flags);
int recv(int socket, char *buffer, int buf_len, int flags);

The first operation sends the given message over the specified socket, while the second operation receives a message from the specified socket into the given buffer. Both operations take a set of flags that control certain details of the operation.

Example Application¶

We now show the implementation of a simple client/server program that uses the socket interface to send messages over a TCP connection. The program also uses other Linux networking utilities, which we introduce as we go. Our application allows a user on one machine to type in and send text to a user on another machine. It is a simplified version of the Linux talk program, which is similar to the program at the core of instant messaging applications.

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

  /* translate host name into peer's IP address */
  hp = gethostbyname(host);
  if (!hp) {
    fprintf(stderr, "simplex-talk: unknown host: %s\n", host);
    exit(1);
  }

  /* build address data structure */
  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);

  /* active open */
  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");
    close(s);
    exit(1);
  }
  /* main loop: get and send lines of text */
  while (fgets(buf, sizeof(buf), stdin)) {
    buf[MAX_LINE-1] = '\0';
    len = strlen(buf) + 1;
    send(s, buf, len, 0);
  }
}

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

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

int
main()
{
  struct sockaddr_in sin;
  char buf[MAX_LINE];
  int buf_len, addr_len;
  int s, new_s;

  /* build address data structure */
  bzero((char *)&sin, sizeof(sin));
  sin.sin_family = AF_INET;
  sin.sin_addr.s_addr = INADDR_ANY;
  sin.sin_port = htons(SERVER_PORT);

  /* setup passive open */
  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);

 /* wait for connection, then receive and print text */
  while(1) {
    if ((new_s = accept(s, (struct sockaddr *)&sin, &addr_len)) < 0) {
      perror("simplex-talk: accept");
      exit(1);
    }
    while (buf_len = recv(new_s, buf, sizeof(buf), 0))
      fputs(buf, stdout);
    close(new_s);
  }
}