Unix Network Programming Episode 31

#include 
#int listen (int sockfd, int backlog);

This function is normally called after both the socket and bind functions and must be called before calling the accept function.

To understand the backlog argument, we must realize that for a given listening socket, the kernel maintains two queues:

1.An incomplete connection queue, which contains an entry for each SYN that has arrived from a client for which the server is awaiting completion of the TCP three-way handshake. These sockets are in the SYN_RCVD state (Figure 2.4(See 7.2.6)).
2.A completed connection queue, which contains an entry for each client with whom the TCP three-way handshake has completed. These sockets are in the ESTABLISHED state (Figure 2.4(See 7.2.6)).

There are several points to consider regarding the handling of these two queues.

• The backlog argument to the listen function has historically specified the maximum value for the sum of both queues.
• Berkeley-derived implementations add a fudge factor to the backlog: It is multiplied by 1.5 (p. 257 of TCPv1 and p. 462 of TCPV2).
• Do not specify a backlog of 0, as different implementations interpret this differently (Figure 4.10). If you do not want any clients connecting to your listening socket, close the listening socket.
• Assuming the three-way handshake completes normally (i.e., no lost segments and no retransmissions), an entry remains on the incomplete connection queue for one RTT, whatever that value happens to be between a particular client and server. Section 14.4 of TCPv3 shows that for one Web server, the median RTT between many clients and the server was 187 ms. (The median is often used for this statistic, since a few large values can noticeably skew the mean.)
• Historically, sample code always shows a backlog of 5, as that was the maximum value supported by 4.2BSD. This was adequate in the 1980s when busy servers would handle only a few hundred connections per day. But with the growth of the World Wide Web (WWW), where busy servers handle millions of connections per day, this small number is completely inadequate (pp. 187–192 of TCPv3). Busy HTTP servers must specify a much larger backlog, and newer kernels must support larger values.
  Many current systems allow the administrator to modify the maximum value for the backlog.
• A problem is: What value should the application specify for the backlog, since 5 is often inadequate? There is no easy answer to this. HTTP servers now specify a larger value, but if the value specified is a constant in the source code, to increase the constant requires recompiling the server. Another method is to assume some default but allow a command-line option or an environment variable to override the default. It is always acceptable to specify a value that is larger than supported by the kernel, as the kernel should silently truncate the value to the maximum value that it supports, without returning an error (p. 456 of TCPv2).

void List(int fd, int backlog)
{
    char *ptr;

    if((ptr=getenv("LISTENQ"))!=NULL)
        backlog=atoi(ptr);

    if(listen(fd, backlog)<0)
        err_sys("listen error");
}

Wrapper function for listen that allows an environment variable to specify backlog