Rahul,

Backlog is explained in man listen(2):

The backlog argument defines the maximum length to which the queue of pending connections for sockfd
may grow. If a connection request arrives when the queue is full, the client may receive an error
with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the
request may be ignored so that a later reattempt at connection succeeds.
...
If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is
silently truncated to that value; the default value in this file is 128. In kernels before 2.4.25,
this limit was a hard coded value, SOMAXCONN, with the value 128.