Re: why does tcp_v[46]_conn_request not inc MIB stats

[David Miller <davem@davemloft.net>]

From: Rick Jones <rick.jones2@hp.com>
Date: Mon, 10 Sep 2007 11:42:18 -0700

> I've been digging around to see about inducing /proc/net/tcp to show 
> some "interesting" things for listen sockets (eg backlog depth, its max, 
> and dropped connection requests).  While there I've noticed that both 
> tcp_v[46]_syn_recv_sock and tcp_v[46]conn_request both check that the 
> listen queue is full, but only tcp_v[46]_syn_recv_sock increments some 
> mib stats for dropped connection requests.

They are checking two different things.

tcp_v{4,6}_conn_request is checking whether we are hitting the limit
for allowing the initial SYN and creating a new embryonic mini-socket.
Exceeding that is not a listen overflow.

tcp_v{4,6}_syn_recv_sock() is processing the end of the 3-way
handshake and wants to create a full established state socket to queue
into the listening parent.  This is checking the listening socket
queue limits, and indeed is a listen queue overflow if exceeded.