netdev - Re: [PATCH] IP: Increment INADDRERRORS if routing for a packet is not successful

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Date:	Wed, 2 Jun 2010 11:12:14 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	netdev@...r.kernel.org, Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH] IP: Increment INADDRERRORS if routing for a packet is
 not successful

On Wed, 2 Jun 2010, Eric Dumazet wrote:

> Le mercredi 02 juin 2010 à 10:27 -0500, Christoph Lameter a écrit :
>
> > Yes but they are not increment any counter. If packets are dropped because
> > of the rp_filter setting interfering f.e. then the packets vanish without
> > any accounting.
> >
>
> But packets are vanishing either way.

Its important to know why drops occur (any drops for that matter, drops
mean retransmit which means latency). The symptom here was that
multicast traffic on secondary interfaces was not being forwarded to the application.
The rp_filter just dropped them and there was no way to easily track down
the issue for the people experiencing the problem.

In 2.6.31 the rp_filter was fixed to work properly with multicast and now
it considers multicast traffic to secondary interfaces to have the wrong
reverse path even though the multicast subscription occurred on the
secondary interface. So it drops multicast traffic. The rp_filter has to
be switched off when using multiple NICs for multicast load balancing.

> > LINUX_MIB_INROUTEERRORS? Does it mean I can create a series of new
> > counters that allow us to diagnose and distinguish all the different
> > causes of packet loss? We would love to have that.
> >
>
> For an example, you could take a look at commit 907cdda5205b
> (tcp: Add SNMP counter for DEFER_ACCEPT)

Well these are all TCP counters. I would add IP and UDP counters?