netdev - Re: [PATCH] neigh: replace unres_qlen by unres_qlen

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Message-Id: <20111109.001843.1753987502002673227.davem@davemloft.net>
Date:	Wed, 09 Nov 2011 00:18:43 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] neigh: replace unres_qlen by unres_qlen_bytes

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 09 Nov 2011 01:14:16 +0100

> unres_qlen is the number of frames we are able to queue per unresolved
> neighbour. Its default value (3) was never changed and is responsible
> for strange drops, especially if IP fragments are used, or multiple
> sessions start in parallel. TCP initial congestion window is now bigger
> than 3.

BTW, it has been observed in practice that if a long living connection
suddently sends a burst of traffic after a very long idle period
(hitting ARP expiry) or something invalidates the ARP entry in use, we
will drop frames.  Because even if the ARP reply comes "fast" it's
never quick enough to beat the burst of frames.

And if this happens in a scenerio where such lost packets potentially
mean lost money...
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