netdev - [RFC 0/4] net: enable timestamps on a per-socket basis

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Date:	Sun, 20 Apr 2008 22:34:02 -0700
From:	Jason Uhlenkott <juhlenko@...mai.com>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	netdev@...r.kernel.org,
	Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: [RFC 0/4] net: enable timestamps on a per-socket basis

Currently, if anyone enables the SOCK_TIMESTAMP flag on any socket
(via the SO_TIMESTAMP socket option or the SIOCGSTAMP ioctl), we start
recording timestamps for *every* packet coming into the system.  We do
this because timestamps are recorded very early by the net core,
before the upper protocol layers have a chance to look up the
socket(s) associated with each incoming packet.

A number of common apps enable timestamps, including ping, tcpdump,
and named.  named probably has the biggest impact -- most people don't
leave ping running all the time, and expect that leaving tcpdump
running will cost a bit of performance, but it's unfortunate that the
mere presence of a local caching nameserver has a global negative
effect on incoming packet overhead.

This patchset creates a mechanism for protocols to opt in to handling
timestamps at a higher layer, and uses this mechanism to implement
per-socket SOCK_TIMESTAMP support for several common protocols.
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