netdev - Re: [PATCH] net/ipv4, linux-2.6.30.4

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Message-Id: <20090812.145549.228391386.davem@davemloft.net>
Date:	Wed, 12 Aug 2009 14:55:49 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	slot.daniel@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] net/ipv4, linux-2.6.30.4

From: Daniel Slot <slot.daniel@...il.com>
Date: Wed, 12 Aug 2009 20:47:44 +0200

> RFC 4653 specifies Non-Congestion Robustness (NCR) for TCP.
> In the absence of explicit congestion notification from the network, TCP
> uses loss as an indication of congestion.
> One of the ways TCP detects loss is using the arrival of three duplicate
> acknowledgments.
> However, this heuristic is not always correct,
> notably in the case when network paths reorder segments (for whatever
> reason), resulting in degraded performance.

Linux's TCP stack already has sophisticated reordering detection.

> TCP-NCR is designed to mitigate this degraded performance by increasing the
> number of duplicate acknowledgments required to trigger loss recovery,
> based on the current state of the connection, in an effort to better
> disambiguate true segment loss from segment reordering.

We already have code in the stack which tries to detect packet
reordering with a high level of sophistication.
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