netdev - Re: tc filter mask for ACK packets off?

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Date:	Tue, 03 Jan 2012 08:31:49 +0100
From:	Michal Kubeček <mkubecek@...e.cz>
To:	netdev@...r.kernel.org
Cc:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
Subject: Re: tc filter mask for ACK packets off?

On Saturday 31 of December 2011 21:30EN, John A. Sullivan III wrote:
> Hello, all.  I've been noticing that virtually all the documentation
> says we should prioritize ACK only packets and that they can be
> identified with match u8 0x10 0xff.  However, isn't the actual flag
> field only 6 bits longs and the first two belong to a previous 6 bit
> reserved field?

It's even worse, those two bits are in fact used for ECN (RFC 3168).

> If that is true, if ever those bits are set, our filters will
> unnecessarily break.  Shouldn't it be match u8 0x10 0x3f?

I think so.

However, by a "ACK only" packet (worth prioritizing), I would rather 
understand a packet with ACK flag without any payload, not a packet with 
ACK as the only flag. For many TCP connections, all packets except 
initial SYN and SYN-ACK and two FIN packets have ACK as the only flag. 
So my guess is you should rather prioritize all TCP packets with no 
application layer data.

                                                         Michal Kubecek

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