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From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net-next] fq_codel: report congestion
 notification at enqueue time
Date: Thu, 28 Jun 2012 22:29:34 -0700 (PDT)
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Fri, 29 Jun 2012 07:24:08 +0200

> By the way, I am not sure NET_XMIT_CN is correctly used in RED.
> 
> Or maybe my understanding of NET_XMIT_CN is wrong.
> 
> If a the packet is dropped in enqueue(), why use NET_XMIT_CN instead of 
> NET_XMIT_DROP ?
> 
> This seems to mean : I dropped _this_ packet, but dont worry too much, I
> might accept other packets, so please go on...

I am pretty sure the behavior in RED is intentional.

It's a soft push back on TCP.

We're taking this path when we are unable to sucessfully ECN mark a
packet.  But our intention was to do so.