Re: [Bloat] shaper team forming up

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 14 mars 2011 à 21:55 +0200, Jonathan Morton a écrit :
> On 14 Mar, 2011, at 9:26 pm, Dave Täht wrote:
> 
> > Over the weekend, Dan Siemons uncovered a possible bad interaction
> > between ECN and the default pfifo_fast qdisc in Linux.
> > 
> > http://www.coverfire.com/archives/2011/03/13/pfifo_fast-and-ecn/
> 
> This seems to be more complicated that it appears.  It looks as though
> Linux has re-used the LSB of the old TOS field for some "link local"
> flag which is used by routing.
> 
> It's not immediately obvious whether pfifo_fast is using this new
> interpretation though.  If it isn't, the fix should be to remove the
> RTO_ONLINK bit from the mask it's using on the tos field.  The other
> half of the mask correctly excludes the ECN bits from the field.
> 

CC netdev, where linux network dev can take a look.

I would say that this is a wrong analysis : 

1) ECN uses two low order bits of TOS byte

2) pfifo_fast uses skb->priority


skb->priority = rt_tos2priority(iph->tos);

#define IPTOS_TOS_MASK            0x1E
#define IPTOS_TOS(tos)            ((tos)&IPTOS_TOS_MASK)

static inline char rt_tos2priority(u8 tos)
{
	return ip_tos2prio[IPTOS_TOS(tos)>>1];
}

No interference between two mechanisms, unless sysadmin messed up things
(skb_edit)