[Bloat] shaper team forming up

[Bloat] shaper team forming up

[Dave Täht]

There are multiple people on these lists looking into existing and
potential shaper designs. I'd like it very much if we could co-ordinate
our efforts. I've had several folk volunteer their efforts, could use more.

Also...

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/

Perhaps some people could look into this, determine the extent that this
is a real problem (it seems very bad to me that acks would get tossed
into different queues based on interpreting mmc instead of ecn)

and figure out where the right place(s) are for a patch?

-- 
Dave Taht
http://nex-6.taht.net

Re: [Bloat] shaper team forming up

[Jonathan Morton]

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.

 - Jonathan

Re: [Bloat] shaper team forming up

[Eric Dumazet]

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)