[Bloat] Bloat on Layer 2 Was: ECN & AQM Hall of Fame?

[Jim Gettys]

On 01/31/2011 03:41 AM, Florian Lohoff wrote:
> On Sun, Jan 30, 2011 at 11:40:05PM +0100, Richard Scheffenegger wrote:
>> Hi,
>>
>> wasn't there talk about getting a Hall of Fame for networks / operators,
>> which are using and actively supporting AQM and ECN in their administrative
>> domain?
>>
>
> The problem is not only in the IP Path.
>
> In the old days i was used to be able to work interactive on a Serial
> connection with 33600 Bit/s shared with 10 Dialup users.
>
> After reading Jims blog i started to dig in our Network and found
> huge buffers on the Huawei MA5600 DSLAMs which are Ethernet based
> DSLAMs. These DSLAMs are basically invisible to the user as they
> are only in the L2 PPPoE path for the customer.
>
> I found the Buffer to be roughly 1MByte per Line. With a 1MBit/s DSL
> line this is roughly 10s worth of Buffer which i can observe on my line
> in the real world.

Yes, Dave Clark first ran into bufferbloat on a DSLAM he runs, and it 
was 6 seconds.

Even a gigabit ethernet switch needs to have buffering (which will work 
out to 8ms, worst case).


>
> There is no way of tuning this buffer based on the speed or even shrink
> it so its difficult to fix.

DOCSIS has the same issue: it's not currently tunable for the speed, and 
different people are provisioned greatly different bandwidth.

>
> ANCP + a shaper on the BRAS would be a solution which is basically how i solved
> it for me - As i have a fixed rate 1MBit/s DSL i attached a 1MBit/s rate
> limiter to my profile on the BRAS which immediatly fixed the problem for me.
>
> And the Hall of Shame probably is a good idea - but wont fix it. The
> ISPs are not aware of the problem as testing happens on empty lines with
> 30cm of wire. I sent the link to Jims blog to certain people and the DSLAM
> guys immediatly promised to contact Huawei about the non tunable buffersize.
>

Yes, please do.  And yes, it is the lack of "latency under load" tests 
that mean that most won't see it in typical testing.

That we're also trying to fix, by getting the formal tests done by 
government agencies and testing services to add such a test.  I think 
we're likely to succeed for speedtest.net and for the FCC broadband 
test.  Those who are helping other government's agencies out with 
similar tests should also be educated.

I believe in the meanwhile, and because having the tests for development 
lab and corporate environments is also really necessary and those tests 
typically aren't available in those environments, we should try to build 
some better tests here and make them available to everyone.  This is 
part of the purpose of bufferbloat.net.

And please, please make sure everyone treads gently: we're all bozos on 
this glass bus, and don't pick up stones.  Everyone (myself included) 
has been making the same class of mistakes, again and again; so do be 
gentle.  The problem is truly counter intuitive to most people.

I will be presenting at the IETF transport area meeting in Prague.
			Best regards,
				- Jim