[Bloat] I am unable to pinpoint the source of bufferbloat

[Jonathan Morton]

Latency caused by bufferbloat always appears at the bottleneck device.
Usually that is the modem, and you've given no alternative that it could
plausibly be. The modems you mention are slightly different model numbers,
but that can hide substantial differences in internal configuration.

For a typical drop-tail queue, the induced latency under load is the size
of the buffer divided by the speed of the link draining it. Assuming both
modems have a 4Mbit uplink, 550ms is consistent with a 256KB buffer, and
220ms is consistent with a 48KB buffer - neither of which would seem
excessively large to a modem builder who hasn't heard of bufferbloat.
However with a shared cable infrastructure, it is possible that the uplink
is constrained by other users on the same segment, which will skew this
calculation.

To cure it without modifying the modem, you need to move the bottleneck to
a point where you can control the buffer. You do this by introducing
traffic shaping at slightly below the advertised modem uplink speed on one
of your own machines and directing all upstream traffic through it.

- Jonathan Morton
 On Feb 9, 2013 7:27 PM, "Forums1000" <forums1000 at gmail.com> wrote:

> Hi Jonathan and Dave
>
> My entire LAN-network is gigabit. My cable subscription is 60 megabit down
> and 4 megabit up.
> Now, both my routers' WAN-port and the cable modems' LAN port are also
> gigabit. The router can route LAN to WAN and the other way around (with NAT
> and connection tracking enabled) in excess of 100 megabit.
>
> Now my cable modem is a Motorola Surfboard SV6120E and hers is a Motorola
> Surfboard CV6181E. My upload lag is 550ms and hers is only 220ms. Moreover,
> at her place there are Powerplugs in the path limiting her download to 30
> megabit instead of 60 megabit. Yet, the upload lag is much lower than mine.
> There, it also did not matter where I ran Natalyzr, the result was always
> 220ms of bufferbload.
>
> Could this still be only the modem?
>
>
> On Sat, Feb 9, 2013 at 10:52 AM, Forums1000 <forums1000 at gmail.com> wrote:
>
>> Hi everyone,
>>
>> Can anyone give some tips on how to diagnose the sources of bufferbloat?
>> According to the Netalyzr test at http://netalyzr.icsi.berkeley.edu/, I
>> have 550ms of upload bufferbloat. I tried all kinds of stuff on my Windows
>> 7 laptop:
>>
>> - For the Intel(R) 82567LF Gigabit Network Connection, I put receive and
>> transmit buffers to the lowest value of 80 (80 bytes? 80 packets? I don't
>> know). I also disabled interrupt moderation.
>> Result? Still 550ms.
>> - Then I connected my laptop directly to my cable modem, bypassing my
>> Mikrotik 450G router. Result? Still 550ms of bufferbloat.
>> - Then I put a 100 megabit switch between the cable modem an the laptop
>> (as both cable modem and Intel NIC are gigabit). Result? Still 550ms of
>> upload bufferbloat.
>>
>> I'm out of ideas now. It seems I can't do anything at all to lower
>> bufferbloat. Or the Netalyzr test is broken?:-)
>>
>> many thanks for your advice,
>> Jeroen
>>
>>
>
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