[ntp:questions] Detecting bufferbloat via ntp?

Dave Täht d at taht.net
Wed Feb 16 14:13:02 UTC 2011


Terje Mathisen <"terje.mathisen at tmsw.no"> writes:

> Rick Jones wrote:
>> Kevin Oberman<oberman at es.net>  wrote:
>>
>>> No, you probably won't. Both theoretical and empirical information
>>> shows that overly large windows are not a good thing. This is the
>>> reason all modern network stacks have implemented dynamic window
>>> sizing.
>>
>>> As far as I know, Linux, MacOS (I think), Windows, and BSD (at least
>>> FreeBSD) all do this and do it better then it is possible to do
>>> manually. N.B. Windows XP probably does not qualify as "modern".
>>
>> Sadly, I see Linux's dynamic window sizing take the window to 4MB when
>> 128KB would do.  I'm not familiar with the behaviour of the other
>> stacks'

I did a little testing with rick a couple days ago. It turned out his
problem was not in his end nodes, but somewhere in his path between his
two sites is something rather bloated.

I suggested he try tcp vegas or veno as those attempt to deal with
buffering in their own ways. Vegas is actually sort of malfunctioning
nowadays in that it was designed to cope with sane levels of buffering,
not what we are seeing today.

Actually finding the most bloated device in the path is something of a
hard problem...

> There's a huge difference between the window sizes at the ends of a
> link and those employed at the various nodes in between:
>
> The end points needs at least bandwidth*latency buffers simply to keep
> the flow going, while routers in between should have very little
> buffer space, simply because that will allow the end points to
> discover the real channel capacity much sooner.

Exactly.  Yea! You get it.

>
> You might claim that a little intermediate buffer space is a good
> thing, in that it can allow a short-term burst of packets to get
> through without having to discard other useful stuff, but only as long
> as most links have spare capacity most of the time.

A *little* is just fine. Bloated buffers - containing hundreds,
thousands, tens of thousands of packets - which is what we are seeing
today - is not.

>
> Terje

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




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