[AusNOG] Bufferbloat - Queue management vs Buffer filling

[Andrew Fort]

On Sun, Jan 16, 2011 at 12:02 PM, John Edwards <john at netniche.com.au> wrote:
> On 16/01/2011, at 2:47 AM, Andrew Cox wrote:
>
> One of the places I have seen this occur is where there is some kind of
> peering standoff between major carriers. The link is congested, but the
> dance of peering politics has not yet resulted in a link upgrade for
> whatever reason.

Sometimes it's politics, sometimes it's bureaucracy, but whatever the
cause, this always helps:

interface chocka 11/0
 random-detect

While I won't say everyone has that turned on, it's been a best
practice for a long time.  I know we made sure it was in "tier 1"
eyeball or content world when I lived in such a hell :-).  You can
also influence RED with DSCP or precedence marks on most platforms.

If the router buffer is always full (say as on our chocka interface),
then it will be dropping from somewhere (by default, the tail).  TCP
will be affected negatively without RED, because the ACK has to get
all the way to the end of the buffer before it gets dropped, meaning
the far end TCP speaker has to resend more traffic.  This is where big
buffers have a negative side effect, because they mess most with TCP
traffic, making it bursty and reducing overall throughput.  Just
making the buffer smaller may give you more problems than turning RED
on, because it is more likely to drop if there is a pause on any of
the interfaces that traffic is going to, which will tend to influence
TCP by limiting the maximum window size (i.e., maximum possible
throughput), because there are a lot of drops.

The takeaway from Gettys' work is that one needs to consider buffers
and importantly, their management.  There's no point managing a 1-byte
or 1-packet buffer, so naturally you tend to think of it when the
buffers are large.

To me, the fact that a networking problem recently got slashdotted
means it's been a problem for at least a decade.  Corollary: Ask the
vendors how well their software engineers understand networking. :-)

> It's hard to see it becoming relevant on most switched NBN networks. The
> likely risk will occur where some smaller players use Linux routers to
> terminate NBN connections, which will have gobs of memory and 64 bit
> architectures that can handle and auto-tune big buffers to treat every
> packet as sacred - all while re-arranging other application memory to swap
> space.

To paraphrase Mr. Bush: "I encourage competitors to build their
routers in this way".  Does this actually happen?  I could be wrong, I
thought space used for sk_buff buffers was pre-carved and finite?

> John
>
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-- 
Andrew Fort (afort at choqolat.org)