How to troubleshoot wide area network performance problem?

[Ben Scott]

On Fri, Jul 11, 2008 at 11:30 AM, Hewitt_Tech <hewitt_tech at comcast.net> wrote:
> One of my colleagues ran into a Comcast throttling problem while
> doing an rsync at a different location. He said the rsync ran at full speed
> for about 30 seconds and then basically dropped to about ten percent
> performance after that.

  That sounds like Comcast's "Speed Boost" feature.  Basically, they
give you a burst of higher bandwidth for 30 seconds when you first
start pulling packets, and then clamp back to nominal rate.  After
some inactivity (5 seconds?), they reset for the next burst.  This
makes web browsing go a lot faster, since web browsing is very bursty
(Click, load, read.  Click, load, read.).  Obviously, it's not useful
for bulk data transfers.  Unlike, say, their TCP-port-blocking or
torrent-thorttling activities, Comcast is pretty up-front about this
feature.  They even advertise it heavily on TV.

  If you want to sustain a large file transfer at steady rate, use
something that limits your file transfer to the nominal rate (not the
burst rate).  You'll get a slower initial rate, but it won't plunge
after 30 seconds.  Also, make sure your send rate is limited to the
upload speed of the Comcast feed.  Since the feed is asymmetric, it
can pull data in faster than it can send it out.  That can lead to
congestion, which leads to dropped packets, which leads to TCP
back-off, which causes your transfer rate graph to look like a bandsaw
blade.

-- Ben