[Bloat] DSLReports Speed Test has latency measurement built-in

jb justin at dslr.net
Tue Apr 21 06:35:21 EDT 2015

As I understand it (I thought) SO_SNDBUF and SO_RCVBUF are socket buffers
for the application layer, they do not change the TCP window size either
send or receive. Which is perhaps why they aren't used much. They don't do
much good in iperf that's for sure! Might be wrong, but I agree with the
premise - auto-tuning should work.

Regarding my own equipment, I've seen a 2012 topic about the Billion 7800N
I have, complaining it has buffer bloat. The replies to the topic suggested
using QOS to get round the problem of uploading blowing up the latency sky
high. Unfortunately it is a very popular and well regarded DSL modem at
least in Australia AND cannot be flashed with dd-wrt or anything. So I
think for me personally (and for people who use our speed test and complain
about very choppy results on upload), this is the explanation I'll be
giving: experiment with your gear at home, it'll be the problem.

Currently the servers are running at a low maximum receive window. I'll be
switching them back in a day, after I let this one guy witness the
improvement it make for his connection. He has been at me for days saying
the test has an issue because the upload on his bonded 5mbit+5mbit channel
is so choppy.


On Tue, Apr 21, 2015 at 7:37 PM, Jonathan Morton <chromatix99 at gmail.com>

> I would explain it a bit differently to David. There are a lot of
> interrelated components and concepts in TCP, and its sometimes hard to see
> which ones are relevant in a given situation.
> The key insight though is that there are two windows which are maintained
> by the sender and receiver respectively, and data can only be sent if it
> fits into BOTH windows. The receive window is effectively set by that
> sysctl, and the congestion window (maintained by the sender) is the one
> that changes dynamically.
> The correct size of both windows is the bandwidth delay product of the
> path between the two hosts. However, this size varies, so you can't set a
> single size which works in all our even most situations. The general
> approach that has the best chance of working is to set the receive window
> large and rely on the congestion window to adapt.
> Incidentally, 200ms at say 2Mbps gives a BDP of about 40KB.
> The problem with that is that in most networks today, there is
> insufficient information for the congestion window to find its ideal size.
> It will grow until it receives an unambiguous congestion signal, typically
> a lost packet or ECN flag. But that will most likely occur on queue
> overflow at the bottleneck, and due to the resulting induced delay, the
> sender will have been overdosing that queue for a while before it gets the
> signal to back off - so probably a whole bunch of packets got lost in the
> meantime. Then, after transmitting the lost packets, the sender has to wait
> for the receiver to catch up with the smaller congestion window before it
> can resume.
> Meanwhile, the receiver can't deliver any of the data it's receiving
> because the lost packets belong in front of it. If you've ever noticed a
> transfer that seems to stall and then suddenly catch up, that's due to a
> lost packet and retransmission. The effect is known as "head of line
> blocking", and can be used to detect packet loss at the application layer.
> Ironically, most hardware designers will tell you that buffers are meant
> to smooth data delivery. It's true, but only when it doesn't overflow - and
> TCP will always overflow a dumb queue if allowed to.
> Reducing the receive window, to a value below the native BDP of the path
> plus the bottleneck queue length, can be used as a crude way to prevent the
> bottleneck queue from overflowing. Then, the congestion window will grow to
> the receive window size and stay there, and TCP will enter a steady state
> where every ack results in the next packet(s) being sent. (Most receivers
> won't send an ack for every received packet, as long as none are missing.)
> However, running multiple flows in parallel using a receive window tuned
> for one flow will double the data in flight, and the queue may once again
> overflow. If you look only at aggregate throughput, you might not notice
> this because parallel TCPs tend to fill in each others' gaps. But the
> individual flow throughputs will show the same "head of line blocking"
> effect.
> - Jonathan Morton
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