Hello all once again,
I tried running the attached ping sweeper yesterday evening as is and
didnt get particularly plausible looking results. I therefore decided
to increase the upper limit of the size of ping packets sent and let the
script run over night while the connection was quiet.
Here is a screen shot of the resulting graph which does appear to have a
stepped appearance, but perhaps not as expected?
http://imgur.com/RjmT8Qh
This test was ran on a BT Infinity VDSL/FTTC connection with the modem
plugged directly into a CentOS 6 machine which is doing PPPoE. The
connection is synced at 80mbit down and 20mbit up. BT restrict
downstream speed to 77.44Mbps IP traffic.
I can run the test on a slower BT connection over the week end if anyone
is interested in the results?
Alan
On 21/09/14 19:35, Sebastian Moeller wrote:
Hi Dave, hi Andy,
On Sep 20, 2014, at 19:55 , Dave Taht <dave.taht@xxxxxxxxx> wrote:
We'd had a very long thread on cerowrt-devel and in the end sebastian
(I think) had developed some scripts to exaustively (it took hours)
derive the right encapsulation frame size on a link. I can't find the
relevant link right now, ccing that list…
I am certainly not the first to have looked at ATM encapsulation effects on DSL-links, e.g. Jesper Dangaard Brouer wrote a thesis about this topic (see http://www.adsl-optimizer.dk) and together with Russel Stuart (http://ace-host.stuart.id.au/russell/files/tc/tc-atm/) I believe they taught the linux kernel about how to account for encapsulation. What you need to tell the kernel is whether or not you have ATM encapsulation (ATM is weird in that each ip Packet gets chopped into 48 byte cells, with the last partially full cell padded) and the per packet overhead on your link. You can either get this information from your ISP and/or from the DSL-modem’s information page, but both are not guaranteed to be available/useful. So I set out to empirically deduce this information from measurements on my own link. I naively started out with using ICMP echo requests as probes (as I easily could generate probe packets with different sizes with the linux/macosx ping binary), as it tur
ned out,
this works well enough, at least for relatively slow ADSL-links. So ping_sweeper6.sh (attached) is the program I use (on an otherwise idle link, typically over night) to collect ~1000 repetitions of time stamped ping packets spanning two (potential) ATM cells. I then use tc_stab_parameter_guide.m (a matlab/octave program) to read in the output of the ping_sweeper script and process the data. In short if the link runs ATM encapsulation the plot of the data needs to look like a stair with 48 byte step width, if it is just smoothly increasing the carrier is not ATM. For ATM links and only ATM links, the script also tries to figure out the per packet overhead which always worked well for me. (My home-link got recently a silent upgrade where the encapsulation changed from 40 bytes to 44 bytes (probably due to the introduction of VLAN tags), which caused some disturbances in link capacity measurements I was running at the time; so I ran my code again and lo and behold the overhead
had incre
ased, which caused the issues with the measurements, as after taking the real overhead into account the disturbances went away, but I guess I digress ;) )
Best Regards
Sebastian
On Sat, Sep 20, 2014 at 7:17 PM, Andy Furniss <adf.lists@xxxxxxxxx> wrote:
Alan Goodman wrote:
Hi,
I am looking to figure out the most fool proof way to calculate stab
overheads for ADSL/VDSL connections.
ppp0 Link encap:Point-to-Point Protocol inet addr:81.149.38.69
P-t-P:81.139.160.1 Mask:255.255.255.255 UP POINTOPOINT RUNNING NOARP
MULTICAST MTU:1492 Metric:1 RX packets:17368223 errors:0 dropped:0
overruns:0 frame:0 TX packets:12040295 errors:0 dropped:0 overruns:0
carrier:0 collisions:0 txqueuelen:100 RX bytes:17420109286 (16.2 GiB)
TX bytes:3611007028 (3.3 GiB)
I am setting a longer txqueuelen as I am not currently using any fair
queuing (buffer bloat issues with sfq)
Whatever is txqlen is on ppp there is likely some other buffer after it
- the default can hurt with eg, htb as if you don't add qdiscs to
classes it takes (last time I looked) its qlen from that.
Sfq was only ever meant for bulk, so should really be in addition to
some classification to separate interactive - I don't really get the
Hmm? sfq separates bulk from interactive pretty nicely. It tends to do
bad things to bulk as it doesn't manage queue length.
A little bit of prioritization or deprioritization for some traffic is
helpful, but most traffic is hard to classify.
bufferbloat bit, you could make the default 128 limit lower if you wanted.
htb + fq_codel, if available, is the right thing here....
http://www.bufferbloat.net/projects/cerowrt/wiki/Wondershaper_Must_Die
The connection is a BT Infinity FTTC VDSL connection synced at
80mbit/20mbit. The modem is connected directly to the ethernet port
on a server running a slightly tweaked HFSC setup that you folks
helped me set up in July - back when I was on ADSL. I am still
running pppoe I believe from my server.
I have similar since May 2013 and I still haven't got round to reading
up on everything yet :-)
I have extra geek score for using mini jumbos = running pppoe with mtu
1500 which works for me on plusnet. You need a recent pppd for this and
a nic that works with mtu >= 1508.
As for overheads, initial searching indicated that it's not easy or
maybe even truly possible like adsl.
The largest ping packet that I can fit out onto the wire is 1464
bytes:
# ping -c 2 -s 1464 -M do google.com PING google.com (31.55.166.216)
1464(1492) bytes of data. 1472 bytes from 31.55.166.216: icmp_seq=1
ttl=58 time=11.7 ms 1472 bytes from 31.55.166.216: icmp_seq=2 ttl=58
time=11.9 ms
# ping -c 2 -s 1465 -M do google.com PING google.com (31.55.166.212)
1465(1493) bytes of data. From
host81-149-38-69.in-addr.btopenworld.com (81.149.38.69) icmp_seq=1
Frag needed and DF set (mtu = 1492) From
host81-149-38-69.in-addr.btopenworld.com (81.149.38.69) icmp_seq=1
Frag needed and DF set (mtu = 1492)
You can't work out your overheads like this.
On slow uplink adsl it was possible with ping to infer the fixed part
but you needed to send loads of pings increasing in size and plot the
best time for each to make a stepped graph.
Based on this I believe overhead should be set to 28, however with 28
set as my overhead and hfsc ls m2 20000kbit ul m2 20000kbit I seem
to be loosing about 1.5mbit of upload...
Even if you could do things perfectly I would back off a few kbit just
to be safe. Timers may be different or there may be OAM/Reporting data
going up, albeit rarely.
No traffic manager enabled:
http://www.thinkbroadband.com/speedtest/results.html?id=141116089424883990118
HFSC traffic manager:
http://www.thinkbroadband.com/speedtest/results.html?id=141116216621093133034
Am I calculating overhead incorrectly?
VDSL doesn't use ATM I think the PTM it uses is 64/65 - so don't specify
atm with stab. Unfortunately stab doesn't do 64/65.
As for the fixed part - I am not sure, but roughly starting with IP as
that's what tc sees on ppp (as opposed to ip + 14 on eth)
IP
+8 for PPPOE
+14 for ethertype and macs
+4 because Openreach modem uses vlan
+2 CRC ??
+ "a few" 64/65
That's it for fixed - of course 64/65 adds another one for every 64 TBH
I didn't get the precice detail from the spec and not having looked
recently I can't remember.
BT Sin 498 does give some of this info and a couple of examples of
throughput for different frame sizes - but it's rounded to kbit which
means I couldn't work out to the byte what the overheads were.
Worse still VDSL can use link layer retransmits and the sin says that
though currently (2013) not enabled, they would be in due course. I have
no clue how these work.
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Dave Täht
https://www.bufferbloat.net/projects/make-wifi-fast
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