On Sep 20, 2014, at 19:55 , Dave Taht <dave.taht@gmail.com> 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 turned 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 increased, which caused the issues with the measurements, as after taking the real overhead into account the disturbances went away, but I guess I digress ;) )