Dave Taht
2014-Sep-20 17:55 UTC
Re: Correctly calculating overheads on unknown connections
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... On Sat, Sep 20, 2014 at 7:17 PM, Andy Furniss <adf.lists@gmail.com> 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 theHmm? 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. > > > > -- > To unsubscribe from this list: send the line "unsubscribe lartc" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html-- Dave Täht https://www.bufferbloat.net/projects/make-wifi-fast _______________________________________________ Cerowrt-devel mailing list Cerowrt-devel@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/cerowrt-devel