Has the path MTU been measured for all vantage point pairs? Is it known to be 1500 or just the end-point MTUs?
That could affect your results very differently. Owen On Aug 28, 2013, at 02:26 , Emile Aben <emile.a...@ripe.net> wrote: > On 28/08/2013 08:05, Tore Anderson wrote: >> * Owen DeLong >> >>> On Aug 27, 2013, at 07:33 , valdis.kletni...@vt.edu wrote: >>> >>>> Saku Ytti and Emile Aben have numbers that say otherwise. And there must >>>> be a significantly bigger percentage of failures than "pretty close to 0", >>>> or Path MTU Discovery wouldn't have a reputation of being next to useless. >>> >>> No, their numbers describe what happens to single packets of differing >>> sizes. >>> >>> Nothing they did describes results of actually fragmented packets. >> >> Yes, it did. >> >> Hint: 1473 + 8 + 20 > > For Saku: yes. For me: that was my intention, but later I discovered the > Atlas ping does include the ICMP header in it's 'size' parameter so what > I did in effect was 1473 + 20 = 1493 (and not the 1501 I intended). > > Redid the tests to a "known good" destination where I knew interface MTU > (1500) and could tcpdump which confirmed that I was looking at > fragmentation. I also took an offline recommendation to do different > packet sizes to try to distinguish fragmentation issues from general > corruption-based packet loss. > > Results: > size = ICMP packet size, add 20 for IPv4 packet size > fail% = % of vantage points where 5 packets where sent, 0 where received. > #size fail% vantage points > 100 0.88 2963 > 300 0.77 3614 > 500 0.88 1133 > 700 1.07 3258 > 900 1.13 3614 > 1000 1.04 770 > 1100 2.04 3525 > 1200 1.91 3303 > 1300 1.76 681 > 1400 2.06 3014 > 1450 2.53 3597 > 1470 3.01 2192 > 1470 3.12 3592 > 1473 4.96 3566 > 1475 4.96 3387 > 1480 6.04 679 > 1480 4.93 3492 [*] > 1481 9.86 3489 > 1482 9.81 3567 > 1483 9.94 3118 > > There is a ~5% difference going up from 1480 to 1481. > > As to interpreting this: Leo Bicknell's observations (this is to a > "known good" host, and the RIPE Atlas vantage points may very well have > a clueful-operator bias) stand, so interpret with care. Also: roughly > 2/3 of these vantage points are behind NATs that may also have some > firewall(ish) behaviour. > > Hope this data point helps interpreting the magnitude of IPv4 > fragmentation problems. > > Emile Aben > RIPE NCC > > [*] redid the 'size 1480' experiment because the first time around it > had significantly less vantage points.
