On 28/08/2013 08:05, Tore Anderson wrote:
* Owen DeLong
On Aug 27, 2013, at 07:33 , Valdis.Kletnieks@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.