I have been graphing ping (IPV4) responses times/vs packet size. I was expecting to see a discontinuity of around 2*MTU/BandWidth in response time around the MTU boundary (actually, around 1464 byte, MTU is 1492 on this particular segment). The rationale for my expectation is that, 2*MTU/BW being the roundtrip time of a packet with 1 MTU size, packet fragmentation will increase roundtrip times in steps of twice that amount.
Alas, that's not what I see. Using
# ping -f -c 30 -s $sz my.host
I see a linear dependence between packet size and roundtrip like follows (size, min avg max, ms.), but no discontinuities of sort:
1159 40.575 41.778 47.200 1220 41.392 42.145 45.420 1281 41.921 43.461 46.974 1342 42.498 43.840 52.638 1403 42.813 44.037 49.272 1464 43.741 45.455 49.795 1525 45.382 50.406 59.891 1586 45.579 55.605 66.309 1647 47.498 53.464 64.518 1708 49.373 73.681 84.820 1769 49.726 80.030 101.057
So I do not know if I am being tripped by something in the way packets are being sent/reported or if my reasoning is wrong some way or the other (and in these case where is the error).
It's and ADSL llink to the internet that goes through 100MB wired ethernet, crosses a FGT60 and a zyxel on its way out, and tracepath (also ping -M do) report 1492 as MTU. I am testing from a FC22 linux box. I do not know, but it seems to me that the outside details of the network should not matter (much) viz. the qualitative result I am missing, because fragmentation should happen (at least) before leaving the local interface past the 1500 MTU mark (that's the ethernet connection MTU)
Edit: And the (hidden) faulty piece of reasoning lies in equating the minimum transmission unit (which for tcpV4 over ethernet would be something around 64 bytes), lets call it the mTU, to the MTU - ie, in assuming that all packets will be padded to the MTU. That not being the case, the discontinuity due to fragmentation should be around mTU/Bw, around 20 times smaller than MTU/Bw and likely to be swallowed by the jitter due to varying network conditions