MTU/MSS testing IPv6

[Mikael Abrahamsson]

On Fri, 27 May 2016, Matthew Luckie wrote:

> Pass it an http or https URL of an object to test against.

This is great! Good work!

tbit from 2001:df0:4:4000::1:115 to 2001:67c:2b24:1000::30
  server-mss 1380, result: pmtud-fail
  app: http, url: http://www.tullverket.se/
  [  0.010] TX SYN             64  seq = 0:0
  [  0.331] RX SYN/ACK         64  seq = 0:1
  [  0.332] TX                 60  seq = 1:1
  [  0.332] TX                236  seq = 1:1(176)
  [  0.652] RX                 60  seq = 1:177
  [  0.664] RX                608  seq = 1:177(548)       ECT
  [  0.664] RX                970  seq = 549:177(910)     ECT
  [  0.664] TX                 60  seq = 177:549
  [  0.664] TX                 60  seq = 177:1459
  [  0.665] RX               1428  seq = 1459:177(1368)   ECT
  [  0.665] RX                299  seq = 2827:177(239)    ECT
  [  0.665] TX PTB           1280  mtu = 1280
  [  0.669] TX                 60  seq = 177:1459
  [  0.985] RX               1428  seq = 3066:177(1368)   ECT
  [  0.985] RX               1428  seq = 4434:177(1368)   ECT
  [  0.985] RX               1428  seq = 5802:177(1368)   ECT
  [  0.985] RX               1428  seq = 7170:177(1368)   ECT
  [  0.990] RX               1428  seq = 8538:177(1368)   ECT
  [  1.943] RX               1428  seq = 1459:177(1368)
  [  1.943] TX PTB           1280  mtu = 1280
  [  3.863] RX               1428  seq = 1459:177(1368)
  [  3.863] TX PTB           1280  mtu = 1280
  [  7.704] RX               1428  seq = 1459:177(1368)
  [  7.704] TX PTB           1280  mtu = 1280
  [ 15.384] RX RST             60  seq = 9906:177

Here it's clear (to me) that they're not acting on the PTB message (just 
keep resending large packtes).

Question is how could this be translated into a more easily understandable 
test with some suggestions on what might be wrong? Because if I send the 
above to someone who isn't an IPv6 expert, I'd have to include 10-20 lines 
of text to explain why above behaviour is wrong.

-- 
Mikael Abrahamsson    email: swmike at swm.pp.se