isc.org unreachability (Re: Test your connectivity for World IPv6 Day)

[Bill Owens]

On Mon, Jun 06, 2011 at 04:25:09PM +0200, Jeroen Massar wrote:
> [Bcc to folks @ ISC so that they are aware of this thread, peek into it
> and most very likely also resolve it]
> 
> On 2011-Jun-06 16:13, Gert Doering wrote:
> > Hi,
> > 
> > On Mon, Jun 06, 2011 at 12:50:13PM +0000, Bjoern A. Zeeb wrote:
> >> ISC looks like an IO issue given the socket opens and GET / then hangs.
> >> Given them another hour or two to wake up;-)
> > 
> > Now that would be typical behaviour for PMTU problems...
> > 
> > OTOH, www.isc.org works from my home link which has an IPv6 MTU of 
> > 1400 bytes, so it's not exactly "typical"...
> 
> Seems something is dropping ICMP and thus most very likely also
> PacketTooBig...:

I did a quick scamper test earlier in the morning and it succeeded; just repeated it and the same result:

[cookiemonster:~] owens% sudo /usr/local/bin/scamper -F ipfw -I "tbit -M 1280 -u 'http://www.isc.org/' 2001:4f8:0:2::d"
tbit from 2001:468:901:1:21b:63ff:fea4:4d92 to 2001:4f8:0:2::d
 server-mss 1440, result: pmtud-success
 app: http, url: http://www.isc.org/
 [  0.049] TX SYN             64  seq = 0:0             
 [  0.135] RX SYN/ACK         64  seq = 0:1              ECT
 [  0.135] TX                 60  seq = 1:1             
 [  0.149] TX                230  seq = 1:1(170)        
 [  0.299] RX               1500  seq = 1:171(1440)     
 [  0.299] TX PTB           1280  mtu = 1280
 [  0.299] RX               1500  seq = 1441:171(1440)  
 [  0.385] RX               1280  seq = 1:171(1220)     
 [  0.385] RX               1280  seq = 1221:171(1220)  
 [  0.385] RX                500  seq = 2441:171(440)   
 [  0.385] TX                 60  seq = 171:1221        
 [  0.385] TX FIN             60  seq = 171:2441        
 [  0.385] TX FIN             60  seq = 171:2881        
 [  0.471] RX               1280  seq = 2881:171(1220)  

It goes on from there, but the shift from 1500 to 1280 is the key, and I think guarantees that PMTUD is working on the path.

Bill.