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[6bone] problem with ftp.netbsd.org
- Subject: [6bone] problem with ftp.netbsd.org
- From: [email protected] (Stephen Stuart)
- Date: Fri, 14 Jun 2002 12:17:13 -0700
- In-reply-to: Your message of "Fri, 14 Jun 2002 20:13:31 +0200." <[email protected]>
> On Thu, Jun 13, 2002 at 08:51:12PM -0700, Stephen Stuart wrote:
> | If anyone is still having problems with TCP sessions to/from
> | ftp.netbsd.org, please send me traceroutes.
> The MTU and tunnel setttings are unrelated. A TCP handshake uses
> Syn/SynAck/Ack packets without payload so these should be around 50
I understand that.
ISC receives routes from two sources: peers on the PAIX Palo Alto
switch fabric, and a tunnel to ISI. I would like to determine if the
people having problems share some property relating to that, like
"people who still report problems all show the ISI tunnel in their
> Stephen, I still cannot reach the site, from various Dutch ISPs, such as
> SURFnet (AS1103), Intouch (AS8954), BIT (AS12859) and Concepts
The traceroutes you sent were helpful, as they all show the ISI tunnel
in the traceroute.
The problem showed up as the result of two variables in our network
changing: our Palo Alto router changed from Cisco to Juniper, and our
Redwood City router changed from Cisco to FreeBSD/zebra (this is not
to be taken as a reflection on Cisco, the change was necessary for
To further comment on your observation regarding TCP - yes, the
handshake is small, and in an environment where MTU mismatch would
cause problems with larger packet sizes, a TCP session would tend to
start and then hang, as the window size ramps up and datagram sizes
approach interface MTU size. As was correctly noted by itojun, when
the *interior* tunnel MTU sizes did not match, everyone suffered
(including the iBGP session between the routers); when the MTUs were
aligned in the manner that itojun noted (the Juniper was brought down
to FreeBSD's setting) connectivity for some improved.
Remaining problems *seem* to have the ISI tunnel in common. Interface
MTU size on the tunnel interface toward the ISI router has now been
matched, just as with the interior tunnel interface.
> As Ronald has also witnessed, I see Syn, then SynAck, then I send Ack
> and the line goes dead (does this Ack ever reach the ftp6 server?).
That is an excellent question. I am not a member of the NetBSD
development team, and I do not have access to their box to determine
conclusively whether the ack really gets there. I can run tcpdump on
the router immediately upstream to see if it is attempting to deliver
it. If someone still having issues would like to coordinate a
debugging session to look at that, please contact me privately.