[OpenAFS] ADS communications issue?

Eric Chris Garrison ecgarris@iupui.edu
Fri, 11 Sep 2009 13:46:35 -0400

Okay, we "solved" the ADS/MTU problem for one supercomputer... what it
turned up was that our AFS servers were only able to do 1500 MTU, but the
router they were on accepted jumbo (9000 MTU) frames and passed them on,
so they fragmented.  We had them set up our VLAN to not accept jumbo
frames, and that supercomputer cluster was up and running fine.

However, there's another supercomputer that had similar problems that were
NOT solved by this.  In fact, the problem there turns out to be
fragmentation during aklog when talking to the AD servers, not when
talking to our AFS servers.

The traceroute shows that the DF (do not fragment) flag is set, and a
packet of 2441 was being sent, which is bigger than 1500.  It's
fragmenting somewhere closer to the ADS servers, which are themselves set
to 1500 MTU, according to their admins.

So is the DF flag necessary?  If not, how can we change that?

I'm suspecting the solution's in the network, not in AFS, however.  We
just have to convince our network engineers (for the three networks the
packets cross) to believe that. :)

As an aside, the AD admins answered a question of mine about making the
ADS tickets smaller as suggested here... by changing the flag immediately,
rather than just saying whether they could or could not.   When they did
so, it made getting new tokens impossible, aklog saying that the key
number didn't match.   The AD admins were puzzled, since they didn't think
this would generate a new key/kvno, but when they unchecked the box,
everything was okay again.

We'll have to test that before we try it again.  If they're correct, and
flipping that switch doesn't generate a new key, why does it break aklog?
 Would a server restart be necessary?  Would I need a new keytab generated
to update the KeyFile with asetkey?


Eric Chris Garrison             | Principal Mass Storage Specialist
ecgarris@iupui.edu              | Indiana University - Research Storage
W: 317-278-1207 M: 317-250-8649 | Jabber IM: ecgarris@iupui.edu