Network Connects Slowly In Kernel 2.6 Using IPv6

[Jeff Pitman]

Hi all:

The following is just a "word of warning" about IPv6 causing initial 
connect slowness in Kernel 2.6. Target audience: packagers of kernel 
2.6 in Fedora Core 2 (maybe provide a work around or configurable shut 
off mechanism) or any kernel hackers or someone with additional 
familiarity. I'm writing this purely out of my personal experience and 
hope that it might be helpful for those on this list.

Without disabling IPv6 in /etc/modprobe.conf with "alias net-pf-10 off", 
in certain environments that evidently do not support "it"--"it" being 
quite a nebulous definition since I am far from understanding any piece 
of IPv6--initial network connections will be obnoxiously slow, both DNS 
lookups and simple "telnet www.google.com 80" tests.

The following threads provide some additional background information 
using Mandrake 10+ with Kernel 2.6:
http://www.linuxquestions.org/questions/showthread.php?s=&threadid=151050&perpage=15&pagenumber=3
http://www.linuxquestions.org/questions/showthread.php?s=&threadid=158683

And fedora test list (the second gives netstat, interface, route 
information):
http://www.redhat.com/archives/fedora-test-list/2004-March/msg00350.html
http://www.redhat.com/archives/fedora-test-list/2004-March/msg00349.html

Some websites that normally take 3 to 4 seconds to download, take up to 
20 seconds under 2.6 with IPv6 enabled.  Disabling it causes dramatic 
speed ups.

The intriguing thing is that this problem doesn't occur in all network 
topologies.  On my ADSL at home using Fedora Core 1 as a gateway, I 
have no problems at all with IPv6 running.  However, in my office (same 
machine cause it's a laptop) it's slower than dog going through an 
Intel NetStructure VPN box.  I could see Redhat, OSDL, et al. having a 
difficult time replicating and finding this since they most likely have 
a more homogenous network (vendor?) topology: Linux based servers, 
gateways, etc.  

My current system is an IBM T30 laptop running the following kernels 
under Redhat 9 (sorry i'm not using fc2-test outside of chroot 
environments yet, please forgive me..):

						Speed Rating (using feels-good(tm) benchmark)
kernel-2.4.20-28.9 (distro)		nominal
kernel-2.4.24-1.ll.rh90.ccrma	nominal
kernel-2.6.2-1.156 (arjan)		slow with IPv6; nominal without
kernel-2.4.20-30.9 (dag)		nominal
kernel-2.6.5-1.332 (arjan)		slow only on DNS with IPv6; nominal without 
(had to use links because my mouse and /udev stuff doesn't work yet)

I will be installing fc2-test on another partition quite soon so I can 
test again with my laptop under the latest environment using the tools 
mentioned in the second thread on the fedora-test list.  Sorry if I 
piss anyone off for mentioning running on an old distribution and/or if 
this off-topic.  I hope this email will be useful to those who've yet 
seen the issue and might see it pop up again.

take care,
-- 
-jeff

PS. Disclaimer: I'm just joining the list.  There's a bunch of lists, 
and I've tried to do a preliminary search 
(http://www.google.com/search?q=site%3Aredhat.com%20kernel%202.6%20network%20slow) 
to see if this has been discussed; I apologize for any redundancy if 
one exists.