Package: general
Severity: important

Hello,

I encountered a problem while using IPv6 : on non-local networks, the bandwidth
is dramatically low, around 8 Kb, while having around 10Mb if deactivating
ipv6. Problem is the following : the bandwidth drop is due to the fact that
when a packet is sent, the next one is sent only if the ACK of the previous
packet has been transfered (as if there was no congestion window at all).
This does not affect only ipv6 connexions, as ipv4 connexions are also
presenting the same syndrom, if ipv6 is active.
On some local transmissions, there seem to be no problem, but I could not figure
why.

The following does not locate the problem, but removes some potential solutions:
Problem is not directly on tcp implementation of IPv6, as I could make it work
on local network.
Problem only happens while the IPv6 module is loaded. If ipv6 is disabled, then
there is no bandwidth problem (for example by setting "blacklist ipv6" in
/etc/modprobe.d/blacklist.conf).
Problem is not directly on the MTU, because fixing a lower MTU on the interface
(1400 for example) does not help.


Here is a sample of wireshark traces (ip have been truncated) :
address starting with 2001: is the client debian
address starting with 2a01: is the server (not running debian)
Exchange is a standard scp transfer between the two computers

93 26.585633 2001: 2a01: TCP 55611 > ssh [SYN] Seq=0
Win=5760 Len=0 MSS=1440 TSV=2652463 TSER=0 WS=7
94 26.638203 2a01: 2001: TCP ssh > 55611 [SYN, ACK] Seq=0
Ack=1 Win=65535 Len=0 MSS=1420 WS=3 TSV=730156014 TSER=2652463

After the establishment of the connexion, here are the first transmissions :

161 31.751915 2a01: 2001: SSHv2 Encrypted response
packetlen=1408
162 31.789951 2001: 2a01: TCP 55611 > ssh [ACK] Seq=2345
Ack=3617 Win=13440 Len=0 TSV=2653765 TSER=730161116
163 31.850036 2a01: 2001: SSHv2 Encrypted response
packetlen=1408
164 31.850052 2001: 2a01: TCP 55611 > ssh [ACK] Seq=2345
Ack=5025 Win=16256 Len=0 TSV=2653780 TSER=730161215

I did quite a few checks, on different connexions, and all the time the ACK
is corresponding to the previous packet.

This goes on as long as the transfer is not stopped or finishes : one packet,
and the corresponding acquitement. over and over.

Hope this helps,
Fabrice Schuler




-- System Information:
Debian Release: 6.0
APT prefers stable
APT policy: (500, 'stable')
Architecture: amd64 (x86_64)

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On Wed, Feb 16, 2011 at 05:22:42PM +0100, frenchippy@free.fr wrote:
> I encountered a problem while using IPv6 : on non-local networks, the
> bandwidth is dramatically low, around 8 Kb, while having around 10Mb if
> deactivating ipv6.

The "general" package won't get you help from the people who
actually know about IPv6 issues. I'm by no means an expert, so
please take this advice as it is, it's not necessarily specific to
your problem.

The most likely explanation for what you are seeing is that there
is a problem with your local network. We can't really help you
with that.

One known source source of slowdowns with misconfigured IPv6
setups is slow name resolution, which slows down making a
connection:

https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/313218
https://bugzilla.redhat.com/show_bug.cgi?id=459756

However, in your case this is probably not applicable; the above
won't affect the speed of a connection once established.

Are your IPv4 and IPv6 connections of the same speed? If not,
and the v6 connection is tunnelled, throttling at the tunnel
endpoint might be an issue. I've certainly had fairly
slow/unresponsive connections at times due to this.

If it's geninely a problem with networking on Linux, I think you
should get in touch with the linux-kernel folks. The netdev list
at vger.kernel.org might be a good place to start.


Regards,
Roger

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