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[Xen-users] network hangs / packet losses - can't find a solution

  • To: xen-users@xxxxxxxxxxxxxxxxxxx
  • From: Tomasz Chmielewski <mangoo@xxxxxxxx>
  • Date: Fri, 03 Feb 2006 13:33:13 +0100
  • Delivery-date: Fri, 03 Feb 2006 12:43:54 +0000
  • List-id: Xen user discussion <xen-users.lists.xensource.com>

I saw some other users on the list also had "network hangs" / packet losses, but I didn't find a solution.

What happens?

There are several symptoms:

1. When logged into a Guest from some other host in LAN via SSH, every few minutes (sometimes seconds), you can see that the Guest doesn't respond (i.e., you type, and it doesn't appear on the screen); after a few seconds it appears

2. Can's SSH from domain0 to guest domains:

# ssh -v
OpenSSH_4.2p1, OpenSSL 0.9.7g 11 Apr 2005
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to [] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: identity file /root/.ssh/identity type -1
debug1: identity file /root/.ssh/id_rsa type -1
debug1: identity file /root/.ssh/id_dsa type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_4.2
debug1: match: OpenSSH_4.2 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.2
debug1: SSH2_MSG_KEXINIT sent

(Here it waits forever)

I can log in from one guest domain to the other without problems.

3. Packet losses (with big packet size):

a) from domain0 to other hosts in LAN, and the way around - no losses:

ping -f -c 1000 -s 20000

b) from/to Guest domains - about 7% losses:

# ping -f -s 20000 -c 1000
PING ( 20000(20028) bytes of data.
--- ping statistics ---
1000 packets transmitted, 928 received, 7% packet loss, time 10817ms
rtt min/avg/max/mdev = 6.155/10.317/695.355/38.685 ms, pipe 3, ipg/ewma 10.828/7.298 ms

No losses with smaller packet sizes though.

The issue 1) is particularly annoying ("hangs" when SSHing), but the issue 2) is also interesting (no ability to SSH from domain0).

It all reminds me some mysterious MTU issues, but I tried setting (guessing?) different values on both domain0 and guestes, but it didn't help.

I use Xen 3.0 + bridging.

Tomasz Chmielewski

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