[ovs-discuss] Unexpected retransmissions

[Jesse Gross]

On Tue, Aug 14, 2012 at 10:37 AM, Abhishek Chanda
<abhishek.lists at gmail.com> wrote:
> Hi all,
>
> I have a number of VMs connected over OVS. One of the VM hosts a web server
> and another VM connects to it. I am trying to debug an issue where it seems
> that the client VM sends out a http packet which is received twice at the
> destination. I ran wireshark on the client VM and the switch, wireshark on
> the client's outgoing interface shows that the http packet was transmitted
> once but wireshark on the switch's corresponding incoming interface shows a
> tcp retransmission for the same packet. I assumed that the switch was
> dropping the packet. When can this happen?

If the packet was dropped somewhere in the network then you would see
the retransmission on the client VM, since it is the one running the
TCP stack.  I would run tcpdump at each interface in the path and see
what exactly is happening to the packets in between each segment.