Hello,
Yes I agree with your observation.
I am currently looking into the problematic scenario when both the host as well
as guest checksum offloading are enabled.
What seems strange here is on the Rx side, the tcpdump log shows that the first
two UDP packets come with wrong checksum (0x0000).
From the third instance, the checksum is correct (0xffff). I am showing this in
the log again (which I had shared earlier):
Tcpdump from Rx Log captured from PM2 running ubuntu-11.10,
root@user-desktop:~/kernel_sources/linux-2.6# tcpdump -i eth0 udp and
src 198.162.0.4 -x
tcpdump: verbose output suppressed, use -v or -vv for full protocol
decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
19:17:03.932414 IP 198.162.0.4.44467 > user-desktop.local.6895: UDP,
length 2
0x0000: 4500 001e 0000 4000 4011 ad81 c6a2 0004
0x0010: c6a2 0005 adb3 1aef 000a a9e9 0000 0000
<csum,data>
0x0020: 0000 0000 0000 0000 0000 0000 0000
19:17:15.581176 IP 198.162.0.4.44467 > user-desktop.local.6895: UDP,
length 2
0x0000: 4500 001e 0000 4000 4011 ad81 c6a2 0004
0x0010: c6a2 0005 adb3 1aef 000a 0000 a9e9 0000
<csum,data> (This
should be 0xffff, here checksum recieved as 0000)
0x0020: 0000 0000 0000 0000 0000 0000 0000
19:17:16.581620 IP 198.162.0.4.44467 > user-desktop.local.6895: UDP,
length 2
0x0000: 4500 001e 0000 4000 4011 ad81 c6a2 0004
0x0010: c6a2 0005 adb3 1aef 000a ffff a9e9 0000
<csum,data> (This
seems to be fine)
0x0020: 0000 0000 0000 0000 0000 0000 0000
19:17:17.581817 IP 198.162.0.4.44467 > user-desktop.local.6895: UDP,
length 2c
0x0000: 4500 001e 0000 4000 4011 ad81 c6a2 0004
0x0010: c6a2 0005 adb3 1aef 000a ffff a9e9 0000
<csum,data> (This
seems to be fine)
0x0020: 0000 0000 0000 0000 0000 0000 0000
4 packets captured
4 packets received by filter
0 packets dropped by kernel
Please see that from the third packet, the checksum value is correct. Can you
please throw some light on this behaviour?
Thanks & Regards,
Rakesh
________________________________
From: Jesse Gross <[email protected]>
To: Rakesh B.K <[email protected]>
Cc: "[email protected]" <[email protected]>; "[email protected]"
<[email protected]>; "[email protected]" <[email protected]>
Sent: Saturday, August 24, 2013 6:20 AM
Subject: Re: [ovs-discuss] udp checksum issue with openvswitch
On Thu, Aug 22, 2013 at 4:06 AM, Rakesh B.K <[email protected]> wrote:
> Hello,
>
> When I disable the checksum offload of ethx (both in guest and host)I am not
> able to reproduce the problem(received correct checksum 0xffff).
> When using normal linux bridge instead of OVS, the issue is not reproducible
> (I am getting proper 0xffff checksum) when I either enable or disable tx
> checksum offload on guest/host eth interfaces.
I reproduced this and it is a general problem with Linux software
emulation of UDP checksum offloading. If you keep checksum offloading
on in the guest and off in the host then you should see the problem at
all times. The issue is that when the UDP stack directly computes the
checksum it takes this into account as does the NIC. However, if it
hits the offload emulation code then nothing special is done.
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