Re: Wrong TCP checksum of packets sent by Linux guest (virtIO/vhost)

["Xie, Huawei" <huawei.xie-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

> -----Original Message-----
> From: Franck Baudin [mailto:franck.baudin@qosmos.com]
> Sent: Wednesday, September 03, 2014 10:13 PM
> To: Xie, Huawei; Gray, Mark D; Thomas Monjalon
> Cc: dev@dpdk.org; dpdk-ovs@lists.01.org
> Subject: Re: [dpdk-dev] Wrong TCP checksum of packets sent by Linux guest
> (virtIO/vhost)
> 
> Hi,
> 
> On 09/03/14 13:13, Xie, Huawei wrote:
> > Looping in the dpdk-ovs list.
> >
> > * Does the new vhost API allow a user to know if all the relevant offloads have
> > been
> > turned on/off for that interface? It seems that this is possible through the
> > virtio_net
> > structure but it would be good to get some feedback from the relevant person
> > working on DPDK (Huawei?).
> >
> > * If this is the case, then it is probably in the realm of the vswitch do the actual
> > checksum (for VM-VM) or correctly configure the NIC when sending out
> through
> > the physical interface.
> >
> > Comments?
> >
> > Mark:
> > So far not supported. This is important as well in VxLan case. For the packet
> flow
> > Guest A->  virtio -> ..->OVDK->.. -> Guest B.
> > 1) If guest A and B are on different host machines, say A and B respectively,
> and if the nic on A supports
> > vxlan checksum offload, then both guest and host needn't generate checksum,
> the nic will
> > generate checksum for both inner and outer packet.
> > 2) In VM2VM case, as it is trusted communication channel, could we negotiate
> with the guest tcp stack not to verify checksum
> > for received packet?
> The problem is that any TCP packet send by a vanilla Linux guest through
> vhost is incorrect (VM to anything, including other colocalied VMs). In
> other words, the VM cannot use TCP. QEMU options and ethtool -K csum off
> tso off ("TCP stack negociation") have no effect, maybe because the
> vhost backend is misbehaving.
> 
> Franck

Hi Franck:
I checked your original thread.
root at linux-native:~# tcpdump -vnei eth0
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
17:27:09.262926 00:1b:21:b9:9b:2c > 52:54:00:51:51:12, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 64, id 47743, offset 0, flags [DF], proto TCP (6), length 60)
   1.1.1.3.34272 > 1.1.1.2.5555: Flags [S], cksum 0x0435 (incorrect -> 0xb2dd), seq 1963818356, win 14600, options [mss 1460,sackOK,TS val 49530635 ecr 0,nop,wscale 7], length 0
17:27:09.263220 52:54:00:51:51:12 > 00:1b:21:b9:9b:2c, ethertype IPv4 (0x0800), length 60: (tos 0x0, ttl 64, id 3367, offset 0, flags [DF], proto TCP (6), length 40)
    1.1.1.2.5555 > 1.1.1.3.34272: Flags [R.], cksum 0x1db4 (correct), seq 0, ack 1963818357, win 0, length 0

The packet from the guest, received on the native machine, has "correct" checksum 0x1db4. The TCP connection is refused is due to this packet has R flag. Could you check if it is the firewall that blocks the 
connection?

Besides, I did ssh tcp test between two guest VMs,  the ssh connection could be established correctly with tx checksum off. I didn't use OVDK, but set the arp table and route table manually.
The packet flow is
	Guest A(virtio)<->vhost example->Guest B(virtio)