Hi all:
I found a question when using ovs userspace datapath without DPDK in
mininet.
I found the tcp checksum is incorrect in userspace. That may means the
NIC checksum offload option is useless.
Here is the what I do:
```
apt-get install mn # install the mininet and openvswitch
mn --switch ovs,datapath='user' #start mn using userspace,
datapath_type=netdev
mininet > iperf #send flow from h1 to h2
```
Than I found the tcp packet both in h1 and h2, has a incorrect checksum.
Then, I try to close the NIC’s offload option.
```
Mininet > h1 ethtool –offload h1-eth0 rx off tx off #disable offload
Mininet > h1 ethtool –offload h2-eth0 rx off tx off #disable offload
```
Now the h1 and h2 can rx/tx correct-check packet. But the performance
is poor.
So can anyone tell me why the checksum is bad when nic offload option
is enabled in userspace.
By the way, I tried kernel datapath in my computer and that work fine.
```
mn --switch ovs,datapath='kernel'
```
Thanks!
Nacht Zhuo
2017.11.27
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