I think I've found the issue ...
On Esxi I do have :
~ # dmesg | grep vmnic
2014-05-22T16:15:52.013Z cpu0:33268)<6>tg3 : vmnic0: Link is up at 1000 Mbps, full duplex
2014-05-22T16:15:52.013Z cpu0:33268)<6>tg3 : vmnic0: Flow control is on for TX and on for RX
~ # ethtool -a vmnic0
Pause parameters for vmnic0:
but on diskstation I do have :
[ 2.339313] e1000 0000:02:00.0: eth0: (PCI:66MHz:32-bit) 00:0c:29:30:6d:ff
[ 2.339321] e1000 0000:02:00.0: eth0: Intel(R) PRO/1000 Network Connection
[ 11.247262] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None <-------------------- Issue
[ 11.250943] ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
So the Flow control is Off on the VM ...
I will have to check how to change this and check the difference.
Thanks for the idea