Very slow and inconsistent internal network speed (between, VM's on the same host) for FreeBSD 11.0+ as guest on, XCP-ng/XenServer

[Eugene Grosbein]

26.06.2019 15:11, Christian M wrote:

> Running tcpdump on the host while running iperf3 between the 12.0 VM's
> results in a lot of incorrect cksum like this.
> 
> tcpdump -i vif54.0 -v -nn| grep -i incorrect
> 172.31.16.125.63013 > 172.31.16.126.5201: Flags [.], cksum 0x7f08
> (incorrect -> 0x030f), seq 223153882:223155330, ack 1, win 1026, options
> [nop,nop,TS val 4104002274 ecr 3926764642], length 1448

Incorrect checksum shown by tcpdump is normal and expected for outgoing packets
if txcsum offload is not disabled because networking stack skips checksum calculation
offloading it to "hardware". It still indicates a bug at sending side if seen for outgoing packets
when txcsum is disabled.

Incorrect checksum shown by tcpdump for incoming packets always indicates a problem
at sending side or intermediate network (if any).

You should check output of "netstat -sp ip" and "netstat -sp tcp" for non-zero
and growing checksum error counters at receiving side.