No ACK on SYN on debian buster in the Cloud

edit

Dear community,

I have a host (jenkins-builder) running in the Google Cloud Platform.

If I execute many HTTP requests to some external host, for instance, http://example.com - source host networking will be "blocked" for a certain time == 30 seconds.

I've captured traffic via tcpdump, and now I can see that there are no ACK packets during the "blocked" time: http://app.ep4sh.cc/paste/MLxciufIus

I do see multiple [S] but lack of [ACK] - I've double-checked the behavior on the neighbor hosts - there is no such issue on them. So in my opinion it's a particular host issue.

Could somebody suggest how to find out the root cause? I'm running on Debian buster. Thank you in advance!