Troubleshooting networking (between nodes)

I think this is one of the hardest (maybe I don't know enough). But here is a good start:

https://gravitational.com/blog/troubleshooting-kubernetes-networking/

Basically, curl is a very important tool since most pods have very limited commands you can run.

And here are some very basic services and how it works within pods:
https://kubernetes.io/docs/concepts/services-networking/service/#virtual-ips-and-service-proxies

For discovering services, check out this DNS docs:
https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/