Networking on AWS works fine with no additional networking as well.

Overlay networking is not required if you're running within a bunch of nodes that can see each other. Only if you get more complex will you require something, and there are quite a few solutions (Flannel, Weave, Calico, etc)

Full disclosure: I work at Google on Kubernetes

As others have mentioned there are multiple options. If you want micro-segmentation (one network per app-tier) with fine-grain access control you can use the OpenContrail plugin https://github.com/Juniper/contrail-kubernetes. It has the added advantage that you have a tenant network span k8s, openstack, vlans, or anything else you can plug into a reasonable mid-tier router.

[Disclosure: i'm currently working on this project]

I think you just need to provide a flat networking infrastructure where all nodes get their own IP and can reach each other. So you can swap flannel for project calico or your own setup if you like.