Remote resource discovery using coap

I am trying to create a small academic project in the area of home automation. I explored various protocols and standards in IOT domain and find COAP close to my requirements.

The intent is pretty simple: To have a peripheral at home (like bulb, temperature sensor etc) which act as COAP server and expose certain resources and services. Since COAP uses an addressing scheme similar to web, any client having the address of this server shall be able to talk to this COAP server (i.e. peripheral) and control its attributes.

I have certain doubts about how a client not present in the same LAN as the server, can discover this server and the resources exposed by it. I understand that COAP facilitates resource discovery using CoRE link format but it seems that it primarily uses multicast which can only work within a LAN. I don't want to put a resource directory server at home and various COAP servers registering with it and then this directory server talking to a server sitting in the cloud. I want to have the ability to just install a peripheral at the home which can communicate to a remote client (like COPPER) using wifi home network. But how to discover the COAP server sitting at home from a remote location and then talk to it considering that COAP server will be on a private IP address. Is this really possible using just COAP?

As you're sitting behind a NAT, that will allow neither direct discovery nor connections; CoAP will not try to magically pierce that.

Popular ways out are:

• Use IPv6 (obliviating the use of a NAT) and (at least selectively) disable the firewall. You will still only be able to do discovery locally, but once discovered, you can connect to your devices from anywhere.
• Use a VPN to make your mobile device act as if it were in the NAT.
• Run a CoAP proxy at the router with a port open to the outside, and tell your mobile device to connect through that proxy. This does not give you local discovery either.

For the proxy and IPv6 cases, you can then still run a resource directory in your local network. That does neither need to connect to nor accept registrations from any cloud service, but it can help you do the very discovery that you'd like to do with multicast but can't because your mobile device is too far away. Both the resource directory and the proxy should be configurable to not talk to devices outside the local network unless they are authenticated.