How To Send One-to-one Upstream Messaging Using Not Not Using Fcm Without Hosting A Server?

Sending messages from one device directly to another device it not supported through Firebase Cloud Messaging. -- Frank van Pufflenen - JUNE 23 2016 That was quite some time

Solution 1:

There have been lots of changes.

But:

1. Sending a message from one device to another still requires the use of a server key, which means you will need a trusted server.

2. Recommending services is off-topic here on Stack Overflow.

3. The Firebase Server SDK is for use on a trusted server.

4. No, sending device to device messages was only possible with Google Cloud Messaging by using a trusted server.

Also see my answer here for the architecture of sending downstream messages: How to send one to one message using Firebase Messaging.