Kris brought up a question regarding sending channelIds in the hello message. The current implementation will save these channels and they will be available. If the channel was registered previously it will not be registered again.

The question about what should happen if a new channelId is sent with the hello message, should it be registered?
This is what the spec says about the channelIds:

channelIDs list of strings REQUIRED
If the UserAgent has a list of channelIDs it wants to be notified of, it must pass these, otherwise an empty list.

If I just read this and don't take into consideration any existing clients, it sounds like it would be ok to pass new channelIds.

This task should look into this and also look at how the Mozilla Go implemention handles this (though keep in mind that it might not follow the spec 100%).