Once a player joins, he has access to multiplayer.get_peers() to get the peer ids.
However, almost every online game requires player ids at some point. Most commonly in videogames, there is a player table, with their IDs to the left (think pressing Tab in FPS, or RTS lobbies before starting a scenario/map)
The usage of a player id should be included with netfox, and stored in a singleton, let's name it NetworkCommonData

It currently has:

var local_player_id: int

# This is for converting peer ids to player ids, either using this or a get function
# <peer_id, player_id>
var peer_ids_to_player_ids: Dictionary
var player_ids_to_peer_ids: Dictionary
var local_session_id: int

player_id is also useful in serialization and deserialization, as it's only 1 byte (may I say, half a byte, since 0-16 is the common)
compared to sending peer_id (4 bytes, u_32)

A good practical example is also sharing IDs. See #242 and its solution which requires having the player id.

session_id is useful for the server, to store replays, or save files. Personally for the replay manager I have made, I use this to store replays in the following path user://gamenamefolder/Session_ID/replaydata.replay
and I plan to implement a similar system to netfox, and session id is required.

The variable player_joined_at_tick: int is required for the replay system, and for implementing state diffs. So our above singleton now looks like this, in its final form:

var local_player_id: int
var local_session_id: int
var local_player_joined_at_tick: int
var local_avatar: Node #You can change the type, but basically you can reference your local avatar from any class

# This is for converting peer ids to player ids, either using this or a get function
# <peer_id, player_id>
var peer_ids_to_player_ids: Dictionary
var player_ids_to_peer_ids: Dictionary

var players_joined_at_tick: Dictionary #<player_id, tick>

This singleton's values are filled before/after NetworkTime.is_initial_sync_done()