Launch with parameter +connect_lobby [duplicate]

I am trying to join my friend's Garry's Mod server and Steam is telling me that Garry's Mod is trying to launch with optional parameters. I don't know why, and he is telling me that he does not know.

The code it is trying to run is:

+connect p2p:(a long string of numbers is here).

What does this mean exactly? Should I be concerned? As of now, I have not connected yet.

Update: I tried to join a friend's Borderlands 2 game and it gave me a similar thing.

It tried to run:

+connect_lobby (a long string of numbers)

Is Steam just displaying something it normally doesn't?

Update: It has stopped loading them up with optional parameters, or at least showing me that it has been.

Perfectly normal.

The game has to know somehow that you intend to join your friend. Thats what the parameters are for, those allow you to be connect to the same multi-player session.

Looking at the SteamAPI documentation, this does appear to be normal:

If the user outside a game chooses to join, your game will be launched with the parameter +connect_lobby <64-bit lobby id>, or with the callback GameLobbyJoinRequested_t if they're already in-game.

I can't find anything in the documentation for +connect p2p, but I'm sure its something along the same lines as the other optional parameter you've seen.