Coop mod for Allied Assault + Breakthrough v1.21 file

Just download it man, it's almost 15 years old and noone cares, especially since devs don't work at EA anymore and they won't get a single cent from anyone's purchase anyway.

It is still sold on gog and origin. Pirating is theft

True, but since you appear to be a a by the book guy, how do you call it when your boss and the politicians rip you off ? Happens probably every day, best option buy it on Good old games ;)

man wtf

There is another mission where we are all on a truck (on rails) driving around shooting eachother while aiming at enemies. Then we spawn in the truck and die again.
Great mod!

I don't think so, but it works great in LAN if you have a couple of laptops.

It might give some lag in multiplayer, depending on the connections. There have been many optimisations in the network settings in the previous release. I haven't found someone to play it with since, so I can't tell you if it works smoothly. But I know for a fact there is still plenty room for optimisation.

You both need to adjust your in-game Network settings.
The Mod is best played via LAN or if you have a very good low ping high-speed internet this will do as well.

The lag you are having is not related to the video settings, it is related to the network settings. The missions are from singleplayer, they where never designed for usage over a network, so they are very intense on the network.

\mainta\pak9_coop_SH.pk3 and \maintt\pak9_coop_AA_BT.pk3 have a file that forces your network settings: autoexec.cfg
If you extract that file from the pk3 with any Archiver like 7-Zip into the game its /mainta/ or /maintt/ folder you can make changes to that cfg file, by opening it with a decent text editor like Notepad++.

In that autoexec.cfg are several cvar (configuration variable) settings, you are adviced to leave them as they are, except for the ones that are listed below.

You should try as the HOST and as the CLIENT which settings are best for you. Notice that some settings will not work as expected if the server limits it. Cvars with sv_ prefixed are server settings.

sv_fps
Recommendation: 30 (in most cases)
Default value: 20
This sets the frames per second the server is doing calculations and sending updates over the network to the clients.
Default is 20 if you have issues try 30.
Player snaps cvar should match for the best expiriance.

snaps
Recommendation: 30 (in most cases)
Default value: 20
Sets how often your game should try to get gameworld updates from the server.
Should match sv_fps, higher will use more bandwidth.

sv_maxrate
Recommendation: 16000 (in most cases)
Default: 18000
Sets the maximum rate for the clients.
Player rate cvar should match for the best expiriance.

rate
Recommendation: 14000 to 16000 (in most cases)
Default: 14000
Sets data exchange rate, lower rate can have a positive effect.
You need to try several settings from 10000 to 18000 and see which one works best for you.
Can not be higher than sv_maxRate

cl_timenudge
Recommendation: +/- half value of your ping shown on the scoreboard (in most cases)
Predicts for the client what is happing on the server, keep it to a low 2 digit number if possible.
Should to some extend match your average ping.
This might be changed for clients that have a bad ping to higher values, but this might also have false predictions as consequence.

cl_maxpackets
Recommendation: 60 (if you have 60 fps (frames per second))
Default: 30
Sets maximum number of data packets your game send/upload to the server per second.
Higher numbers usually provide a smoother expiriance.
It should match your fps, but it should not be higher than your fps.
If you have a high fps you can use half. For example 120 fps = cl_maxpackets 60, 90 fps = 90 or 45.

sv_flood_persecond
Recommendation: 10-15
Default: 8
This sets how many events a client can send to the server per second, this includes movement usage and chat commands.
This should usually never go over 8 but it is difficult to tell with the singleplayer elements used in this coop mod.

net_noipx
Recommendation: (maybe try 0)
I am not sure what this one really does and if it is even necessary or having a impact at all.