Playing FeverLink3 (version 0.2.1)

FeverLink3 lets two or more people play the same Transport Fever 3 game together. One player hosts, the others join with a code. It is built into the game: there is no launcher or program to run. Players connect through Steam: no port forwarding, no router settings, no VPN and nothing to open in the firewall. FeverLink3 is in beta: keep backups of the saves you play with, and tell the maintainer (Discord azalshehri) what happens.

Licence: FeverLink3 is free for noncommercial use (PolyForm Noncommercial 1.0.0, see LICENSE); commercial use needs a separate licence from the maintainer.

What you need

• Windows 10 or 11, and Transport Fever 3 on Steam, the same game version as the host (shown at the bottom right of the main menu).
• Steam running and logged in (it is, if you start the game from Steam).

1. Install (once per PC): copy two things into the game folder

You install FeverLink3 by hand only once. After that it updates itself. There is no installer:

1. Unzip FeverLink3-<version>.zip: right-click it, Extract All. Open the unzipped folder: it holds winhttp.dll, a folder called mods, and some text files. Do not run anything from inside the zip file itself.
2. Close Transport Fever 3 if it is running.
3. Open the game’s folder: in Steam, right-click Transport Fever 3, choose Manage, then Browse local files. A folder opens that contains TransportFever3.exe and a folder called mods.
4. In the unzipped folder select winhttp.dll and the mods folder, copy them (Ctrl+C), go to the game’s folder and paste (Ctrl+V).
   • Windows asks about the mods folder that already exists: choose Yes / Merge. Nothing of the game is replaced; the mod is added as mods\release\feverlink3.
   • If Windows asks for administrator permission to paste, click Continue.
   • If Windows says winhttp.dll already exists, stop: another mod uses the same file name. Do not replace it; tell the maintainer.
5. Check: the game’s folder now contains winhttp.dll next to TransportFever3.exe, and mods\release\feverlink3 exists.

Updates come by themselves. Once per game start (never during a session) FeverLink3 asks GitHub whether a newer version exists. If so, it downloads it inside the game, only from the official GitHub releases over HTTPS, and checks every file’s SHA-256 against the release. The window then says “restart the game to finish”: after one restart you play the new version. Outside a session the window and the pop-up also show Save and quit: it saves your world as feverlink3-restart (your own saves are not touched) and closes the game. Start the game again from Steam and load feverlink3-restart (Continue or Load Game); FeverLink3 cannot load it for you, because it has no code in the main menu. If the save does not work, the game stays open and nothing is lost. You can turn automatic updates off in the window’s Troubleshooting section. Releases are not code-signed until 1.0.0.

To update by hand: close the game and copy the new files over the old ones in the same way. If the game’s folder still has mods\release\feverlink3_1 (the mod’s folder name in version 0.1.0), delete that folder, or the game loads two copies of the mod.

To remove FeverLink3: close the game and delete winhttp.dll and the folder mods\release\feverlink3 from the game’s folder. Steam’s “Verify integrity of game files” leaves both in place.

2. Start the game

Start Transport Fever 3 from Steam as usual.

• Starting a new game: on the settings page open the Add Mods tab, click Activate under FeverLink3 Multiplayer (it is listed with the game’s own mods), then Start Game.
• In the game, click FeverLink3 in the bar at the bottom of the screen (next to the item counters). A window called FeverLink3 Multiplayer opens.
• The mod does nothing until you host or join: without a session, the game plays as it always does.
• If the window says “The FeverLink3 module is not running”, winhttp.dll is not next to TransportFever3.exe: close the game, check step 1, start the game from Steam again.

3. Hosting and joining

The host:

1. Start a game (a new game, or load one) with FeverLink3 Multiplayer activated, and click FeverLink3 in the bottom bar.
2. Press Host a game. The window shows a join code. Press Copy code (the code is put on your clipboard; the game’s text fields cannot copy by themselves) and paste it into a message to your friend with Ctrl+V (chat, Discord). Treat it like a password for the session.
3. When your friend appears under Players, press Start the session. Your world is sent to the other player, whose game reloads into it; yours keeps running after a short pause, without a loading screen. The games compare a checksum before the session runs, so a difference would stop it instead of going unnoticed. The same happens for a player who joins later.
4. While you play, Pause, 1x, 2x and 3x in the window’s Session section change the speed in every game, and the chat line under them sends a message to the others. Every player can do both.

The player who joins:

1. Start any game with FeverLink3 Multiplayer activated (a new game is fine; it is replaced) and click FeverLink3 in the bottom bar.
2. Click the field paste the join code here, press Ctrl+V to paste the host’s code, and press Join. The window shows what it is doing (contacting the host, finding a route, connected).
3. Keep the window open. When the host presses Start the session, the host’s world is sent to you (usually well under a minute), your game reloads into it, and the session runs on both games. Open the window again after the reload (the reload closes it).

Joining through a Steam invite. When the host invites you through Steam (or you click Join game on the host in your friends list), a game that already has a world loaded (with FeverLink3 Multiplayer activated) joins by itself, without opening the window. If the invite reaches you in the main menu, it waits: load any world (a new game is fine; it is replaced) and you join automatically. Until then the window shows “ invited you. Load any world and you join automatically" with **Join** and **Not now**. Transport Fever 3 has no documented way for a mod to load a save from the main menu, so a world must be loaded first. **A fix for joining by itself is in 0.2.1 and has not yet been tested between two PCs; the join code always works.**

Only join people you know. When you join, your game loads the host’s world, and a Transport Fever savegame can carry game scripts, just as a mod can. Joining a session means trusting the host the way you would trust the author of a mod.

If joining fails, the window says why (wrong code, the host is not hosting, no route found). Send the maintainer a screenshot and the log files below.

4. The Steam connection test (for problems)

Press Connection test… in the window (under the join code field) to open the Steam connection test. It checks that two players can reach each other through Steam, without hosting: your Steam ID (17 digits) is shown; paste the other player’s Steam ID into paste the other player’s Steam ID and press Test the connection. Within about 20 seconds the window shows connected with the round-trip time and whether the route is direct or through a Steam relay. A Steam ID identifies your Steam account, so share it only with the person you are testing with.

5. For testers: the Steam speed test

This tells us whether a save of about 80 MB can be sent over Steam, and how long it takes. It needs two Steam accounts on two PCs (one account can only talk to itself). Both of you do steps 1 to 3; only one of you does step 4.

1. Both start Transport Fever 3 from Steam, start a game with the mod active, and open the window (FeverLink3 in the bottom bar). Stay in the game for the whole test; do not pause it with the main menu.
2. Both press Connection test…. Each sees their own Steam ID. Send yours to the other (any chat), and paste the other’s into paste the other player’s Steam ID.
3. Optional: to try a faster send rate for the reliable mode, both start the game with the environment variable FEVERLINK3_STEAM_SENDRATE set (for example 4194304 for 4 MiB/s; the default is 1048576). Without it you measure the default. The datagram mode does not depend on it: it raises the connection’s Steam rate itself while it runs and puts it back after.
4. One of you picks the size (8 MB, 32 MB or 80 MB; start with 32 MB) and presses Run the speed test. The other does nothing. The note under the button shows the progress (“sending to your friend: 12.4 of 32 MB”), then the result, four lines: MB/s each direction and total seconds; the route and round trip; the queue and any stall; and how long an 80 MB save would take if you host and if your friend hosts. The test sends the data both ways, so it takes about twice the size divided by the speed (a minute or two for 32 MB at 1 MB/s).
5. Send us the result: the lines under the button, or the lines starting steam speed test in session.log (the file %LOCALAPPDATA%\FeverLink3\live\session.log on the PC that pressed the button). Then swap roles and run it once the other way, and once with 80 MB.

Two ways to move the data. Under the size buttons choose how the data travels: Reliable (Steam’s reliable stream, what a resync uses today), Datagrams (our own protocol over Steam’s unreliable messages, see the Steam transport notes, section 7.2; it has not run on real Steam yet), Both (reliable, then datagrams, with the same size), Parallel (the reliable stream on 1, 2 and 4 connections at once, to see whether the limit is per connection) or Auto (scaled) (the way a synced world is sent: several Steam connections at once, as many as the size needs, 3 for 32 MB and 8 for 80 MB, added one by one while the speed keeps growing; the result says how many connections were used, MB/s, how long it took and why it stopped adding connections; it has not run on real Steam yet). A series shows one block per run, each under a line == reliable (32 MB) ==; a run that fails stops the series. The reliable result keeps its four lines (plus a fifth, “Steam’s own view”, with the extremes of what Steam reported while it ran). A datagram block has: the two rates; the route and any route change during the run; for each direction how many datagrams were lost and sent again, the round trip when idle and the extra delay under load (the “queue”, the number that tells whether a voice call would suffer), the rate the controller settled at and the most datagrams in flight; the longest pause; and the time of an 80 MB save without the base64 growth.

Suggested order with a friend: Reliable 32 MB, then Auto (scaled) 32 MB and Auto (scaled) 80 MB (each player presses it once, so both directions are measured by both), then, if you like, Datagrams 32 MB. While it runs, a voice call on the same line shows whether the datagram mode leaves room for it. The detailed Steam status (steam speed test status ...) and one sample line per second go to session.log; send those lines too.

If it says Speed test failed: ... the reason follows (no route to the other PC, the connection ended, no data for 60 s). A line saying STALLED means the data stopped for more than 5 seconds at some point; say when it happened.

How a synced world travels. When the host presses Sync everyone to my world, the host’s game saves the world and FeverLink3 sends the file (about 70 to 80 MB) to every player. A player whose FeverLink3 has announced it (bulk-par-1, the default in this version) receives it over several Steam connections at once, as raw bytes written straight to the save file: the host opens as many connections as the file’s size needs (7 for 75 MB, 8 for 80 MB, none below 6 MB, at most 16) and adds them one by one only while each really adds speed and the ping of the session stays low. The window shows the progress: “Syncing: Receiving the host’s world: 63% (7 connections, 6.9 MB/s)”. The lockstep and the chat keep using the session’s own connection. If the extra connections cannot be opened, the file does not arrive complete, or nothing moves for 10 seconds, the host sends that player the world in the old way instead (slower, in pieces on the session connection) and says so in session.log (bulk transfer to <name> failed (<why>); sending the chunks instead); the other players are not affected. FEVERLINK3_BULK=off (chunks only), FEVERLINK3_BULK_CONNS=<n> (a fixed number of connections) and FEVERLINK3_BULK_MAX=<n> (another cap) are set in the host’s environment. This has not run on real Steam yet; the speed test Auto (scaled) above runs the same code between two real PCs.

Without Transport Fever 3

A friend who does not own the game can take part with the small program feverlink3-speedtest.exe, which needs only Steam (running and logged in). It has not run on real Steam yet; it is tested only against the fake Steam of the test suite. It runs the same speed test as the window between two PCs, using Valve’s test app id 480 (Spacewar) on both. Its default run takes about 10 minutes: the idle round trip; the rate ladder (1, 2 and 4 MiB/s on one connection); 2, 4, 8 and 16 connections at 1 MiB/s each (a direction skips the larger counts after one that did not scale); Auto (scaled) with 80 MB each way (exactly how a world is sent in a sync: the number of connections the size needs, the connections used, MB/s and time are in the result); and a datagram test. --classic lists the older modes, including Auto (scaled) with 8, 32 and 80 MB.

• The maintainer builds two packages with a local script: the tester package (made with -PeerSteamId <the other player's SteamID64>, which writes peer.txt) and the responder package (made without it). Each is a folder and a zip next to the repository, holding the exe, Valve’s steam_api64.dll, steam_appid.txt (480), peer.txt (tester only), a README.txt and a SHA-256 file. The Steam ID is written nowhere except peer.txt.
• Unzip, double-click feverlink3-speedtest.exe. With a peer.txt next to it the program is the tester: it connects to that Steam ID (“waiting for the other PC to start its program…” for up to 120 seconds, then once more), runs the tests one after another and writes speedtest-results.txt (date, program version, both Steam IDs, route and ping, the four-line result of each run); at the end it says which file to send. Without a peer.txt it is the responder: it prints its own Steam ID, waits, answers every tester that connects (several, one after another) and logs each test to speedtest-responder.txt. Start the responder first, then the tester.
• feverlink3-speedtest.exe --peer <SteamID64> and feverlink3-speedtest.exe --listen override the files.
• If it cannot start it says why in plain words: Steam is not running, not logged in or in Offline Mode, steam_api64.dll missing from the folder, or Steam’s own message.
• The program needs no administrator rights and only talks to the Steam network and the one other PC. To remove it, delete the folder.

What works and what does not (yet)

Seen working in the real game (played over the internet with a friend through Steam, in sessions of up to 3.5 hours, and on the maintainer’s PC):

• Two players on two PCs connect over Steam with a join code; the host’s world is sent to the friend.
• Building together: roads, rail, stations, stops, depots, lines and vehicles.
• The full game: towns grow, traffic flows, ratings change, as in single player. Nothing is switched off.
• World checks run all the time; when the worlds drifted apart, an automatic repair sync gave everyone the host’s world back (about 15 s for an 80 MB world over Steam).
• Shared speed, pause and chat.
• The in-game interface: the bar icon with its colours, the pop-up and the window.
• Report a problem over Steam: one zip with both players’ logs arrived.
• Automatic updates, on the maintainer’s PC: 0.2.1 downloaded, checked and installed itself, finished after one restart.

Not yet seen in the real game (tested automatically): putting the previous version back when an update does not start, and the switch that turns automatic updates off.

Known gaps:

• Terrain editing, ground paint and trees are not shared yet; the terrain tools are switched off during a session.
• Some extra repair syncs: the game sometimes orders its internal object ids differently on two PCs. FeverLink3 finds this within seconds and repairs it automatically.
• Mods that change the world on their own (for example an automatic line namer) cause extra repair syncs. Turn their automatic changes off while playing together. Every player needs the same mods as the host.
• Two players building at the same spot within about a second: one of the two builds can be refused (the worlds stay equal).
• All players share one company; separate companies and a dedicated server are planned.

If something goes wrong

Press Report a problem: it is in the pop-up of the bar icon while the icon is red, at the bottom of the window’s Players tab during a session, and under Troubleshooting on the start screen. If someone else’s report is still being made, the window says so: that report reaches the player who asked for it. Type a line about what you were doing, if you like. In a session the host collects the logs of every player (up to a minute; the other players see a notice that their logs were sent to the host) and you get one zip, FeverLink3-report-<date>-<id>.zip, on your Desktop (or in Documents\FeverLink3\reports). Its folder opens, and so does a GitHub page with the bug report form filled in: drag the zip onto it and say what happened. Nothing is sent anywhere by itself. What the zip holds and what is taken out of it: Privacy. This was used over Steam with a friend.

If the button does not work, send the maintainer these files instead (they hold no personal data besides your Windows user name and folder names):

• %LOCALAPPDATA%\FeverLink3\native.log
• %LOCALAPPDATA%\FeverLink3\live\mod.log and ctl_status.txt
• the game’s own log, in Steam’s folder: userdata\<number>\3493540\local\crash_dump\stdout.txt

Paste %LOCALAPPDATA%\FeverLink3 into the Explorer address bar to open the folder.

The game shows a red “Reload UI” error: note the text, send a screenshot, click Back to Main Menu, and remove FeverLink3 (step 1, “To remove”) if you want to play normally again.

Contact: Discord azalshehri.