Analyzing Disconnects

When you build an online multiplayer game, you have to be aware that sometimes connections between clients and servers fail.

Disconnects might be caused by software or hardware. If any link of a connection fails, messages get delayed, lost or corrupted and the connection need to be shut down.

If this happens frequently you can usually do something about it.

Common Issues

Timeout Disconnect

The timeout disconnect is the most frequent issue, aside from problems to connect "at all".

A timeout means that an established connection fails to deliver or to acknowledge reliable messages and gets closed.

Both sides of a Photon connection (client and server) monitor if their sent reliable commands get acknowledged by the other side. If there is no ACK "immediately" (based on the current round-trip time), a reliable command gets resent.

If there is no reaction for several repeats, this means the other side can't reply properly and the connection times out.

Handling Timeouts

There is no single point of failure when you run into frequent timeouts, but there are a few common scenarios that cause issues and some ways to fix them.

If you encounter timeout disconnects frequently, you should check the amount of data you are sending. If there are spikes or if your messages/sec rate is very high, this can affect the connection quality.

Check if you can reproduce the issue on other hardware and on another network.

You can adjust the number of resends and the timing of resends. See "Solution: Tweak Resends".

Have a look at your ResentReliableCommands. Read: "Check: Resent Reliable Commands".

Traffic Issues and Buffer Full

Buffer Full issues are more or less a variant of "Out of Memory" issues. Photon servers and clients usually buffer some commands before they are actually put into a package and sent via the internet. This allows us to aggregate multiple commands into (fewer) packages.

If some side produces a lot of commands (e.g. by sending lots of big events), then the buffers might run out.

Filling buffers will also cause additional Lag: You will notice that events take longer to arrive on the other side. Operation responses are not as quick as usual.

Typical Solutions

Solution: Tweak Resends

The latest DotNet Photon library has two properties which allow you to tweak the resend timing: QuickResendAttempts and SentCountAllowance.

The QuickResendAttempts speed up repeats of reliable commands that did not get acknowledged by the receiving end. The result is a bit more traffic for a shorter delays if some message got dropped.

The SentCountAllowance defines how often the client will repeat an individual, reliable message. If the client repeats faster, it should also repeat more often.

In some cases, you see a good effect when setting QuickResendAttempts to 3 and SentCountAllowance to 7.

Set PhotonNetwork.QuickResends.

Check: Resent Reliable Commands

You should begin to monitor ResentReliableCommands. This counter goes up for each resend of a reliable command (because the acknowledgement from the server didn't arrive in time).

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If this value goes through the roof, the connection is unstable and UDP packets don't get through properly (in either direction).

Solution: Enable CRC Checks

Sometimes, packages get corrupted on the way between client and server. This is more likely when a router or network is especially busy. Some hardware or software is outright buggy corruption might happen anytime.

Photon has an optional CRC Check per package. As this takes some performance, we didn't activate this by default.

You enable CRC Checks in the client but the server will also send a CRC when you do.

Set PhotonNetwork.CrcCheckEnabled = true before you connect

Photon clients track how many packages get dropped due to enabled CRC checks.

You could monitor PhotonNetwork.PacketLossByCrcCheck.

Solution: Send Less

You can usually send less to avoid traffic issues. Doing so has a lot of different approaches:

Don't Send More Than What's Needed

Exchange only what's totally necessary. Send only relevant values and derive as much as you can from them. Optimize what you send based on the context. Try to think about what you send and how often. Non critical data should be either recomputed on the receiving side based on the data synchronized or with what's happening in game instead of forced via synchronization.

Examples:

  • In an RTS, you could send "orders" for a bunch of units when they happen. This is much leaner than sending position, rotation and velocity for each unit ten times a second. Good read: 1500 archers.

  • In a shooter, send a shot as position and direction. Bullets generally fly in a straight line, so you don't have to send individual positions every 100 ms. You can clean up a bullet when it hits anything or after it travelled "so many" units. No need to instantiate and destroy each bullet.

  • Don't send animations. Usually you can derive all animations from input and actions a player does. There is a good chance that a sent animation gets delayed and playing it too late usually looks awkward anyways.

  • Use delta compression. Send only values when they changes since last time they were sent. Use interpolation of data to smooth values on the receiving side. It's preferable over brute force synchronization and will save traffic.

Don't Send Too Much

Optimize exchanged types and data structures.

Examples:

  • Make use of bytes instead of ints for small ints, make use of ints instead of floats where possible.
  • Avoid exchanging strings at all costs and prefer enums/bytes instead.
  • Avoid exchanging custom types unless you are totally sure about what get sent.

  • Typically avoid exchanging a Vector3 for rotation of your character as most likely it only rotates around the vertical axis. So make extra code to extract the vertical rotation and only send this you save 2/3 of the size just for that parameter.

  • Don't try to combine data into arrays. Split everything into individual using stream.SendNext() for every single variable.
  • Be careful of over-using Custom Properties. If you set a lot of them in a long running game, joining players will have a lot to catch up with. When your clients drop a lot while joining a room, check this.

Use another service to download static or bigger data (e.g. maps). Photon is not built as content delivery system. It's often cheaper and easier to maintain to use HTTP-based content systems. Anything that's bigger than the Maximum Transfer Unit (MTU) will be fragmented and sent as multiple reliable packages (they have to arrive to assemble the full message again).

Don't Send Too Often

  • Lower the send rate, you should go under 10 if possible. This depends on your gameplay of course. This has a major impact on traffic. You can also use adaptive or dynamic send rate based on the user's activity or the exchanged data, this is also helping a lot.

  • Send unreliable when possible. You can use unreliable messages in most cases if you have to send another update as soon as possible. Unreliable messages never cause a repeat. Example: In an FPS, player position can usually be sent unreliable.

Things to Check

Try: Another Connection

In some cases, specific hardware can make the connection fail. Try another WiFi, router, etc. Check if another device runs better.

Try: Another Server or Region

Using the Photon Cloud, you can also use another region easily.

Hosting yourself? Prefer physical over virtual machines. Test minimum lag (round-trip time) with a client near the server (but not on the same machine or network). Think about adding servers close to your customers.

Try: Another Project

All client SDKs for Photon include some demos. Use one of those on your target platform. If the demo fails too, an issue with the connection is more likely.

Try: Lower MTU

With a setting on the client-side, you can force server and client to use an even smaller maximum package size than usual. Lowering the MTU means you need more packages to send some messages but if nothing else helped, it makes sense to try this.

The results of this are unverified and we would like to hear from you if this improved things.

Set PhotonNetwork.networkingPeer.MaximumTransferUnit = 520;.

Try: More Repeats

By default, Photon clients send each reliable command up to 6 times. If there is no ACK for it after the 5th re-send, the connection is shut down.

You can experiment with more repeats by setting PhotonNetwork.MaxResendsBeforeDisconnect.

More repeats don't guarantee a better connection though and definitely allow longer delays.

Check: The Logs

All clients have some callback to provide log messages about internal state changes and issues. You should log these messages and access them in case of problems.

You can usually increase the logging to some degree, if nothing useful shows up. Check the API reference how to do this.

Check: Traffic Stats

On some client platforms, you can enable Traffic Statistics directly in Photon. Those track various vital performance indicators and can be logged easily.

In C#, the Traffic Stats are available in the LoadBalancingPeer class as TrafficStatsGameLevel property. This provides an overview of the most interesting values.

As example, use TrafficStatsGameLevel.LongestDeltaBetweenDispatching to check the longest time between to consecutive DispatchIncomginCommands calls. If this time is more than a few milliseconds, you might have some local lag. Check LongestDeltaBetweenSending to make sure your client is frequently sending.

The TrafficStatsIncoming and TrafficStatsOutgoing properties provide more statistics for in- and outgoing bytes, commands and packages.

Tools

Wireshark

This network protocol analyzer and logger is extremely useful to find out what is actually happening on the network layer of your game. With this tool, we can have a look at the facts (networking wise).

Wireshark can be a bit intimidating but there are only a few settings you have to do when we ask you to log our game's traffic.

Install and start. The first toolbar icon will open the list of (network) interfaces.

PhotonServerSettings in Inspector
Wireshark Toolbar

You can check the box next to the interface which has traffic. In doubt, log more than one interface. Next, click "Options".

PhotonServerSettings in Inspector
Wireshark - Capture Interfaces

We don't want all your network traffic, so you have to setup a filter per checked interface. In the next dialog ("Capture Options"), find the checked interface and double click it. This opens another dialog "Interface Settings". Here you can setup a Filter.

PhotonServerSettings in Inspector
Wireshark - Interface Settings

A filter to log anything Photon related looks like so:

When you press "Start", the logging will begin when you connect. After you reproduced an issue, stop the logging (third toolbar button) and save it.

In best case, you also include a description of what you did, if the error happens regularly, how often and when it happened in this case (there are timestamps in the log). Attach a client console log, too.

Mail the .pcap and other files to us and we take a look.

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