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= Capture Replay



Sometimes the problem with industry protocols is, that the most interesting protocols live in places that are not very welcoming to IT folks.

Either too hot or too cold and usually no good coffee around.

The `Capture Replay` tool is a little helper, that can possibly help with this.

Especially when it comes to developing passive mode drivers this tool can come in very handy.

It allows to replay recorded network traffic and to directly intercept this traffic in any passive mode drivers.

Possibly it could also work with non passive drivers, but I expect synchronization to be tricky.

== Getting a Capture

In order to create a capture I usually connect a device running `WireShark` to the network.

Ideally to the replication port of a network switch so I can record all the traffic.

An alternative would be to run `WireShark` on one of the PCs/Servers having access to the traffic I'm interested in.
So if for example I wanted to work on a driver for control system `X`, capturing the traffic on one of the `X` servers is probably the simplest way to do it.
If complicance rules prevent ths a third option would be to use a network tap to record the capture.

== Replaying the Capture

Now you need to copy the `pcapng` file ideally to your development system.

The capture can be replayed by the tool you find in `plc4j/tools/capture-replay`.

Here simply look for the `target/plc4j-capture-replay-{current-last-released-version}-uber-jar.jar` and run:

[subs=attributes+]
----
   java -jar target/plc4j-capture-replay-{current-last-released-version}-uber-jar.jar --input-file {path to capture} --output-device {name of the network device} --replay-speed 1 --loop true
----

The options `input-file` and `output-device` are mandatory, the rest is optional:

- `input-file` specifies the path to the capture file (pcapng)
- `output-device` name of the network device that will be used for output (name listed when running `ifconfig` on Linux and Mac or `ipconfig` on Windows)
- `replay-speed` Speed the replay will be played back. 1 is real time, 0 or less is `as fast as possible`, 0.5 is 50% slower than the original and 2 is twice as fast
- `loop` if set to true it will automatically start replaying the capture from the start as soon as the end is reached

TIP: The replay-speed is ideal to see how your driver can cope with different volumes of traffic. So even if your driver is currently working fine, consider using this tool in order to find out if your system could cope with 50% more nodes or increased polling intervals etc.