Just looking at the code and I think this is probably not optimal to use.

boolESP8266WiFiSTAClass::stationKeepAliveEnabled (){return _keepStationAliveUs !=0;}voidESP8266WiFiSTAClass::stationKeepAliveStop (){    _keepStationAliveUs = -1;// will be set to 0 at recurrent call}boolESP8266WiFiSTAClass::stationKeepAliveSetupMs (int ms){if (_keepStationAliveUs !=0 || ms <=0)returnfalse;[...]

So on other words, you can only change the value by calling stop and then wait for the next scheduled interval before you can even set a new value.
Worst case scenario is to wait for as long as the previous set interval.
It would make sense to set the interval to anything > 0 and if you want to stop it, call the stop function.
Now you just have to poll as long as you receive false to set a new interval.

Also I do miss to send (or restart the scheduler) when a wifi connection to the AP is established.
That's the most important one to send these ARP packets. If it was set to for example 10 seconds and you make a connection right after the scheduler tried sending these ARP packets, you may be unable to make a connection to another host, for about 10 seconds after successful wifi connection was established.

@TD-er please review the API change

@TD-er the API has changed, based on your use case. Setting the interval will:

• cancel everything that was planned
• immediately send a packet
• schedule the next ones based on the new interval

Novice question, is there a simple way I can test this branch in platformio?

If you are using git for this repository in your platformIO setup, then yes.
You first need to checkout this PR (help with this) then rebuild using PIO.

Sorry for replying sooo late. We had no equipment until now to reproduce the issue (of not beeing reachable via http after a while) in our wifi setups. Since a few days i have a OpenWRT AP which has this issue...
@s-hadinger implemented your changes as backport in a Tasmota test versionhttps://github.com/s-hadinger/Tasmota/tree/arp_dev
Using this version (which sends every minute Gratiuitous ARP) device keeps beeing reachable.
The issue is solved (at least for OpenWRT)

It would be great if you merge this PR that all users can benefit from this option to solve issues
of devices not reachable because ARP seems to get lost if no Gratiuitous ARP is done.

I confirm this feature is useful with some wifi routers. I ported this feature in Tasmota, and it would be great for it to be integrated natively in Arduino Core. Some other projects might benefit from it.

Per internal review comment:

• use a Ticker, but with adding a new ticker method using recurrent scheduled function once so:
  when tick is fired, function is executed once at nextyield() (instead of at nextloop())
  This is needed to keep regularity the most we can in cont space.
• move to a newexperimental::ESP8266WiFiGratuitous class

@ivankravets@platformio I need help withthis issue:

Following change does not help (because in that case arduino does not compileTicker.cpp):

-#include <Ticker.h>+#include "../../Ticker/src/Ticker.h" // instead of <Ticker.h> (platformio can't find it otherwise)

ThisTicker is ours and does not appear inhttps://platformio.org/lib/search?query=header:Ticker.h .
This is a case where a local library uses another local library.
This isnot a problem with arduino.

Moving#include <Ticker.h> from.cpp to.h didn't help

Hi@d-a-v ! Looks like these changes are going to affect a lot of PlatformIO users sinceLibrary Dependency Finder in default mode doesn't look for dependencies in source files. Probably an optimal solution would be to adddepends field to thelibrary.properties file forESP8266WiFi library, e.g.:

name=ESP8266WiFiversion=1.0author=Ivan Grokhotkovmaintainer=Ivan Grokhtkov <ivan@esp8266.com>sentence=Enables network connection (local and Internet) using the ESP8266 built-in WiFi.paragraph=With this library you can instantiate Servers, Clients and send/receive UDP packets through WiFi. The shield can connect either to open or encrypted networks (WEP, WPA). The IP address can be assigned statically or through a DHCP. The library can also manage DNS.category=Communicationurl=architectures=esp8266dot_a_linkage=truedepends=Ticker

What about just leaving thestationKeepAliveNow () function and leaving the timer on the client side? In Tasmota that's easy to add a timer for that, and it would avoid bringing yet another library to projects that don't need it.

@valeros good idea but as@s-hadinger says, it may include Ticker for nothing if gratuitous is not used (unless it is moved in a separate directory ?).
Well, I chose to short-circuit the library and use the core api instead.