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In the software I used, same "node" needs to join an IPv4 multicast group (to respond to the network protocol requests) but also needs to send requests to the same IPv4 multicast group (in order to get responses from other nodes).

With some WiFi routers, this gets broken. Once an Android process running on a phone (connected to the WiFi router) successfully joins the multicast group, it gets blocked for about 5 minutes from broadcasting. When trying to send a request, there is no error in the programmatic layer, but the routers seem to block the traffic. [The way I prove it: I see the logs of the other nodes and they get nothing]. After 5 minutes, it starts working again. If I disable the group joining code, there is no issue with the requests at all.

The interesting thing is that it depends on the router. I have 2 TPLink routers, one works fine and one blocks.

  • How can this be explained? debugged further? resolved?
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What's relevant here is the "switch" part of your device. A dumb switch process multicast as broadcast. A smart switch will do IGMP snooping, restricting multicast distribution to live subscribers.

Cheap consumer grade mart switches are known to misbehave regarding the IGMP management. I would look here first. Open the administration console of your device, and look for IGMP settings.

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  • Thanks. My TPLink router used to reproduce the issue has some IGMP settings under "Network" -> "IPTV". By default, "IGMP Proxy" is turned ON and set to "V2". It's possible to change it to "V3", or turn OFF. No other IGMP related configuration I was able to find. From the initial tests, I don't see any behavior change.
    – Max
    Commented Mar 29, 2018 at 5:30
  • Did you turn it off entirely? On the downside, the router should send multicast to all ports and all wifi interfaces (as broadcast packets). But the five minutes delay should go away, if it was related to an IGMP incompatibility.
    – Strangelovian
    Commented Mar 29, 2018 at 12:47
  • Yep, I disable the "IGMP Proxy" via the Management GUI, and restarted the router.
    – Max
    Commented Apr 2, 2018 at 7:41

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