When using a fritzbox with attached phones in bridge mode behind another router, manufacturer avm suggests to enable the option Keep port forwarding of the Internet router enabled for telephony, instead of adding static port forwarding in the NAT.

I wanted to see whether my router supports this, and what timeout to choose (the fritzbox has a sub-option Keep port forwarding enabled every [0.5-5] minutes.

So, I would need to find out

  1. what is the underlying technology on that option? Googling just led me to reverse port forwarding, but from what I read this is completely unrelated and just refers to reverse ssh tunnels.

  2. how can I configure the timeout in question in my Mikrotik Router (RouterOS 6.33)?

migrated from networkengineering.stackexchange.com Jan 7 '16 at 10:06

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  • Your FritzBox is consumer-grade equipment, and those questions are specifically off-topic here. You should ask on Super User. – Ron Maupin Jan 6 '16 at 15:12
  • I got a different impression when reading meta.stackexchange.com/questions/88303/…. Maybe clarify there? The router (not the fritzbox) in question is a smart router (Mikrotik), not consumer grade. – bebbi Jan 6 '16 at 18:57
  • If the question is about the other router, you should edit your question to clarify that and put in the router model and configuration. It doesn't help to guess about what you need. – Ron Maupin Jan 6 '16 at 19:03
  • I can see what you mean. But wouldn't SU say, go to networking? I had a similar experience before: being sent back and forth between programmers and codereview. In the end, the question dropped out of both due to scope 'underlap'. This scope thing can be a bit intimidating. Or is there a simple move-between-sites feature I've missed? – bebbi Jan 7 '16 at 9:40

The technology used is UDP hole punching. A keep-alive packet is sent to keep the UDP connection open.

SIP proxies seem to mostly send OPTIONS or NOTIFY messages for this.

Still have to find out whether UDP state expiration can be configured on a smart router.

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