Many of today’s commercial routers implement SIP ALG (Application-level gateway), coming with this feature enabled by default. While ALG could help in solving NAT related problems, the fact is that many routers’ ALG implementations are wrong and break SIP.
The article goes on to explain why the implementation is broken, and how to disable it in several brands of routers. Certain VoIP adapter manufacturers also recommend disabling this feature if you are having problems with SIP registration, not being able to receive a call or one-way audio. But note that this issue can affect any type of SIP-based communications, regardless of hardware or software used.
EDIT (May, 2018): For information on another issue that may cause problems when you switch routers, see this DSLReports thread: SIP registration times.
I actually just discovered this a couple of weeks ago when I swapped out my SPA3102 with the new Obihai. I had the SPA3102 between my DSL modem (set to full bridging) and my D-Link DGL-4300 router because I gave up on fighting to get it working behind the router – where I would rather have had it.
I’d been testing the Obihai in the office connected to a cable drop at my desk via a Motorola Surfboard SBG6580 modem/router. It worked beautifully with no port forwarding or tweaking needed – which I attributed to some special juju in the Obihai unit that is lacking in the SPA3102. Based on this I took it home, took the SPA3102 out of the mix, reconfigured the D-Link router w/ my IP information and plugged the Obihai into one of the LAN ports on the router. I was able to make calls without issue – but incoming calls just weren’t happening.
After digging through the settings on the router I found a list of “ALG” items checked by default. One of those was SIP. After unchecking that one and rebooting the router the Obihai worked as expected on all incoming and outgoing calls. I then spent some time Googling ALG and found information explaining what you’ve mentioned above. What it’s meant to do it does ineffectively, or even breaks.
I wish this information had been more prevalent 6 years ago when I initially set up VoIP at my house – it would have saved me days of aggravation … and potentially a few grey hairs 🙂
This stuff kills me all the time. What I don’t get is why they added it to begin with. I don’t know anyone who needed it, but it’s ALWAYS broken stuff.
I saw one where they advertised that with QoS and SIP ALG that it was perfect for voip. When you go to the FAQ on their website, it had something like “Why does VoIP not work with this router?” “The SIP ALG feature is currently broken and corrupts VoIP traffic. Fixing this is on our roadmap. There is no way to disable this feature in the interim.”
The damn thing had heuristics to find it on any port I ran it on, so switching to an alternate port wasn’t an option. Piece of crap.
Paul –
I just ordered a new NETGEAR WNDR3700 ( http://www.newegg.com/Product/Product.aspx?Item=N82E16833122326 ) a couple of days ago. Your tale of being unable to disable that “feature” if it’s available and enabled now has me worried … as I did no research into that aspect in regard to this router.
I guess worst case scenario I can throw DD-WRT on it but – argh. My time and patience for home networking / computer repair issues have been at an all-time low lately – especially after banging my head on those kinds of things all day every day at work.
I’ll just keep my fingers crossed that if it’s there and enabled I can disable it!
Sorry, I posted the link wrong in the last post and I can’t edit my comment so just don’t aprove it.
The reason you can’t connect calls or receive one way audio is down to SIP not being able to work with NAT. To get around this it uses a STUN server but even STUN has issues with symetric NATs. You have probably heard of this before regarding SIP but from all the resources I found NONE OF THEM tell you why and they all say you can’t get it to work with a symetric NAT. Well you can, but it took me ages to find out how it all works. I ended up writing One Way Audio Fix which explains exactly the cause and how to fix it…even with ANY type of NAT.
You may be surprised but this solution is nowhere to be found on the internet which is what prompted me to write it.
NOTICE: All comments above this one were imported from the original Michigan Telephone Blog and may or may not be relevant to the edited article above.