priller
join:2000-10-20
Gainesville, VA
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| reply to priller
Re: How multiple ATAs can be behind same NAT router?
Packet on the inside | 
Packet after PAT |
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priller
join:2000-10-20
Gainesville, VA
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| reply to artisticcheese
said by artisticcheese  :I thought UDP are packets are sent and received on the same port number so on router side it shall be receiving also on 5060? That's what you see on the "inside" of your network. But, the port is being changed before it leaves the WAN interface.
The combination of the IP address being translated, as well as the port, is what allows you to have multiple ATA's register to the same VoIP provider and not get confused. |
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artisticcheese
join:2004-11-09
Carrollton, TX | reply to priller
I thought UDP are packets are sent and received on the same port number so on router side it shall be receiving also on 5060? |
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priller
join:2000-10-20
Gainesville, VA
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1 edit | reply to DracoFelis
A piece you're missing is that it's really PAT. When the translation is done, your return port number is also changed.
For example, leaving the ATA the UDP SIP packet connects to the host on 5060 and says talk back to me on 5060. Now when the xlate is done, the return port is changed. Here's the connection table for one of my ATA's ....
"UDP PAT from inside:zoom5801/5060 to outside:69.255.118.xxx/1061"
... so the VoIP provider is actually talking back to me on 1061. When the packet passes back through the PAT process, that gets changed back to 5060.
The combination of the IP address and the port number being changed is why you can have multiple ATA's registered to the same provider ... all on what you "think" is 5060. But the router is smarter than we are. 
The translation table should never get confused. If it does, get a new router.
FWIW, I've got 5 VoIP lines living well together. |
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DracoFelis
Premium
join:2003-06-15
| reply to artisticcheese
said by artisticcheese :How on IP level it'll work if you have several ATA on internal network with different VOIP providers using the same SIP port number. If you make sure both the SIP (call setup) and RTP (voice) ports are different, than it works quite well. Because in that case, you use the ports to uniquely identify the adapter.
OTOH you asked about the same ports being used. In that case, it's much more of a YMMV thing. Because when you are using the same ports, the ONLY THING that is keeping the mapping (of port to adapter) correct is the NAT tables in your router (in much the same way that the NAT tables in your router allow multiple PCs on your LAN to web browse on port 80 at the same time). This will sometimes work, but there is many things that could go wrong (which is why I have my multiple adapters on different ports).
For example, if you have each adapter (that is using the same ports) registered with a different provider, the NAT tables (in your router) may know (if it's a smart enough router) that inbound calls from the IP address of provider x go to the adapter that is "registered" with provider x. But how is the router supposed to know which adapter to send a call to, when that "provider" is registered on multiple adapters behind your LAN? And, even worse, how is your router supposed to know which adapter to send an anonymous (call from a non-registered party) call supposed to (in the case of different ports, the port numbers keep this straight, but the NAT tables in the router don't have such queues when different port numbers are used).
Bottom line:
I personally prefer to keep my VoIP adapters (behind my home NAT router, and a single public IP) all on different ports, to avoid problems. Yes, I realize that things might work OK with just my router's NAT, but that would still be "asking for trouble". But by keeping the ports (I use for VoIP) unique, I keep things unambiguous (and therefore avoid possible headaches as to which adapter which signal is for). |
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