rradina
join:2000-08-08
Chesterfield, MO
| Re: Port 80? I don't believe it's an incoming problem. Most of these ATA units work behind NAT firewalls that block all incoming ports. They do this by establishing an outbound socket connection to the VOIP provider and keeping that socket open. Once open, the VOIP provider can send information to the ATA through the already-open socket. I believe the return port on this socket is random and part of the standard TCP where the two hosts agree upon an unused port.
I think VOIP has trouble when certain types of protocols are filtered (RDP,SIP) or the well-known VOIP setup ports and TFTP are outbound blocked. This prevents the VOIP ATA from initiating service when it's booted.
In my opinion, the VOIP provider SIP back-ends could be configured to accept connections on a wide variety of seemingly random or even well-known ports (how about 100 or 1000 different ports, including 80, 443, 21, 23, etc.) If they could then program the ATA units to methodically walk that port list until a connection is established, it would be difficult for the ISP to block VOIP traffic by blocking a single port. Since the ATA accepts firmware upgrades, the VOIP provider could periodically change the list of ports to keep ahead of the ISP.
Perhaps I'm oversimplifying it but it seems like they could make the blocking process very painful for ISPs. Of course the ISP could just blatantly block traffic to all of the VOIP provider's IPs. Naturally this could become a challenge if the VOIP provider could program the ATA to not only walk a port list but also a large list of different IP addresses (perhaps proxies located throughout the country...)
I read somewhere that a Mexican ISP is introducing random packet delays that play hell with VOIP quality. Of course the random delays are evident with every application but this approach would probably be impossible for the VOIP providers to overcome. |
