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Blocking VoIP Calls?

December 2, 2005

Apparently, the Israeli government has ordered ISPs to block VoIP traffic on their networks.  The government is concerned (like the Telcos here) that VoIP traffic is being used to by-pass the international gateway facility. The ISPs have responded by saying that they’re not aware their networks are being used for this purpose.

I’m not a technical guy but I think what the ISPs are saying is that they can’t stop VoIP traffic.  I once talked to a network admin in an Asean University and he said he has found no way to block the use of Skype on their network.  The reason is that Skype uses all sorts of ports in the course of one call.  So, the most they can do is monitor the fact that a Skype conversation is in progress but they can’t stop it.

I think the same is true for VoIP calls.  How can the ISP determine which packets are being used for VoIP calls?  Aren’t VoIP IP packets identical to email or WWW IP packets? 

In this country, I know the telcos are up in arms about what they call International Simple Resale (ISR) which simply means the business of offering international calls using the Internet instead of the international gateway facility.  But the telcos have so far failed to stop the IP traffic and have relied instead on “primitive” investigation techniques such as caller ID to detect ISR activity. 

The telcos here have all the incentive to block VoIP traffic and to my knowledge, they’ve never done that.  So, maybe the Israeli government is asking too much from their ISPs. 

If I’m wrong, please set me straight.

Posted by JJ Disini at 8:00 am | permalink

Previous Comments

JJ, you can track VoIP calls by looking for SIP or H.323 packets, especially during the call setup negotiation phase. H.323 is in binary format so additional sw is needed to decode H.323 protocols across. However, there are devices avaialble through major vendors which can decode VoIP packets and stop thier transit. The cable operators in the US tried doing this for a while until the FCC put a stop to it. Essentially, the FCC says that the in the US, there should is no distinction regarding which apps run on a public IP network.

Posted by Mikko at December 3, 2005, 8:18 am

JJ, you can track VoIP calls by looking for SIP or H.323 packets, especially during the call setup negotiation phase. H.323 is in binary format so additional sw is needed to decode H.323 protocols across. However, there are devices available through major vendors which can decode VoIP packets and stop thier transit. The cable operators in the US tried doing this for a while until the FCC put a stop to it. Essentially, the FCC says that the in the US, there should is no distinction regarding which apps run on a public IP network.

Posted by Mikko at December 3, 2005, 8:19 am

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Posted by Rizalist at December 4, 2005, 7:49 am

[4] JJ, You can block essentially any VoIP calls. It's not rocket science. One way would be to buy a VoIP firewall that is designed to scan all H.323 and SIP messages as well as look for specific traffic pertinent to Skype or Vonage. Then, you can actually redirect the calls to a separate server and just drop the packets while sending a "busy signal" to the source IP address of the call.

Posted by Mikko at December 5, 2005, 1:26 pm

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