The Broadband Internet Technical Advisory Group (BITAG) recently issued some guidelines for carriers to suggest ways to improve the delivery of VoIP. BITAG is a technical advisory group that makes recommendations about ways to make the Internet function better. They say that a substantial portion of global voice traffic now uses VoIP, but that there are numerous problems that stop it from working as well as it might.
The report looks in detail at how VoIP works and ways that it can be impaired or restricted. It goes on to make specific recommendations on methods for mitigating restrictions and then describers steps that ISPs, software developers and equipment vendors can take to make VoIP work better.
The primary impairment to VoIP is port blocking where the sending and receiving end of a call are unable to make the desired connection. A VoIP calls requires the synchronization of the source IP address, the destination IP address, the transport protocol being used, the source port and the destination port. The failure of both ends of the call to agree on these parameters will impair or block a VoIP call.
Another way that VoIP calls fail is when a connection is being made into or out of a network that sits behind a Network Address Translation (NAT) device which allows multiple devices in the network to share the same public IP address. These devices often don’t give any priority to VoIP and make it hard to make and keep a connection. Often there is a device sitting behind the NAT, a Application-Level Gateways (ALG), which is used to try to find a path for each kind of traffic hitting a NAT. But the ALGs often do a poor job in identifying and allowing for VoIP. particularly for VoIP that was not provided by the ISP deploying the ALG.
The other major cause of VoIP failure is compatibility problems between programs or applications that end up restricting some portion of the VoIP functionality.
BITAG has made specific recommendations that they hope will ease these problems:
- They recommend that ISPs should avoid impairing or restricting VoIP applications unless there is no technical alternative. ISPs often take steps to block certain ports on their network in an attempt to kill spam or other unwanted traffic, and some of these network management techniques also end up blocking valid VoIP applications.
- If ISPs use techniques or have policies that might impair VoIP they should provide full disclosure on-line of these policies on their web site. They should also provide a way for customers to communicate with them concerning such polices when they result in blocking ports or in other ways restricting VoIP. Interestingly, the original Net Neutrality rules issued by the FCC contained similar disclosure requirements for ISPs, but I think those rules stopped being effective when the courts overturned the Net Neutrality order.
- BITAG recommends that the port selection in consumer equipment should allow for user configuration of the ports. Many devices or firewalls make it impossible for a consumer to modify the specific port assignment needed for their VoIP application.
- They also recommend that VoIP-related ALGs should minimize the impact on VoIP other than the service provider’s VoIP. For example, a cable modem might be set to allow the cable company’s VoIP but will cause problems with VoIP from other sources.
- They recommend that VoIP applications be designed to be port-agile, meaning that the application does not require a specific port but would allow finding a port that will work at either the sending or receiving end of a VoIP call.