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Overview of SIP and Networking Issues


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SIP Overview



SIP stands for "Session Initiation Protocol" and was created by the International Engineering Task Force under the name "RFC 2543". It allows computers (or other Internet-enabled devices) to connect to each other and communicate multimedia information, such as voice or video. SIP itself does not handle the transmission of the multimedia information, it relies on other protocols to handle this (e.g. RTP, SDP, RTSP). SIP is purely responsible for the establishment and termination of connections between devices.


In the area of IP telephony, SIP can be used in applications such as Instant Messaging, Internet phone calls, voice messaging, and as part of PBX systems. Development for such applications is being carried out in both software and hardware environments.



Express Talk and Networking



There are a few issues that come into play when you are using Express Talk on a computer that is behind a firewall or a NAT (Network Address Translator). In some scenarios, Express Talk can automatically resolves all the issues for you. In other cases, you may need to do some configuration yourself.





A firewall protects your computer from many of the dangers of the Internet by blocking your computer's Internet access ports and thus not allowing a malicious computer to contact you without your knowledge (please do not confuse the ports mentioned here with the USB or serial ports on the back of your computer, they are completely different things).


Default Ports used by Express Talk -


SIP: 5070

RTP: 8000, 8002, 8004, 8006


If you a have firewall running (like the Windows firewall or a hardware firewall), you will probably be prompted to "open" or "unblock" the aformentioned ports when you first run Express Talk. If you do not do this, you won't be able to make any phone calls!


How to unblock ports varies from setup to setup, so please refer to your firewall help documentation for further assistance.





If you have a network of computers setup and they are connecting to the Internet through a computer running Windows Internet Connection Sharing, this means you have a NAT installed and you need to read this section carefully. This also applies to owners of hardware NAT devices.


There are a couple of different ways to ensure that Express Talk works properly with your network setup. The first way is called "uPnP" and the second is called "STUN".


uPnP (Universal Plug and Play) is a technique where a networked computer can ask the NAT how to let computers outside the network find itself. The NAT will then respond by giving the computer an IP address and port number.


uPnP runs as a service by default on Windows systems. In many cases, uPnP should resolve your networking issues, but it will not work in some scenarios. For example, if you are running multiple networks using multiple NATs, uPnP will not work. Also, a lot of hardware NATs do not support uPnP, so this option cannot be used.


STUN (Simple Traversal of UDP through NAT) has the same basic concept of uPnP, but executes the concept differently. The computer on the network who wants to communicate outside the network queries a server on the public Internet. That server then queries the NAT and discovers how the private computer is viewed publicly, as well as the type of NAT (or firewall) configuration employed on the network. It then tells the private computer all this information, allowing the computer to successfully conduct two-way communication with a computer on the public Internet.


STUN has the advantage that it does not requires NATs to support any particular type of service or configuration (unlike uPnP). It has the disadvantage of not supporting all types of NATs, however. Information obtained by the STUN server from a full cone NAT is guaranteed to be accurate. Restricted Cone and Port Restricted Cone NATs may be accurate only some of the time. Information from a symmetric NAT will always be unreliable.


"How do I know what type of NAT I have?"


Download the following application made by the folks at Free World Dialup:




It should correctly determine your NAT type - although take the other information it gives with a grain of salt (it is not always accurate).

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