How Mediaproxy works ?? how opensips and mediaproxy works together to solve NAT problems

This NAT traversal solution operates by placing a media relay in the middle between 2 SIP user-agents. It mangles the SDP messages for both of them in a way that will make the parties talk with the relay while they think they talk directly with each other.

Mediaproxy consists of 2 components:

• The OpenSIPS mediaproxy module
• An external application called MediaProxy which employs a dispatcher and multiple distributed media relays. This is available from http://ag-projects.com/MediaProxy.html (version 2.0.0 or newer is required by this module).

The mediaproxy dispatcher runs on the same machine as OpenSIPS and its purpose is to select a media relay for a call. The media relay may run on the same machine as the dispatcher or on multiple remote hosts and its purpose is to forward the streams between the calling parties. To find out more about the architecture of MediaProxy please read the documentation that comes with it.

To be able to act as a relay between the 2 user agents, the machine(s) running the module/proxy server must have a public IP address.

OpenSIPS will ask the media relay to allocate as many ports as there are media streams in the SDP offer and answer. The media relay will send back to OpenSIPS the IP address and port(s) for them. Then OpenSIPS will replace the original contact IP and RTP ports from the SDP messages with the ones provided by the media relay. By doing this, both user agents will try to contact the media relay instead of communicating directly with each other. Once the user agents contact the media relay, it will record the addresses they came from and will know where to forward packets received from the other endpoint. This is needed because the address/port the NAT box will allocate for the media streams is not known before they actually leave the NAT box. However the address of the media relay is always known (being a public IP) so the 2 endpoints know where to connect. After they do so, the relay learns their addresses and can forward packets between them.

The SIP clients that will work transparently behind NAT when using mediaproxy, are the so-called symmetric clients. The symmetric clients have the particularity that use the same port to send and receive data. This must be true for both signaling and media for a client to work transparently with mediaproxy without any configuration on the NAT box.

engage_media_proxy()

Trigger the use of MediaProxy for all the dialog requests and replies that have an SDP body. This needs to be called only once for the first INVITE in a dialog. After that it will use the dialog module to trace the dialog and automatically call use_media_proxy() on every request and reply that belongs to the dialog and has an SDP body. When the dialog ends it will also call automatically end_media_session(). All of these are called internally on dialog callbacks, so for this function to work, the dialog module must be loaded and configured.

This function is an advanced mechanism to use a media relay without having to manually call a function on each message that belongs to the dialog. However this method is less flexible, because once things were set in motion by calling this function on the first INVITE, it cannot be stopped, not even by calling end_media_session(). It will only stop when the dialog ends. Until then it will modify the SDP content of every in-dialog message to make it use a media relay. If one needs more control over the process, like starting to use mediaproxy only later in the failure route, or stopping to use mediaproxy in the failure route, then the use_media_proxy and end_media_session functions should be used, and manually called as appropriate. Using this function should NOT be mixed with either of use_media_proxy() or end_media_session().

This function can be used from REQUEST_ROUTE.

Example 1.8. Using the engage_media_proxy function

...
if (method==INVITE && !has_totag()) {
    # We can also use a specific media relay if we need to
    #$avp(media_relay) = "1.2.3.4";
    engage_media_proxy();
}
...