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Understanding SIP and RTP

To understand the following subsections, you should understand some things about SIP and RTP. First, SIP is a signaling protocol that controls the call with methods such as INVITE, BYE, and CANCEL. The SIP protocol includes in the INVITE request information about the session (audio/video/text) using a protocol called SDP (Session Description Protocol). The information contained in the SDP describes one or more media flows configured between two user agents.

A SIP proxy never participates in the media flow, thus it is media agnostic. In other words it supports calls with whichever media are specified by UA and gateways. However, sometimes a B2BUA (back to back user agent) such as mediaproxy can be installed at the same server to treat RTP audio (that is, NAT traversal mechanism). The SDP protocol works in an Offer/Answer model. The SDP offer is embedded in the INVITE request and the answer in the 200 OK response.

Example: Excerpt from Ethereal:

Understanding SIP and RTP

The packet described in the preceding figure is an INVITE request. The INVITE request embeds the SDP packet that describes the session information. We can see there the INVITE generated on an eyeBeam phone. It is offering to use the Codec G.729 at the UDP port 8558 for Audio (I conceal the IP address for security reasons). The attribute rtpmap:101 telephone-event/8000 describes the DTMF forward method being used (RFC2833). The other device, in this case a gateway, answers the offer in the 200 OK reply.

Understanding SIP and RTP

Summary

In this chapter we have learned what OpenSER is and its main characteristics. Now you can identify the openser.cfg configuration file and its configuration blocks such as global definitions, load modules, module's parameters, main routing block, routing blocks, reply routing block, and failure routing block. Each request accepted by the proxy is processed according to the openser.cfg script. The script is organized almost in the same sequence as the SIP stateful proxy processing. Usually OpenSER operates as a loose router (SIP version 2). At the end we presented concepts about SIP and SDP.

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