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VoIP Hacks by Theodore Wallingford

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Hack #82. Dig into SDP

No SIP-based telephone call happens without Session Description Protocol, and there's an SDP message inside every SIP Invite.

Knowing what SDP messages look like will help you identify the cause of failed calls that are rooted in incompatible phones. That's right; if the phones (or softphones) calling each other with SIP don't have (or aren't configured to use) the same media codecs, they won't be able to talk to each other! Of course, the phone isn't going to say, "Hey dude, I don't have the right codec!" It's more than likely just going to give you a busy signal. So you'll need to dig into SDP to find out if a mismatch of media capabilities is to blame.

SDP is an essential part of SIP call signaling. Its elements are text tokens sent in SIP packets with the SDP content-type header. These tokens advertise the capabilities and requirements of each endpoint according to the parameters of the application, be it a telephone call, instant message, or something else.

During call setup, specifically during the SIP INVITE method, the SDP payload is sent from one endpoint to the other. A SIP 200 OK response indicates agreement with the SDP parameters, and a 4xx response indicates disagreement or incapability. For a much deeper discussion on SIP, have a peek inside my book, Switching to VoIP (O'Reilly).

Inspect Successful Capabilities Negotiation

Using Ethereal configured with the same filter string from "Peek Inside of SIP Packets" [Hack #81] , you can capture a successful ...

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