Switching to VoIP

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SIP Packet Inspection

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To spot the incorrect username, highlight packet 4 and scroll down the bottom pane

until you see the SIP User header in the ASCII-encoded column of the packet con-

tents on the right side.

Similar capture and analysis techniques can be used to troubleshoot access control

and dial-plan configuration problems on the SIP server, or misconfigurations on SIP

phones. If you’re daring enough to decode ASN.1, you can even use this technique to

troubleshoot H.323 signaling.

Project 11.2. Inspect SDP Capabilities Negotiation

What you need for this project:

• A SIP-compatible softPBX such as Asterisk

• A SIP phone such as X-Lite

• Ethereal software

• LAN

The Session Description Protocol is an essential part of SIP call signaling. Its ele-

ments 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, while a 4xx response indicates disagreement or incapabil-

ity. If you need a refresher on SIP signaling, ...