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Spirent KB Article
Doc ID: FAQ15028
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Spirent TestCenter: Why does the capture of RTCP and RTP frames only seen as UDP packets?
Wireshark needs some assistance to know how deep to decode a packet.
In this case RTCP and RTP is transmitted after a SIP call.
If Wireshark sees these SIP messages, it will look deeper into the UDP packets and decode accordingly.
If the SIP messages have already transpired prior to the capture taking place then wireshark doesn't know about any SIP protocol, and thus will only show RTCP and RTP packets as UDP packets.
Product : Spirent TestCenter,SIP,ALP