VoIP glossary · Real-time Transport Protocol
What is RTP?
RTP (Real-time Transport Protocol) carries the actual audio of a VoIP call as a stream of small UDP packets, separate from the SIP messages that set the call up.
RTP explained
SIP handles the call setup; RTP handles the voice. Each RTP packet has a sequence number and timestamp so the receiver can reorder packets and spot losses. RTP uses a range of UDP ports, commonly 10000–20000, that must be open on your firewall.
Because SIP and RTP travel separately, a call can connect fine while the audio fails. That is the classic sign of a NAT or firewall issue affecting RTP rather than a problem with the trunk itself.
Example
A firewall allows UDP 5060 for SIP but blocks the RTP range. Calls ring and answer, but both sides hear silence until UDP 10000–20000 is opened.
Put it into practice
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