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What is SRTP (Secure Real-time Transport Protocol)?

In the following section you can read about the SRTP protocol that is for message integrity, authentication and data encryption as well. Check out the definition of SRTP and why it is an important companion of RTP protocol. Furthermore, you can get to know how it is connected to UDP protocol and how the authentication process works.

The SRTP protocol specifies a profile of RTP. It provides message integrity and authentication and also encryption (Figure 1). In the picture the VoIP phones are part of the Ozeki Phone System. The usage of SRTP is optional to the usage of RTP or RTCP. SRTP protocol is good for protecting VoIP traffic so it is an advantage for voice traffic.

Figure 1 - What is SRTP?

Definition of Protocol

Protocol is a standard that specifies how the endpoints communicate with each-other. For audio and video transmission via the Internet, RTP protocol is used.

RTP (Real-time Transport Protocol)

RTP protocol is a packet format that transmits multimedia data (audio and video) over the Internet. It transmits data with real-time features. The transmission can happen over a multicast or unicast network. Real-time transport protocol is one of the important elements of VoIP technology because RTP is used in phoning, video-conferencing, etc. SRTP secures the transmissions.

UDP (User Datagram Protocol)

RTP packets are basically UDP packets. UDP is part of the communication protocols on the Internet. With the help of UDP, computer applications can send different messages, so called datagrams. UDP reveals any unreliability in a network. It is good for query-response processes and for high number of clients.


To authenticate and integrate a message, a special algorythm is used that is called HMAC-SHA1 (message authentication code).

These days VoIP traffic is delivered without being encrypted, it is not protected, so it can be disturbed and monitored. Basically, SRTP protects RTP that transports media data. VoIP technology uses RTP to transport media data and since RTP is protected, VoIP calls are protected too.

The Ozeki Phone System can encrypt voice and video data with the help of SRTP protocol.

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What is SRTP?

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