SRTP-RTP and SRTP-SRTP Transcoding

The device supports transcoding between SRTP and RTP. The device can also enforce specific SBC legs to use SRTP and/or RTP. The device’s handling of SRTP/RTP is configured using the IP Profile parameter [SBCMediaSecurityBehaviour], which provides the following options:

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SBC passes the media as is, regardless of whether it’s RTP or SRTP (default).

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SBC legs negotiate only SRTP media lines (m=); RTP media lines are removed from the incoming SDP offer-answer.

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SBC legs negotiate only RTP media lines; SRTP media lines are removed from the incoming offer-answer.

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Each SDP offer-answer is extended (if not already) to two media lines for RTP and SRTP.

If after SDP offer-answer negotiation, an SBC leg uses RTP while the other uses SRTP, the device performs RTP-SRTP transcoding. To translate between RTP and SRTP, the following prerequisites must be met:

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At least one supported SDP "crypto" attribute.

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SRTP must be enabled - [EnableMediaSecurity] parameter configured to [1].

Transcoding where both legs are configured for SRTP is typically required to trans-encrypt and trans-decrypt. This is relevant when the MKI and Symmetric MKI parameters are enabled. In other words, both sides need to both encrypt and decrypt the outgoing and incoming SRTP packets, respectively.