When SRTP is configured with a Pre-Shared Key (PSK), the device expects all incoming RTP packets to be encrypted. The required key and salt specifications are defined by the selected encryption algorithm, as described in RFC 3711 for AES-CM with HMAC-SHA1 and in RFC 7714 for AES-GCM modes. To support SRTP decryption, the RTP Roll-Over Counter (ROC) must be conveyed reliably. This is achieved using an RTP header extension formatted according to the one-byte header extension mechanism defined in RFC 8285. Because RTP header extensions are included in the SRTP authentication tag, this approach ensures integrity and guards against tampering. For detailed information on SRTP packet structure and the use of extension headers, refer to Chapter 9 of the Streaming Specification. The frequency of ROC transmission is determined by the streaming source, but it is recommended to include the ROC at least once every few seconds to maintain synchronization and enhance robustness.
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