While I understand the differences between DTLS-SRTP and pure DTLS, I cannot find much information on why exactly is DTLS-SRTP really "optimized" over generic DTLS 1.2.
RFC 5764's Introduction makes an obvious statement but without specifics anywhere in that document:
Datagram Transport Layer Security (DTLS) [RFC4347] is a channel security protocol that offers integrated key management, parameter negotiation, and secure data transfer. Because DTLS data transfer protocol is generic, it is less highly optimized for use with RTP than is SRTP, which has been specifically tuned for that purpose.
I can see that in DTLS-SRTP the RTP sequence number eliminates the need to maintain another sequence number which DTLS requires. And yes, there are certain aspects like key generation for RTP, RTCP which is provided by DTLS-SRTP but in such functionality I do not see any optimization, merely convenience.
Can someone direct me to some source to read further? I am also surprised that there is no separate tag for SRTP or even DTLS on this security-specific site. I cannot create one because of lack of points.