Offer duplicated SDP media line, secured and unsecured version, in SRTP optional mode (thanks Bogdan Krakowski for the suggestion)

[nanang]

Currently, SRTP optional mode is implemented by putting 'crypto' attributes in unsecured media line (with RTP/AVP transport) in the SDP offer. This non-standard approach is somehow getting popular.

The suggested approach is offering duplicated media line, for secured and unsecured version, in the SDP. This approach is syntactically standard-friendly, however it may lack the way to tell UAS to choose/activate only one of the duplicated media.