Function negotiate 

pub fn negotiate(
    session: &SessionDescription,
    media: &MediaDescription,
) -> Negotiation

Read one stream of a peer’s description (RFC 8839 §5.3, §6).

session is the whole description because three of the four inputs are session-level or may be: a=ice-lite is session-level only, the credentials default to the session’s, and the c= line a stream inherits when it has none of its own is the session’s.