Function fingerprint_of 

pub fn fingerprint_of(
    offer: &SessionDescription,
    stream: &MediaDescription,
) -> Option<Fingerprint>

The fingerprint that applies to a stream: the media-level one, or the session-level fallback.

RFC 8122 §5 allows the attribute at either level, and a session-level value applies to every stream that does not override it. Reading only the media level is the reason a stack fails against a peer that puts one a=fingerprint at the top and none on the m= lines — which is what a browser does.