Mangling a=extmap and a=mid before setLocalDescription

Hi,

I know that the idea is to not have to mangle the SDP to get some cool
things, but before that happens in the real world, is it allowed to
mangle the ID of a=extmap and the value of a=mid between createOffer()
and setLocalDescription()?

Are current browsers supposed to honor those new values given in
setLocalDescription() and use them within the RTP packets?

-- 
Iñaki Baz Castillo
<ibc@aliax.net>