How to set the video quality sent to a peer

I need a bit of help with this new amazing technology called webRTC.

Let's consider this scenario for a WebRTC mesh network with three peers.
The Peer A has an HD stream from its cam and it is sharing with the other 2
peers B and C. B and C are not sending any data. The connection between A
and B is rather good and the peer B can enjoy the video at full HD. While
the connection between A and C is not as good as that between  A and B and
it does not support an HD stream, thus B is not having a smooth video
stream. Nevertheless, the connection between  C and A  would smoothly
support a lower quality video, for instance 640x480.

I would like to provide a smooth video experience to both clients. I could
reduce the quality video of the source calling getUserMedia with a lower
quality but this is not a good solution because it would reduce the quality
of the video even for the peers that support HD (B) .

What I would like to have is that the source peer ( A ) gets a stream at
its maximum quality, in this case HD. Then the quality of the video to be
sent to a peer is chosen according to the connection characteristic between
the source and that peer. In our previous example :

                   HD (1280x720)
              /------------------->     B
     A       /
  stream HD /
 (1280x720) \
             \    HD (640x480)
              \------------------->     C

 How can I achieve this result? Can I modify the SDP descriptor to set the
video resolution just for that session?