Tim Wicinski <tjw.ietf@gmail.com> performed the opsdir review.

Quick question: 
My initial understanding was that newer systems usually support RTP/RTCP multiplexing, while older systems that do not support RTP/RTCP multiplexing will also not be able to understand this new attribut.  So I guess that's not the use case. That means the use case if for newer system that do know about  RTP/RTCP multiplexing as well as the 'rtcp-mux-only' attribute but don't support it for some reason. What are these reasons? Maybe you can provide some more background about the intended usage here!

Minor: 
Some sentences are a little hard to pharse as they are quite long, e.g.

OLD:
"When an offerer sends the initial offer, if the offerer wants to
   indicate exclusive RTP/RTCP multiplexing for RTP-based media, the
   offerer MUST associate an SDP 'rtcp-mux-only' attribute with the
   associated SDP media description ("m=" line)."

MAYBE:
"If the offerer wants to
   indicate exclusive RTP/RTCP multiplexing for RTP-based media, the
   offerer MUST send the initial offer with an associated SDP 'rtcp-mux-only' attribute and the
   associated SDP media description ("m=" line)."

In Section 3:

   The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'rtcp-
   mux-only' attribute is 'IDENTICAL', which means that the attribute,
   if used within a BUNDLE group
   [I-D.ietf-mmusic-sdp-bundle-negotiation], must be associated with all
   multiplexed RTP-based media descriptions within the BUNDLE group.

This sounds pretty normative, so I think the following 2 references should be Normative:

   [I-D.ietf-mmusic-sdp-mux-attributes]
              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-12
              (work in progress), January 2016.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-31 (work in progress), June 2016.