In this specification, we define a framework for specifying constraints on RTP streams in the Session Description Protocol. This framework defines a new "rid" SDP attribute to unambiguously identify the RTP Streams within a RTP Session and constrain the streams' payload format parameters in a codec-agnostic way beyond what is provided with the regular Payload Types. This specification updates RFC4855 to give additional guidance on choice of Format Parameter (fmtp) names, and on their relation to the constraints defined by this document.