I have reviewed this document as part of the security directorate's 
ongoing effort to review all IETF documents being processed by the 
IESG.  These comments were written primarily for the benefit of the 
security area directors.  Document editors and WG chairs should treat 
these comments just like any other last call comments.

This document is "Ready with nits".

This document defines two RTCP XR Report Blocks that allow the
reporting of concealment metrics for audio applications of RTP.
In layman's terms, this allows an audio receiver to report back
on how much the audio is being mangled by packet loss.

>From a security perspective, this document is fine. The security
considerations section says that this document introduces no new
security considerations beyond those described in [RFC3611].
I agree.

I do have one nit that I wanted to ask about. At the very end of
section 3.2, the Mean Playout Interrupt Size is defined to be
32 bits long. However, the second paragraph of this definition
says:

      If the measured value exceeds 0xFFFD, the value 0xFFFE MUST be
      reported to indicate an over-range measurement.  If the
      measurement is unavailable, the value 0xFFFF MUST be reported.

Shouldn't those constants be 0xFFFFFFFD, 0xFFFFFFFE, and 0xFFFFFFFF?

Thanks,

Steve

P.S. I apologize for sending this review late. However, I believe
that it's still before the IESG telechat on this document so I hope
that it will have some value.