[Ballot comment]
The way requirements and validation requirements are set out in this document is hard to follow. I suggest the following changes:

Section 3:

OLD:
Metrics in this block report on Burst/Gap Loss in the stream arriving
at the RTP system. The measurement of these metrics are made at the
receiving end of the RTP stream. Instances of this Metrics Block
refer by Synchronization source (SSRC) to the separate auxiliary
Measurement Information block [RFC6776] which describes measurement
periods in use(see RFC6776 section 4.2). This Metrics Block relies
on the measurement period in the Measurement Information block
indicating the span of the report and MUST be sent in the same
compound RTCP packet as the measurement information block. If the
measurement period is not received in the same compound RTCP packet
as this Metrics Block, this Metrics Block MUST be discarded.
NEW:
Metrics in this block report on Burst/Gap Loss in the stream arriving
at the RTP system. The measurement of these metrics are made at the
receiving end of the RTP stream. Instances of this Metrics Block
refer by Synchronization source (SSRC) to the separate auxiliary
Measurement Information block [RFC6776] which describes measurement
periods in use(see RFC6776 section 4.2).

This Metrics Block relies on the measurement period in the
Measurement Information block indicating the span of the report.
Senders MUST send this block in the same compound RTCP packet
as the measurement information block. Receivers MUST verify that the
measurement period is received in the same compound RTCP packet
as this Metrics Block. If not, this Metrics Block MUST be discarded.

In section 3.2, the text is needlessly repetitive, and also doesn't say what must be discarded; I suspect that it should be changed to something like this:

OLD:
In this document, Burst/Gap Loss Metrics can only be measured over
definite intervals, and cannot be sampled. Accordingly, the value
I=01, indicating a sampled value, MUST NOT be sent, and MUST be
discarded when received. In addition, the value I=00 is reserved
and also MUST NOT be sent, and MUST be discarded when received.
NEW:
In this document, Burst/Gap Loss Metrics can only be measured over
definite intervals, and cannot be sampled. Also, the value I=00 is
reserved for future use. Senders MUST NOT use
the values I=00 or I=01. If a block is received with I=00 or
I=01, the receiver MUST discard the block.

In section 3.3, there's a very awkward parenthetical note which refers back to the first paragraph of section 3. This is the text that prompted my DISCUSS; on review I see that rather than actually adding requirements to the sender and receiver, it's merely reiterating them. The easy fix is to just take it out:

OLD:
The metrics described here are intended to be used as described in
this section, in conjunction with information from the Measurement
Information block [RFC6776] (which MUST be present in the same RTCP
packet as the Burst/Gap Loss block (see section 3,1st paragraph) )
and also with the metric "cumulative number of packets lost" provided
in standard RTCP [RFC3550].
NEW:
The metrics described here are intended to be used as described in
this section, in conjunction with information from the Measurement
Information block [RFC6776] and also with the metric "cumulative
number of packets lost" provided in standard RTCP [RFC3550].

However, of course the authors had some reason for putting this note into section 3.3; if removing it entirely feels uncomfortable, how about something like this?

OLD:
The metrics described here are intended to be used as described in
this section, in conjunction with information from the Measurement
Information block [RFC6776] (which MUST be present in the same RTCP
packet as the Burst/Gap Loss block (see section 3,1st paragraph) )
and also with the metric "cumulative number of packets lost" provided
in standard RTCP [RFC3550].
NEW:
The metrics described here are intended to be used as described in
this section, in conjunction with information from the Measurement
Information block [RFC6776], which, as mentioned previously, must
be present, and also with the metric "cumulative number of packets
lost" provided in standard RTCP [RFC3550].

I prefer the former edit, but something like the latter edit would also satisfy me. I've removed my DISCUSS on this because now that I understand the context better, I don't think this _has_ to be fixed. However, I would appreciate it if the authors would consider fixing it; I think it would improve the readability of the document.