[rtcweb] Benjamin Kaduk's No Objection on draft-ietf-rtcweb-fec-08: (with COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Sun, 17 February 2019 22:33 UTC

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Date: Sun, 17 Feb 2019 14:33:05 -0800
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Subject: [rtcweb] Benjamin Kaduk's No Objection on draft-ietf-rtcweb-fec-08: (with COMMENT)
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Benjamin Kaduk has entered the following ballot position for
draft-ietf-rtcweb-fec-08: No Objection

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COMMENT:
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Section 3

nit: The subsequent discussion seems to indicate that at least some of
these mechanisms are already specified and not new in this document; (if
so) it would be nice to have the exposition reflect that.

Section 3.3

   For Opus, packets deemed as important are re-encoded at a lower
   bitrate and added to the subsequent packet, allowing partial recovery

Is "added" supposed to be something other than "appended" (which strongly
resembles the "redundant encoding" of the previous section)?

Section 4.1

Does it make sense to give subsection backreferences when talking about
(e.g.) redundant encoding?

Section 5.2

   Support for a SSRC-multiplexed flexfec stream to protect a given RTP
   stream SHOULD be indicated by including one of the formats described
   in [I-D.ietf-payload-flexible-fec-scheme], Section 5.1, as an

nit: Since this Section 5 is solely for video, is it more appropriate to
refer to Section 5.1.2 ("Registration of video/flexfec") of
draft-ietf-payload-flexible-fec-scheme?

Section 7

   To support the functionality recommended above, implementations MUST
   be able to receive and make use of the relevant FEC formats for their
   supported audio codecs, and MUST indicate this support, as described
   in Section 4.  Use of these formats when sending, as mentioned above,
   is RECOMMENDED.

Just to double-check: this is explicitly only mandating FEC for audio and
ignoring video entirely?

Section 8

   Because use of FEC always causes redundant data to be transmitted,
   and the total amount of data must remain within any bandwidth limits
   indicated by congestion control and the receiver, this will lead to
   less bandwidth available for the primary encoding, even when the
   redundant data is not being used.  This is in contrast to methods
   like RTX [RFC4588] or flexfec [I-D.ietf-payload-flexible-fec-scheme]
   retransmissions, which only transmit redundant data when necessary,
   at the cost of an extra roundtrip.

This seems to imply that "FEC" is a specific usage and that flexfec is not
a subset of generic FEC.  If so, this could probably be reworded to be
less confusing to the reader (though my suspicion is that the "always
causes redundant data to be transmitted" is only intended to apply to
specific mechanisms from Section 3).

Section 9

This document seems to be agnostic on the question of RTP vs. SRTP; I would
consider referencing their respective security considerations as well as
what is already covered here.

   As described in [RFC3711], Section 10, the default processing when
   using FEC with SRTP is to perform FEC followed by SRTP at the sender,
   and SRTP followed by FEC at the receiver.  This ordering is used for
   all the SRTP Protection Profiles used in DTLS-SRTP [RFC5763], as
   described in [RFC5764], Section 4.1.2.

I was going to comment about the lack of clarity here, but I see that Ekr
has already gotten the core points, and that the secdir review has already
resulted in some chanegs in the editor's copy.  It would be nice to have
the result of the (merged) edits available to look at, though.