Re: TSVART telechat review of draft-ietf-payload-flexible-fec-scheme-16

[Bernard Aboba <[e-mail]>]

Some additional notes:

1. The spec says that absence of a ToP value means that any ToP is
allowable, but it doesn't explicitly say that implementations needs to
support all ToP types.  So I was unclear whether it might be necessary to
Offer multiple potential flexible FEC configurations so as to be able to
negotiate what ToP values each side can handle, and if so, how this would
work. Personally, things would be simpler if the spec were to mandate
support for as many features as possible so as to avoid the need to put
multiple potential configurations into an Offer.

2. With respect to Offer/Answer, Section 5.2.1 strikes me as potentially
quite complex:

      Each combination of the L and D parameters produces a different
      FEC data and is not compatible with any other combination.  A
      sender application may desire to offer multiple offers with
      different sets of L and D values as long as the parameter values
      are valid.  The receiver SHOULD choose the offer that has a
      sufficient amount of interleaving.  If multiple such offers exist,
      the receiver may choose the offer that has the lowest overhead or
      the one that requires the smallest amount of buffering.  The
      selection depends on the application requirements.

[BA] By "multiple Offers" I presume you are not talking about multiple
rounds of O/A or multiple Offers sent at once,
but rather multiple SDP lines describing potential configurations.  In
this paragraph, "choosing" the offer
presumably refers to the configurations that are provided in the
Answer?  What if the Answerer wants to utilize a very
configuration from what is in the Offer?





On Sun, Feb 3, 2019 at 10:29 PM Bernard Aboba <[e-mail]>
wrote:

> Reviewer:  Bernard Aboba
> Review result:  Needs clarifications
>
> This document has been reviewed as part of the transport area review team's
> ongoing effort to review key IETF documents. These comments were written
> primarily for the transport area directors, but are copied to the
> document's
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>
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>
> Document: draft-ietf-payload-flexible-fec-scheme-16
>
> My reading of the document raised questions relating to implementation
> requirements as well as the configuration and use of the Flexible Mask mode
> (R=0, F=0).  Presumably, this mode can be used to choose arbitrary packets
> to protect. There is not much discussion of flexible mode early in the
> document, and no use cases are presented relating to this mode.  However,
> it would appear to me that flexible mode can be used to implement scenarios
> such as differential protection for Scalable Video Coding.
>
> For example, the sender could use flexible mode to only protect base layer
> packets by using a flexible mask to select only packets sent with TID = 0
> and SID = 0.  Since with flexible mode the mask is not negotiated and thus
> can be varied on the fly, it would appear to me that differential
> protection can be provided even in situations where the number of layers
> encoded (and even the temporal/spatial encoding mode) vary on the fly.
>
> If this interpretation is correct, I would suggest adding a section after
> 1.1.4 covering the flexible mask mode and a differential protection use
> case for it.
> It also would appear to me that flexible mode could be used to implement
> dynamic FEC, but I'll leave it to the authors to decide whether to mention
> that use case.
>
> With respect to SDP parameters (L, D, ToP) defined in Section 5.1.1, I was
> unclear on several points:
>
> 1. Is it possible to configure a ToP value to indicate that the sender
> desires to utilize both FEC and retransmission?  Or must the sender choose
> to utilize this payload for one or the other but not both?
>
> 2. What happens if both RTX and flexible FEC with retransmission are
> Offered in SDP?  Could this result in the sender being allowed to send both
> types of retransmission (though presumably only one at a time)?  Are the
> type(s) of retransmission used determined by which retransmission schemes
> are provided in the Answer?
>
> 3. If L and D are not specified, does this imply that the sender will
> operate in flexible mode?  Are implementations of the specification
> required to support all of the modes except for the F=1, R=1 mode that is
> forbidden?  If not, how does an Answerer indicate that it doesn't support
> the mode that is Offered?
>
> 4. Does the negotiation of L, D and ToP in SDP imply that the sender
> cannot switch to use of another configuration without renegotiation?  Since
> the flexible FEC format is self-describing, it would appear to me that
> switching should be possible as long as the implementation requirements are
> clear.  For example, do all implementations needs to support all mask
> sizes?
>
>
>