Hello,
For these two specific comments, the editors’ response is enclosed below. There will be a separate email regarding the other TSVART questions on FLEX FEC.
-Giri Mandyam
>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.
It may not be possible to support all ToP types for arbitrary repair windows for all classes of FLEX FEC endpoints, so mandating support does not look
feasible. An Offerer can choose to offer multiple m-lines (streams) to allow for ToP flexibility, and the Answerer can choose to reject unsupported ToP’s (see
https://tools.ietf.org/html/rfc3264#section-6, “An offered stream MAY be rejected in the answer …”).
>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?
Yes, multiple SDP lines was the intent. Choosing the offer corresponds to accepting/rejecting m-lines as per RFC 3264. The Answerer can choose to initiate a FLEX FEC session based on the initial Offer, but modify the session by making
its own Offer if the chosen configuration is undesirable. In addition, non-SDP signaling is always possible between the endpoints if the Answerer does not find the offered streams suitable. But such signaling is beyond the scope of this specification.
From: Bernard Aboba <bernard.aboba@xxxxxxxxx>
Sent: Sunday, February 3, 2019 9:36 PM
To: Giridhar Mandyam <mandyam@xxxxxxxxxxxxxxxx>
Cc: IETF discussion list <ietf@xxxxxxxx>;
draft-ietf-payload-flexible-fec-scheme@xxxxxxxx;
payload@xxxxxxxx;
tsv-art@xxxxxxxx
Subject: Re: TSVART telechat review of draft-ietf-payload-flexible-fec-scheme-16
Meant very different. For example, in a conferencing scenario a participant sends SVC with differential protection via flexible mode and the conferencing server sends a single layer with 2D.
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?
Reviewer: Bernard Aboba
Review result: Needs clarifications
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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?
