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 authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC tsv-art@ietf.org if you reply to or forward this review. 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?