Summary: Has a DISCUSS. Needs 6 more YES or NO OBJECTION positions to pass.
I'm confused about some parts of how I'd implement this. It's quite possible this is just my error, but I'm including this point in the Discuss section in case it's not. This basically relates to how multiple recovery packets from a given FEC block get encoded and identified on the wire, but also how to populate the source block when multiple SSRCs are included. In short: suppose that I have D=3 and L=2. I should expect 5 repair packets for the six source packets in a block; the scheme for determining what order to generate them in and what their contents are is fairly clear to me. But how do I identify them on the wire? I'm assuming that the D and L on the wire are fixed values, since there's the possibility to only send zero on the wire and negotiate their values out of band. It's a little less clear whether the "SN base" fields are expected to be the same for all 5 recovery packets based on a given block, but if they do change then I'm not sure how I tell whether a given recovery packet is for a row or a column. Is this supposed to be using the sequence number from the outer RTP header for packet ordering, and the implicit order for row/column FEC packets? (It seems that in case of very bad packet loss and dynamic L+D, the receiver could then get out of sync as to what the sequence number is that corresponds to the start of a new batch of recovery blocks.) I also don't see how, for the case when there are multiple SSRCs, I know how many source packets to include from each SSRC in order to make up the D x L source block -- since Section 6.2's discussion lumps all the "source packets" together into a single set that get mutually xor'd, that seems to imply that the encoding is not "do recovery for SSRC1, do recovery for SSRC2, ..., concatenate them all". There are perhaps some other scenarios to worry about, such as interleaved recovery within a single block, but I'm happy to focus on the single 2-D case for purposes of illustration. Any insight into what I'm missing would be appreciated. A couple other points to check on: I'm not sure I'm following the procedures in Section 6.3.2 properly (see COMMENT) -- is the text correct as written? I also think there are a couple more factors worth mentioning in the security considerations (see COMMENT).
It's a little odd to see so much content in Section 1.1 before we get to requirements notation and defintions/notations. I think I'm a bit confused about current best practices for multiplexing, as RFC 3550 Section 5.2 says "separate audio and video streams SHOULD NOT be carried in a single RTP session and demultiplexed based on the payload type or SSRC fields", but we seem to be not only recommending using SSRC for demultiplexing repair packets, but also suggesting that the FEC can cover multiple different audio and/or video streams with different SSRCs. I guess RFC 8108 is supposed to clarify when it's okay to use multiple SSRCs in the same RTP session, so maybe the answer is just "3550 was overly cautious and we don't worry about that anymore". Section 4.2.1 Version (V) 2 bits: This MUST be set to 2 (binary 10), as this specification requires all source RTP packets and all FEC repair packets to use RTP version 2. The reason for this restriction is the first 2 bits of the FEC header contain other information (R and F bits) rather than recovering the RTP version field. nit: is it better to say that the FEC mechanism does not recover this value, rather than talking about how the first 2 bits of the FEC header are used for something else? (The FEC header's structure need not bear any relation to the 12-byte RTP header, AFAICT.) Payload Type: The (dynamic) payload type for the FEC repair packets is determined through out-of-band means. [...] Is "(e.g., SDP)" applicable here? Sequence Number (SN): The sequence number follows the standard definition provided in [RFC3550]. definition. Therefore it must nit: drop separate "definition." multiplex multiple repair streams in an RTP session. The repair streams' SSRC's CNAME SHOULD be identical to the CNAME of the source RTP stream(s) that this repair stream protects. An FEC stream that protects multiple source RTP streams with different CNAME's uses the CNAME associated with the entity generating the FEC stream or the CNAME of the entity on whose behalf it performs the protection operation. In cases when the repair stream covers packets from multiple source RTP streams with different CNAME values, any of these CNAME values MAY be used. I'm not sure I'm parsing this properly; the penultimate sentence says that the CNAME to use is determined by nature of the entity producing the repair stream, but the last sentence says that there is a nondeterministic choice. Section 4.2.2 Any reason not to include "retransmit" and "fixed block" mnemonics for the 'R' and 'F' bits? Please include a note here that several fields (e.g., P, PT, etc.) in the FEC header are not meant to be interpreted directly but are instead actual FEC parity data akin to the following "payload". (Absent such an indication, the reader could see that these fields are "used to determine" values when they appear to contain values directly, and get confused.) I would suggest adding a forward-reference to Section 6 since that describes how the Repair Payload is calculated. Section 220.127.116.11 Should implementations set bounds on L and D that are smaller than the maximum encodable value (255)? If L=0, D=0, use the optional payload format parameters for L and D. What is the behavior when those payload format parameters were not provided? The L=1 case seems to imply that some full packet retransmission will be used; is it worth calling that out as a consequence of such a parameter choice? Section 18.104.22.168 nit: The "P|X" bits in Figure 15 seem indented by one too many spaces. Section 5.1 (all subsections) Having the ToP values for interleaved and non-interleaved 1-D protection presented in a different order than virtually all of the body text (that presents non-interleaved first) is needlessly hard on the reader. What is the interaction between rate, repair-window, and the L and D values? That is, if we set L and D to be large, and rate to be small, can we end up claiming a repair window that is too small to accumulate the necessary L*D source packets and compute recovery packets? Section 5.2.1 o The value for the repair-window parameter depends on the L and D values and cannot be chosen arbitrarily. More specifically, L and D values determine the lower limit for the repair-window size. The upper limit of the repair-window size does not depend on the L and D values. Per my above remark, this consideration seems generally applicable and not limited to SDP Offer/Answer. o Any unknown option in the offer MUST be ignored and deleted from the answer. If FEC is not desired by the receiver, it can be deleted from the answer. This sounds like it is restating an existing normative requirement (in which case a reference and descriptive, non-normative, text seems appropriate). Section 6.2 o The first 16 bits of the RTP header (16 bits). Maybe note here that we'll actually ignore the first 2 bits? Section 6.3.2 2. For the repair packet in T, compute the FEC bit string from the first 80 bits of the FEC header. I'm scratching my head a bit at this. Is this operation something other than "take the first 80 bits of the FEC header"? (If not, the length and sequence number base seem to be in different places in the source packets and FEC bit string, if I'm reading things right.) 11. Set the SN field in the new packet to SEQNUM. Skip the next 16 bits in the recovered bit string. To be clear, we're skipping over the xor of the reconstructed length field with the seqnum field of the source packets? 13. Take the next 16 bits of the recovered bit string and set the new variable Y to whatever unsigned integer this represents (assuming network order). Convert Y to host order. Y represents the length of the new packet in bytes minus 12 (for the fixed RTP header), i.e., the sum of the lengths of all the following if present: the CSRC list, header extension, RTP payload and RTP padding. I don't see how this matches up with the bit string construction in Section 6.2. Section 6.3.3 1. Append Y bytes to the new packet. [...] 5. Append the recovered bit string (Y octets) to the new packet generated in Section 6.3.2. I think a different verb than "append" should be used in step 1, perhaps "allocate Y additional bytes for the new packet", as the text as-written has us appending 2*Y bytes, only Y of which have a value specified. Section 9 The main security considerations for the RTP packet carrying the RTP payload format defined within this memo are confidentiality, integrity and source authenticity. Confidentiality is achieved by encrypting the RTP payload. Integrity of the RTP packets is achieved through a suitable cryptographic integrity protection mechanism. [...] This phrasing of "is achieved by" implies that the mechanisms for doing so are defined in this document, but that's not the case. Don't we really mean things like "Confidentiality can be provided by encrypting the RTP payload"? Given that FLEX FEC enables the protection of multiple source streams, there exists the possibility that multiple source buffers may be created that may not be used. An attacker could leverage unused source buffers to as a means of occupying memory in a FLEX FEC endpoint. Moreover the application source data may not be perfectly matched with FLEX FEC source partitioning. If this is the case, there is a possibility for unprotected source data if, for instance, the FLEX FEC implementation discards data that does not fit perfectly into its source processing requirements. I don't think this text quite covers the risks when interacting with an adversarial endpoint -- an attacker could try to advertise FEC schemes with large D and L and/or large repair windows, that cause the receiver to consume a lot of resources buffering packets that may be used as repair inputs. Endpoints need to be aware of the risk when deciding whether to accept FEC streams, e.g., via SDP Offer/Answer. Similarly, a network attacker could modify the recovery fields corresponding to packet lengths (when integrity protection is not in play), to force large allocations on the receiver. It's fairly likely that this doesn't even require knowing which source packet(s) will be lost, since length is a 16-bit field and the expected input values are not likely to have the high bit(s) set. The need for integrity protection on the SDP Offer/Answer exchange is probably sufficiently well-documented elsewhere that we don't need to reiterate it here.
I do have one question - the IESG has approved https://datatracker.ietf.org/doc/draft-ietf-tsvwg-fecframe-ext/, which updates RFC 6363 to support sliding encoding window codes, in addition to block codes, and it seems like that would be useful for real-time payload FEC here. Is that something that people have looked at?
I found 2-D description confusing as well.