Re: [payload] Benjamin Kaduk's Discuss on draft-ietf-payload-flexible-fec-scheme-17: (with DISCUSS and COMMENT)

[Giridhar Mandyam <[e-mail]>]

Thanks to Magnus for the partial response, and Benjamin for the careful review.

>> It's a little odd to see so much content in Section 1.1 before we get to requirements notation and defintions/notations.

I don't know what to make of this comment.  Is there a concrete suggestion that you may have in mind?

> 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".

I think the confusion lies around audio and video source streams.  The repair packets corresponding to bundled protection are not really source streams, which is what the 3550 guidance was addressing.  I think 3550 did not contemplate bundled protection.

> 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.)

Proposal:  remove the sentence starting with "The reason for this restriction ...".

>
>       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."
>

Good catch.  Will drop in revised I-D.

>       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.
>

Proposed change to last sentence:  "in cases when the repair stream covers packets from multiple source RTP stream with difference CNAME values and none of these CNAM values can be associated with the entity generating the FEC stream, then any of these CNAME values MAY be used."

> Section 4.2.2
>
> Any reason not to include "retransmit" and "fixed block" mnemonics for 
> the 'R' and 'F' bits?
>

Can you explain further?  The R/F definitions already include mention of retransmission and fixed FEC L/D.

> 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.

I am sorry.  I do not understand what is meant by "used to determine" values.  Can you explain?

> Section 4.2.2.2
>
> Should implementations set bounds on L and D that are smaller than the 
> maximum encodable value (255)?

Yes.  This is assumed.

> 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?
>

Optional payload format parameters may be provided in SDP (see Sec. 5.1).

> 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?

I am not sure that I understand this statement.  L=1 does not imply anything about the value of D.


> Section 4.2.2.3
>
> nit: The "P|X" bits in Figure 15 seem indented by one too many spaces.

Thanks.  Will revise on next I.-D.

> 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 would you suggest?  Would sub-bullets help?

> 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?

Yes, it is possible.  We expect that specific uses of FLEX FEC will bound the appropriate values for repair window, L and D.  It is difficult to establish these bounds in this specification, however, since the applications that are currently making use of FLEX FEC are varied (e.g. WebRTC, 3GPP MTSI).  We expect these specification to provide concrete guidance on the expected repair windows, L and D, based on the target application.

> 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.

This is also covered in Sec. 1.1, which provides the general guidance.

>    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).

This requirement is specific to SDP O/A. Can you explain further as to why you think there is a different normative requirement?

> 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?

Why?  The FEC repair is covering all relevant parts of the first 16 bits of the RTP header.

> 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.)

Yes, this is simply the first 80 bits as per the header formats in Sec.'s 4.2.2.1 and 4.2.2.2.  The wording as it stands seems to be accurate.  Did you have a suggestion in mind?

>    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?

Yes.

>    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.

As per Sec. 6.2,
"The rest of the FEC bit string, which contains everything after
      the fixed 12-byte RTP header of the source packet, is written into
      the Repair "Payload" following the FEC header, where "Payload"
      refers to everything after the fixed 12-byte RTP header, including
      extensions, CSRC list, true payloads, and padding."

> 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.

Agreed.  Proposed new wording for steps 1 and 5:

"1.  Allocate Y additional bytes for the new packet generated in Section 6.3.2."
"5.  Set the last Y octets in the new packet to the recovered bit string." 

> 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"?

Proposed new wording as recommended:  "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.

The Section 8 guidance covers scenarios where the use of a given FEC configuration can result in no benefits in performance (or even degradation in performance).  If a receiver detects FEC parameters that are inconsistent with the nature of the source data or the transmission conditions, then the receiver could reject the offer (as per Sec. 6 of RFC 3264, "An offered stream MAY be rejected in the answer, for any reason").  However, additional guidance could improve the current text.

"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.  ***In addition, an attack against the FEC parameters themselves (e.g. repair window, D or L values) can result in a receiver having to allocate source buffer space that may also lead to excessive consumption of resources.***  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. " 


-----Original Message-----
From: payload <[e-mail]> On Behalf Of Magnus Westerlund
Sent: Thursday, February 21, 2019 7:09 AM
To: Benjamin Kaduk <[e-mail]>; The IESG <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; [e-mail]
Subject: Re: [payload] Benjamin Kaduk's Discuss on draft-ietf-payload-flexible-fec-scheme-17: (with DISCUSS and COMMENT)

-------------------------------------------------------------------------
CAUTION: This email originated from outside of the organization.
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Hi Benjamin,

I am not one of the authors, but one that have helped beating on this document in the WG, so I think I can answer your questions. I think the authors should check what I say and check the last part of the comments.

On 2019-02-18 22:53, Benjamin Kaduk wrote:
> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-payload-flexible-fec-scheme-17: Discuss
>
> When responding, please keep the subject line intact and reply to all 
> email addresses included in the To and CC lines. (Feel free to cut 
> this introductory paragraph, however.)
>
>
> Please refer to 
> https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
>
>
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-payload-flexible-fec-schem
> e/
>
>
>
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
>
> 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 
> L+number
> is that corresponds to the start of a new batch of recovery blocks.)

So, if one are going to do a 2-D FEC code and have indicated that in the signaling, each repair packet is still either a row or column FEC. So a Row packet for your D=3 and L=2 2-D FEC configurations are going to say:

Sn base= i, L=2 D=1

The rest of the Row packets for this block are going to have:

Sn base=i+2, L=2 D=1

Sn base= i+4, L=2 D=1

Then the column packets

Sn base=i, L=2 D=3

Sn base=i+1, L=2 D=3

>From a receiver perspective you are not actually not caring about what
block structures the sender uses. You anyway only can store received FEC packet in a receiver buffer for the stipulated time. When a repair packet comes in one checks if that repairs any loss, otherwise stores it in the buffer.


> 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".

Well, for each SSRC one follows the SN base and L and D parameters. This results in a number of packets that the XOR is performed over.

Does this help?

Cheers

Magnus


> 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).
>
>
> ----------------------------------------------------------------------
> 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 4.2.2.2
>
> 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 4.2.2.3
>
> 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.
>
>
> _______________________________________________
> payload mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/payload
>

-- 

Magnus Westerlund 

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