Re: [secdir] Benjamin Kaduk's Discuss on draft-ietf-payload-tsvcis-03: (with DISCUSS and COMMENT)

[Benjamin Kaduk <[e-mail]>]

Hi Victor,

On Tue, Nov 19, 2019 at 03:14:21PM -0500, [e-mail] wrote:
> Hi Ben,
> 
> Sorry I overlooked sending you a response.  I would like to address the two
> concerns you have by explaining what the speech coders are doing.

Thanks for the extra clarifications.  To supply one of my own: I'm not
concerned that the protocol doesn't work as implemented, but just want to
make sure that the document includes enough information to admit new
implementations without guesswork.  That is to say, "either tell me how to
do it or tell me where to look that tells me how to do it".

> WRT to 600 bps MELP, there is one TSVCIS mode that uses one bit beyond the
> 54-bit frame for MELP 600 as a frame sync which alternates between frames.
> With two or more MELP 600bps frames in one RTP packet, if any frame
> indicates 600 bps by CODA being 0 and CODB being 1, then we know the stream
> is 600bps.  If there is a single frame in an RTP packet, you can still
> deduce this by looking at every other RTP packet (every other MELP 600bps
> frame) and by the timestamp advance.  Most likely the two ends would
> negotiate 600 bps in SDP anyways so there really should not be a problem.  I
> know it's not pretty but its workable.  I hope this explanation helps you
> with the concerns for this issue.

In this case, the use as an "end-to-end framing bit" (i.e., the alternating
behavior you describe above) is not explicitly stated; one might imagine a
scheme where the framing usage is to have the bit cycle through 1, 1, 0,
and 0, or some other scheme.  I'd suggest to note in the document that if
any instance of (CODA, CODB) == (0, 1) is observed, then the 600bps mode is
in use.  It might also be helpful to include the observation that two
successive MELPe payloads with CODA == CODB == 0 indicates the 2400bps mode
(and that seeing them in a single RTP packet is decisive, whereas
additional information about packet non-loss would be needed in the
one-MELPe-frame-per-RTP-packet case), but that would be a fair bit of
additional text and might be diminishing returns.  (Or, of course, the use
of CODB as an alternating 1/0 bit as the framing usage could be documented
instead.)

> As for the TSVCIS parameter packing/unpacking, this is really simple.  There
> is exactly on three bit parameter, exactly one five bit parameter and a
> variable number of eight bit parameters.  In our view, the speech coder
> itself (or a wrapper for it) is responsible for preparing the block of
> octets.  RTP then just transports it.  On receive, the complementary wrapper
> reverses the packing operation.  I hope this clarifies and explains the
> simplicity.

That's exactly what I expected to happen; however, it's not what I believe
the current text of the document is describing.  Specifically, I think that
the current text implies that the "preparing the block of octets" and
"complementary wrapper reverses the packing operation" are supposed to be
part of the RTP payload format that this document describes, but this
document does not have enough information to actually perform those
operations reversibly.  If the packing is to be done in the speech coder,
then this document doesn't need to talk about the packing at all (e.g., at
the end of Section 2); if we need to keep the packing/wrapper in this
document then we need to indicate that there's a defined priority order for
the (8-octet) TSVCIS parameters in the TSVCIS references, to allow the
packing/unpacking to be deterministic.

Thanks,

Ben

> 
> -----Original Message-----
> From: Benjamin Kaduk <[e-mail]> 
> Sent: Thursday, October 31, 2019 8:12 PM
> To: Barry Leiba <[e-mail]>
> Cc: [e-mail]; Roni Even (A) <[e-mail]>; The
> IESG <[e-mail]>; Catherine Meadows <[e-mail]>;
> IETF SecDir <[e-mail]>; [e-mail]; Ali Begen
> <[e-mail]>; [e-mail]; [e-mail]; Dave
> Satterlee (Vocal) <[e-mail]>; IETF discussion list
> <[e-mail]>; [e-mail]
> Subject: Re: Benjamin Kaduk's Discuss on draft-ietf-payload-tsvcis-03: (with
> DISCUSS and COMMENT)
> 
> I don't think so, unfortunately.
> 
> I do see the clarification about CODB's potential for deviation from Table
> 1, that only the 600 bps MELPe is allowed to deviate, and that CODA gets us
> to "it's one of 2400 or 600 bps" and the RTP timestamp disambiguates that
> 600 bps is in use.  But, it seems that this means that the recipient in
> general should not rely on CODB to differentiate 600 from 2400 bps, and
> instead is more robustly implemented by *always* using the RTP timestamp to
> detect 600 bps, since that will always work and CODB will sometimes not work
> under conditions not fully specified here.  So, if we are unwilling or
> unable to clarify what those conditions are (e.g., whether at a minimum
> mutual agreement is required), then I think we need to describe this
> procedure of consulting the RTP timestamp as the default behavior and avoid
> giving the impression that CODB should be used to do so.
> 
> Additionally, I don't see anything to address my concern about TSVCIS
> parameter decoding.  To be clear, the procedure I see this document
> describing is that:
> - TSVCIS gives parameters (and their lengths in bits) to the codec
>   described in this document
> - this document specifies how to densely encode those parameters into a
>   byetstream
> - RTP transmits that encoded bytestream to the peer
> - the codec specified by this is responsible for turning that encoded
>   bystream back into a list of TSVCIS parameters (and their length in bits)
> 
> I don't see how that last step is attainable with only the information
> provided by this document.  I *assume* that one of the TSVCIS specifications
> has a canonical (ordered) listing of parameters, and that the list of
> parmeters given to this codec in the first step will always be an initial
> prefix of that list, but that's just me guessing at how to make sense of the
> stated procedure given insufficient information.  I don't think it's
> appropriate to make the reader of an RFC guess at what to do; we need to
> either say how to do it or give a pointer to an external reference that
> does.
> 
> -Ben
> 
> On Tue, Oct 29, 2019 at 02:26:09PM -0400, Barry Leiba wrote:
> > Ben, does the -04 version address everything?
> > 
> > Barry
> > 
> > On Thu, Oct 24, 2019 at 1:42 PM <[e-mail]> wrote:
> > >
> > > I forgot to address security comments in one email.  The changes are:
> > >
> > > Section 8, second paragraph - Suggested edit by reviewer
> > >
> > > (was)
> > >    This RTP payload format and the TSVCIS decoder do not exhibit any
> > >    significant non-uniformity in the receiver-side computational
> > >    complexity for packet processing and thus are unlikely to pose a
> > >    denial-of-service threat due to the receipt of pathological data.
> > >    Additionally, the RTP payload format does not contain any active
> > >    content.
> > >
> > > (now)
> > >    This RTP payload format and the TSVCIS decoder, to the best of our
> > >    knowledge, do not exhibit any significant non-uniformity in the
> > >    receiver-side computational complexity for packet processing and thus
> > >    are unlikely to pose a denial-of-service threat due to the receipt of
> > >    pathological data. Additionally, the RTP payload format does not
> > >    contain any active content.
> > >
> > >
> > > Section 8, third paragraph - Suggested edit by reviewer
> > >
> > > (was)
> > >    Please see the security considerations discussed in [RFC6562]
> > >    regarding VAD and its effect on bitrates.
> > >
> > > (now)
> > >    Please see the security considerations discussed in [RFC6562]
> > >    regarding Voice Activity Detect (VAD) and its effect on bitrates.
> > >
> > > Victor
> > >
> > > -----Original Message-----
> > > From: [e-mail] <[e-mail]>
> > > Sent: Thursday, October 24, 2019 10:05 AM
> > > To: 'Roni Even (A)' <[e-mail]>; 'Benjamin Kaduk' 
> > > <[e-mail]>; 'The IESG' <[e-mail]>
> > > Cc: [e-mail]; 'Ali Begen' 
> > > <[e-mail]>; [e-mail]; [e-mail]; 
> > > 'Dave Satterlee (Vocal)' <[e-mail]>
> > > Subject: RE: Benjamin Kaduk's Discuss on 
> > > draft-ietf-payload-tsvcis-03: (with DISCUSS and COMMENT)
> > >
> > > Hi Everyone,
> > >
> > > First we want to thank everyone for their review and comments for this
> draft RFC.  We believe we reviewed all the comments and suggestions and
> incorporated them adequately in the next draft (04).  We'd like to send out
> this list of exact changes in case anyone has additional comments or thinks
> the clarifications are inadequate.  We would be most happy to address
> concerns before publishing draft 04 tomorrow.
> > >
> > > With so many emails from a half dozen or more reviewers, we apologize
> that we cannot address each sender individually.  We hope this detail is
> sufficient for everyone.
> > >
> > > Again, many thanks to all.
> > >
> > > Victor & Dave
> > >
> > > --------------------------------------------------------------------
> > > --------------------------
> > >
> > > Section 1.1 - Suggested reference to RFC 8088 added.
> > >
> > > (was)
> > >    Best current practices for writing an RTP payload format
> > >    specification were followed [RFC2736].
> > >
> > > (now)
> > >    Best current practices for writing an RTP payload format
> > >    specification were followed [RFC2736] [RFC8088].
> > >
> > >
> > > Section 2, paragraphs 3 and 4 - Suggested edits by reviewers
> > >
> > > (was)
> > >    In addition to the augmented speech data, the TSVCIS specification
> > >    identifies which speech coder and framing bits are to be encrypted,
> > >    and how they are protected by forward error correction (FEC)
> > >    techniques (using block codes).  At the RTP transport layer, only the
> > >    speech coder related bits need to be considered and are conveyed in
> > >    unencrypted form.  In most IP-based network deployments, standard
> > >    link encryption methods (SRTP, VPNs, FIPS 140 link encryptors or Type
> > >    1 Ethernet encryptors) would be used to secure the RTP speech
> > >    contents.  Further, it is desirable to support the highest voice
> > >    quality between endpoints which is only possible without the overhead
> > >    of FEC.
> > >
> > >    TSVCIS augmented speech data is derived from the signal processing
> > >    and data already performed by the MELPe speech coder.  For the
> > >    purposes of this specification, only the general parameter nature of
> > >    TSVCIS will be characterized.  Depending on the bandwidth available
> > >    (and FEC requirements), a varying number of TSVCIS specific speech
> > >    coder parameters need to be transported.  These are first byte-packed
> > >    and then conveyed from encoder to decoder.
> > >
> > > (now)
> > >    In addition to the augmented speech data, the TSVCIS specification
> > >    identifies which speech coder and framing bits are to be encrypted,
> > >    and how they are protected by forward error correction (FEC)
> > >    techniques (using block codes).  At the RTP transport layer, only the
> > >    speech-coder-related bits need to be considered and are conveyed in
> > >    unencrypted form.  In most IP-based network deployments, standard
> > >    link encryption methods (SRTP, VPNs, FIPS 140 link encryptors or Type
> > >    1 Ethernet encryptors) would be used to secure the RTP speech
> > >    contents.
> > >
> > >    TSVCIS augmented speech data is derived from the signal processing
> > >    and data already performed by the MELPe speech coder.  For the
> > >    purposes of this specification, only the general parameter nature of
> > >    TSVCIS will be characterized.  Depending on the bandwidth available
> > >    (and FEC requirements), a varying number of TSVCIS-specific speech
> > >    coder parameters need to be transported.  These are first byte-packed
> > >    and then conveyed from encoder to decoder.
> > >
> > >
> > > Section 3, last sentence paragraph 3 - Suggested edit by reviewer
> > >
> > > (was)
> > >    When more than one codec data frame is
> > >    present in a single RTP packet, the timestamp is, as always, that of
> > >    the oldest data frame represented in the RTP packet.
> > >
> > > (now)
> > >    When more than one codec data frame is
> > >    present in a single RTP packet, the timestamp specified is that of
> > >    the oldest data frame represented in the RTP packet.
> > >
> > >
> > > Section 3.1, last paragraph - Clarified permission for MELP 600 
> > > end-to-end framing bit
> > >
> > > (was)
> > >    It should be noted that CODB for both the 2400 and 600 bps modes MAY
> > >    deviate from the values in Table 1 when bit 55 is used as an end-to-
> > >    end framing bit.  Frame decoding would remain distinct as CODA being
> > >    zero on its own would indicate a 7-byte frame for either rate and the
> > >    use of 600 bps speech coding could be deduced from the RTP timestamp
> > >    (and anticipated by the SDP negotiations).
> > >
> > > (now)
> > >    It should be noted that CODB for MELPe 600 bps mode MAY deviate from
> > >    the value in Table 1 when bit 55 is used as an end-to-end framing
> > >    bit. Frame decoding would remain distinct as CODA being zero on its
> > >    own would indicate a 7-byte frame for either 2400 or 600 bps rate and
> > >    the use of 600 bps speech coding could be deduced from the RTP
> > >    timestamp (and anticipated by the SDP negotiations).
> > >
> > >
> > > Section 3.2, first paragraph - Clarifications requested by reviewers
> > >
> > > (was)
> > >    The TSVCIS augmented speech data as packed parameters MUST be placed
> > >    immediately after a corresponding MELPe 2400 bps payload in the same
> > >    RTP packet.  The packed parameters are counted in octets (TC).  In
> > >    the preferred placement, shown in Figure 6, a single trailing octet
> > >    SHALL be appended to include a two-bit rate code, CODA and CODB,
> > >    (both bits set to one) and a six-bit modified count (MTC).  The
> > >    special modified count value of all ones (representing a MTC value of
> > >    63) SHALL NOT be used for this format as it is used as the indicator
> > >    for the alternate packing format shown next.  In a standard
> > >    implementation, the TSVCIS speech coder uses a minimum of 15 octets
> > >    for parameters in octet packed form.  The modified count (MTC) MUST
> > >    be reduced by 15 from the full octet count (TC).  Computed MTC = TC-
> > >    15.  This accommodates a maximum of 77 parameter octets (maximum
> > >    value of MTC is 62, 77 is the sum of 62+15).
> > >
> > > (now)
> > >    The TSVCIS augmented speech data as packed parameters MUST be placed
> > >    immediately after a corresponding MELPe 2400 bps payload in the same
> > >    RTP packet.  The packed parameters are counted in octets (TC).  The
> > >    preferred placement SHOULD be used for TSVCIS payloads with TC less
> > >    than or equal to 77 octets, is shown in Figure 6.  In the preferred
> > >    placement, a single trailing octet SHALL be appended to include a
> > >    two-bit rate code, CODA and CODB, (both bits set to one) and a six-
> > >    bit modified count (MTC).  The special modified count value of all
> > >    ones (representing a MTC value of 63) SHALL NOT be used for this
> > >    format as it is used as the indicator for the alternate packing
> > >    format shown next.  In a standard implementation, the TSVCIS speech
> > >    coder uses a minimum of 15 octets for parameters in octet packed
> > >    form.  The modified count (MTC) MUST be reduced by 15 from the full
> > >    octet count (TC).  Computed MTC = TC-15.  This accommodates a maximum
> > >    of 77 parameter octets (maximum value of MTC is 62, 77 is the sum of
> > >    62+15).
> > >
> > >
> > > Section 3.3, first paragraph - Suggested edit by reviewer
> > >
> > > (was)
> > >    A TSVCIS RTP packet consists of zero or more TSVCIS coder frames
> > >    (each consisting of MELPe and TSVCIS coder data) followed by zero or
> > >    one MELPe comfort noise frame.  The presence of a comfort noise frame
> > >    can be determined by its rate code bits in its last octet.
> > >
> > > (now)
> > >    A TSVCIS RTP packet payload consists of zero or more consecutive
> > >    TSVCIS coder frames (each consisting of MELPe 2400 and TSVCIS coder
> > >    data), with the oldest frame first, followed by zero or one MELPe
> > >    comfort noise frame.  The presence of a comfort noise frame can be
> > >    determined by its rate code bits in its last octet.
> > >
> > >
> > > Section 3.3, fourth paragraph - Clarification requested by reviewers
> > >
> > > (was)
> > >    TSVCIS coder frames in a single RTP packet MAY be of different coder
> > >    bitrates.  With the exception for the variable length TSVCIS
> > >    parameter frames, the coder rate bits in the trailing byte identify
> > >    the contents and length as per Table 1.
> > >
> > > (now)
> > >    TSVCIS coder frames in a single RTP packet MAY have varying TSVCIS
> > >    parameter octet counts.  Its packed parameter octet count (length) is
> > >    indicated in the trailing byte(s).  All MELPe frames in a single RTP
> > >    packet MUST be of the same coder bitrate.  For all MELPe coder
> > >    frames, the coder rate bits in the trailing byte identify the
> > >    contents and length as per Table 1.
> > >
> > >
> > > Section 4.1 - Editor note removed
> > >
> > >
> > > Section 4.1 - Change controller is now
> > >
> > > (now)
> > >    Change controller: IETF, contact <[e-mail]>
> > >
> > >
> > > Section 5, first paragraph - Suggested edits by reviewers
> > >
> > > (was)
> > >    A primary application of TSVCIS is for radio communications of voice
> > >    conversations, and discontinuous transmissions are normal.  When
> > >    TSVCIS is used in an IP network, TSVCIS RTP packet transmissions may
> > >    cease and resume frequently.  RTP synchronization source (SSRC)
> > >    sequence number gaps indicate lost packets to be filled by PLC, while
> > >    abrupt loss of RTP packets indicates intended discontinuous
> > >    transmissions.
> > >
> > > (now)
> > >    A primary application of TSVCIS is for radio communications of voice
> > >    conversations, and discontinuous transmissions are normal.  When
> > >    TSVCIS is used in an IP network, TSVCIS RTP packet transmissions may
> > >    cease and resume frequently.  RTP synchronization source (SSRC)
> > >    sequence number gaps indicate lost packets to be filled by Packet
> > >    Loss Concealment (PLC), while abrupt loss of RTP packets indicates
> > >    intended discontinuous transmissions.  Resumption of voice
> > >    transmission SHOULD be indicated by the RTP marker bit (M) set to 1.
> > >
> > >
> > > Section 10 - Added reference
> > >
> > > (added)
> > >    [RFC8088]  Westerlund, M., "How to Write an RTP Payload Format",
> > >               RFC 8088, DOI 10.17487/RFC8088, May 2017,
> > >               <http://www.rfc-editor.org/info/rfc8088>.
> > >
> > > --------------------------------------------------------------------
> > > -----------------------------
> > >
> > >
> > > -----Original Message-----
> > > From: Roni Even (A) <[e-mail]>
> > > Sent: Sunday, October 6, 2019 2:09 AM
> > > To: [e-mail]; 'Benjamin Kaduk' <[e-mail]>; 
> > > 'The IESG' <[e-mail]>
> > > Cc: [e-mail]; 'Ali Begen' 
> > > <[e-mail]>; [e-mail]; [e-mail]; 
> > > 'Dave Satterlee (Vocal)' <[e-mail]>
> > > Subject: RE: Benjamin Kaduk's Discuss on 
> > > draft-ietf-payload-tsvcis-03: (with DISCUSS and COMMENT)
> > >
> > > Hi,
> > > About the reference to TSVCIS.
> > > The RTP payload is about how to encapsulate the payload in an RTP
> packet. The objective is to define how an RTP stack can insert the tsvcis
> frames and  extract the tsvcis frames from the RTP packet. Typically it is
> not required to understand the payload structure in order to be able to
> perform the encapsulation.
> > > This is why the reference to the payload is Informational and we did 
> > > not require to have it publically available.  If there is a need to 
> > > understand the payload itself for the encapsulating than we need 
> > > more information in the RTP payload specification and a publically 
> > > available normative reference. I think this is not the case here
> > >
> > > Roni Even
> > >
> > > AVTCore co-chair (ex Payload)
> > >
> > > -----Original Message-----
> > > From: [e-mail] 
> > > [mailto:[e-mail]]
> > > Sent: Saturday, October 05, 2019 12:18 AM
> > > To: 'Benjamin Kaduk'; 'The IESG'
> > > Cc: [e-mail]; 'Ali Begen';
> [e-mail]; [e-mail]; 'Victor Demjanenko, Ph.D.'; 'Dave
> Satterlee (Vocal)'
> > > Subject: RE: Benjamin Kaduk's Discuss on 
> > > draft-ietf-payload-tsvcis-03: (with DISCUSS and COMMENT)
> > >
> > > Everyone,
> > >
> > > Thanks for the comments.  I think I mis-understood the ambiguity with
> respect to to changing rates within a RTP packet.  That was not plan.  An
> RTP packet must have MELP speech frames of the same rate.  What is possible
> is that the amount of augmented TSVCIS speech data may vary from one speech
> frame to the next.  This allows for a dynamic VDR as suggested by the NRL
> paper.  So an RTP packet may have varying TSVCIS data but must always have
> MELPe 2400 data.
> > >
> > > Again backwards parsing is necessary but the timestamp uniformly
> increments 22.5msec per combined MELP/TSVCIS speech frame.
> > >
> > > The NRL is a good public reference on the VDR aspects.  The actual
> TSVCIS spec we had was FOUO so we could not replicate its detail.  (I
> believe a later spec is public or at least partially public.  I am trying to
> get this.)  The opaque data is pretty obvious with the TSVCIS spec in hand.
> > >
> > > We will address the issues/concerns raised next week.  Other business
> had priority.
> > >
> > > Thank you and enjoy the weekend.
> > >
> > > Regards,
> > >
> > > Victor & Dave
> > >
> > > -----Original Message-----
> > > From: Benjamin Kaduk via Datatracker <[e-mail]>
> > > Sent: Wednesday, October 2, 2019 10:40 PM
> > > To: The IESG <[e-mail]>
> > > Cc: [e-mail]; Ali Begen 
> > > <[e-mail]>; [e-mail]; 
> > > [e-mail]; [e-mail]
> > > Subject: Benjamin Kaduk's Discuss on draft-ietf-payload-tsvcis-03: 
> > > (with DISCUSS and COMMENT)
> > >
> > > Benjamin Kaduk has entered the following ballot position for
> > > draft-ietf-payload-tsvcis-03: 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-tsvcis/
> > >
> > >
> > >
> > > --------------------------------------------------------------------
> > > --
> > > DISCUSS:
> > > --------------------------------------------------------------------
> > > --
> > >
> > > I support Magnus' point about the time-ordering of adjacent frames in a
> packet.
> > >
> > > Additionally, I am not sure that there's quite enough here to be
> interoperably implementable.  Specifically, we seem to be lacking a
> description of how an encoder or decoder knows which TSVCIS parameters, and
> in what order, to byte-pack or unpack, respectively.  One might surmise that
> there is a canonical listing in [TSVCIS], but this document does not say
> that, and furthermore [TSVCIS] is only listed as an informative reference.
> (I couldn't get my hands on my copy, at least on short notice.)  If we
> limited ourselves to treating the TSVCIS parameters as an entirely opaque
> blob (codec, convey these N octets to the peer with the appropriate one- or
> two-byte trailer for payload type identification and framing), that would be
> interoperably implementable, since the black-box bits are up to some other
> codec to interpret.
> > >
> > > In a similar vein, we mention but do not completely specify the
> potential for using CODB as an end-to-end framing bit, in Section 3.1 (see
> Comment), which is not interoperably implementable without further details.
> > >
> > >
> > > --------------------------------------------------------------------
> > > --
> > > COMMENT:
> > > --------------------------------------------------------------------
> > > --
> > >
> > > Where is [TSVCIS] available?
> > >
> > > Is [NRLVDR] the same as
> > > https://apps.dtic.mil/dtic/tr/fulltext/u2/a588068.pdf ?  A URL in the
> references would be helpful.
> > >
> > > Is additional TSVCIS data only present after 2400bps MELPe and the first
> thing to get dropped under bandwidth pressure?  The abstract and
> introduction imply this by calling out MELPe 2400 bps speech parameters
> explicitly, but Section 3 says that TSVCIS augments standard 600, 1200, and
> 2400 bps MELP frames.
> > >
> > > It's helpful that Section 3.3 gives some general guidance for decoding
> this payload type ("[t]he way to determine the number of TSVCIS/MELPe frames
> is to identify each frame type and length"), but I think some generic
> considerations would be very helpful to the reader much earlier, along the
> lines of "MELPe and TSVCIS data payloads are decoded from the end, using the
> CODA and CODB (and, if necessary, CODC and others) bits to determine the
> type of payload.  For MELPe payloads the type also indicates the payload
> length, whereas for TSVCIS data an additional length field is present, in
> one of two possible formats.  A TSVCIS coder frame consists of a MELPe data
> payload followed by zero or one TSVCIS data payload; after the TSVCIS
> payload's presence/length is determined, then the preceding MELPe payload
> can be determined and decoded.  Per Section 3.3, multiple TSVCIS frames can
> be present in a single RTP packet."  This (or something like it) would also
> serve to clarify the role of the COD* bits, which is otherwise only
> implicitly introduced.
> > >
> > > Section 1.1
> > >
> > > RFC 2736 is BCP 36 (but it's updated by RFC 8088 which is for some
> reason an Informational document and not part of BCP 36?!).
> > >
> > > Section 2
> > >
> > >    In addition to the augmented speech data, the TSVCIS specification
> > >    identifies which speech coder and framing bits are to be encrypted,
> > >    and how they are protected by forward error correction (FEC)
> > >    techniques (using block codes).  At the RTP transport layer, only the
> > >    speech coder related bits need to be considered and are conveyed in
> > >    unencrypted form.  In most IP-based network deployments, standard
> > >
> > > Am I reading this correctly that this text is just summarizing what's in
> the TSVCIS spec in terms of what needs to be in unencrypted form, so the
> "only the speech coder related bits[...]" is not new information from this
> document?  I'm not sure I agree with the conclusion, regardless -- won't the
> (MELPe) speech coder bits be enough to convey the semantic content of the
> audio stream, something that one might desire to keep confidential?
> > >
> > >    link encryption methods (SRTP, VPNs, FIPS 140 link encryptors or Type
> > >    1 Ethernet encryptors) would be used to secure the RTP speech
> > >    contents.  Further, it is desirable to support the highest voice
> > >    quality between endpoints which is only possible without the overhead
> > >    of FEC.
> > >
> > > I think I'm missing a step in how this conclusion was reached.
> > >
> > >    TSVCIS will be characterized.  Depending on the bandwidth available
> > >    (and FEC requirements), a varying number of TSVCIS specific speech
> > >    coder parameters need to be transported.  These are first byte-packed
> > >    and then conveyed from encoder to decoder.
> > >
> > > Per the Discuss point, how do I know which parameters need to be
> transported, and in what order?
> > >
> > >    Byte packing of TSVCIS speech data into packed parameters is
> > >    processed as per the following example:
> > >
> > >       Three-bit field: bits A, B, and C (A is MSB, C is LSB)
> > >       Five-bit field: bits D, E, F, G, and H (D is MSB, H is LSB)
> > >
> > >            MSB                                              LSB
> > >             0      1      2      3      4      5      6      7
> > >         +------+------+------+------+------+------+------+------+
> > >         |   H  |   G  |   F  |   E  |   D  |   C  |   B  |   A  |
> > >         +------+------+------+------+------+------+------+------+
> > >
> > >    This packing method places the three-bit field "first" in the lowest
> > >    bits followed by the next five-bit field.  Parameters may be split
> > >    between octets with the most significant bits in the earlier octet.
> > >    Any unfilled bits in the last octet MUST be filled with zero.
> > >
> > > I agree with Adam that this is very unclear.  A is the MSB of the
> three-bit field but the LSB of the octet overall?
> > > We probably need an example of splitting a parameter across octets as
> well, to get the bit ordering right.
> > >
> > > Section 3.1
> > >
> > >    It should be noted that CODB for both the 2400 and 600 bps modes MAY
> > >    deviate from the values in Table 1 when bit 55 is used as an end-to-
> > >    end framing bit.  Frame decoding would remain distinct as CODA 
> > > being
> > >
> > > Where is the use of CODB as an end-to-end framing bit defined?  If we're
> going to provide neither a complete description of how to do it nor a
> reference to a better description, we probably shouldn't mention it at all.
> > >
> > > Section 3.2
> > >
> > >    RTP packet.  The packed parameters are counted in octets (TC).  In
> > >    the preferred placement, shown in Figure 6, a single trailing octet
> > >    SHALL be appended to include a two-bit rate code, CODA and CODB,
> > >
> > > I'd consider saying something about this being the preferred format
> > > ("placement") due to its shorter length than the alternative, and say
> that it "SHOULD be used for TSVCIS payloads with TC less than or equal to 77
> octetes".
> > >
> > > Section 3.3
> > >
> > > When a longer packetization interval is used, is that indicated by
> signaling or RTP timestamps or otherwise?
> > >
> > >    TSVCIS coder frames in a single RTP packet MAY be of different coder
> > >    bitrates.  With the exception for the variable length TSVCIS
> > >    parameter frames, the coder rate bits in the trailing byte identify
> > >    the contents and length as per Table 1.
> > >
> > > Maybe also note that the penultimate octet gives the length there?
> > >
> > >    Information describing the number of frames contained in an RTP
> > >    packet is not transmitted as part of the RTP payload.  The way to
> > >    determine the number of TSVCIS/MELPe frames is to identify each frame
> > >    type and length thereby counting the total number of octets within
> > >    the RTP packet.
> > >
> > > terminology nit: if a frame is the combination of MELPe and TSVCIS
> payload data units then there are two layres of decoding to get a length for
> the frame, since we have to get the TSVCIS length and then the MELPe length.
> > >
> > > Section 4.2
> > >
> > >    Parameter "ptime" cannot be used for the purpose of specifying 
> > > the
> > >
> > > nit: missing article ("The parameter")
> > >
> > >    will be impossible to distinguish which mode is about to be used
> > >    (e.g., when ptime=68, it would be impossible to distinguish if the
> > >    packet is carrying one frame of 67.5 ms or three frames of 22.5 ms).
> > >
> > > So how is the operating mode determined, then?
> > > (I think this is the same question I asked above)
> > >
> > > Section 4.4
> > >
> > >    For example, if offerer bitrates are "2400,600" and answer bitrates
> > >    are "600,2400", the initial bitrate is 600.  If other bitrates are
> > >    provided by the answerer, any common bitrate between the offer and
> > >    answer MAY be used at any time in the future.  Activation of these
> > >    other common bitrates is beyond the scope of this document.
> > >
> > > It seems important to specify whether this requires a new O/A exchange
> or can be done "spontaneously" by just encoding different frame types.
> > > (It seems like the latter is possible, on first glance, and this is 
> > > implied by Section 3.3's discussion of mixing them in a single 
> > > packet.)
> > >
> > > Section 5
> > >
> > > Please expand PLC at first use (not second).
> > >
> > > Section 6
> > >
> > > I don't understand the PLC usage.  Is the idea that a receiver, on
> seeing an SSRC gap, constructs fictitious PLC frames to "fill the gap"
> > > and passes the resulting stream to the decoder?
> > >
> > > Section 8
> > >
> > >    and important considerations in [RFC7201].  Applications SHOULD use
> > >    one or more appropriate strong security mechanisms.  The rest of this
> > >    section discusses the security-impacting properties of the payload
> > >    format itself.
> > >
> > > I thought we described TSVCIS itself (much earlier in the document) as
> requiring encryption for some data; wouldn't that translate to a "MUST"
> > > here and not a "SHOULD"?
> > >
> > >
> > >
>