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

<victor.demjanenko@vocal.com> Tue, 19 November 2019 20:27 UTC

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From: <victor.demjanenko@vocal.com>
To: "'Benjamin Kaduk'" <kaduk@mit.edu>, "'Barry Leiba'" <barryleiba@computer.org>
Cc: "'Roni Even \(A\)'" <roni.even@huawei.com>, "'The IESG'" <iesg@ietf.org>, "'Catherine Meadows'" <catherine.meadows@nrl.navy.mil>, "'IETF SecDir'" <secdir@ietf.org>, <draft-ietf-payload-tsvcis@ietf.org>, "'Ali Begen'" <ali.begen@networked.media>, <avtcore-chairs@ietf.org>, <avt@ietf.org>, "'Dave Satterlee \(Vocal\)'" <Dave.Satterlee@vocal.com>, "'IETF discussion list'" <ietf@ietf.org>, <draft-ietf-payload-tsvcis.all@ietf.org>, <victor.demjanenko@vocal.com>
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Date: Tue, 19 Nov 2019 15:14:21 -0500
X-ASG-Orig-Subj: RE: Benjamin Kaduk's Discuss on draft-ietf-payload-tsvcis-03: (with DISCUSS and COMMENT)
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Subject: Re: [secdir] Benjamin Kaduk's Discuss on draft-ietf-payload-tsvcis-03: (with DISCUSS and COMMENT)
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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.

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.

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.

Regards,

Victor

-----Original Message-----
From: Benjamin Kaduk <kaduk@mit.edu>; 
Sent: Thursday, October 31, 2019 8:12 PM
To: Barry Leiba <barryleiba@computer.org>;
Cc: victor.demjanenko@vocal.com; Roni Even (A) <roni.even@huawei.com>;; The
IESG <iesg@ietf.org>;; Catherine Meadows <catherine.meadows@nrl.navy.mil>;;
IETF SecDir <secdir@ietf.org>;; draft-ietf-payload-tsvcis@ietf.org; Ali Begen
<ali.begen@networked.media>;; avtcore-chairs@ietf.org; avt@ietf.org; Dave
Satterlee (Vocal) <Dave.Satterlee@vocal.com>;; IETF discussion list
<ietf@ietf.org>;; draft-ietf-payload-tsvcis.all@ietf.org
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 <victor.demjanenko@vocal.com>; 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: victor.demjanenko@vocal.com <victor.demjanenko@vocal.com>;
> > Sent: Thursday, October 24, 2019 10:05 AM
> > To: 'Roni Even (A)' <roni.even@huawei.com>;; 'Benjamin Kaduk' 
> > <kaduk@mit.edu>;; 'The IESG' <iesg@ietf.org>;
> > Cc: draft-ietf-payload-tsvcis@ietf.org; 'Ali Begen' 
> > <ali.begen@networked.media>;; avtcore-chairs@ietf.org; avt@ietf.org; 
> > 'Dave Satterlee (Vocal)' <Dave.Satterlee@vocal.com>;
> > 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 <avt@ietf.org>;
> >
> >
> > 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) <roni.even@huawei.com>;
> > Sent: Sunday, October 6, 2019 2:09 AM
> > To: victor.demjanenko@vocal.com; 'Benjamin Kaduk' <kaduk@mit.edu>;; 
> > 'The IESG' <iesg@ietf.org>;
> > Cc: draft-ietf-payload-tsvcis@ietf.org; 'Ali Begen' 
> > <ali.begen@networked.media>;; avtcore-chairs@ietf.org; avt@ietf.org; 
> > 'Dave Satterlee (Vocal)' <Dave.Satterlee@vocal.com>;
> > 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: victor.demjanenko@vocal.com 
> > [mailto:victor.demjanenko@vocal.com]
> > Sent: Saturday, October 05, 2019 12:18 AM
> > To: 'Benjamin Kaduk'; 'The IESG'
> > Cc: draft-ietf-payload-tsvcis@ietf.org; 'Ali Begen';
avtcore-chairs@ietf.org; avt@ietf.org; '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 <noreply@ietf.org>;
> > Sent: Wednesday, October 2, 2019 10:40 PM
> > To: The IESG <iesg@ietf.org>;
> > Cc: draft-ietf-payload-tsvcis@ietf.org; Ali Begen 
> > <ali.begen@networked.media>;; avtcore-chairs@ietf.org; 
> > ali.begen@networked.media; avt@ietf.org
> > 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
> >
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> > 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"?
> >
> >
> >