RTP Payload Format for VC-2 High Quality (HQ) Profile
draft-ietf-payload-rtp-vc2hq-08

[Ballot comment]
Thanks for the work on this document.

Given that the underlying format doesn't appear to be resilient to loss, I'm a
little surprised to see no discussion of FEC; and, in particular, no treatment
of the allocation of unequal error protection to the various packet types. For
example, it sounds like the transform parameters packet is significantly more
important than, e.g., a picture fragment that contains slices. I suspect there
is a general prioritization among the various types that would useful to call
out for implementors.

[Ballot comment]
Thanks for the clear Security Considerations section.
I am a little uncertain about the privacy properties of the padding
data, though, largely due to my uncertainty about the details of how
the padding works.  (This is, perhaps, in a similar vein to Eric's
general concerns on interoperability.)

In particular, Section 4.2 says that the Data Length for a Padding
Data Unit "may have any value" and "indicates the size of the
recommended padding".  There is also an "Optional Payload Data" in
Figure 6, and I failed to find a description of what its contents
are for padding data units.  Section 4.5.1's coverage of the Padding
Data Parse Info Header suggests that the "native VC-2" and RTP
padding elements are essentially distinct, with the RTP one being
essentially a recommendation to add a VC-2 one, but giving no
mandatory guidance on how much padding to apply.  In this scenario I
don't know what the purpose of the "optional payload data" in Figure
6 be, though.  Padding is of course ignored by the actual VC-2
decoder, so the concern would mostly be if the (RTP) bits on the
wire include a side channel or "stegangraphic channel" (not exactly
the normal meaning of that one) where identifying information could
be inserted, unbeknownst to the recipient.  This could come into
play if media encryption is not used, or when a middlebox/mixer is
used, or probably in other scenarios as well.  The specification of
all-zeros padding along with the Padding Data Parse Info Header of
course removes any such channel at that point, but I didn't see
a real confirmation that there was no channel in the RTP bits on the
wire.

Some additional section-by-section comments follow.

Section 4.1

  Timestamp: 32 bits  If the packet contains transform parameters or
        coded slice data for a coded picture then the timestamp
        corresponds to the sampling instant of the coded picture.  A
        90kHz clock SHOULD be used.  A single RTP packet MUST NOT
        contain coded data for more than one coded picture, so there is
        no ambiguity here.

Is this a new requirement or quoting a preexisting one?  If a new
requirement, I suggest replacing "so there is no ambiguity here"
with "in order to eliminate any chance for ambiguity".

Section 4.2

        If the receiver does not receive a transform parameters packet
        for a picture then it MAY assume that the parameters are
        unchanged since the last picture, or MAY discard the picture.

Those seem like two very different options!  How would I choose
between them?


We only get the starting x- and y-coordinates of a slice for the
first slice in a packet; it sounds like the main VC-2 spec specifies
the order in which the following slices are laid out?
(Do we need to say anything about what "x coordinate" and "y
coordinate" mean?  I seem to recall that over history there have
existed pixel identifying schemes that put the origin at both the
top left and bottom left of the display.)

Section 4.5.1

There is some text here and elsewhere that seems to imply reusing a
Parse Info Header for various data received in different RTP
packets, potentially even from different coded pictures.  The Parse
Info Header contains "next" and "prev parse offset"s, though --
when would those offsets need to be updated?

  o  A receiver MAY combine all fragment data units (with parse code
      0xEC) and the same picture number into a single picture data unit
      with parse code 0xE8.  If the stream is required to comply with
      major versions 1 or 2 of the VC-2 Spec then this MUST be done.

The "and" in "and the same picture number" seems to be an editing
error; maybe "with" is better?

  o  Once a data unit has been assembled, whether a Sequence Header,
      Coded Picture Fragment, Coded Picture, or Auxiliary Data Unit, the
      next parse offset and previous parse offset values in its Parse
      Info Header should be filled with the offset between the start of
      the header and the start of the next or previous.

This text could probably be tightened up with respect to which
next/previous fields are updated when, and what values go in them.
E.g., do we have enough information to fill in the "previous" field
when we start assembling a data unit, and the "next" when we finish
assembling that data unit?

Section 6.1

Perhaps "RFC XXXX" makes more sense as the "published specification"
than ST 2042-1?  That is, this is where the (mandatory) RTP framing
is required, so it may be a better starting point for an
implementor.

[Ballot comment]
Rich version of this review at:
https://mozphab-ietf.devsvcdev.mozaws.net/D4183


I am not sure that this specification can be interoperably
implemented. I have noted a number of points below. I believe these
are largely minor and so have not made this a DISCUSS, but it is
important they be resolved.

This document would also benefit from significant editorial work.
Based on S  4.5.1, I take the structure to be that you start with the
VC-2 stream and then use RTP headers to contain the information in
Parse Info blocks. If that is correct, it would be much clearer if
stated upfront.



IMPORTANT
S 4.2.
> 
>      The fields of the extended headers are defined as follows:
> 
>      Extended Sequence Number: 16 bits  MUST Contain the high-order
>            16-bits of the 32-bit packet sequence number, a number which
>            increments with each packet.  This is needed since the high

Increments by one?


S 4.2.
>            data rates of VC2 Sequences mean that it is highly likely that
>            the 16-bit sequence number will roll-over too frequently to be
>            of use for stream synchronisation.
> 
>      B: 1 bit  MUST be set to 1 if the packet contains the first byte of
>            an Auxiliary Data or Padded Data Unit.

And otherwise must be 0?


S 4.2.
> 
>      B: 1 bit  MUST be set to 1 if the packet contains the first byte of
>            an Auxiliary Data or Padded Data Unit.
> 
>      E: 1 bit  MUST be set to 1 if the packet contains the final byte of
>            an Auxiliary Data or Padded Data Unit.

And otherwise must be 0?


S 4.2.
>            from a new picture until all the coded data from the current
>            picture has been sent.
> 
>            If the receiver does not receive a transform parameters packet
>            for a picture then it MAY assume that the parameters are
>            unchanged since the last picture, or MAY discard the picture.

How does this interact with packet loss?

COMMENTS
S 3.
>      the decoder.
> 
>      Each Sequence consists of a series of 13-octet Parse Info headers and
>      variable length Data Units.  The Sequence begins and ends with a
>      Parse Info header and each Data Unit is preceded by a Parse Info
>      Header.  Data Units come in a variety of types, the most important

This text isn't very clear to me. Is the following valid: PI | Data |
PI? How about PI | PI | Data | PI.  PI | PI | PI | Data | PI?




S 3.
>      should not be assumed.
> 
>      The High Quality (HQ) profile for VC-2 restricts the types of Parse
>      Info Headers which may appear in the Sequence to only:
> 
>      o  Sequence Headers,

The text above says that Sequence Headers are a type of Data Unit. So
I'm confused by this text.


S 4.
> 
>  4.  Payload format
> 
>      This specification only covers the transport of Sequence Headers,
>      High Quality Fragments, Auxiliary Data, and (optionally) End of
>      Sequence Headers and Padding Data.

So it doesn't include Parse Info?


S 4.
>        Picture (Figure 2),
> 
>      o  A Picture Fragment containing VC-2 Coded Slices (Figure 3) for a
>        picture,
> 
>      o  The end of a VC-2 Sequence (Figure 4)

It would be helpful if you cited specific sections of VC-2 for these.


S 4.
>      .                                                              .
>      +---------------------------------------------------------------+
> 
>                  Figure 6: RTP Payload Format For Padding Data
> 
>      All fields in the headers longer than a single bit are interprted as

Nit: interpreted.


S 4.2.
> 
>      Data Length: 32 bits  For an auxiliary data unit this contains the
>            number of bytes of data contained in the uncoded payload
>            section of this packet.  For a Padding Data Unit this field may
>            have any value and simply indicates the size of the recommended
>            padding.

This seems like a very large field given that RTP datagrams are almost
never this large, so I am suspecting the "uncoded payload" means pre-
compressed? Can you be clearer..

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Services Operator has completed its review of draft-ietf-payload-rtp-vc2hq-05. If any part of this review is inaccurate, please let us know.

The IANA Services Operator understands that, upon approval of this document, there are two actions which we must complete.

First, in the video namespace of the Media Types registry located at:

https://www.iana.org/assignments/media-types/

a single, new registration will be made as follows:

Name: vc2
Template: [ TBD-at-Registration ]
Reference: [ RFC-to-be ]

Second, in the RTP Payload Format media types registry on the Real-Time Transport Protocol (RTP) Parameters registry page located at:

https://www.iana.org/assignments/rtp-parameters/

a single, new registration will be made as follows:

Media Type: video
Subtype: vc2
Clock Rate (Hz):
Channels (audio):
Reference: [ RFC-to-be ]

The IANA Services Operator understands that these are the only actions required to be completed upon approval of this document.

Note:  The actions requested in this document will not be completed until the document has been approved for publication as an RFC. This message is meant only to confirm the list of actions that will be performed.

Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2018-05-16):

From: The IESG
To: IETF-Announce
CC: ben@nostrum.com, ali.begen@networked.media, payload-chairs@ietf.org, payload@ietf.org, draft-ietf-payload-rtp-vc2hq@ietf.org
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (RTP Payload Format for VC-2 HQ Profile Video) to Proposed Standard


The IESG has received a request from the Audio/Video Transport Payloads WG
(payload) to consider the following document: - 'RTP Payload Format for VC-2
HQ Profile Video'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2018-05-16. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the beginning of
the Subject line to allow automated sorting.

Abstract


  This memo describes an RTP Payload format for the High Quality (HQ)
  profile of SMPTE Standard ST 2042-1 known as VC-2.  This document
  describes the transport of HQ Profile VC-2 in RTP packets and has
  applications for low-complexity, high-bandwidth streaming of both
  lossless and lossy compressed video.

  The HQ profile of VC-2 is intended for low latency video compression
  (with latency potentially on the order of lines of video) at high
  data rates (with compression ratios on the order of 2:1 or 4:1).




The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-payload-rtp-vc2hq/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-payload-rtp-vc2hq/ballot/


No IPR declarations have been submitted directly on this I-D.

(1) What type of RFC is being requested (BCP, Proposed Standard,
Internet Standard, Informational, Experimental, or Historic)?  Why
is this the proper type of RFC?  Is this type of RFC indicated in the
title page header?

This document will be a standard track RFC, it specifies and RTP payload format for the VC2 profile of the SMPTE Standard ST 2042-1. RTP payload format are standard track documents. The type is indicated on the title page.

(2) The IESG approval announcement includes a Document Announcement
Write-Up. Please provide such a Document Announcement Write-Up. Recent
examples can be found in the "Action" announcements for approved
documents. The approval announcement contains the following sections:

Technical Summary

This memo describes an RTP Payload format for the High Quality (HQ) profile of SMPTE Standard ST 2042-1 known as VC-2.  This document describes the transport of HQ Profile VC-2 in RTP packets and has applications for low-complexity, high-bandwidth streaming of both lossless and lossy compressed video.

Working Group Summary

  Was there anything in WG process that is worth noting? For
  example, was there controversy about particular points or
  were there decisions where the consensus was particularly
  rough?

The document was discussed in the meetings,  and on the mailing list. The open issues were addressed and there are no remaining open issues, there was consensus on the content of the document.


Document Quality

  Are there existing implementations of the protocol? Have a
  significant number of vendors indicated their plan to
  implement the specification? Are there any reviewers that
  merit special mention as having done a thorough review,
  e.g., one that resulted in important changes or a
  conclusion that the document had no substantive issues? If
  there was a MIB Doctor, Media Type or other expert review,
  what was its course (briefly)? In the case of a Media Type
  review, on what date was the request posted?

According to the author, there are existing implementations (BBC and Barco's).

The request for a media type review was posted on March 22nd, 2017. A review was provided on April 8th.


Personnel

  Who is the Document Shepherd? Who is the Responsible Area
  Director?

Ali C. Begen is the Document Shepherd.
The responsible AD is Ben Campbell.


(3) Briefly describe the review of this document that was performed by
the Document Shepherd.  If this version of the document is not ready
for publication, please explain why the document is being forwarded to
the IESG.

The shepherd reviewed the document and believes that the document is ready for publication.


(4) Does the document Shepherd have any concerns about the depth or
breadth of the reviews that have been performed?

No.

(5) Do portions of the document need review from a particular or from
broader perspective, e.g., security, operational complexity, AAA, DNS,
DHCP, XML, or internationalization? If so, describe the review that
took place.

None.

(6) Describe any specific concerns or issues that the Document Shepherd
has with this document that the Responsible Area Director and/or the
IESG should be aware of? For example, perhaps he or she is uncomfortable
with certain parts of the document, or has concerns whether there really
is a need for it. In any event, if the WG has discussed those issues and
has indicated that it still wishes to advance the document, detail those
concerns here.

None.

(7) Has each author confirmed that any and all appropriate IPR
disclosures required for full conformance with the provisions of BCP 78
and BCP 79 have already been filed. If not, explain why.

Yes. The author has confirmed, as of today there are no IPR disclosures on the draft.


(8) Has an IPR disclosure been filed that references this document?
If so, summarize any WG discussion and conclusion regarding the IPR
disclosures.

No.

(9) How solid is the WG consensus behind this document? Does it
represent the strong concurrence of a few individuals, with others
being silent, or does the WG as a whole understand and agree with it? 

The WG understands the document and agrees with it.


(10) Has anyone threatened an appeal or otherwise indicated extreme
discontent? If so, please summarise the areas of conflict in separate
email messages to the Responsible Area Director. (It should be in a
separate email because this questionnaire is publicly available.)

No.

(11) Identify any ID nits the Document Shepherd has found in this
document. (See https://www.ietf.org/tools/idnits/ and the Internet-Drafts
Checklist). Boilerplate checks are not enough; this check needs to be
thorough.

There is a possible downref to a non-IETF document. It is an SMPTE standard, so the WG is fine with it.

(12) Describe how the document meets any required formal review
criteria, such as the MIB Doctor, media type, and URI type reviews.

Media type was reviewed. Refer to https://mailarchive.ietf.org/arch/msg/media-types/ttAU0VqYtFD2ZM0xntSdM0aIsWU/?qid=fa64b66637ab7b55ff3db3326e4b021c


(13) Have all references within this document been identified as
either normative or informative?

Yes.

(14) Are there normative references to documents that are not ready for
advancement or are otherwise in an unclear state? If such normative
references exist, what is the plan for their completion?

No.

(15) Are there downward normative references references (see RFC 3967)?
If so, list these downward references to support the Area Director in
the Last Call procedure.

There is a normative reference to the SMPTE standard.

(16) Will publication of this document change the status of any
existing RFCs? Are those RFCs listed on the title page header, listed
in the abstract, and discussed in the introduction? If the RFCs are not
listed in the Abstract and Introduction, explain why, and point to the
part of the document where the relationship of this document to the
other RFCs is discussed. If this information is not in the document,
explain why the WG considers it unnecessary.

No.

(17) Describe the Document Shepherd's review of the IANA considerations
section, especially with regard to its consistency with the body of the
document. Confirm that all protocol extensions that the document makes
are associated with the appropriate reservations in IANA registries.
Confirm that any referenced IANA registries have been clearly
identified. Confirm that newly created IANA registries include a
detailed specification of the initial contents for the registry, that
allocations procedures for future registrations are defined, and a
reasonable name for the new registry has been suggested (see RFC 5226).

IANA section is OK.


(18) List any new IANA registries that require Expert Review for future
allocations. Provide any public guidance that the IESG would find
useful in selecting the IANA Experts for these new registries.

No new IANA registries.


(19) Describe reviews and automated checks performed by the Document
Shepherd to validate sections of the document written in a formal
language, such as XML code, BNF rules, MIB definitions, etc.

None was needed.