Rapid Synchronisation of RTP Flows
draft-ietf-avt-rapid-rtp-sync-13

[Ballot comment]
What is the impact of some of the elements in the multimedia session not being updated to support these extensions? One of the cases mentioned in the document is middleboxes that may not understand the header extensions and may remove them, another would be if some of the receivers or the sender are not updated. I understand that the protocol extensions are designed robustly and with backwards compatibility in mind so that nothing breaks, but what are the tools at hands of a manager to understand which elements in the network support the extensions and which ones do not?

From the discussions with the editors I understand that these cases are known and understood and that there are methods for a nertwork operator to detect them like  logging which receivers are generating rapid resynchronisation requests or inspecting the signalling exchange (for in-band delivery of synchronisation metadata). It would be good if these were documented in the RFC.

[Ballot discuss]
This is a well-written document, but before approving it I have a question concerning the operational and deployment model. According to the response to my question this may be a DISCUSS-DISCUSS or may require some clarification text.

What is the impact of some of the elements in the multimedia session not being updated to support these extensions? One of the cases mentioned in the document is middleboxes that may not understand the header extensions and may remove them, another would be if some of the receivers or the sender are not updated. I understand that the protocol extensions are designed robustly and with backwards compatibility in mind so that nothing breaks, but what are the tools at hands of a manager to understand which elements in the network support the extensions and which ones do not?

[Ballot comment]
1. Have the identifiers "urn:ietf:params:rtp-hdrext:ntp-56" and "urn:ietf:params:rtp-hdrext:ntp-64" passed Expert Review in accordance with RFC 5285?

2. Please check some of the lower-case conformance language ("may", "should", "must"). Here are some examples where the all-caps versions might be appropriate:

  The RTP extension
  for unicast-based rapid acquisition of multicast RTP sessions
  [16] may be used to reduce the time taken to receive the access
  points in some scenarios.

  If the use of non-compound RTCP [5] was
  previously negotiated, both the feedback request and the RTCP SR
  response may be sent as non-compound RTCP packets.

  The media source may ignore RTCP-SR-REQ
  packets if its regular schedule for transmission of synchronisation
  metadata can be expected to allow the receiver to synchronise the
  media streams within a reasonable time frame.

  Once a receiver has synchronised the components of a layered, multi-
  description, or multi-view flow using the RTP header extensions as
  described in Section 4.1, it may then derive a decoding order based
  on the synchronised timestamps as follows (or it may use information
  in the RTP payload header to derive the decoding order, if present
  and desired).

  Flows may also include packets corresponding to additional sampling
  instants that are not present in the flows on which they depend.

  The
  decoding order of the RTP flow hierarchy may be indicated by
  mechanisms defined in [8] or by some other means.

  These timestamps may be derived using
  the NTP format timestamp provided in the RTCP sender reports or as
  shown in the figure directly from the NTP timestamp contained in the
  RTP header extensions as indicate by the timestamp in "".

  If a feedback
  target receives an RTCP-SR-REQ feedback message in such a case, the
  request should be forwarded to the media source.

  Receivers should use both the information
  contained in RTCP SR packets and the in-band mapping of RTP and NTP-
  format timestamps as input to the synchronisation process,

  For H.264 SVC, the Empty NAL unit packet [9] should be used.

  The receiver should decode the packets in all the component RTP flows
  as follows:

  Accordingly, a receiver must
  estimate the skew on the NTP-format clock in order to align RTP
  timestamps across sessions.

  o  The sender must negotiate the use of the RTP header extensions
      described in Section 3.3, and must periodically and synchronously
      insert such header extensions into all the RTP flows forming the
      separate components of the layered, multi-description, or multi-
      view flow.

For other uses of the lowercase words, you might consider changing "may" to "might" or "can", "should" to "ought" or "will", and "must" to "needs to" or some other appropriate phrase.

[Ballot comment]
Questions about 2119 language (i.e., should these be capitalized?):

1) and media security keys should have been ...
2) the request should be forwarded to the media source ...
3) Receivers should use both the ...
4) The sender must negotiate the use of the RTP header extensions described in Section 3.3, and must periodically ...
5) For H.264 SVC, the Empty NAL unit packet [9] should be used.
6) The receiver should decode ...

[Ballot discuss]
Sec 2, says it's a MUST for a common reference clock but then later says it's out of scope.  How are we supposed to get to interoperable implementations?  Can't you just point to NTP?

[Ballot comment]
A good document, thanks.

I found "can be synchronised immediately" jarred a little. I guess it is immediate from a certain point of time :-)

IANA comments:

Action 1:

Upon approval of this document, IANA will make the following assignment
in the "FMT Values for RTPFB Payload Types" registry at
http://www.iana.org/assignments/rtp-parameters

Value Name Long Name Reference
----- ---- --------- --------------------------------------
TBD[5] RTCP-SR-REQ RTCP Rapid Resynchronisation Request
[RFC-avt-rapid-rtp-sync-10]


Action 2:

Upon approval of this document, IANA will make the following assignments
in the "RTP Compact Header Extensions" registry at
http://www.iana.org/assignments/rtp-parameters

Extension URI Description Contact Reference
------------- ---------- ------- ----------
urn:ietf:params:rtp-hdrext:ntp-64 Synchronisation metadata: Thomas
Schierl  [RFC-avt-rapid-rtp-sync-10]
64-bit timestamp format IETF Audio/Video Transport Working Group

urn:ietf:params:rtp-hdrext:ntp-56 Synchronisation metadata: Thomas
Schierl  [RFC-avt-rapid-rtp-sync-10]
56-bit timestamp format IETF Audio/Video Transport Working Group


We understand the above to be the only IANA Actions for this document.

Document Shepherd Write-Up

    (1.a)  Who is the Document Shepherd for this document?  Has the
          Document Shepherd personally reviewed this version of the
          document and, in particular, does he or she believe this
          version is ready for forwarding to the IESG for publication?

The document shepherd is Roni Even. I have reviewed the document, 
and believe it is ready for publication.

    (1.b)  Has the document had adequate review both from key WG members
          and from key non-WG members?  Does the Document Shepherd have
          any concerns about the depth or breadth of the reviews that
          have been performed?

The document went through  a WGLC. The comments were minor and were
addressed. Before the WGLC, the document started as a merger of two
proposals and was in the WG for eight months as a WG document in order to
let people review it. The document shepherd has no concerns about the review
process.



    (1.c)  Does the Document Shepherd have concerns that the document
          needs more review from a particular or broader perspective,
          e.g., security, operational complexity, someone familiar with
          AAA, internationalization or XML?

No concerns.

    (1.d)  Does the Document Shepherd have any specific concerns or
          issues 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.  Has an IPR disclosure related to this document
been filed?  If so, please include a reference to the disclosure and
summarize the WG discussion and conclusion on this issue.



No concerns. There is an IPR statement
(https://datatracker.ietf.org/ipr/1140/) . There were no concerns about the
IPR in the WG.

    (1.e)  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 document has good consensus from the WG there was a good mailing list
discussion on the individual drafts that resulted in a WG document.  It was
presented in AVT sessions and the editors (Colin Perkins and Thomas Schierl)
are very familiar with RTP and RTCP.

    (1.f)  Has anyone threatened an appeal or otherwise indicated 
          extreme discontent?  If so, please summarize the areas of
conflict in
          separate email messages to the Responsible Area Director. 
        (It  should be in a separate email because this questionnaire is
          entered into the ID Tracker.)

No.

    (1.g)  Has the Document Shepherd personally verified that the
          document satisfies all ID nits?  (See
          http://www.ietf.org/ID-Checklist.html and
          http://tools.ietf.org/tools/idnits/).  Boilerplate checks are
          not enough; this check needs to be thorough.  Has the 
          document  met all formal review criteria it needs to, such as the
MIB          Doctor, media type and URI type reviews?


The idnits tool reports no nits.

    (1.h)  Has the document split its references into normative and
          informative?  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
          strategy for their completion?  Are there normative
          references that are downward references, as described in
[RFC3967]?  If
          so, list these downward references to support the Area
          Director in the Last Call procedure for them [RFC3967].

References have been split. There are no normative references to 
internet-drafts which are in progress.

    (1.i)  Has the Document Shepherd verified that the document IANA
          consideration section exists and is consistent with the body
          of the document?  If the document specifies protocol
          extensions, are reservations requested in appropriate IANA
          registries?  Are the IANA registries clearly identified?  If
          the document creates a new registry, does it define the
          proposed initial contents of the registry and an allocation
          procedure for future registrations?  Does it suggest a
          reasonable name for the new registry?  See [RFC2434].  If the
          document describes an Expert Review process has Shepherd
          conferred with the Responsible Area Director so that the IESG
          can appoint the needed Expert during the IESG Evaluation?

The IANA considerations section exists; the registries are identified there
are no new registries.

    (1.j)  Has the Document Shepherd verified that sections of the
          document that are written in a formal language, such as XML
          code, BNF rules, MIB definitions, etc., validate correctly in
          an automated checker?

No such sections

    (1.k)  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

              Relevant content can frequently be found in the abstract
              and/or introduction of the document.  If not, this may be
              an indication that there are deficiencies in the abstract
              or introduction.


"This memo outlines how RTP sessions are synchronised, and discusses how
rapidly such synchronisation can occur.  We show that most RTP sessions can
be synchronised immediately, but that the use of video switching multipoint
conference units (MCUs) or large source specific multicast (SSM) groups can
greatly increase the synchronization delay.  This increase in delay can be
unacceptable to some applications that use layered and/or multi-description
codecs.

This memo introduces three mechanisms to reduce the synchronisation delay
for such sessions.  First, it updates the RTP Control Protocol (RTCP) timing
rules to reduce the initial synchronisation delay for SSM sessions.  Second,
a new feedback packet is defined for use with the Extended RTP Profile for
RTCP-based Feedback (RTP/AVPF), allowing video switching MCUs to rapidly
request resynchronisation.  Finally, new RTP header extensions are defined
to allow rapid synchronisation of late joiners, and guarantee correct
timestamp based decoding order recovery for layered codecs in the presence
of clock skew."

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?

There is nothing specific to note.


          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?


This document explains and updates the timing consideration for RTCP in RFC
3550. It also defines new protocol that is referenced by other documents
like payload specification for H.264 SVC
http://tools.ietf.org/html/draft-ietf-avt-rtp-svc-21

          Personnel
              Who is the Document Shepherd for this document?  Who is 
              the Responsible Area Director?


Roni Even is the document shepherd.
The responsible area director is Robert Sparks.