Duplicating RTP Streams
draft-ietf-avtext-rtp-duplication-06

[Ballot comment]
I want to talk about this on the call on Thursday.

Like others (incl. Suresh) I am concerned about Section 7 language. It is important that the section indicates who the MUST/SHOULD requirements are placed at, and how those entities know the situation.

One interpretation is that the requirements are placed on operator (the human). In that case, maybe some language such as "The network operator MUST NOT …" should be used.

On the other hand, if you are talking about things that devices should do, they have no knowledge of congestion, as Suresh pointed out in his Gen-ART review. In this case I'd recommend a change along the following lines:

OLD:
  Duplication is RECOMMENDED only to
  be used for protection against network outages due to a temporary
  link or network element failure and where it is known that there is
  sufficient network capacity to carry the duplicated traffic.
NEW:
  Duplication is RECOMMENDED only to
  be used for protection against network outages due to a temporary
  link or network element failure and where it is known (e.g., through explicit
  operator configuration) that there is
  sufficient network capacity to carry the duplicated traffic.

[Ballot comment]
From section 4.2:
  As specified in Section 3.2 of [RFC7104], it is advisable that the
  SSRC listed first in the "a=ssrc-group:" line (i.e., SSRC of 1000) is
  sent first, with the other SSRC (i.e., SSRC of 1010) being the time-
  delayed duplicate.

This is a rather odd thing to say, given that the text goes on to say that it doesn't really matter.  Either it matters or it doesn't.  If it doesn't, you might want to say this instead:

  Section 3.2 of [RFC7104] states that it is advisable that the
  SSRC listed first in the "a=ssrc-group:" line (i.e., SSRC of 1000) is
  sent first, with the other SSRC (i.e., SSRC of 1010) being the time-
  delayed duplicate.

If this is stated in anticipation that some implementations will erroneously take the order from the ordering of the two SSRC lines, then you might as well just require them to be specified in the intended order.

[Ballot comment]
I agree with Benoit that the Abstract (and I think second paragraph of the intro) should be rewritten to indicate that this technique is only applicable to non-congestive packet loss.

In the intro, one additional sentence would have helped me. After this:

  One technique to recover from packet loss without incurring unbounded
  delay for all the receivers is to duplicate the packets and send them
  in separate redundant streams.
 
Add:

  As described later in this document, the probability that two copies
  of the same packet are lost in cases of non-congestive packet loss is
  quite small.

In fact, nowhere in the document do you actually say something along the lines of "the techniques we describe actually address the kinds of non-congestive losses we see in the field." When chatting with Richard, he suggested some text like:

  Time-delay duplication and spatial duplication deal with different
  patterns of loss.  Time-delay duplication helps with transient loss
  (within the duplication window), while spatial duplication can help
  with longer-term loss that affects only one of the two redundant
  paths.

Saying that, and saying, "Those are the kinds of losses we actually see, so these techniques will help" would be useful.

[Ballot comment]

- Please see the additional security considerations text added
to [1] version -03 [2] after my discuss on that. I'm not sure
that text is prefect but isn't something similar warranted
here too? (And since we've common authors, this should be easy
enough:-) Doing that by reference is fine if that's easier.

- BTW: I thought I was reading the same document as [1] for a
while;-)

[1] https://datatracker.ietf.org/doc/draft-ietf-mmusic-delayed-duplication/
[2] http://www.ietf.org/rfcdiff?url2=draft-ietf-mmusic-delayed-duplication-03

[Ballot comment]
I have a number of comments below, and an overarching concern.

The over arching concern is the assumption that using different flow
identifiers will cause the use of diverse paths, and this is by no means
assured. If you want diverse paths we should look at methods of
implementing that such as segment routing and MPLS TE.

=====

From a routing perspective, two streams are considered identical if
  the following two IP header fields are the same, since they will be
  both routed over the same path:

Not really, we look at the IP type and transport ports to determine
flow equivalence.

=====

When two routing-plane identical streams are used, the two
      streams will have identical IP headers.  This makes it
      impractical to forward the packets onto different paths. 

Er well they could have different flow labels.

========
Due to the possible presence of network address and port
  translation (NAPT) devices, load balancers, or other middleboxes, use
  of anything other than an identical 5-tuple might also cause spatial
  redundancy

... and flow label

========

When using spatial redundancy, the duplicate RTP stream is sent using
  a different source and/or destination address/port pair.

This does not guarantee spacial redundancy of course.

[Ballot comment]
  This loss might be due to congestion,
  it might also be a result of an unplanned outage caused by a flapping
  link, link or interface failure, a software bug

I just have an amusing image of Martian packets on a flapping link.  Don't mind me....

[Ballot comment]
Not a DISCUSS, but I would appreciate of this one would be solved.
I believe the abstract is misleading:

  Packet loss is undesirable for real-time multimedia sessions, but can
  occur due to congestion, or other unplanned network outages.  This is
  especially true for IP multicast networks, where packet loss patterns
  can vary greatly between receivers.  One technique that can be used
  to recover from packet loss without incurring unbounded delay for all
  the receivers is to duplicate the packets and send them in separate
  redundant streams.  This document explains how Real-time Transport
  Protocol (RTP) streams can be duplicated without breaking RTP or RTP
  Control Protocol (RTCP) rules.

The abstract tells me: here is mechanism to avoid packet loss,
which can occur due to congestion of other unplanned network outage
However, from section 7:

  First of all, RTP duplication MUST NOT be used
  in cases where the primary cause of packet loss is congestion since
  duplication can make congestion only worse.

So this spec. only applies to "other unplanned network outage".
This should be clear from the beginning, i.e. the abstract.

Interestingly, this is in line with my DISCUSS on draft-ietf-mmusic-delayed-duplication-02.
At least, I'm consistent with myself.


Thank for addressing Linda's and David's OPS-DIR reviews

[Ballot comment]
Not a DISCUSS, but I would appreciate of this one would be solved.
I believe the abstract is misleading:

  Packet loss is undesirable for real-time multimedia sessions, but can
  occur due to congestion, or other unplanned network outages.  This is
  especially true for IP multicast networks, where packet loss patterns
  can vary greatly between receivers.  One technique that can be used
  to recover from packet loss without incurring unbounded delay for all
  the receivers is to duplicate the packets and send them in separate
  redundant streams.  This document explains how Real-time Transport
  Protocol (RTP) streams can be duplicated without breaking RTP or RTP
  Control Protocol (RTCP) rules.

The abstract tells me: here is mechanism to avoid packet loss,
which can occur due to congestion of other unplanned network outage
However, from section 7:

  First of all, RTP duplication MUST NOT be used
  in cases where the primary cause of packet loss is congestion since
  duplication can make congestion only worse.

So this spec. only applies to "other unplanned network outage".
This should be clear from the beginning, i.e. the abstract.

Interestingly, this is in line with my DISCUSS on draft-ietf-mmusic-delayed-duplication-02.
At least, I'm consistent with myself.

IESG/Authors/WG Chairs:

IANA has reviewed draft-ietf-avtext-rtp-duplication-04, which is currently
in Last Call, and has the following comments:

We understand that, upon approval of this document, there are no IANA
Actions that need completion.  IANA requests that the IANA Considerations section of the document remain in place upon publication.

If this assessment is not accurate, please respond as soon as possible.

The following Last Call announcement was sent out:

From: The IESG
To: IETF-Announce
CC:
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (Duplicating RTP Streams) to Proposed Standard


The IESG has received a request from the Audio/Video Transport Extensions
WG (avtext) to consider the following document:
- 'Duplicating RTP Streams'
  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 2014-02-04. 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


  Packet loss is undesirable for real-time multimedia sessions, but can
  occur due to congestion, or other unplanned network outages.  This is
  especially true for IP multicast networks, where packet loss patterns
  can vary greatly between receivers.  One technique that can be used
  to recover from packet loss without incurring unbounded delay for all
  the receivers is to duplicate the packets and send them in separate
  redundant streams.  This document explains how Real-time Transport
  Protocol (RTP) streams can be duplicated without breaking RTP or RTP
  Control Protocol (RTCP) rules.




The file can be obtained via
http://datatracker.ietf.org/doc/draft-ietf-avtext-rtp-duplication/

IESG discussion can be tracked via
http://datatracker.ietf.org/doc/draft-ietf-avtext-rtp-duplication/ballot/


The following IPR Declarations may be related to this I-D:

  http://datatracker.ietf.org/ipr/1656/

As required by RFC 4858, this is the current template for the Document
Shepherd Write-Up.

Changes are expected over time. This version is dated 24 February 2012.

(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?

A Proposed Standard RFC is being requested.  The document specifies normative behavior necessary for interoperability of systems using RTP stream duplication.  The title page indicates "Standards Track".


(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

  Packet loss is undesirable for real-time multimedia sessions, but can
  occur due to congestion, or other unplanned network outages.  This is
  especially true for IP multicast networks, where packet loss patterns
  can vary greatly between receivers.  One technique that can be used
  to recover from packet loss without incurring unbounded delay for all
  the receivers is to duplicate the packets and send them in separate
  redundant streams.  This document explains how Real-time Transport
  Protocol (RTP) streams can be duplicated without breaking RTP or RTP
  Control Protocol (RTCP) rules.


Working Group Summary

The document went through a working group last call.  There were comments and the document was updated to resolve all comments.

The work was originally presented in the AVTCORE working group, but following chair and AD discussion was adopted as a work item of the AVTEXT group instead.


Document Quality

The document got good reviews from several AVText members, notably Magnus Westerlund, who brought up several important topics about the document's congestion control, and limitations of its source-association mechanisms, which were resolved during working group last call.

Ali Begen says that Cisco uses duplicated streams "more or less" according to the guidelines in this document, as do several other implementations from the SMPTE community.

Personnel

The Document Shepherd is Jonathan Lennox.  The Responsible Area Director is Gonzalo Camarillo.


(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 document shepherd read the submitted version of the document fully, as well as reviewing several earlier versions of the document.


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

This document was reviewed by multiple people from AVText and all comments
were addressed.


(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.

No broader reviews are needed.


(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.

No concerns.


(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.

Both authors have confirmed this.  Ali Begen has referred to disclosure 1656 (see below); Colin Perkins confirmed that he had no IPR disclosures that needed to be made.


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

One IPR disclosure has been filed, , by Cisco, on an earlier version of the individual submission that was adopted as the working group draft.  Ali Begen has confirmed that the same disclosure applies to the working group document.  This disclosure did not occasion any comment in the working group.


(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? 

While only a relatively small number of people have commented on the draft, this is not atypical for the AVTEXT working group, and most of the group's most active (and expert) participants have indicated agreement with the document.


(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 http://www.ietf.org/tools/idnits/ and the Internet-Drafts
Checklist). Boilerplate checks are not enough; this check needs to be
thorough.

http://tools.ietf.org/idnits?url=http://tools.ietf.org/id/draft-ietf-avtext-rtp-duplication-04.txt

Two of the document's normative references reference previous versions of Internet-Drafts that were revised following IESG review, but the changes that were made do not affect the contents of this draft.


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

Not applicable.


(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.

No.


(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).

The document has no requirements of IANA.


(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.

The document has no requirements of IANA.


(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.

The document contains no sections written in a formal language.