Re: [AVTCORE] Comments on draft-ietf-avtcore-feedback-supression-rtp-07

Roni Even <Even.roni@huawei.com> Mon, 24 October 2011 14:45 UTC

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Date: Mon, 24 Oct 2011 16:42:25 +0200
From: Roni Even <Even.roni@huawei.com>
In-reply-to: <4E9E93A0.10701@ericsson.com>
To: 'Magnus Westerlund' <magnus.westerlund@ericsson.com>, 'Qin Wu' <bill.wu@huawei.com>
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Cc: 'IETF AVTCore WG' <avt@ietf.org>, draft-ietf-avtcore-feedback-supression-rtp@tools.ietf.org
Subject: Re: [AVTCORE] Comments on draft-ietf-avtcore-feedback-supression-rtp-07
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Hi Magnus,
Maybe we can add a recommendation for SSM to use the round trip sub report
block specified in section 7.1.6 of RFC 5760
(http://tools.ietf.org/html/rfc5760 ) to distribute the RTT from the
distribution source to the receivers.

Roni

> -----Original Message-----
> From: avt-bounces@ietf.org [mailto:avt-bounces@ietf.org] On Behalf Of
> Magnus Westerlund
> Sent: Wednesday, October 19, 2011 11:09 AM
> To: Qin Wu
> Cc: IETF AVTCore WG; draft-ietf-avtcore-feedback-supression-
> rtp@tools.ietf.org
> Subject: Re: [AVTCORE] Comments on draft-ietf-avtcore-feedback-
> supression-rtp-07
> 
> Hi,
> 
> Sorry for the delay in following up.
> 
> I have removed the issues which don't need further discussion.
> 
> 
> On 2011-10-11 03:40, Qin Wu wrote:
> > Hi, Magnus,
> > Thank for your valuable review, please see my replies belows.
> > ----- Original Message -----
> > From: "Magnus Westerlund" <magnus.westerlund@ericsson.com
> 
> > 2. Section 3.
> >   "The timer value shall be
> >    based on the observed round-trip time.  A receiver should compute
> an
> >    estimate of the round-trip time (RTT) to the sender of TPLR from
> RTCP
> >    report round-trip time if available, or its reception time of the
> >    reflected RTCP FB message (e.g.,NACK), or any other means."
> >
> > We hade the discussion before but now that you clarified which RTT to
> be
> > used, I think it is worth having a discussion if that RTT is the
> right
> > one. So you suggest that the RTT between the sender of the TPLR and
> the
> > receiver of the message is the most appropriate to use when
> suppressing
> > the message. I am not certain that is the case.
> >
> > Lets consider the use cases. In the case of an intermediary in a
> > distribution tree sending the TPLR and at the same time sending a
> NACK
> > upstream. Then the time until the retransmission reaches the
> suppressed
> > receiver is the time from the sending of the NACK up to the
> > retransmitting entity and all the way down past the TPLR sender to
> the
> > suppressed receiver. The receiver may not have a true RTT to the
> > original media sender as that source appears to live at the TPLR
> sender,
> > not further upstream.
> >
> > Yes, the sender of the TPLR and the receiver might at least have
> mutual
> > knowledge of each other, thus the TPLR sender is able to determine
> the
> > lowest RTT in the set of receviers thus determining how often to
> repeat
> > it to cover additional time requried for the upstream part of the
> > request. But I do note that these times aren't matching what appear
> to
> > be the delays in the systems.
> >
> > Can we find a better solution?
> >
> >
> > [Qin]: It looks to me the round trip time between media source and
> >
> >  intermediary is trivial if we assume the intermediary is much
> >
> > closer to the source, that is why I put the last paragraph in the
> >
> > section 3 as follows:
> >
> > “
> >
> > In order not to incur a lot of NACK requests due to additional TPLR
> >
> > described above, it is recommended that the RTP system sending
> >
> > TPLR should be implemented more closer to the source. Also when
> >
> >  the loss was detected and repair initiated much closer to the
> source,
> >
> > the delay for the receiver to recover from packet loss can be reduced
> >
> > through the combination of intermediary feedback to the source and
> >
> > Third Party Loss Report downstream.
> >
> > ”
> >
> >
> >
> > If we look for a better solution, I think we have two possible ways:
> >
> > The first way is to rely on sender report from media sender to
> >
> > calculate the more approximate RTT from media sender to the receiver.
> >
> > RTT measurement method follows the same way described in the
> >
> > figure 2 of RFC3550.
> >
> >
> >
> > The second way is to we can allow the intermediary tell the receiver
> the
> >
> > RTT from media source to intermediary itself by sending a new message
> >
> > since the intermediary can measure such RTT based on past observation
> >
> > and estimation. However for the second way we need to define new
> >
> > message or extend TPLR message to carry such RTT.
> >
> >
> >
> > What do you think?
> >
> 
> From follow up email:
> > Hi,Magnus: As regarding if there is better solution, I have some
> > corrections and additional thoughts.
> > I think If the media source is the repair server, the receiver should
> > set timer to T1=max(the RTT from media source to middlebox sending
> > TPLR, 1/2 of RTT from middlebox to receiver) when the receiver
> > receives first TPLR. When the receiver receive addtional TPLR within
> > T1, the receiver should reset the timer to the RTT from media source
> > to middlebox sending TPLR.
> >
> > If the middlebox is the repair server, the receiver should set the
> > timer to T2=1/2 of RTT from middlebox to receiver when the receiver
> > receives first TPLR. When the receiver receive addtional TPLR within
> > T2, the receiver should reset the timer to the T2.
> >
> 
> Can you express these rules for the receiver as something that is based
> on what is observable from the RTP and RTCP traffic? Is when media
> server being repair server equal to TPLR sender SSRC = Media SSRC the
> TPLR applies on? And if they are not the same it is the second case?
> 
> 
> 
> >
> >
> > 3. Section 3:
> >   "To increase the robustness to the loss of a TPLR or of a
> transmission
> >    packet, a receiver is allowed to receive additional TPLR for the
> same
> >    packet.  In the case the first TPLR is lost and the additional
> TPLR
> >    arrives at the receiver, the receiver should immediately refresh
> the
> >    timer."
> >
> > In the second sentence I don't see how the receiver can know that it
> is
> > a retransmission. It is simply a TPLR from its perspective if that is
> lost.
> >
> > Robustness reasons is only one reason for repeating them the other is
> > clearly to renew the timer. Thus I think the formulation should be
> more
> > generic to make it clear that all receivers should upon reception of
> a
> > TPLR refresh the timer for the packets indicated.
> >
> >
> >
> > [Qin]: If the receiver does not keep state for the times the TPLR is
> >
> >  retransmitted, we can add repetition count field in each FCI entry
> >
> >  of TPLR feedback message to indicate the times TPLR is
> retransmitted.
> >
> >  Particularly in PSLEI Feedback Message, we can reuse seq nr. Field
> to
> >
> >  indicate if additional TPLR is retransmitted, which one you think
> better?
> >
> >
> 
> How, can the receiver keep state for something it has not seen due to
> the initial packet not having been received?
> 
> I don't think we need new packet extensions, just clarified rules that
> correctly describe what the RTP end-point should do when it sees the
> TPLR message.
> 
> 
> >
> > 7. Section 4.2:
> >    Seq nr:8bits  Command sequence number.  It is used by the Command
> >       receiver to check if the Command is repeated.  The sequence
> number
> >       space is unique for each pairing of the SSRC of command sender
> and
> >       the SSRC of the command receiver.  The sequence number SHALL
> >       increase by 1 modulo 256 for each new Command.  A repetition
> SHALL
> >       NOT increase the sequence number.  The initial value is
> arbitrary.
> >
> > Based on that it is defined that upon receiving one of the PSLEI
> > request, one shouldn't send a FIR or PLI message for a time interval
> > equal to the RTT between the receiver and the sender of the PSLEI
> > message does the sequence number field have any meaning? From my
> > perspective it doesn't matter if it is a new request or not. You are
> > suppressed from the reception of the message, nothing else.
> >
> > [Qin]: I think we can use seq nr field to indicate the times the TPLR
> is
> > retransmitted, e.g.,
> >
> > If the additional TPLR with the same SSRC and different Seq Number is
> > received, the
> >
> > receiver knows that it is a retransmission if the receiver keeps
> state
> > for the previous
> >
> > TPLR with the same SSRC.
> >
> > Also we can replace seq nr. Field with Repetition count field to
> > indicate how many time
> >
> > the TPLR is retransmitted.
> >
> >
> 
> That is possible, but is the repetition count really providing
> anything?
> What is the simplest most straight forward that can work? I think that
> is that one simple suppress the source for X time after having receives
> a TPLR message indicate that source. So, as long as you get repeat
> messages more often than X you continue to be suppressed. Will the
> receiver work better to know that it is now suppressed for Y times in a
> go because of the same event?
> 
> cheers
> 
> Magnus Westerlund
> 
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