Subject: [AVTCORE] Retransmission interval for RTCP FIR packet.

From: "Padma Priya (padpriya)" <[email-masked]>
Date: Mon, 24 Sep 2012 16:20:34 +0000

Hi Experts,
         I am working on a call flow where one of the video endpoints joins the video call while the call is in progress. RTP is used for media.
So we trigger RTCP FIR packet to the existing video endpoints in the call when a new user joins.
To mitigate UDP packet loss, we are planning to have a retransmission mechanism for the RTCP FIR packet.

I am looking for inputs on what should be the frequency and how many times we might have to retransmit the RTCP FIR packet.
I know it would be greatly dependent on the network as such, but wanted some kind of default value that would mostly suffice.

Regards,
Priya

---

From: Magnus Westerlund <[email-masked]>
Date: Thu, 27 Sep 2012 10:07:01 +0200

Hi,
(as individual)

First of all the frequency of the FIR retransmissions are limited by the
rules of transmitting RTCP packets.

Secondly, as soon as one RTCP packet in each direction has been sent for
a session participant that is sending media will have an RTT estimate to
the participant. That should be used to avoid unnecessary retransmissions.

I note that RFC 5104 says the following:

4.3.1.3.  Timing Rules

   The timing follows the rules outlined in section 3 of [RFC4585].  FIR
   commands MAY be used with early or immediate feedback.  The FIR
   feedback message MAY be repeated.  If using immediate feedback mode,
   the repetition SHOULD wait at least one RTT before being sent.  In
   early or regular RTCP mode, the repetition is sent in the next
   regular RTCP packet.

I think there are two strategies that can be employed here. Either one
repetition per RTT until a reaction is received. Or one sends (likely
more) narrowly spaced repetitions that have a really high probability of
reaching the media sender. Thus the interval needs to be longer than
likely burst drops and allowed by the transmissions rules.

Cheers

Magnus

On 2012-09-24 18:20, Padma Priya (padpriya) wrote:
> Hi Experts,
> 
>          I am working on a call flow where one of the video endpoints
> joins the video call while the call is in progress. RTP is used for media.
> 
> I am looking for inputs on what should be the frequency and how many
> times we might have to retransmit the RTCP FIR packet.
> 
> I know it would be greatly dependent on the network as such, but wanted
> some kind of default value that would mostly suffice.
> 
>  
> 
> Regards,
> 
> Priya
> 
>  
> 

-- 

Magnus Westerlund

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