Subject: [AVT] Retransmission Suppression for NACK Implosion

From: "Qin Wu" <[email-masked]>
Date: Mon, 26 Jul 2010 20:48:30 +0200

Hi, folks:
May I draw your attention again to the new version of draft-wu-avt-retransmission-supression-rtp-02 (http://tools.ietf.org/id/draft-wu-avt-retransmission-supression-rtp-02.txt), which follows the SSM model defined in RFC5760 to address NACK implosion issue. The concept for why we can not use NACK for suppression indication is highlighted as follows:
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NACK Suppression
NACK lacks semantics for suppression indication. 
---NACK is designed to request sender to compensate the missing packet
---NACK is unidirectional message sent from receiver to the sender.
---However in NACK suppression, one message used for suppression indication need to be sent from 
     sender (i.e., DS) to the receiver.

Even NACK is compulsively used for suppression indication,
---- Distribution Source does not define the behavior for handling this NACK suppression indication message
-----In Simple Feedback Model, DS does not know NACK used for suppression indication. NACK sent from receiver may be forwarded directly to media sender.
-----In Feedback Summary Model, DS also does not know NACK used for suppression indication and may just forward NACK to the media sender.
 ----The receiver can not understand this NACK message is used for suppression indication.
-----Even the receiver can receive the NACK, the receiver may just drop it.
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This draft will be discussed on Friday session, if people can read the draft before the meeting, it will be great to have a efficient discussion.

Regards!
-Qin