Subject: [AVT] IESG comments on draft-ietf-avr-rtp-retransmission-11.txt - Issue 1: "acceptable packet loss" / retransmission during congestion

From: "Jose Rey" <[email-masked]>
Date: Wed, 25 May 2005 12:37:29 +0200

Jon, Mark,

I tidied up the previous email a bit.  For each issue, I give you a piece of text that (hopefully) brings you back to the issue (instead of copying all text) and then list text pieces proposed. Used different emails for each issue or unanswered comment, will follow.. 

Looking forward to your comments!

José

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Issue 1:  "little guidance is given how an implementer migh assert that the packet loss level is acceptable"
Answer:  IMO, this should not be specified in this document.  Acceptable packet loss level is application dependent. Likewise, determining when congestion started and when it ended, or when is still a congestion episode is not in scope.  Therefore, it is proposed to change the text and say that "if congestion is not kept under control, retransmission SHOULD NOT be used".  The SHOULD is justified by examples below. 

OLD:
   These congestion control mechanisms should keep the packet loss 
   rate within acceptable parameters.  Packet loss is considered 
   acceptable if a TCP flow across the same network path and 
   experiencing the same network conditions would achieve, on a 
   reasonable timescale, an average throughput, that is not less than 
   the one the RTP flow achieves.  If the packet loss rate exceeds an 
   acceptable level, it SHOULD be concluded that congestion is not 
   kept under control and retransmission SHOULD NOT then be used.  It 
   may further be necessary to adapt the transmission rate (or the 
   number of layers subscribed for a layered multicast session), or 
   to arrange for the receiver to leave the session.  

NEW:

   These congestion control mechanisms should keep the packet loss 
   rate within acceptable parameters.  In the context of congestion control, packet loss is considered 
   acceptable if a TCP flow across the same network path and 
   experiencing the same network conditions would achieve, on a 
   reasonable timescale, an average throughput, that is not less than 
   the one the RTP flow achieves.  If congestion is not
   kept under control, then retransmission SHOULD NOT be used.  

   Retransmissions MAY still be sent in some cases, e.g., 
   in wireless links where packet losses are not caused by congestion, or if the 
   server (or the client that makes the retransmission request) estimates that a particular
   packet or frame is important to continue play out, or if an RTSP PAUSE has been 
   issued to allow the buffer to fill up (RTSP PAUSE does not affect
   the sending of retransmissions.)  

   Finally, it may further be necessary to adapt the transmission rate (or the 
   number of layers subscribed for a layered multicast session), or 
   to arrange for the receiver to leave the session.  

---------------------------------------------
Jose Rey     mailto:[email-masked]
P R D C G - Panasonic R&D Center Germany GmbH
Phone/Fax: +49(0)6103766-134/166
--------------------------------------------- 

---------------------------------------------
Jose Rey     mailto:[email-masked]
P R D C G - Panasonic R&D Center Germany GmbH
Phone/Fax: +49(0)6103766-134/166
--------------------------------------------- 

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Audio/Video Transport Working Group
[email-masked]
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