Subject: [AVT] IESG comments on draft-ietf-avr-rtp-retransmission-11.txt - Issue 4: more parameters needed for RTX?

From: "Jose Rey" <[email-masked]>
Date: Wed, 25 May 2005 12:34:32 +0200

Hi Mark, all,

You say:

"What I am afraid of in a few cases here is that if someone is handed
this document code from without a reference implementation to go by,
some of the lack of details in these sections would end up in: (1)
several questions posted to a list asking for help, (2) a lot of guesses
made, or (3) an unusable number of configuration commands to tweak each
and every possible parameter."

==>Yes, the client/server has limited means to set certain parameters
that may help in defining when / how to use RTX.  There is just one
parameter defined on this regard and that would be the server buffer
time, "rtx", which specifies the time a server keeps the packet in its
buffers.  One could add additional parameters, i.e., parameter(s) that:

1) allow server to tell the receiver qualitative how to use NACKs, e.g.:
a) minimum / maximum number of retransmissions per packet; b) what timer
value to use for the retransmission requests; c) etc...or,
2) allow the server to do the same in a more detailed form, e.g.:
bitrate share that should be allocated to the retransmission stream,
number of packets that should be reported in each NACK compound et
cetera.  

On the other hand, these settings may sometimes be dynamic and even not
apply to all packets equally, so it would be very difficult to express
them in, say, SDP.   Like the number of rtxs per packet depends on how
important it is. Also O/A would be needed... 

I think that this is too detailed and complex; using the RTCP bandwidth
modifiers (RFC3556) should enable to play with these settings at a
fraction of the complexity.  Therefore I would leave it as is and
include the following text in the draft section 6.3 "Retransmission
requests" :

"The server may indirectly control the number of retransmission requests
per packet by specifying the RTCP bandwidth of the original RTP stream,
e.g., by using the RTCP bandwidth modifiers as per RFC 3556 [ref].
Although this is an approximate approach, it is preferred to defining
parameters for each type of appplication because it is simpler.  If
actual experience proves this not to be sufficient, more detailed
parameters shall be defined in the future."

Comments?

---------------------------------------------
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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