All things related to multicast

[Bill Nickless <>]

At 12:26 PM 3/4/2002 -0800, David Richardson wrote:
> - Less than best effort QoS might be helpful. Unfortunately, there are
>   social challenges to deploying QoS mechanisms that attempt to provide
>   better than best effort -- who gets to have their packets preferred?
>   Less than best effort allows an application to choose how it should
>   be degraded, and avoids some of the social problems. An MPEG app would
>   mark P and B frames less than best effort, hoping that at least audio
>   and I frames make it through whole.

Fundamentally, the idea of QoS is to move congestion control out of the 
forwarding plane into the control plane (or above!).  That's obviously the 
right thing to do for those folks focused on the forwarding plane, but not 
so obvious if you're worried about the 
control/management/political/religious plane.

There are some situations where an application can tell you what packets 
are less valuable than others.  I agree with David: our experience is that 
people are willing to tolerate a certain level of video stream loss (lower 
framerate, etc), but cannot tolerate any loss in the audio.  The loss of a 
single word in the middle of a sentence causes human-speech-time requests 
for retransmission.  A preferred queuing strategy for audio packets can 
help avoid this situation in a constrained-bandwidth context.

Perhaps QoS efforts would find more success focusing on intra-application 
problems rather than inter-application or inter-user "fairness" or 
"prioritization"?

Maybe the question we should ask about QoS approaches is "what packets can 
we throw away?" rather than "what packets should be 
protected?"  Audio/video conferencing applications can answer the "throw 
away" question rationally.  Also consider non-TCP bulk-data transfer 
protocols; perhaps they should be allowed to blast UDP traffic without TCP 
back-off behavior if they're queued at a lower priority than TCP packets?
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Bill Nickless    http://www.mcs.anl.gov/people/nickless      +1 630 252 7390
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