Subject: SIMA's tagging scheme

From: Laurent Toutain <[email-masked]>
Date: Fri, 29 Aug 1997 15:14:57 +0200

Hi,

Within SIMA the priority a packet receives depends on the current NBR /
actual bit rate ratio. This could be problematic for :

. VBR flow : there is no way to send it with constant priority,

. adaptive applications : the priority field cannot be set by the user,
  so as to reflect the flow semantics.

Maybe another tagging scheme (which is sort of dual to the original one)
could be better: Define a token bucket whose rate and depth are used
as a basis for charging (details have to be worked out). Then let the
application define a priority for each packet, and let the number of
tokens drawn from the bucket be a function of the requested priority
(and of the packet's length of course), so that high priority packets
consume more tokens than lower priority ones.

With the NBR-based scheme, you get high priority if the actual bit rate
is low. With the new scheme, you get low bit rate if the priority is
high
(because the bucket is emptied faster when sending high priority
packets).
But proper parametering allows to send high priority bursts.

Cheers,

Laurent Toutain                         [email-masked]
Francois Toutain                        [email-masked]
Octavio Medina                          [email-masked]

-- 
http://www.rennes.enst-bretagne.fr/~toutain

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From: [email-masked] (Kilkki Kalevi NRC/Hki)
Date: Fri, 12 Sep 1997 15:16:36 +0200

Hi,

I am aware of these problems which arise from the basic principle used in   
SIMA (that is, the priority of each packet depends on the instantaneous   
traffic process). There are a certain cases where the user wants to   
define the priority itself, or at least mark some packets more important   
than some other.

As to the VBR flow with constant priority, I do not think that users   
wants exactly that, because the priorities are relevant only inside the   
network. Moreover, although the priority were constant during a session   
it does not mean that the quality of the flow will be constant (as the   
quality of certain priority level depends on the actual load situation in   
the network).

I believe that the user is interested on two issues: overall quality of   
the application and price. So, although different tagging schemes are   
possible, the result shall be better taking into account the quality of   
application and price. Especially with real-time video transmission it   
seems to be very difficult to construct a control scheme which guarantees   
small delay variation and high utilization at the same time. You may try   
:-)

In SIMA one of the key properties is that the packet priority of a   
connection drops if the transmission rate of the connection increases   
significantly. In that way the traffic load of a certain priority level   
(and quality of the priority level, as well) is much more stable than if   
the user can keep the priority unchanged even though the traffic load is   
changed. Therefore, my opinion is that the packet priority should be   
changed when the transmission rate changes essentially.

Another issue that the network may try to take into account the   
information about the relative importance of different packets in one   
flow. This kind of scheme could be possible, but it seems that it makes   
the priority determination much more complicated than in the basic SIMA.   
One solution could be that the user has two different flows with proper   
NBRs, although that approach has some disadvantages.

regards, Kalevi

 ----------
From:  Laurent Toutain
Sent:  29. Augustta 1997 3:14 PM
To:  kalevi.kilkki
Cc:  int-serv; Francois.Toutain; medina
Subject:  SIMA's tagging scheme

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Hi,

Cheers,