Subject: Mechanisms and bit-semantics

From: Wu-chang Feng <[email-masked]>
Date: Wed, 10 Dec 1997 10:08:45 -0500 (EST)

This is in response to one of Dave Clark's comments during the intserv
WG.  He used this example to show how people erroneously assume a particular 
mechanism for a given bit semantic.  While the general point is well taken, 
I thought the particular example he used about implementing multiple drop 
preferences could use some qualifications/clarifications.

He made the claim that with multiple drop preferences, you do NOT need
to keep a large queue in order to differentiate between the higher
levels of traffic.  In particular, you don't need distinct thresholds
based on queue occupancy to do DP differentiation.  Instead, you could
vary the "drop level" of the queue in an attempt to keep the queue
lengths at around the same average length.

The problem I have with the latter technique is that it cannot insure
that high DP packets will always get less loss than low DP packets
over an arbitrary arrival stream of variably-marked DP packets.
Packet arrivals such as those described in

  I. Cidon, R. Guerin, and A. Khamisy.  "Protective Buffer Management
  Policies."  IEEE/ACM Trans. Netw., Vol. 2, No. 3, June 1994, pp. 240--246.

can continually defeat the service differentiation mechanism.  The
problem is made worse when considering schemes based on RED (WRED,
RIO, ERED, etc) in which the triggers are on average queue length rather 
than instantaneous queue length.  With the threshold based scheme, you can, 
in fact, insure high DP traffic will always get better performance than low 
DP traffic over an arbitrary stream.  However, as discussed in previous
postings, this implies longer queueing delays when providing service
differentiation between the highest levels of DP.

Only if you make some assumptions on the smoothness of the arriving DP
traffic, can you loosen the threshholds and/or come up with
alternative techniques such as the adaptive drop-level scheme above
which performs as expected with low queueing delay.

Wu