Subject: [Tsvwg] Other RT-ECN Marking Methods

From: "Jozef Babiarz" <[email-masked]>
Date: Sun, 28 Aug 2005 11:27:54 -0400

[David's Concern]
(4) Need to look at alternative marking.  At least the admission control
	results in draft-babiarz-rtecn-marking-00.pdf are not a
surprise,
	as the odds of 2 packets not getting marked when a router is
	marking only 10% of packets are quite good (81%).  For the
	functionality desired by RT-ECN, having routers switch operation
	among 3 marking modes (using RFC 3168 ECN values) based on
congestion
	should be considered:
		a) Mark no packets (no congestion)
		b) Mark 50% of packets (1st level congestion)
		c) Mark all packets (2nd level congestion)
	If a 4-packet probe sequence of ECT(0) and ECT(1) packets is
used,
	the odds of failing to detect a 50%-marking router is 1 in 16
	(about 6%).  In general, marking functions other than the
strictly
	linear one tested (mark x% of packets to indicate x% overload)
	should be considered (e.g., if low levels of overload are
important,
	ramp marking percentage faster [e.g., 5x ramp would yield 50%
marking
	at 10% overload, after which ramp could be more gentle], or
start
	marking at a level other than 0 [e.g., jump to 50% as above]).

[Response]
We are considering different methods of marking for the indication of
the level where admission of new flows should stop and the level where
already admitted flows selectively may be preempted.
There are various tradeoffs that need to be analyzed and discussed so an
optimum marking approach can be proposed. Currently, we are
investigating different mechanism of "ramp" marking to be used for
preemption.

We think there are two approaches to ECN marking, first approach is to
use the same ECN marking as defined in RFC 3168 with 11 as CE and
depending on when or how frequently CE is marked RT-ECN end-system can
determine congestion level. The marking as proposed by David or defined
in Section 6.1 "Alternate RT-ECN Marking" in RT-ECN draft are methods
that potentially could be used.

The one issue that we see is that this approach does not allow for
detection of RFC 3168 routers in the path. Routers that implement RFC
3168 use some form of AQM method for marking and routers that perform
RT-ECN marking need to use some form of rate measurement, be it token
bucket or other method. Secondly, the response by the end-system to CE
marking is different. The expected response of a RFC 3168 flows is treat
this CE marked packet as if it was dropped, for TCP, reduce your sending
rate in half. For real-time flows dropping of a packet normally causes
no reaction to flow control.  The detection of onset of congestion for
real-time traffic needs to be different as well the expected response
will be different, do not admit or preempt the flow, depending on the
state of the flow. Also some real-time flows may be able to change
codecs when received CE marked packets to reduce their sending rate.

This raises some questions:
With 3168 and RT-ECN end-systems requiring different end-system
behaviors, should they use the same marking?

If they use the same marking, does RT-ECN end-systems need to be able to
detect RFC 3168 routers in the path? 

The other approach that we currently chose was with the objective that
we try to detect conformance of the path to RFC 3168 or RT-ECN expected
behavior. This approach makes use of different ECN markings, allowing
for detection of the two different mechanisms.

Discussions, advices, suggestions are welcomed.
  
Regards, 
-- Joe Babiarz, Kwok Ho Chan, Corey Alexander -

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