Subject: [Tsvwg] RT-ECN and safe co-existence with 3168

From: "Jozef Babiarz" <[email-masked]>
Date: Sun, 28 Aug 2005 11:24:44 -0400

[David's Concern]
(3) RT-ECN's CE(1) -> CE(2) marking is dangerous.  This increase in
	congestion marking for RT-ECN is the same as a CE -> ECT(1)
change
	in RFC 3168, enabling an RT-ECN router to erase RFC 3168 ECN
	congestion indications.  If such traffic arrives at a receiver
	that is looking for RFC 3168 indications, this is bad.  At a
	minimum, RT-ECN's CE(1) and CE(2) values ought to be swapped.

[Response]
The marking was selected so that RFC 3168 capable flow that uses cheater
detection as per RFC 3540 can detect the cheater in the path, in this
case the cheater is a router that marks as per RT-ECN.  This works when
a RFC 3168 compliant sender sends a packet marked with ECN 10 and
receives 01, caused by a RT-ECN router congested at the 2nd level in the
path.

Swapping RT-ECN's CE(1) and CE(2) marking is possible but there are some
side effects. For one, the cheater detection as per RFC 3540 would not
detect RT-ECN routers that are at the 2nd level of congestion.

Regards,
-- Joe Babiarz, Kwok Ho Chan, Corey Alexander -

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