Subject: [bmwg] draft-ietf-bmwg-ipg-dataplane-conv-term-02.txt

From: "Hickman, Brooks" <[email-masked]>
Date: Tue, 25 May 2004 17:33:44 -0700

I have been reviewing the draft-ietf-bmwg-ipg-dataplane-conv-term-02.txt and
have some questions/comments.

3.5 Convergence Packet Loss

	Definition:
	The amount of packet loss produced by a Convergence Event
	until Route Convergence occurs.

	Discussion:
	Packet loss can be observed as a reduction of forwarded 
	traffic from the maximum forwarding rate.

	Measurement Units:
	number of packets

This term is the one I have been struggling with the most. During a
convergence event, the forwarding rate on the Preferred Egress Interface
will drop to zero and the forwarding rate on the Next-Best Egress Interface
will ramp up(Hopefully, very fast) to the offered load. The aspect of this
term that I am having difficulty with is equating a drop in forwarding rate
to a specific "number of packet" lost. A lost packet is a packet offered to
the DUT/SUT that was not forwarded. However, what if buffering occurs as a
result of the convergence event? The number of lost packets would then
include the number of packets that are truly lost AND the number of packets
still in the DUT/SUT buffer unless such a measurement is made after traffic
is stopped being offered to allow the buffer to empty. Is there something I
am overlooking or misinterpreting?  

3.8 Rate-Derived Convergence Time

	(eq 1)	Rate-Derived Convergence Time = Convergence Recovery Instant
- Convergence Event Instant.  

The Convergence Recovery Instant term that is used to calculate the
Rate-Derived Convergence Time notes that it is measurable from the data
plane as the time when the DUT achieves Full Convergence. But neither the
Convergence Recovery Instant,  nor Full Convergence terms explicitly define
what externally observable condition determines full convergence(Ending
Goalpost)? 
Suggest adding to the discussion of section 3.7 "Convergence Recovery
Instant is externally observable from the data plane when the forwarding
rate on the Next-Best Egress Interface equals the offered rate." 

3.6 Convergence Event Instant

	Definition:
	The time instant that a Convergence Event occurs.

	Discussion:
	Convergence Event Instant is observable from the data 
	plane as the precise time that the device under test begins 
	to exhibit packet loss.  

Inconsistency in the wording between the definition and discussion sections.
A Convergence Event may involve a route flap(Control plane event). The
wording in the "definition" section and the "discussion" sections imply, at
least to me, two different goalposts. The definition references a control
plane event and the discussion references a data plane event. While I do not
have a complete solution, I would recommend making the definition explicitly
note the data plane event. I basically paraphrased the discussion section.

"The time instant that the DUT/SUT begins to exhibit packet loss as a result
of a Convergence Event" 

*** See first comment with regards to Convergence Packet Loss.

3.18 Preferred Egress Interface

	Suggest changing the "discussion" to:

	"The preferred Egress Interface is the egress interface that data
plane traffic is forwarded on prior to the occurrence of a Convergence Event
Instance."

3.19 Next-Best Egress Interface

	"The Next-Best Egress Interface is the egress interface that data
plane traffic is forwarded on after the occurrence of a Convergence Event
Instance."

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