Re: Comment on draft-ogier-manet-ospf-extension-00.txt

["Spagnolo, Phillip A" <phillip.a.spagnolo at BOEING.COM>]

Richard,

Section 6.1 of your draft states:
   A router can decide to suppress ACKs for a given LSA if a new
   instance of the LSA (with a larger sequence number) is usually
   received every RxmtInterval seconds (e.g., 5-10 seconds).  An
   exponential moving average of the time between LSA updates, or of the
   number of LSA updates during the last RxmtInterval seconds, can be
   maintained for each LSA. Two thresholds can be defined to employ
   hysteresis in deciding whether to suppress ACKs for a given LSA.

We thought the above mechanism would effectively reduce the amount of
overhead due to useless acks.  It potentially allows for a hybrid
operation between reliable flooding during periods of low link change
and periodic flooding under high link change.  Therefore, we decided to
take a look at this technique in isolation, as applied to OSPF.
Specifically, we looked at a QualNet OSPFv2 simulation with
Point-to-Multipoint interfaces on 802.11b network, a random waypoint
mobility model, and only the above suggested modifications (no other
wireless-oriented changes such as flooding optimizations).

However, somewhat to our surprise, the simulation results suggest that
the performance gains may be elusive, at least in the mobility model we
tested.  Specifically, 

- the approach adapts to the link changes passively by predicting the
future based upon the history.  However, we found it difficult to
predict based on the past, looking at both fixed and exponentially
weighted windows with different window sizes and weights.  In fact, it
looked almost as if the interarrivals were exponentially distributed
(modulo the MinLSInterval).  Not only was there a degree of
unpredictability in the actual generation of the LSAs, the practice of
OSPF flooding further filters the interarrival process from the
perspective of a given node that may be multiple hops downstream of an
originator.  Due to inaccurate estimates, the overall overhead went up
in all simulations, and the delivery ratio went down.   The LSAcks are
reduced, but LSUs are increased (more retransmissions).  

- even if each node suppresses the ACKs, the LSAs are flooded through
out the network during rapid link changes around the originator.  These
LSAs generate much higher overhead than the ACKs (overhead due to LSA
transmissions dominates that of the ACKs).  

- the approach assumes senders use the same RxmtInterval as its own
value and requires sender uses same RxmtInterval for all neighbors. This
requirement may force the RxmtInterval to be interchanged between
neighbors like the hello and dead intervals, or make it into an area
parameter.

We are preparing a draft that will provide more details about this and
related topics, but just wanted to note that the early results were not
promising.

Phil