Subject: RE: [manet] OLSR - link hysteresis questions

From: "Dearlove, Chris (UK)" <[email-masked]>
Date: Mon, 23 Feb 2004 09:36:10 +0000

> I have implemented detection of "a long period of silence" in 
> this way:
> A link tuple records the following in addition to the fields specified
> in the RFC:
> next_hello_timer - a timer describing withing what time a new HELLO
> should be received on this link.
> last_htime - last recorded htime value from the 
> neighbor(which this link
> connects us to).
> 
> Upon receiving a HELLO the next_hello_timer is set to now + received
> htime(+ some slack) and the last_htime is set to the received htime.
> the link set is constantly checked for timed out entris with 
> regards to
> the next_hello_timer. If a links next_hello_timer is timed 
> out - a "long
> period of silence" is considered detected and the instabillity rule is
> applied to that links link_quality. The next_hello_timer is the set to
> now + last_htime to be able to detect another "long periode 
> of silence".
> I assume this is the correct way to do this? Or should this 
> test not be
> "reset" once a long period of silence is detected?

In my (not publicly available) implementation I do something essentially
as this, except that when what you've called next_hello_timer times out
I only increment it by htime, I don't add the "slack" (which I have my
own version of) again.

Before reading your next paragraph I then typed the following comment

If also tracking packet numbers for loss, you may want to adjust that,
as otherwise a single lost packet containing a HELLO message gets double
counted.

And then found that was your point. I increment the "next expected
packet number" when applying lost HELLO correction. I must admit I
haven't discussed this externally, but not doing it is seriously
wrong, as you have also found.

I did suggest formally adding these fields with names to OLSR, but
my comments to the authors were too late for the RFC.

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