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Re: TSV-DIR Review of draft-ietf-shim6-protocol-09.txt

2007-11-25 07:55:10
My concern is about wireless media which can experience large variations
in signal/noise ratio, in the process generating transient "link down"
indications.   This could cause those connections to migrate to other
media/interfaces.

Wouldn't that be something that should be fixed in the driver for that
interface? Declaring a link to be down has significant implications on many
systems, this shouldn't be done at the drop of a hat for links where this
determination isn't easily made. Having drivers declare links down too soon
and then having the next layer ignore that is not a good solution, especially
because there are also link layers which can determine their up/down status
much more accurately.

Drivers vary widely in their behavior.  I have seen drivers that can take 
as long as 30 seconds to notice that the point of attachment is gone after 
pulling the plug.  Other drivers notice the missing network announcements 
or low SNR on incoming data and react much sooner.  However, even after 
sending a "link down", a "link up" can come soon after if a new point of 
attachment is discovered.  So rather than treating a "link down" as a 
trigger, it is probably best to treat it as a "hint", lowering the timers. 

If the host has implemented the strong host model, then
when the transient "link down" is resolved, the connection won't resume
using the prior outbound interface.  This could lead to applications
experiencing sub-optimal conditions  long-term  based on a transient
event.

Hm, I must say that I don't know off the top of my head if shim6 will
automatically rehome to the primary address pair after some time. I'll have to
reread the specifications. Or does anyone else remember this?

I don't think it will automatically re-home in the strong host model. 


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