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Re: failure detection
On Sun, 14 Aug 2005, Iljitsch van Beijnum wrote:
An implementation that uses upper layer advice would only initiate
a probe/reply sequence when the upper layer is unhappy.
This kind of thing worries me. Assuming it will be possible to have
the upper layer 'direct' shim6 is assuming:
- OS maintainers will be prepared fundamentally change their network
stacks to support shim6
- Worse, assuming upper layers even /have/ a notion of being
unhappy or happy.
I don't think designing shim6 to accomodate either of those
assumptions is a good idea at this time, or anywhere in the near to
mid term future.
It's a data-layer: Provide best-effort packet delivery in a low-order
*bounded* magnitude of time. Bounded time is possibly more important
If there are multiple paths available, let shim6 do heartbeats of
them all at startup and X interval thereafter, pick whichever one
seems best for sending packets down; Keep a note of which other paths
seem to be available; if a path starts failing, pick another within
some bounded time.
Don't try get clever, cause the upper layer knows /more/ than shim6
- It might have a list of addresses it wants to try, don't delay it
- At the /application/ or even *user* layer, there might be other
- VoIP: if the users call to foo@multi-site doesnt initiate
the user might know they should try joe@other-site
It is *much* better for the data-layer to signal *up*:
"sorry, it's unreachable"
as soon as it can, and let the transport or the application or maybe
even the *user* get on with trying potential alternatives than for
the data-layer to try aimlessly continue on for a significant period
of time with complicated reachability tests.
Shim6 will have the *least* amount of information about where the
user is trying to get packets to.
Adding overcomplicated multi-pathing reachability/failover to shim6,
when it will be the *least* informed part of the stack, is daft.
Failure *IS* an option, and it's preferable to signal failure up
*sooner* rather than later.
(and again, I hope this WG will disabuse itself of the notion that
extra signals /down/ from transports to shim6 can even be considered
at this early stage in shim6's life).
Paul Jakma firstname.lastname@example.org email@example.com Key ID: 64A2FF6A
You will be married within a year, and divorced within two.