Re: failure detection

[Paul Jakma <paul@clubi.ie>]

On Tue, 30 Aug 2005, Erik Nordmark wrote:

> The problem is, as preferred/valid is defined and used in RFC 2462, a host 
> would interpret the prefix becoming deprecated as

> 	There is some graceful renumbering going on.

>       Make sure that new
> 	communication avoid the deprecated address. But existing
> 	communication can continue to use the deprecated address until
> 	the valid lifetime expires.

Other than that first line, those are the semantics we want surely? 
Transport whose state is inexorably entwined with that prefix is 
hosed (eg TCP). Shim6 shouldn't be ;). (Nor transports using shim6, 
eg TCP on top of shim).

Note that for internal communication, TCP continues to work, as long 
as the prefix stays valid.

> The behavior we'd want in multihoming is instead

> 	The address prefix has failed (at least for external
> 	communication). If there is a mechanism (like shim6) which can
> 	be used to quickly failover to some other address, then it makes
> 	sense to invoke it. Also, new communication which picks a source
> 	address should avoid using the failed prefix.

Well, as only preferred prefixes may be considered for 'new' 
connections, that happens automatically, right - at least for 
datagramme-like transports which dont specify their source..

>       But for existing
> 	communication when there isn't an easy way to switch to another 
> address, it isn't clear what to do. (In some cases it might make
> 	sense to reset the ULP connection and recreating it, which will
> 	make it get a non-failed source address, but in other cases it
> 	would be better to wait for the failed address to start working
> 	again.)

Ah, you're working on a "shim6 has intimate knowledge of ULP" model 
in mind. Yes, things will get tricky that way - which seems a good 
reason to /not/ do that.

> If we overload the "failed" semantics on the "deprecated" notion, 
> then the effect is that during graceful renumbering there will be a 
> storm of shim6 traffic when everybody tries to immediately switch 
> over, even though they might have a week during which the 
> deprecated address will continue to work.

Ah, yes. Good point.

I'll have to mull that one over. :)

>   Erik

regards,
--
Paul Jakma	paul@clubi.ie	paul@jakma.org	Key ID: 64A2FF6A
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