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Re: Status of <draft-hinden-ipv6-global-local-addr-00.txt>



Robert Elz wrote:

>I do "local first" (actually a variation of that, but the effects for this
>purpose are the same) - the implementation waits up to about 100ms for
>a response.  If it gets an answer, it goes on with the local address.  If
>it gets an ICMP it switches to global, if the 100ms passes with no
>response, it switches to global.
>
>This relies on several assumptions about the nature of local addresses.
>
>First, they tend to be local - the RTT to a local address should be
>fairly small (100ms allows for spans of a continent).  Sometimes
>for a diverse organisation, this might result in a global address being
>used where a local one could have been (either because 100ms isn't long
>enough, or because of packet loss - congested local nets) but that's
>no different than "prefer global first" would achieve.   (Work is
>currently being done which should lower the effects of this without
>delaying applications).
>
>Second, if one local address doesn't work, then it is very likely that
>none will (the way we do this, it actually becomes a side effect,
>rather than a deliberate act, but anyway).   That is, if one local address
>fails, then we don't try others, we switch to global (of course, if
>there are no global addresses things are different).   We're going to
>need to revisit this if we end up switching from site local addresses
>to Hinden addresses, as there the reachability of one local address
>really says nothing about that of any other, but for current SLs it does.
>
>The overall effect of this is that the difference to the application
>when preferring local first is almost invisible (but we're doing more
>to make this even smaller).
>
>There doesn't need to be a huge delay before finally trying a global
>address and having it work.
>
Reachability os one thing, but you may have to wait for TCP timeouts.
RFC1122, section 4.2.3.5  TCP Connection Failures
Says that the initial SYN has to wait at least 3 minutes...
Note that I've seen some implementation ignoring this
MUST and setting the timer to 12 or 25 seconds,
but never to 100ms.


>On the other hand, if you try global first, and you're really trying to
>reach a local node, for which the global addresses don't work (have been
>arranged to be dropped by routers intervening, to force the use of
>local addressing) then you really must suffer a long delay - the delay
>waiting for a response has to allow for worst case internet RTTs (or
>you might fail to find the only address that actually works) along
>with retransmits (as while dropped packets locally are usually quite rare,
>across the internet they're normal) - and you cannot draw any conclusions
>about one global address's reachability from that of another, they all
>need to be tried.
>
Same issue here, TCP timeouts.

    - Alain.

>  
>


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