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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  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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