[clug] IPv6

David Austin david at d-austin.net
Sun Jan 16 15:17:23 MST 2011


Well it's all water over the dam now but lets be technically accurate here
least.  So let me fix that for you:


> What you're probably talking about is 'I want my IPv6-only machine to be
> able
> to access IPv4 addresses with no additional hardware, routing or proxies'.
> And the bad news is that that's really just a fantasy.  The IPv4 IP packet
> is
> fairly prescriptive about what goes where, what it all means, and its
> extensions are fairly limited.  You can't just bolt on an option that says
> 'oh, yeah, and the other 96 bits of the source and destination are in this
> other part of the packet'.  The IPv4-only server you're talking to is going
> to
> toss those bits away and you're never going to get your response.

The IPv4 packet header is nicely generalisable, with versioning and more.
The IPv4 packet header is at least 20 bytes and up to 60 bytes long (with
a variable length option field of up to 40 bytes)
So bolting on 96bits/8 bytes is easy.  The trick would be to convince IPv4
hosts to return the special extended-IP-address-option field when
replying to packets.  In the decade plus timeframe that we're talking about,
that should be doable.  Certainly for "important" IPv4 sites (web,smtp,etc)
that'd be quite quick.  Yep it's a kludge.  Perhaps it should use the IPv4
evil bit to make that clear (http://tools.ietf.org/html/rfc3514).


> However, being able to exist with an IPv6-only machine on the internet and
> fetch pages from an IPv4-only host without any network address translation,
> proxying or other network shenanigans is, AFAICS, completely impossible.
>  If
> that's what 'backward compatible' means, then I'm afraid Mr Austin is out
> of luck.

Dr Austin is out of luck, but not for technical reasons.  IPv6 is the path
"forward".  I note that the first author of the IPv6 RFC works/worked at
and Cisco stands to proft the most from IPv6 but perhaps I'm too cynical in
my old age.


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