

IPv6 Netowrk Device Numbering BP

On Thu, 2012-11-01 at 07:07 -0700, Owen DeLong wrote:
> I agree with you that we shouldn't introduce partially decimal format,
but I
> don't see why you say IPv6 addresses are difficult to parse.

They are not simple to parse, but not particularly difficult either.

> 1.	Tokenize (on : boundaries).
> 2.	If n(tokens) < 8, expand null token to 9-n tokens.

It's a bit harder than that. You need to deal with the positioning of
the "::", which may be at the beginning or end. Scope identifiers need
to be handled. On output, you need to handle the requirements of RFC

> You really shouldn't need to parse [mapped addresses] and it's
perfectly valid
> to reject them as invalid input.

No, it's not OK to reject them. You can't just say they are invalid,
they are not.

> Finally, at this point, if you're feeling like you have to write your
own IP address parser,
> you're probably doing something wrong. PLEASE PLEASE PLEASE use the
> libraries whenever possible.

Definitely oh very yes! That said, I have had to write my own three
times now, because of errors or inadequacies in the existing parsers,
and I can confidently say it is slightly tricky, but not hard. The key,
the essential and vital thing, is to unit test that sucker until it is
gasping and limp.

> There are similar library functions for virtually every other language
at this point as well.

Java's is broken, for a start. I have had to replace it for literals,
because it doesn't compress for output, and because it treats a mapped
IPv4 address as an IPv4 address! It's also hard to do some operations on
InetAddress objects. I still use InetAddress where actual names are
concerned, so as not to duplicate the Java DNS functionality.
Unfortunately Java appears to not properly prefer IPv6 addresses. There
is allegedly a system property to control that, but it is either
documented incorrectly or just doesn't work.

Regards, K.

Karl Auer (kauer at biplane.com.au)


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