Subject: RE: new rev. of rfc2462bis will be coming

From: "Manfredi, Albert E" <[email-masked]>
Date: Fri, 3 Sep 2004 15:03:37 -0400

> -----Original Message-----
> From: [email-masked] 

[ ... ]

> Thus, I'd now rather be more concrete on this.  My latest proposal to
> this part is as follows:
> 
>    A link-local address is formed by prepending the well-known link-
>    local prefix FE80::0/10 [RFC3513] to the interface identifier.  If
>    the interface identifier has a length of N bits, the interface
>    identifier replaces the right-most N zero bits of the link-local
>    prefix.  If the interface identifier is more than 118 bits in
>    length, autoconfiguration fails and manual configuration is
>    required.

A slight reword, because I get confused with being told N bits, and no limits on N.

   A link-local address is formed by prepending the well-known link-
   local prefix FE80::0/10 [RFC3513] to the interface identifier.  If
   the interface identifier has a length of N bits, where N <= 118 bits,
   the interface identifier replaces the right-most N zero bits of the
   link-local prefix.  If the interface identifier is more than 118 bits
   in length, autoconfiguration fails and manual configuration is
   required.

Another point. Any reason why autoconfiguration with DAD is not possible even if N is > 118? Maybe this was already discussed.

Bert

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From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Sat, 04 Sep 2004 12:53:37 +0900

>>>>> On Fri, 3 Sep 2004 15:03:37 -0400, 
>>>>> "Manfredi, Albert E" <[email-masked]> said:

>    in length, autoconfiguration fails and manual configuration is

Note that in a later discussion in this thread we've agreed on
different text:

   A link-local address is formed by combining the well-known link-local
   prefix FE80::0 [RFC3513] (of appropriate length) with the interface
   identifier as follows:

   1.  The left-most 'prefix length' bits of the address are those of
       the link-local prefix.

   2.  The bits in the address to the right of the link-local prefix are
       set to all zeroes.

   3.  If the length of the interface identifier is N bits, the
       right-most N bits of the address are replaced by the interface
       identifier.

   If the sum of the link-local prefix length and N is larger than 128,
   autoconfiguration fails and manual configuration is required.

(I did not mention the magic numbers of 10 or 118 deliberately, since
prefix lengths have often confused people and I thought it's better to
just refer to the principal document (RFC3513) without saying a
concrete value).

I hope you can also live with this.

> Another point. Any reason why autoconfiguration with DAD is not possible even if N is > 118? Maybe this was already discussed.

I'm not really sure about the point....first, this is only related to
link-local addresses.  Secondly, the discussion here is irrelevant to
DAD.  So your question should actually be:

  Any reason why autoconfiguration of link-local addresses is not
  possible even if N is > 118?

And if we recall that the link-local prefix requires (at least) 10
leading bits and the length of an IPv6 address is 128 bits, the reason
is too obvious to me to bother to explain...

BTW, if you can live with the above latest text, we don't have to
worry about the magic number of 118.

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]

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From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Sat, 04 Sep 2004 13:57:49 +0900

>>>>> On Sat, 04 Sep 2004 12:53:37 +0900, 
>>>>> JINMEI Tatuya <[email-masked]> said:

> DAD.

Hmm, perhaps your point is something like this:

  if N is > 118, simply use the rightmost 118bits of the interface
  identifier following the link-local prefix fe80::/10.  If the
  shortened interface identifier collides with others, DAD will detect
  it as a duplicate of the entire address.  If DAD completes
  successfully, we can simply use the address with the shortened IFID.

I don't know if this was discussed in the past, but I'd first like to
point out that this (= shortening the long IFID) can be regarded as a
kind of "manual configuration":

   If the interface identifier is more than 118
   bits in length, autoconfiguration fails and manual configuration is
                                               ^^^^^^^^^^^^^^^^^^^^
   required.
(from RFC2462, but rfc2462bis will have a similar sentence)

So, there is nothing that (at least explicitly) prohibits this
operation in RFC2462 or in rfc2462bis.

(I would personally not use this type of "semi-auto" manual
configuration in this case though).

					JINMEI, Tatuya