PPP IPv6 prefix length and stateless autoconfiguration?

[Krejsa, Dan]

Hi Jinmei,

Thanks for your reply!  I'm actually working on an OS which uses
a FreeBSD and Kame-derived stack, very similar in its IPv6 code
to the current FreeBSD.  The PPP code is of a different derivation,
however.  It specifies a 128-bit subnet mask and sets a destination
address for PPP/IPv6 interfaces, and we consequently saw an
issue with IPv6 autoconfiguration.  As a workaround, I did exactly
what you suggest, changed the code to configure the interface with a
64-bit prefix without a destination address (actually, the code
tried but failed to set the destination address, but I didn't notice
it at first).

This appears to make the autoconfiguration work fine, and I
encountered no other connectivity issues in brief testing;
but a coworker of mine noticed that ifconfig no longer showed
the destination address, and I investigated and found the
128-bit enforcement in in6_update_ifa().  This makes me somewhat
nervous; but if configuring a PPP/IPv6 interface without an
IPv6 destination address is the intended method of use,
I'd be more comfortable with this.  Is that the standard
way of doing things?

Thanks,
- Dan

-----Original Message-----
From: JINMEI Tatuya / ???? [mailto:jinmei at isl.rdc.toshiba.co.jp] 
Sent: Monday, October 16, 2006 7:40 PM
To: Krejsa, Dan
Cc: freebsd-net at freebsd.org
Subject: Re: PPP IPv6 prefix length and stateless autoconfiguration?

>>>>> On Mon, 16 Oct 2006 15:19:55 -0700, 
>>>>> "Krejsa, Dan" <dan.krejsa at windriver.com> said:

> Some code in the in6_update_ifa() function in netinet6/in6.c
> enforces that if an IPv6 destination address is specified for
> an interface address, the interface must be point-to-point or
> loopback (fine), and the corresponding prefix length must be
> exactly 128 bits.

> The latter seems (at least naively) to conflict with 
> the definition in

>
http://www.ietf.org/internet-drafts/draft-ietf-ipv6-over-ppp-v2-02.txt

> that the interface identifier length for PPP interfaces is 64 bits,
and
> correspondingly prefixes accepted from a router advertisement
> must also be 64 bits long; see section 5.5.3 in

>
http://www.ietf.org/internet-drafts/draft-ietf-ipv6-rfc2462bis-08.txt

So shouldn't you simply specify the prefix length of 64 without
specifying the *destination* address of the p2p link?

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba
Corp.
					jinmei at isl.rdc.toshiba.co.jp