Subject: RA "requires" DHCPv6 ?

From: Karl Auer <[email-masked]>
Date: Sat, 31 Mar 2012 09:57:28 +1100

In a discussion titled "Stable privacy addresses (upcoming rev)",
Fernando Gont said:
> They could [...] have RAs require you to do DHCPv6 and then have
> DHCPv6 assign you a constant address, etc.

What interests me here is the phrase "have RAs require you to do DHCPv6".

When, if ever, are hosts "required" to obtain an address via DHCPv6?
When the "M" flag is set in an RA?

There was a bunch of stuff about the M and O flags in RFC2462, almost
all of which was removed in RFC4862. In RFC2462, the word
"should" (*not* capitalised) was used, along with phrases like "is to
be".

Then there is RFC 4861 (neighbor discovery) which says:

      M              1-bit "Managed address configuration" flag.  When
                     set, it indicates that addresses are available via
                     Dynamic Host Configuration Protocol [DHCPv6].
[...]
      O              1-bit "Other configuration" flag.  When set, it
                     indicates that other configuration information is
                     available via DHCPv6.

Anyway, I've been working on the basis that the M and O flags are
advisory and not prescriptive. That is, they do not *require* the host
to do anything.

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer ([email-masked])
http://www.biplane.com.au/kauer

GPG fingerprint: AE1D 4868 6420 AD9A A698 5251 1699 7B78 4EEE 6017
Old fingerprint: DA41 51B1 1481 16E1 F7E2 B2E9 3007 14ED 5736 F687

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From: Hesham Soliman <[email-masked]>
Date: Sat, 31 Mar 2012 23:03:04 +1100

Just a quick comment below

>"should" (*not* capitalised) was used, along with phrases like "is to

=> "should" does not need to be capitalised to indicate that it's a
keyword. It's a common misunderstanding.

Hesham

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From: Karl Auer <[email-masked]>
Date: Sat, 31 Mar 2012 23:12:05 +1100

> keyword. It's a common misunderstanding.

Oh, I don't misunderstand that point. But reading RFC2462, and seeing
things like "is to be" and seeing how the word "should" was used in
context, I got the feeling that it was not being treated with the
respect it deserved - i.e., that sometimes it was not being used
prescriptively. In any case, that's all gone from RFC4862, which
obsoletes RFC2462. Leaving me still wondering.

Regards, K.

---

From: Fernando Gont <[email-masked]>
Date: Sat, 31 Mar 2012 14:10:04 +0200

Hi, Karl,

> What interests me here is the phrase "have RAs require you to do DHCPv6".

If the RAs do not include PIOs, and have the "M" bit set, what are your
options:

1) Do DHCPv6, or,
2) Go to the park, and enjoy some sun, because "no internet today".

> advisory and not prescriptive. That is, they do not *require* the host

Exactly: They do not REQUIRE you to do DHCPv6. You MAY want  not to do
it, and just not use the Internet for the day.

Thanks,
-- 
Fernando Gont
SI6 Networks
e-mail: [email-masked]
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492

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From: Karl Auer <[email-masked]>
Date: Sun, 01 Apr 2012 00:12:55 +1100

On Sat, 2012-03-31 at 14:10 +0200, Fernando Gont wrote:
> > advisory and not prescriptive. That is, they do not *require* the
> > host to do anything.
> 

Or you could use one's statically assigned address. Or you could make
addresses using other available prefixes, from the same or other
routers.

Regards, K.

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From: Fernando Gont <[email-masked]>
Date: Sat, 31 Mar 2012 21:43:17 +0200

On 03/31/2012 03:12 PM, Karl Auer wrote:
> Or you could use one's statically assigned address. 

Oh, yeah. And you could have opted to not send the RS in the first
place... and what? :-)

-- However, particularly in the case in which the local router wants you
to use DHCPv6, they could easily block addresses that have not been
leased by the DHCPv6 server. SO, at the end of the day, you're at its mercy.

> Or you could make

We were assuming the "regime" was controlling the local link. If they
do, its up to them whether there are other routers on the link and/or
whether RAs are allowed to flow on that link.

Thanks,

---

From: Karl Auer <[email-masked]>
Date: Sun, 01 Apr 2012 08:46:02 +1000

> Oh, yeah. And you could have opted to not send the RS in the first

Um, I'm not sure why we're arguing. I'll still see unsolicited RAs at
regular intervals, and I'll see RAs sent because of other hosts' RSes.
Also, the RA contains lots of useful stuff even if I am not doing DHCPv6
- for example if I want to do SLAAC I'll still be sending an RS. I don't
think my question was stupid. I'm sorry you do.

> -- However, particularly in the case in which the local router wants
> you to use DHCPv6, they could easily block addresses that have not
> been leased by the DHCPv6 server. SO, at the end of the day, you're at
> its mercy.

Absolutely. The question was just whether or not a host was *required*
"O" flags and do DHCPv6, not whether it would be practical or
appropriate to do so.

> whether RAs are allowed to flow on that link.

My question was unrelated to the discussion about "the regime". That's
why I started a new thread with a new subject line, and quoted only that
exact part of your statement where you said "requires you to do
DHCPv6". 

And I was not saying you were wrong - I was seeking clarification (from
you or anyone), because IMHO it is not clear from the RFCs whether a
host SHOULD or MUST honour the "M" and "O" flags (obviously it MAY :-). 

Regards, K.