Subject: StoredLifetime in RFC 2462

From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Wed, 29 Aug 2001 23:22:27 +0900

I have a quick question about RFC2462 (stateless address
autoconfiguration).  I know we've had a similar discussion before, but
please let me repeat it...

The RFC defines the notion of "StoredLifetime" as follows:

    e) If the advertised prefix matches the prefix of an autoconfigured
       address (i.e., one obtained via stateless or stateful address
       autoconfiguration) in the list of addresses associated with the
       interface, the specific action to perform depends on the Valid
       Lifetime in the received advertisement and the Lifetime
       associated with the previously autoconfigured address (which we
       call StoredLifetime in the discussion that follows):

In my understanding, StoredLifetime is the "remaining" lifetime to
expiration at the time when the new Router Advertisement is received.
So, for example, if we first received a prefix with the valid lifetime
being 7200 seconds, and then received a next advertisement 1800
seconds later, the StoredLifetime corresponding to the address from
the prefix should be 5400 (= 7200 - 1800) seconds.

I believe this interpretation is correct, and I've implemented RFC
2462 in this manner.  However, a user of our implementation said it
was *not* what RFC2462 says, because

> all "Lifetime associated with the address" pseudo-definitions are
> very, very careful not to say the Lifetime is actually being
> decremented.
(cited the user's message)

But is there any other interpretation of StoredLifetime?  I'm now
confused, so I'd like to hear authors' intention.

Thanks,

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[email-masked]
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From: Pekka Savola <[email-masked]>
Date: Wed, 29 Aug 2001 18:13:02 +0300 (EEST)

> was *not* what RFC2462 says, because

Well, this is the only interpretation that works with lifetimes less than
7200, which (partially) triggered the "Jim Bound rule" (Date: Sun, 08 Nov
1998 10:57:59 -0500) thread in the first place.

So, it's obviously the "right" interpretation.

But is it what it actually says?  Wording is IMO very ambiguous:

>	Lifetime

The name StoredLifetime implies it's stored once, not decremented.  Why
else would there even be need for 'StoredLifetime', why just use
'Lifetime' there?

'Lifetime associated with the previously autoconfigured address',
autoconfigured hints at static ValidLifetime, there is no clear definition
of Lifetime, much less the whole deal.

And:

   IPv6 addresses are leased to an interface for a fixed (possibly
   infinite) length of time. Each address has an associated lifetime
   that indicates how long the address is bound to an interface. When a
   lifetime expires [...]

Lifetime is originally derived from advertised ValidLifetime, which is not
decremented but updated.  This doesn't specify whether variable "Lifetime"
is really a decrementing timer or only a value, which is updated by
advertisements, _and_ having separate timer decrementing that.

As above, you can deduce the intention there if you think about it, but
again, I'm not sure that's what the spec _exactly_ says.

(yeah, it was my message :-)

Speaking of which, would it soon be time to move ND/autoconf forward, and
at the same time, clarify the text?

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords

---

From: Thomas Narten <[email-masked]>
Date: Thu, 30 Aug 2001 06:55:28 -0400

> the prefix should be 5400 (= 7200 - 1800) seconds.

Yes, that is how this should be interpreted.

> (cited the user's message)

A quick scan of the text suggests to me that this could be worded a
bit better than it currently is.

> confused, so I'd like to hear authors' intention.

This author agrees with your original intepretation.

Thomas

---

From: JINMEI Tatuya / 神明達哉<[email-masked]>
Date: Thu, 30 Aug 2001 20:23:08 +0900

>>>>> On Thu, 30 Aug 2001 06:55:28 -0400, 
>>>>> Thomas Narten <[email-masked]> said:

> This author agrees with your original intepretation.

I see, thanks for the response.  I also agree that the wording could
be clearer.

					JINMEI, Tatuya