Re: rfc4941bis: trying to keep focus

Lorenzo Colitti <> Fri, 31 January 2020 08:43 UTC

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From: Lorenzo Colitti <>
Date: Fri, 31 Jan 2020 17:43:38 +0900
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Subject: Re: rfc4941bis: trying to keep focus
To: Fernando Gont <>
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I agree with all your points. I don't think it's a good idea to pause or
derail this update because of more general concerns that some participants
have with temporary addresses in general or with SLAAC in general.
Temporary addresses have been extremely widely deployed for many years, and
it seems unlikely that we would degrade them to historic, or that
implementers would stop implementing them. If there are bugs in RFC 4941
that impact user privacy, and we can fix them, then we should absolutely do
so, and do so sooner rather than later.


On Fri, Jan 31, 2020 at 5:51 AM Fernando Gont <> wrote:

> Folks,
> I'm trying to summarize what I've seen as part of the recent discussion
> about (or at least triggered by) rfc4941bis.
> First, let me give the (obvious) context:
> rfc4941bis is meant, for the most part, to address flaws in RFC4941 --
> RFC4941 is a Standards Track document, already. rfc4941bis is not
> proposing or introducing temporary addresses, but simply addressing
> flows in the scheme *we already have* (which is widely deployed).
> Now, let's move to the main topics of this discussion:
> 1) "temporary addresses results in too many addresses"
> This is not a problem araising from RFC4941, but a problem with SLAAC.
> In SLAAC, routers offer network configuration information, and hosts
> do... what they virtually please.
> Routers could also advertise many different prefixes on the same link,
> leading to many addresses. Hosts could also manually configure lots of
> addresses.  An OS might also provide a proprietary interface for
> proprietary apps to request "one address per flow".
> Given a sufficient number of prefixes and hosts, this may get to a point
> where implementation limits such as the maximum number of NCE may be hit.
> If folks are concerned about the maximum number of addresses that may be
> employed in a network (a valid concern), then I guess energy should be
> spent on how to address this general issue of SLAAC, *in SLAAC*, as
> opposed to simply bother about one of the many possible ways in which a
> host may configure addresses.
> I'd note that we have a BCP (RFC7934) about the topic of "number of
> addresses". In the typical/default case, RFC4941 will lead to a maximum
> of 7 addresses per host. In the context of RFC7934, I guess being able
> to handle 7 addresses per host is the least one could expect. I do
> understand that some systems can't handle them (given a sufficient
> number of hosts). Maybe we need to send a signal to router vendors.
> Maybe a few thousand entries in the NC is way too IPv4ish? All these
> issues are worth discussing... but they are issues with SLAAC or ipv6
> network configuration, and not with RFC4941.
> That said, and given recent feedback, it seems sensible to reduce the
> preferred lifetime and valid lifetime of temporary addresses to 1 day
> and 2 days, respectively. This would result in one preferred and one
> deprecated temporary addresses (as opposed to 1 preferred and 6
> deprecated addresses resulting from RFC4941).
> 2) The value of temporary addresses
> As noted above, one would assume that since RFC4941 has not been
> deprecated, there is consensus that RFC4941 provides value in the area
> of privacy.
> Everytime you reuse an identifier, it leaks information. Temporary
> addresses reduce the address lifetime, and hence limit the amount of
> time you reuse an identifier. That's an improvement. And it is a
> middleground between not using temporary addresses (and hence resuse the
> same identifier forever) or doing "one address per flow" which, while
> interesting, would take the number of addresses employed in any network
> to another dimension. As such (a middle-ground), it can't be expected to
> be perfect.
> Temporary addresses are meant employed for outgoing connections. Maybe
> RFC4941 should provide stronger hints or even recommendations that this
> use mode is enforced. For connection-oriented transport protocols, the
> concept is straightforward. For stateless protocols, not so much.
> But even if this mode is enforced for connection-oriented transport
> protocols, this would make temporary addresses reduce host exposure
> quite a lot. This is another area where temporary addresses have value.
> That said, I'm not sure to what extent it makes sense to argue about the
> value of temporary addresses in the context of *this* document. If we
> take the to extreme possible outcomes:
> * we don't publish rfc4941bis, and we keep a flawed RFC4941
> * we publish rfc4941bis, and have an improved version of rfc4941
> Only one of these options will improve RFC4941, and none of them will
> obsolete rfc4941.
> 3) Should temporary addresses be enabled by default?
> One would assume that since RFC4941 has not been deprecated, there is
> consensus that RFC4941 provides value in the area of privacy.
> In that sense, and in the context of RFC7258, I believe it would be a
> hard case to make to have temporary addresses disabled by default.
> I do believe some network might want to disable them. That would require
> the network to be able to convey this policy to hosts. This was tried
> (draft-gont-6man-managing-slaac-policy) and rejected ten years ago.
> I believe that, at least in the absence of policy, and in the context of
> RFC7258, it is sensible for the default to be "on".
> Besides, in a post-Snowden era, I believe we'd nevertheless have a hard
> time recommending OS vendors to disable them by default.
> Thanks!
> Cheers,
> --
> Fernando Gont
> SI6 Networks
> e-mail:
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