Re: [dhcwg] Eric Rescorla's No Objection on draft-ietf-dhc-rfc3315bis-10: (with COMMENT)

Eric Rescorla <ekr@rtfm.com> Tue, 23 January 2018 00:26 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 22 Jan 2018 16:25:15 -0800
Message-ID: <CABcZeBNN1GFBZHo2kbvNKwbh1ehHaCcu4yOjz7cuEHcNGkxcxg@mail.gmail.com>
To: Tomek Mrugalski <tomasz.mrugalski@gmail.com>
Cc: The IESG <iesg@ietf.org>, draft-ietf-dhc-rfc3315bis@ietf.org, Ralph Droms <rdroms.ietf@gmail.com>, dhc-chairs@ietf.org, dhcwg@ietf.org
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Subject: Re: [dhcwg] Eric Rescorla's No Objection on draft-ietf-dhc-rfc3315bis-10: (with COMMENT)
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On Mon, Jan 22, 2018 at 4:14 PM, Tomek Mrugalski <tomasz.mrugalski@gmail.com
> wrote:

> On 01/21/18 20:26, Eric Rescorla wrote:
> > Eric Rescorla has entered the following ballot position for
> > draft-ietf-dhc-rfc3315bis-10: No Objection
> Hi Eric,
>
> Thank you for your review and your favourable ballot. See my answers below.
>
> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> >
> > Document: draft-ietf-dhc-rfc3315bis-10.txt
> >
> > This document was quite clear and well written. A few small comments
> > below.
> >
> > It would have been easier for me to have a bit of intro about why
> > both the 4-message and 2-message rapid commit exchanges exist
> > and maybe some guidance about when to use each one.
> How about this text being added early to Section 5.1:
>   "It is sometimes desired to expedite the configuration process from
>   typical four messages down to two. This may be desired for couple
>   reasons. The first one is when there is only one server available
>   on link and there is no expectation that a second server would become
>   available. The second is when completing of the configuration process
>   as quickly as possible is a priority."
>

Yes, that helps.


> I am finding the guidance on DUIDs a bit confusing. Rather than
> > having a bunch of constructions that produce variable length
> > things that are intended to be unique, why not just take all
> > those values and feed them into a hash function and then you
> > could just have UUIDs?
> I was not participating in the original discussion that led to this
> decision back around 2002, but I can imagine there was a need to
> generate unique identifiers by devices with various capabilities (thus
> with/without timestamp variants). This concept predates usage of UUIDs
> in IETF, which as far as I can tell was first mentioned in RFC4122.
>
> Anyway, this part of the spec hasn't changed in bis. It would be
> extremely difficult to make any changes as there are millions of devices
> that already use those DUID types.
>

Well, we could encourage people to move to a UUID type for sending, no?


> I'm a little sad that the transaction ID is so short. This doesn't
> > seem like really enough to provide uniqueness against guessing
> > attacks.
> This was never meant as protection, simply as a way to differentiate
> between separate transactions that could be happening at the same time.
> Even if the transaction was much larger, it is sent by the client to a
> multicast address.
>

Fair enough.


> We're trying to discourage HMAC-MD5. Do you have any way to
> > transition to something stronger?
> Am afraid not at this time. The whole WG seriously tried very hard, but
> the draft-ietf-dhc-sedhcpv6 was abandoned at revision -21.
>

Yeah, that's what I expected.


> > The description of how to actually do replay detection seems pretty
> > thin. Do you think more detail would be helpful here.
> It would. How about this text added to the end of Section 20.3:
>
> "The client that receives a message with RDM field set to 0x00 MUST
> compare its replay detection field with the previous value sent by
> the same server. If this is the first time a client sees Authentication
> option sent by the server, it MUST record the replay detection value,
> but otherwise skip the replay detection check.
>
> The servers that support reconfigure mechanism MUST ensure the replay
> detection value is retained after restarts. Failing to do that will
> cause the clients to refuse reconfigure messages sent by the server,
> effectively rendering the reconfigure mechanism useless."
>

This seems like it scales badly as you need to keep an infinite replay
list. What happens if you falsely declare something a replay?



>
> > S 1.
> >    DHCPv6 can also provide only other configuration options (i.e., no
> >    addresses or prefixes).  That implies that the server does not have
> >
> > Perhaps "DHCP can also be used just to provide..."
> Sounds good.
>
> > S 2.
> > Nit: do you want to cite 8174.
> Yes.
>
> > S 4.2.
> > When acronyms are used ahead of their definition, it would be good to
> > expand them.
> Ok, will do that.
>
> > S 21.22.
> > What is the part of the IPv6-prefix after prefix-length filled with?
> > Does it matter?
> Should be filled with zeros by sender and ignored by the receiver. Will
> add a text that clarifies that.
>

thanks
-Ekr


>
> Tomek
>