Re: [trill] Eric Rescorla's Discuss on draft-ietf-trill-arp-optimization-08: (with DISCUSS and COMMENT)

[Donald Eastlake <[e-mail]>]

Hi Eric,

A -09 version of draft-ietf-trill-arp-optimization has been posted. Could
you look at it to see if it resolves your comments?

Thanks,
Donald
===============================
 Donald E. Eastlake 3rd   [phone] (cell)
 155 Beaver Street, Milford, MA 01757 USA
 [e-mail]

On Mon, Oct 2, 2017 at 2:12 PM, Eric Rescorla <[e-mail]> wrote:

>
>
> On Mon, Oct 2, 2017 at 4:54 PM, Donald Eastlake <[e-mail]> wrote:
>
>> Hi Eric,
>>
>> My apologies for the delay in response. (For part of the time I was on
>> a cruise...)
>>
>> It looks like we are in agreement on most items.
>>
>> See below.
>>
>> On Fri, Sep 22, 2017 at 9:18 AM, Eric Rescorla <[e-mail]> wrote:
>> >
>> >
>> > On Wed, Aug 23, 2017 at 12:38 PM, Donald Eastlake <[e-mail]>
>> wrote:
>> >>
>> >> Hi Eric,
>> >>
>> >> On Wed, Jul 5, 2017 at 7:38 AM, Eric Rescorla <[e-mail]> wrote:
>> >> > Eric Rescorla has entered the following ballot position for
>> >> > draft-ietf-trill-arp-optimization-08: Discuss
>> >> >
>> >> > ------------------------------------------------------------
>> ----------
>> >> > DISCUSS:
>> >> > ------------------------------------------------------------
>> ----------
>> >>
>> >> As a general comment, the Security Consideration section could use
>> >> clarification and some expansion.
>> >>
>> >> Although one can imagine various mixed modes, to the first
>> approximation
>> >> there are two ways of doing the optimization of various
>> multi-destination
>> >> messages discussed in this document:
>> >>
>> >> + Using data plane learning by observing ARP/ND and similar messages
>> (in
>> >> addition to the learning of MAC addresses from the data plane which is
>> >> enabled by default in TRILL). As described in this document, such data
>> plane
>> >> learning by TRILL switches can be used to sometimes optimize
>> >> ARP/ND/RARP/unknown-destination messages. But the result isn't
>> particularly
>> >> more or less secure than it is when this data plane learning is done
>> by end
>> >> stations rather than by TRILL switches (or bridges or other kinds of
>> >> switches) in the middle doing some optimization of the relevant
>> >> multi-destination messages. Interface address information learned
>> through
>> >> data plane learning by edge TRILL switches can also be propagated via
>> the
>> >> control plane but this is not significantly different from propagating
>> that
>> >> information via the multi-destination messages that are being
>> optimized to
>> >> unicast or optimized away.
>> >> + Using a complete, trusted directory as might be based on an
>> orchestration
>> >> system in a data center. Since the messages between TRILL switches
>> doing
>> >> ARP/ND/RARP/unknown-destination optimization can be secured and the
>> TRILL
>> >> switches can be configured to ignore data plane learning, this enables
>> TRILL
>> >> switches to provide answers that are "correct" in that they correspond
>> to
>> >> the data in this complete, trusted directory. However, there can still
>> be
>> >> various forged messages/addresses on a local link between end
>> station(s) and
>> >> edge TRILL switches. Such a local link can, with TRILL, be a bridged
>> LAN, so
>> >> such forgeries emitted by an end station may be seen by other end
>> stations
>> >> and cause incorrect learning from the data plane.
>> >>
>> >> The existing Security Considerations section is a bit sketchy and
>> >> concentrates more on the data plane case, as that is the one with the
>> >> greatest security challenges.
>> >
>> > Yes, this seems like a reasonable assessment. Alia, does someone have
>> the
>> > action item to make it less sketchy?
>>
>> We are working on a draft revision to reflect the above thinking to a
>> greater extent and decrease sketchiness.
>>
>> >> > It's not clear to me how the security properties of this mechanism
>> >> > compare to existing TRILL. The text says:
>> >> >
>> >> >    Unless Secure ND (SEND [RFC3971]) is used, ARP and ND messages
>> can be
>> >> >    easily forged. Therefore the learning of MAC/IP addresses by
>> RBridges
>> >> >    from ARP/ND should not be considered as reliable. See Section 4.1
>> for
>> >> >    SEND Considerations.
>> >> >
>> >> > "not considered as reliable" seems suboptimal. You need to cover how
>> >> > this mechanism compares to the non-use of this mechanism.
>> >>
>> >> As above, the optimization mechanisms do not make any significant
>> >> difference to the inherent insecurities of data plane learning but
>> when used
>> >> with a complete, trusted directory, it improves considerably over data
>> plane
>> >> learning..
>> >
>> > That had kind of been my impression, but then I don't understand what
>> "not
>> > considered as reliable" is doing here. I'm supposed to be doing
>> something
>> > with it, so while it may be non-secure, I'm still relying on it, no?
>>
>> Well, at some level some elements of your protocol stack are "relying"
>> on it or "trusting" it or whatever term you want, but it's trivially
>> forged unless you use additional security protocols so I guess we
>> could say something about that. TRILL supports different link
>> technologies but if you are using an Ethernet link, I suppose you
>> could use 802.1AE to improve trust in MAC addresses at layer 2. It
>> wouldn't hurt to mention that but it has very little to do with ARP/ND
>> optimization or TRILL.
>>
>
> I think if you just sort of say "you are using it, but it's not
> trustworthy and here's why" you have it covered.
>
> -Ekr
>
>
>> >> > This document seems very sketchy on what you do when you get
>> duplicate
>> >> > IPs.
>> >>
>> >> Why should what you do when you notice a duplicate IP address be
>> >> significantly different just because some multi-destination messages
>> are
>> >> being optimized? Why shouldn't you just do whatever you do now and why
>> is
>> >> that any of TRILL's business?
>> >
>> > Because you're getting the information via different vector, TRILL
>> needs to
>> > specify what you do,
>> > if only to say "do what you would do otherwise".
>>
>> OK.
>>
>> >> >    In the case where the former owner replies, a Duplicate Address
>> has
>> >> >    been detected. In this case the querying RBridge SHOULD log the
>> >> >    duplicate so that the network administrator can take appropriate
>> >> >    action.
>> >> >
>> >> > How does logging solve the problem?
>> >>
>> >> It doesn't but it seems better than not logging.
>> >>
>> >> > What do you reply to ARPs with and/or propagate to other nodes?
>> >>
>> >> If you are asking how you reply to ARPs if the IP address being ARPed
>> for
>> >> has been found to be duplicated, there are the two cases outlined
>> above. If
>> >> you are using a complete, trusted directory, you give the presumed
>> correct
>> >> answer provided by that directory. If you are using data plane
>> learning, you
>> >> use whatever you last learned from the data plane (which could have
>> been a
>> >> forgery over writing good information or good information over writing
>> a
>> >> forgery or a forgery overwriting a different forgery or ...).
>> >>
>> >> > Do you tell the originator of the advertisement you have a duplicate?
>> >>
>> >> As far as I know there is no standardized way to tell a station that
>> you
>> >> think it has an IP address that is duplicated locally and I don't
>> think it
>> >> is the job of TRILL to provide such a mechanism. Certainly this
>> document
>> >> does not propose anything of that sort.
>> >
>> > OK.
>> >
>> >
>> >> >    When a newly connected end-station exchanges messages with a DHCP
>> >> >    [RFC2131] server an edge RBridge should snoop them (mainly the
>> >> >    DHCPAck message) and store IP/MAC mapping information in its
>> ARP/ND
>> >> >    cache and should also send the information out through the TRILL
>> >> >    control plane using ESADI.
>> >> >
>> >> > What happens if the attacker sets up a fake DHCP server and pretends
>> >> > to assign addresses to himself? It seems like maybe that's the same
>> >> > as fake ARPs but maybe not.
>> >>
>> >> This snooping on DHCP messages only applies to the data plane learning
>> >> case. In the case of a complete, trusted directory, edge RBridges
>> already
>> >> know the right answer, so to speak. With data plane learning, if there
>> are
>> >> forged DHCP messages, things are likely to get all screwed up. Of
>> course,
>> >> you could run DHCP over IPsec but, partly due to the localized nature
>> of
>> >> DHCP, I don't think very many people do that. There is also RFC 3118
>> which
>> >> can be used to authenticate DHCP messages, but that assumes the
>> distribution
>> >> of keying information.
>> >>
>> >> There isn't much difference between the various multi-destination
>> messages
>> >> (ARP / DHCP) advertising the addresses and point of attachment of
>> >> interfaces. But, overall, I'm not sure why a document that is just
>> >> describing ways to optimize the distribution of various
>> multi-destination
>> >> messages needs to analyze the security of the protocols implemented by
>> those
>> >> multi-destination when, as in the data plane learning case, it is not
>> >> significantly changing that security.
>> >
>> > Well, in this case, you have a new threat vector: DHCP. Even if it is
>> the
>> > same kind of threats, it's still a new threat vector.
>> > More generally documents need to contain complete security
>> considerations
>> > sections. If the security considerations
>> > are the same as with some other document, then they need to say that.
>>
>> OK.
>>
>> >> > In general, the security considerations need a complete threat
>> >> > analysis per 3552.
>> >>
>> >> I don't think there is that much new in this document. While the
>> Security
>> >> Considerations section needs clarification and some expansion, I don't
>> think
>> >> it is, for example, the job of this document to do a threat analysis
>> of the
>> >> ARP protocol as long as it can make a reasonable case that the
>> security of
>> >> ARP is not significantly changed by the optimizations specified.
>> >
>> > Sure, but it also doesn't make that case.
>>
>> I'll see what we can do.
>>
>> >> > ------------------------------------------------------------
>> ----------
>> >> > COMMENT:
>> >> > ------------------------------------------------------------
>> ----------
>> >>
>> >> For completeness, I've cut and pasted below the COMMENT responses I
>> sent
>> >> to you a while ago.
>> >>
>> >> Thanks,
>> >> Donald
>> >> ===============================
>> >>  Donald E. Eastlake 3rd   [phone] (cell)
>> >>  155 Beaver Street, Milford, MA 01757 USA
>> <https://maps.google.com/?q=155+Beaver+Street,+Milford,+MA+01757+USA&entry=gmail&source=g>
>> >>  [e-mail]
>> >>
>> >> > S 2.
>> >> >
>> >> >    plane on the edge RBridges, it should be possible to completely
>> >> >    suppress flooding of ARP/ND messages in a TRILL Campus, When all
>> end-
>> >> >    station MAC addresses are similarly known, it should be possible
>> to
>> >> >    suppress unknown unicast flooding by dropping any unknown unicast
>> >> >    received at an edge RBridge.
>> >> >
>> >> > Are these "should be possibles" normative? Descriptive?
>> >>
>> >> Section 2 of this document is a general discussion of the optimization
>> >> problem which, while it does mention a couple of possible facets of
>> >> potential solutions, is not intended to be normative.
>> >>
>> >>
>> >> > S 4.
>> >> > This is a sequence of steps, so it would be nice to preface them with
>> >> > a list of the steps. It's also odd to have SEND considerations right
>> >> > in the middle here.
>> >>
>> >> Sorry, I don't understand exactly what you are pointing at as a
>> >> sequence of steps. All of Section 4? Just the text between the Section
>> >> 4 heading and the Section 4.1 heading? Something else?
>> >
>> > Sorry, my point is that if I read this correctly, I do 4.3, then 4.4,
>> etc.
>> > It would be nice if they were listed ahead of this as "this is what you
>> do,
>> > now read the sections"
>>
>> OK. We can add some clarifying material to Section 4 before the 4.1
>> heading.
>>
>> >> > 4.3 Get Sender's IP/MAC Mapping Information for Non-zero IP
>> >> >
>> >> > Please explain what a non-zero IP is and why it's relevant.
>> >> > This graf also needs an introductory sentence or something before
>> >> > the bullets.
>> >>
>> >> Here is a copy of the response that was sent to a similar comment in
>> >> Dale Worley's GENART review:
>> >>
>> >> This section is talking about getting and handling the IP/MAC
>> >> mapping information for a local end station by an edge RBridge. An
>> >> ARP/ND from such a local end station might show a zero source IP
>> >> address if the end station does not have one or is probing for
>> >> duplicates.
>> >>
>> >> Perhaps the title for the Section should be something like
>> >>
>> >>    4.3 Extracting Local End Station IP/MAC Mapping Information
>> >>
>> >> and a new first paragraph could be added something like
>> >>
>> >>    Edge RBridges extract and use information about the correspondence
>> >> between local end station IP and MAC addresses from the ARP/ND
>> >> messages those end stations send as described below. An apparent zero
>> >> source IP address means that the end station is probing for duplicate
>> >> IP addresses and messages with such a zero source IP address are not
>> >> used for the extraction of IP/MAC address mapping information
>> >
>> > LGTM
>> >
>> >
>> >> > S 4.4.
>> >> >    It is not essential that all RBridges use the same strategy for
>> which
>> >> >    option to select for a particular ARP/ND query. It is up to the
>> >> >    implementation.
>> >> >
>> >> > This seems inconsistent with the MUST in arm (b) below, because I
>> >> > can just take some other arm. It's also kind of surprising to be this
>> >> > non-prescriptive.
>> >>
>> >> The wording needs to be clarified. Which lettered item listed below
>> >> applies is determined by the circumstances; however, which of the
>> >> numbered strategies associated with these items is followed is an
>> >> implementation choice in handeling any particular ARP/ND.
>> >
>> > OK.
>> >
>> >
>> >> > S 8.
>> >> >    some other location (MAC/VM Mobility) and gets connected to egde-
>> >> >
>> >> > Nit: edge is mispelled.
>> >>
>> >> OK.
>>
>> Thanks,
>> Donald
>> ===============================
>>  Donald E. Eastlake 3rd   [phone] (cell)
>>  155 Beaver Street, Milford, MA 01757 USA
>> <https://maps.google.com/?q=155+Beaver+Street,+Milford,+MA+01757+USA&entry=gmail&source=g>
>>  [e-mail]
>>
>
>