Re: [lisp] Martin Duke's Discuss on draft-ietf-lisp-rfc6830bis-32: (with DISCUSS and COMMENT)

Albert Cabellos <albert.cabellos@gmail.com> Wed, 09 September 2020 12:56 UTC

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From: Albert Cabellos <albert.cabellos@gmail.com>
Date: Wed, 9 Sep 2020 14:55:54 +0200
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To: Martin Duke <martin.h.duke@gmail.com>
Cc: "Joel M. Halpern" <jmh@joelhalpern.com>, Luigi Iannone <ggx@gigix.net>, draft-ietf-lisp-rfc6830bis@ietf.org, "lisp@ietf.org list" <lisp@ietf.org>, The IESG <iesg@ietf.org>
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Subject: Re: [lisp] Martin Duke's Discuss on draft-ietf-lisp-rfc6830bis-32: (with DISCUSS and COMMENT)
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Hi Martin

Just posted -34 per your comments:

https://www.ietf.org/rfcdiff?url2=draft-ietf-lisp-rfc6830bis-34

1) Removed duplicate paragraph in section 7
2) Added the following sentence for clarification of what happens when N=0
and V=0: "*Finally, when both the N and V-bit are not set (N=0, V=0), then
both the Nonce and Map-Version fields are set to 0 and ignored on receipt*"
3) Removed the IPv4-only requirement in section 7.2. Please note that this
is in -35 since I missed it in the first place.
4) Added the following paragraph (yours, verbatim) at the end of section
7.2:

*Please note that [RFC1191] and [RFC1981], which describe the use of ICMP
packets for PMTU discovery, can behave suboptimally in the presence of ICMP
black holes or off-path attackers that spoof ICMP. Possible mitigations
include ITRs and ETRs cooperating on MTU probe packets ([RFC4821],
[I-D.draft-ietf-tsvwg-datagram]), or ITRs storing the beginning of large
packets to verify that they match the echoed packet in ICMP Frag
Needed/PTB.*

Albert


On Fri, Aug 14, 2020 at 12:17 AM Martin Duke <martin.h.duke@gmail.com>
wrote:

> As promised, here are my reconsidered thoughts about Section 7.2:
>
> 1) as agreed before, delete the restriction to IPv4 and restore the other
> references to ICMPv6 in draft-31.
>
> 2) There is not an IETF consensus document that describes what I feel to
> be the most secure way to do tunnel PMTU management. So the current design
> is acceptable; however, there should be some warning about the robustness
> issues here. Example text:
>
> "Please note that [RFC1191] and [RFC1981], which describe the use of ICMP
> packets for PMTU discovery, can behave suboptimally in the presence of ICMP
> black holes or off-path attackers that spoof ICMP. Possible mitigations
> include ITRs and ETRs cooperating on MTU probe packets ([RFC4821],
> [I-D.draft-ietf-tsvwg-datagram]), or ITRs storing the beginning of large
> packets to verify that they match the echoed packet in ICMP Frag
> Needed/PTB."
>
> Feel free to re-word, of course.
>
> This can either be in the section or mentioned in security considerations
> with a pointer in 7.2.
>
> Martin
>
> On Thu, Aug 6, 2020 at 6:28 PM Joel M. Halpern <jmh@joelhalpern.com>
> wrote:
>
>> Exploring Martin's second comment, I looked at section 7.2 of the draft.
>>   I do not see any obvious reason why this section is restricted to
>> IPv4.  If there is a reason, we need to state it.  If there is no
>> reason, we should allow it for the v6 case as well.
>>
>> Yours,
>> Joel
>>
>> On 8/6/2020 6:24 PM, Martin Duke wrote:
>> > Hi Joel,
>> >
>> > I'm realizing that we may not have a consensus document that provides
>> > good guidance on how to proceed. I'm going to consult with a couple of
>> > SMEs and come up with a reasonable recommendation. This shouldn't take
>> > any more than a couple of days.
>> >
>> > However the "IPv4 only" recommendation is just wrong and should be
>> reverted.
>> >
>> > On Thu, Aug 6, 2020 at 1:48 PM Joel M. Halpern <jmh@joelhalpern.com
>> > <mailto:jmh@joelhalpern.com>> wrote:
>> >
>> >     Martin, I want to check one aspect of your response about MTU
>> handling.
>> >
>> >     The entity which is originating the packets, and receiving the ICMP
>> >     responses, is the ITR.  In most cases, the ITR is a router.  I do
>> not
>> >     know of any tunnel protocol for rotuers that expects the routers to
>> >     store state about the packets it has sent in the tunnels.
>> >     As these are low-state tunnels, and as the packets are those
>> >     provided by
>> >     the sources behind the ITR, I doubt that we can use PLPMTUD,
>> although I
>> >     would be happy to be given enough information to find I am wrong
>> >     about that.
>> >
>> >     I am somewhat confused as to what you would have us do.
>> >     Yours,
>> >     Joel
>> >
>> >     On 8/6/2020 4:35 PM, Martin Duke wrote:
>> >      > Hi Albert,
>> >      >
>> >      > thanks for the edits, and sorry for the delay! We're not quite
>> >     there on
>> >      > a few of the items:
>> >      >
>> >      > Though first, there is now a duplicate paragraph in Section 7.
>> >     Please
>> >      > delete one.
>> >      >
>> >      > On Fri, Jul 31, 2020 at 5:43 AM Albert Cabellos
>> >      > <albert.cabellos@gmail.com <mailto:albert.cabellos@gmail.com>
>> >     <mailto:albert.cabellos@gmail.com
>> >     <mailto:albert.cabellos@gmail.com>>> wrote:
>> >      >
>> >      >
>> >      >     On Fri, Jul 3, 2020 at 9:07 PM Martin Duke via Datatracker
>> >      >     <noreply@ietf.org <mailto:noreply@ietf.org>
>> >     <mailto:noreply@ietf.org <mailto:noreply@ietf.org>>> wrote:
>> >      >
>> >      >          >
>> >      >
>> >      >      > Sec 5.3 What is in the Nonce/Map-Version field if both the
>> >     N and
>> >      >     V bits are
>> >      >      > zero?
>> >      >      >
>> >      >
>> >      >     There is no field then.
>> >      >
>> >      >
>> >      > so the bits are set to zero, or is the LISP header actually
>> >     shorter by 3
>> >      > octets?
>> >      >
>> >      >
>> >      >      >
>> >      >      > Sec 7.2 The stateful MTU design does not incorporate any
>> >     security
>> >      >     measures
>> >      >      > against ICMP spoofing. At the very least, the ITR needs to
>> >     make
>> >      >     sure that some
>> >      >      > fields in the outer IP and UDP headers are hard to guess,
>> and
>> >      >     that this
>> >      >      > information is stored to verify that the ICMP message came
>> >     from
>> >      >     on-path. If
>> >      >      > this is not possible, the design is not safe to use over
>> >     IPv4.  If
>> >      >      > hard-to-guess information is not available to be stored
>> >     deeper in
>> >      >     the packet,
>> >      >      > then it is not safe over IPv6 either.
>> >      >      >
>> >      >
>> >      >     The source UDP port is random. We have therefore added the
>> >     following
>> >      >     statement at the beginning of section 7.7:
>> >      >
>> >      >             An ITR stateful solution to handle MTU issues is
>> >     described
>> >      >         as follows, this solution can only be used with
>> >      >         IPv4-encapsulated packets:
>> >      >
>> >      >
>> >      > This is backwards, and anyway inadequate.
>> >      >
>> >      > An off-path attacker can generate a fairly small number of ICMP
>> >     messages
>> >      > to reduce the MTU to ridiculously low levels (e.g. 68 bytes),
>> which
>> >      > depending on tunneling overhead could render the path unusable.
>> The
>> >      > defense against this is to either ignore ICMP messages (instead
>> >     using
>> >      > PLPMTUD
>> >      >
>> >     <
>> https://datatracker.ietf.org/doc/draft-ietf-tsvwg-datagram-plpmtud/> to
>> >
>> >      > find the MTU) or to compare the echoed information the ICMP
>> message
>> >      > against the stored contents of the packet, where obviously there
>> >     needs
>> >      > to be enough entropy to make it hard to guess. Generally the port
>> >     is not
>> >      > sufficient entropy, since it takes fewer than 2^16 packets to
>> >     take you
>> >      > down, but admittedly there isn't much UDP-based protocols can do
>> >     about this.
>> >      >
>> >      > In IPv6, the router should include as much of the packet as
>> >     possible in
>> >      > the ICMP packet, so the chance of guessing is low. It's therefore
>> >     it's
>> >      > simply a matter of specifying that hosts should store the packet
>> >     payload
>> >      > and do the validation step.
>> >      >
>> >      > In IPv4, the router is required to include the first 8 bytes of
>> >     the IP
>> >      > payload (eg the UDP header), so all you have are the IP and UDP
>> >     headers.
>> >      > Hosts should still do the validation.
>> >      >
>> >      > The main thing is to tell them to do that validation.
>> >      >
>> >      >
>> >      >      >
>> >      >      > Sec 7.2 There is a fourth situation which can arise. If
>> >     the ETR
>> >      >     receives an
>> >      >      > ICMP packet from an EID in its network. I have a couple of
>> >      >     questions about what
>> >      >      > should happen in this case:
>> >      >      >
>> >      >
>> >      >     In this case the EID is locally attached to the xTR.
>> >     Therefore, the
>> >      >     xTR has a locally configured MTU to reach the EID. So what is
>> >      >     written in the section already covers this scenario.
>> >      >
>> >      >      >
>> >      >      > - How is this communicated to the sender of the flow that
>> >      >     triggered the
>> >      >      > message? Is there an "outer" ICMP to the ITR, and "inner"
>> >     ICMP to
>> >      >     the source
>> >      >      > EID, both, or neither?
>> >      >      >
>> >      >      > - Is the ETR responsible for enforcing the MTU to that
>> EID for
>> >      >     subsequent flows?
>> >      >      >
>> >      >
>> >      >
>> >      > I read 7.2 again and I don't see that it does. According to this
>> >      > section, what does the ETR do when it receives a packet from the
>> ITR
>> >      > that exceeds the locally configured MTU?
>> >      >
>> >      > Martin
>> >      >
>> >      > _______________________________________________
>> >      > lisp mailing list
>> >      > lisp@ietf.org <mailto:lisp@ietf.org>
>> >      > https://www.ietf.org/mailman/listinfo/lisp
>> >      >
>> >
>>
>>