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 <[email protected]
<mailto:[email protected]>> 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
> <[email protected] <mailto:[email protected]>
<mailto:[email protected]
<mailto:[email protected]>>> wrote:
>
>
> On Fri, Jul 3, 2020 at 9:07 PM Martin Duke via Datatracker
> <[email protected] <mailto:[email protected]>
<mailto:[email protected] <mailto:[email protected]>>> 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
>
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