Re: Back to work

[Ian Swett <[e-mail]>]

On Sat, Oct 31, 2020 at 4:41 PM Martin Duke <[e-mail]> wrote:

> I want to carefully think through the various scenarios here, two "normal"
> ones and two attack ones. But the summary is that the proposal to always
> use the amp limit is good, if we add a coupe of more recommendations for
> clients (in bold below)
>
> 1) Normal NAT rebinding. The connection is idle for a long period. A good
> server will have reset its congestion controller due to the long idle
> period.
>
> The client then sends a packet (if the server does first, it'll probably
> disappear).  Whether or not that packet is small, the server is going to be
> limited by the amplification limit rather than congestion control. One
> alternative is to kill the connection after an idle period and restart with
> 0RTT, but that's only superior because the client has to send a bunch of
> bytes. The recommendation here, I think, is that *clients SHOULD send one
> or more full-size datagrams when restarting after a long idle period*, *and
> possibly reset its PMTU assumptions. *[I can't remember what DPDLPMTUD
> says about idle periods]
>

I'd suggest a non-normative can in this case, but I think it's worth
pointing out.


>
> If the client does this, the server will be able to validate address and
> MTU in one RTT. Otherwise, it's going to have to take two. But the
> recommendation is sufficient to mitigate the impact of the 3x limit in NAT
> rebinding cases.
>
> 2) Spoofed NAT rebinding. An attacker rewrites source addresses on someone
> else's packets, or opens a valid connection and then switches the source
> address to the victim. The amplification limit will prevent anything bad
> from happening here, then PATH_CHALLENGE will fail and we're done.
>
> 3) Normal Migration. The client suddenly changes the CIDs and source IP.
> (a) If the path is pre-validated, there is no issue. PATH_CHALLENGE and
> PATH_RESPONSE SHOULD have been padded to handle MTU and address validation
> simultaneously.
> (b) If the path is not-prevalidated, the client SHOULD pad PATH_CHALLENGE.
> This will be more restrictive than init-cwnd, unless *the client sends 3
> datagrams or so, padded pings if necessary.*
>

I think this is already a MUST pad PATH_CHALLENGE?

>
> 4) Spoofed Migration: the attacker opens a connection, then sends packets
> with new CIDs and the victim's IP. It is impossible for this to be
> pre-validated. Because the attacker has to send 1 byte for every 3 the
> victim gets, this is safe.
>
> I'll propose some text in review.
>

Besides on 1, are you suggesting any additional changes to the existing
PR?  I agree with your analysis, except I believe the client already MUST
pad PATH_CHALLENGE in these cases.

>
> On Fri, Oct 30, 2020 at 8:08 PM Ian Swett <ianswett=
> [e-mail]> wrote:
>
>> Thanks for all the awesome discussion.
>>
>> It sounds like we're landing on text that requires the 3x limit be
>> enforced until path validation succeeds, even if it's believed it's NAT
>> rebinding.  I don't think that's a huge performance hit for the reasons
>> stated above, but it does add some complexity for some implementations, so
>> I'm not sure if it'll be widely enforced or not?
>>
>> To answer Jana's question(far above) about resetting the congestion
>> controller, my thinking is the following.  In cases when the server
>> believes it's NAT migration, it does not reset the congestion controller
>> and this limits the potential attack to the congestion window built up
>> prior to the migration.  Additionally, the server has to believe it's a NAT
>> migration, which makes the attack unpredictable or useless in the presence
>> of modern NATs and Firewalls.
>>
>> So what I'd argue for is that NAT migrations MAY be treated as validated
>> paths, but the congestion controller MUST NOT be reset in that case.  I
>> think this fits what MT said their implementation currently did and what I
>> believe ours does today as well.
>>
>> Thanks, Ian
>>
>> On Thu, Oct 29, 2020 at 9:28 PM Eric Kinnear <ekinnear=
>> [e-mail]> wrote:
>>
>>> Agreed, I think that’s where the “client needs to pad PATH_CHALLENGE and
>>> any PATH_RESPONSE needs to be padded if the challenge was padded” came
>>> from.
>>>
>>> To the point about it being an error case — that totally makes sense,
>>> and as Christian points out, that is also significantly more likely under
>>> some of the attacks than someone being both idle and transferring lots of
>>> data at the same time.
>>>
>>> I’m pretty sure we did previously have text that allowed the server to
>>> treat the new address as validated if only the port changed, but we took it
>>> out to help with some of the MOTS attacks. Also fully agreed that it would
>>> be really nice to not split the logic across retaining some things (CC/RTT)
>>> some of the time, but not others (Path, MTU validation).
>>>
>>> All this said, I suspect these are all edge cases enough that the more
>>> conservative PR (as currently written) would be totally sufficient.
>>> My current feeling is that if we wanted to carve it out such that a
>>> small packet from an attacker on the side (or an unintentional migration)
>>> didn’t generate MAX(full_size_packet, 3x_what_came_in_from_the_attacker)
>>> sized packets, that would be neat, but not absolutely necessary.
>>>
>>> Thanks,
>>> Eric
>>>
>>>
>>> > On Oct 29, 2020, at 5:09 PM, Martin Thomson <[e-mail]> wrote:
>>> >
>>> > On Fri, Oct 30, 2020, at 10:42, Christian Huitema wrote:
>>> >> That means doing thing differently for a regular migration and for a
>>> NAT
>>> >> rebinding. A regular migration happens starts the client sends a full
>>> >> size packet, with a not yet seen CID, and containing a path challenge.
>>> >> Responding to that with a full size response makes sense. But if the
>>> >> server receives a short packet, with an already used CID, and without
>>> a
>>> >> path challenge, that's probably a NAT rebinding. Responding with full
>>> >> size challenge packets is counter-productive.
>>> >
>>> > Ahh, that seems sensible.  Conveniently, the current PR results in
>>> what you describe, so I have less work to do :)
>>>
>>>