Re: Back to work

[Martin Duke <[e-mail]>]

My original proposal was for additional recommendations on (1) and (3), as
bolded.

I could live with non-normative text. My point is that these optimizations
are, IMO, extremely non-obvious and we would benefit readers to point them
out.

On Mon, Nov 2, 2020 at 4:21 AM Ian Swett <[e-mail]> wrote:

>
>
> 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 :)
>>>>
>>>>