Re: Back to work

[Martin Duke <[e-mail]>]

NAT doesn't require reset, but there are times when it's wise, particularly
when the NAT is closer to the server than the client. (e.g. a NAT might
signal a mobility event on the clientside by changing its port)

On Wed, Oct 28, 2020 at 1:04 PM Ian Swett <ianswett=
[e-mail]> wrote:

> I'll note that this problem is created/worsened by the fact that the
> congestion controller is reset.  If it was not reset, you'd be limited by
> the existing congestion controller.
>
> That would allow you to build up a big window and direct it at another
> path, but creating a larger window is more work on top of completing the
> handshake.
>
> NAT rebinds don't require resetting the congestion controller if my memory
> is correct, so I don't believe they don't need to be covered by this new
> amplification factor.
>
> Ian
>
> On Wed, Oct 28, 2020 at 2:18 AM Mikkel Fahnøe Jørgensen <
> [e-mail]> wrote:
>
>> Rather than a race to the top with padding, would it be possible to do
>> the opposite:
>>
>> Force challenges and responses to occur in their packets and also UDP
>> datagrams. This prevents other traffic until a path is confirmed.
>>
>> The initial handshake has several concerns with padding:
>>
>> - amplification attack mitigation
>> - PMTU discovery
>> - reply capacity for completing handshake
>>
>> Since new paths do not need a handshake, there is less need for large
>> replies. Of course there is the PMTU issue still.
>>
>>
>>
>> Kind Regards,
>> Mikkel Fahnøe Jørgensen
>>
>>
>> On 28 October 2020 at 03.55.46, Eric Kinnear (
>> ekinnear=[e-mail]) wrote:
>>
>> This is an interesting PR, and likely accomplishes the goals at the
>> moment.
>> I do really like how we’ve kept some bidirectionally of the approach and
>> the padding can stay as is.
>>
>> Just thinking things through a little bit:
>> (This is all discussed below by Ian/Magnus/Martin/Kazuho, and others,
>> just restating so we have it in one place)
>>
>> At any point, either endpoint can choose to send a PATH_CHALLENGE.
>> The presence of a PATH_CHALLENGE always evokes a PATH_RESPONSE.
>>
>> Therefore, we assume that in order to restrict folks from being able to
>> spoof a source address when sending a PATH_CHALLENGE and attack the real
>> owner of that source address with the PATH_RESPONSE, we need to make the
>> PATH_CHALLENGE very large as well.
>>
>> However, there’s another situation where PATH_CHALLENGE is sent, and
>> that's whenever we receive a non-probing packet that arrives on a new path
>> without any prior validation, and we send that PATH_CHALLENGE on both the
>> old and the new path.
>>
>> This is where we haven’t fully plugged the amplification hole, since an
>> attacker can use *any other, smaller datagram* to cause the other
>> endpoint to generate full-size datagrams containing PATH_CHALLENGE. This
>> wasn’t previously a huge issue since PATH_CHALLENGE wasn’t meaningfully
>> larger than the smallest packet you’d otherwise be able to send (slash the
>> per-packet costs were potentially higher than the cost of the data inside
>> that packet).
>>
>> ———
>>
>> One other approach we could take here would be to restrict ourselves to
>> only covering the cases where you’re actively generating a PATH_CHALLENGE
>> to validate a new path, not responding to a new non-probing packet on an
>> unvalidated path.
>>
>> In other words:
>> Only the client needs to pad PATH_CHALLENGE and any response to a padded
>> PATH_CHALLENGE should also be padded. That also fits nicely into the
>> unidirectionality of path validation as it stands today.
>>
>>
>> The other option that we haven’t discussed much is if we’d rather live
>> with the previous pre-padding problem and remove the padding.
>> My initial inclination was to avoid this, but actually we’d be returning
>> to a state where the main risk was that the path wasn’t MTU compatible and
>> any implementation migrating is likely already dealing with cases where
>> packets aren’t going through on a path in at least one direction. So, the
>> natural responses to path validation failures (for MTU reasons or
>> otherwise), if you map them all out, generally result in the “correct”
>> behavior. We could then say “any endpoint using a new path is encouraged to
>> do PMTUD or otherwise be careful that the path may not work in at least one
>> direction” and leave it at that.
>>
>> ———
>>
>> Overall, I suspect we’re probably headed in the right direction by making
>> the 3x limit more universal, although it does seem like it introduces some
>> really interesting cases to code around, and that limit and double path
>> validation might be more painful than just checking for “am I client,
>> therefore I should pad” which is annoying because it has a client/server
>> distinction but does likely cause less churn and risk for later fallout.
>>
>> Thanks,
>> Eric
>>
>>
>> On Oct 27, 2020, at 7:41 PM, Martin Thomson <[e-mail]> wrote:
>>
>> Thanks to everyone for the feedback.
>>
>> I've written up a draft pull request here:
>> https://github.com/quicwg/base-drafts/pull/4264
>>
>> This does something like what Magnus suggests below.  It's not pretty,
>> because in some very common cases path validation could take twice as long,
>> and it's more complicated, but I think that it is at least principled.
>>
>> On Wed, Oct 28, 2020, at 04:04, Magnus Westerlund wrote:
>>
>> On Tue, 2020-10-27 at 09:12 -0400, Ian Swett wrote:
>>
>> Thanks for summarizing this issue. I think the above discussion is about
>> immediate migration and repeated immediate migrations, but I also wonder
>> if
>> we've introduced a single packet amplification attack by requiring
>> PATH_RESPONSEs be padded on new paths without a requirement on the size of
>> PATH_CHALLENGE(see first item)?
>>
>> Validating a new path
>> If one receives only a PATH_CHALLENGE on a new path, then the server
>> responds with a full-sized PATH_RESPONSE.  This seems safe.  If a
>> non-padded
>> PATH_CHALLENGE is received on a new path, then the peer is supposed to
>> send a
>> fully padded PATH_RESPONSE on the path, which could be >20x larger.  I'm
>> not
>> sure if we care about this, but I wanted to point it out.
>>
>> Immediately migrating to a new path
>> I think we should remove the text about allowing kMinimumWindow each
>> kInitialRtt after migration and change it to the 3x limit.  I'm actually
>> surprised the text about 2*kInitialWindow still exists, since recovery
>> says
>> "Until the server has validated the client's address on the path, the
>> amount
>> of data it can send is limited to three times the amount of data
>> received, as
>> specified in Section 8.1 of {{QUIC-TRANSPORT}}.".
>>
>> In order to not get deadlocked by the 3x factor, I think we should change
>> the
>> newly added MUSTs to only apply to path validation prior to migration,
>> not the
>> peer responding to migration.
>>
>> My reasoning is that if a peer migrates prior to validating the path, it
>> means
>> it's either unintentional or they have no other choice, so the migrating
>> peer
>> has implicitly decided that validating PathMTU is not a prerequisite to
>> migrating.
>>
>>
>> So some quesitons and ideas as I think it is relevant to resolve this as
>> best as
>> possible.
>>
>> So isn't this recreating the issue that if the client initiates a
>> migration to a
>> new path that is not QUIC compatible, by responding with a minimal size
>> packet
>> and completing the migration and then if the server performs the path
>> verification with 1200 bytes UDP payload it fails. Thus maintaining a
>> broken
>> path.
>>
>> So is there need for the non pre-validated path migration case that one
>> need
>> need to do a two step process where one will ACK with minimal packet while
>> initiating path validation. If path validatation fails then maybe one
>> need to
>> close down the connection as the migration ended up on a path that was
>> unable to
>> support QUIC. The question here is how to avoid the DoS attack this may
>> open up
>> if an attack rewrites source address of packets.
>>
>> So Maybe the path validation needs to be a two step process. First a
>> return
>> routability over the new path to verify that it is bi-directional. When
>> that has
>> been verified one does a test with minimal MTU to prove it to be QUIC
>> compatible. This might even be done with application data if there is
>> some that
>> are available to send.
>>
>> But, I think that one needs to work through the criterias for when the
>> QUIC
>> connection is shut down under the conditions that the path available is
>> not
>> supporting 1200 bytes. Also do we end up in a situation where the client
>> needs
>> to do the second step itself towards the server to verify the path so
>> that it
>> can determine if it needs to try another path if this one doesn't work?
>>
>> Cheers
>>
>> Magnus
>>
>>
>> Attachments:
>> * smime.p7s
>>
>>
>>
>>