Re: Back to work

[Mikkel Fahnøe Jørgensen <[e-mail]>]

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