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 <[email protected]> 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 ( > [email protected]) 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 <[email protected]> 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 > > > >
