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 <[email protected]> wrote: > > > On Sat, Oct 31, 2020 at 4:41 PM Martin Duke <[email protected]> > 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= >> [email protected]> 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= >>> [email protected]> 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 <[email protected]> >>>> 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 :) >>>> >>>>
