Ah fun, another issue in this document. So not only are write epoch lifetimes unspecified and complex with 0-RTT, but read epoch lifetimes *are* specified but *wrong*.
Section 4.2.1 says: > Because DTLS records could be reordered, a record from epoch M may be received after epoch N (where N > M) has begun. Implementations SHOULD discard records from earlier epochs but MAY choose to retain keying material from previous epochs for up to the default MSL specified for TCP [RFC0793] to allow for packet reordering. (Note that the intention here is that implementers use the current guidance from the IETF for MSL, as specified in [RFC0793] or successors, not that they attempt to interrogate the MSL that the system TCP stack is using.) https://www.rfc-editor.org/rfc/rfc9147.html#section-4.2.1 First, it's a bit weird to say you SHOULD discard *records* but MAY retain *keying material*. I assume that meant SHOULD discard records but MAY process records anyway up to MSL. Anyway, this model implies that only one read epoch is active at once, but this isn't true. You basically have to read epoch 1 (early data) as unordered relative to epoches 0 and 2. Consider a DTLS 1.3 server: 1. The server reads ClientHello with early_data extension at epoch 0 and accepts early data. 2. The server sends ServerHello (epoch 0), EE..Finished (epoch 2), and activates write epoch 3 for half-RTT application data. 3. The server reads early data (epoch 1) from the client. The RFC would lead you to think the server can close read epoch 0 now, but... 4. ServerHello gets lost and, if we are to believe https://www.rfc-editor.org/rfc/rfc9147.html#section-7.1-8, the client might send an empty plaintext ACK to trigger a retransmit. This ACK will be at epoch 0. This only works if the server keeps read epoch 0 open! 5. Client eventually gets the ServerHello but now it only gets half of the epoch 2 data. It sends an ACK to trigger another retransmit. This ACK will come at epoch 2. 6. Server receives that ACK at epoch 2 and retransmits. The RFC would lead you to think the server can close read epoch 1 now, but... 7. Let's say that retransmit is lost again, or hasn't arrived yet. From the client's perspective, it has a connection that has yet to reach the 1-RTT point, so any data from the calling application will still be sent as early data. That means the client will continue to send early data at epoch 1. This only works if the server keeps read epoch 1 open! 8. The handshake progresses and the server finally gets 1-RTT data at epoch 3 from the client. *Now* the spirit of the rule in the text applies to epoch 1 and the server can close the epoch (after optionally waiting a spell for reordering) So the rule is actually that we close according to a partially ordered set: - 0 (unencrypted) < 2 (handshake) < 3 (first app data) < 4 < 5 < ... - 1 (early data) < 3 (first app data) < 4 < 5 < ... - 1 is not ordered relative to 0 and 2. On Wed, Sep 18, 2024 at 3:47 PM David Benjamin <david...@google.com> wrote: > One more wriggle if we wish to allow unencrypted ACKs, though it is > fixable. Section 7, says: > > > During the handshake, ACK records MUST be sent with an epoch which is > equal to or higher than the record which is being acknowledged. [...] > Implementations SHOULD simply use the highest current sending epoch, which > will generally be the highest available. After the handshake, > implementations MUST use the highest available sending epoch. > > Taken at face value, that text implies that a client sending 0-RTT data > should send its ACKs at the highest current sending epoch, epoch 1 (0-RTT). > But if the server has rejected 0-RTT data, it will not (and cannot) > instantiate epoch 1 at all, so it won't get the ACKs! That guidance needs a > special case: if you would have ACKed at epoch 1, you should ACK at epoch 0 > instead. > > Alternatively, one might interpret that situation as 0 being the sending > epoch and 1 being some magical epoch on the side. This isn't supported by > the document, but honestly no interpretation is supported by the document > because the document never tells you what a "current sending epoch" even > is. While 4.2.1 gives some rough guidance on when to close out receiving > epochs, I could not find any text on send epoch management at all. > Reasoning through the protocol, you might arrive at this *almost* correct > rule: > > A write epoch may be discarded IF: > 1. It is not the highest available epoch. AND > 2. There are no unacked, outgoing messages at that epoch > > That rule, however, does not work in 0-RTT. If the highest epoch is 1, you > cannot discard 0. The server might reject 0-RTT and then send > HelloRetryRequest, at which point you will need to discard epoch 1 and > reactivate epoch 0, maintaining continuity of sequence numbers. The > 0-RTT/1-RTT transition is also interesting on the write side, though I'll > start a separate thread for that. > > All this is subtle enough that it should not be left as an exercise to the > reader. > > David > > On Wed, Sep 18, 2024 at 12:39 AM Bob Beck <b...@obtuse.com> wrote: > >> >> >> > On Sep 17, 2024, at 5:28 PM, David Benjamin <davidben= >> 40google....@dmarc.ietf.org> wrote: >> > >> > Ah, I just noticed this text at the end of Section 7.1: >> > >> > > Note that in some cases it may be necessary to send an ACK which does >> not contain any record numbers. For instance, a client might receive an >> EncryptedExtensions message prior to receiving a ServerHello. Because it >> cannot decrypt the EncryptedExtensions, it cannot safely acknowledge it (as >> it might be damaged). If the client does not send an ACK, the server will >> eventually retransmit its first flight, but this might take far longer than >> the actual round trip time between client and server. Having the client >> send an empty ACK shortcuts this process. >> > >> > https://www.rfc-editor.org/rfc/rfc9147.html#section-7.1-8 >> > >> > I guess then the intent is indeed that if you receive some random >> encrypted DTLS 1.3 header, even though you don't know it's DTLS 1.3 yet, >> you interpret as activating the ACKing mechanism? But that seems to prompt >> more questions than it answers. For instance, what happens if you do that, >> but then finally receive the ServerHello and it turns out this was just >> some junk packet and we're really negotiation DTLS 1.2? Do you check that >> the ACK mechanism has been activated and return an error? Do you just pause >> the ACK mechanism and hope you're in an OK state? This seems quite prune to >> send the implementation into unexpected and untested states. >> > >> > >> >> >> Yeah, I think this has missed a nasty corner case here for >> implementations that support both. >> >> I think I also lean towards option A) (from below) here. Anyone else who >> has gotten at least their hands mildly dirty in a DTLS implementation that >> supports both 1.2 and 1.3 care to chime in as well? >> >> >> > On Thu, Sep 12, 2024 at 4:31 PM David Benjamin <david...@google.com> >> wrote: >> > Hi all, >> > >> > I noticed another issue with the DTLS 1.3 ACK design. :-) >> > >> > So, DTLS 1.3 uses ACKs. DTLS 1.2 does not use ACKs. But you only learn >> what version you're speaking partway through the lifetime of the >> connection, so there are some interesting corner cases to answer. As an >> illustrative example, I believe the diagram in section 6 is [probably] >> incorrect: >> > https://www.rfc-editor.org/rfc/rfc9147.html#section-6 >> > >> > If the client loses the first packet, it never sees the ServerHello and >> thus learns it's speaking DTLS 1.3. While it does see the second packet, >> that packet only contains ciphertext that it cannot decrypt. Unless it >> decides to say "this looks like a 1.3 record header, therefore I will turn >> on the 1.3 state machine", which isn't supported by the RFC (maybe TLS 1.4 >> will use the same record header but redo ACKs once again), it shouldn't >> activate the 1.3 state machine yet. I expect what will actually happen is >> that the client will wait for the retransmission timeout a la DTLS 1.2. >> > >> > More generally, I believe these are the situations to worry about: >> > >> > 1. If a DTLS 1.2 (i.e. does not implement RFC 9147 at all) >> implementation receives an ACK record for whatever reason, what happens? >> This decision we don't get to change. Rather, it is a design constraint. >> Both OpenSSL and BoringSSL treat unexpected record types as a fatal error. >> I haven't checked other implementations. So I think we must take as a >> constraint that you cannot send an ACK unless you know the peer is >> 1.3-capable. >> > >> > 2. Do plaintext ACKs exist? Or is the plaintext epoch permanently at >> the old state machine? Honestly, I wish the answer here was "no". That >> would have avoided so many problems, because then epochs never change state >> machines. Unfortunately, the RFC does not support this interpretation. >> Section 4.1 talks about how to demux a plaintext ACK, and section 6, though >> wrong, clearly depicts a plaintext ACK. So instead we get to worry about >> the transition within an epoch. Keep in mind that transitions happen at >> different times on both sides. Keep in mind that there is a portion of the >> plaintext epoch that lasts after version negotiation in HelloRetryRequest >> handshakes. >> > >> > 3. If a 1.3-capable server receives half of a ClientHello, does it send >> an ACK? I believe (1) means the answer must be "no". If you haven't read >> the ClientHello, you haven't selected the version, so you don't know if the >> client is 1.3-capable or not. If the client is not 1.3-capable, sending an >> ACK may be incompatible. >> > >> > 4. Is it possible for a 1.3-capable client to receive an ACK before it >> receives a ServerHello? If so, how does the client respond? I believe the >> answer to this question, if plaintext ACKs exist, is unavoidably "yes". >> Suppose the server receives a 1.3 ClientHello and then negotiates DTLS 1.3. >> That is a complete flight, so Section 7.1 discourages ACKing explicitly >> (you can ACK implicitly), but it does not forbid an explicit ACK. An >> explicit ACK may be sent if the server cannot generate its responding >> flight immediately. That means a server could well send ACK followed by >> ServerHello. Now suppose ServerHello is lost but the ACK gets through. Now >> the client must decide what it's doing. Rejecting the ACK would result in >> connection failure, so we must either drop the ACK on the floor, or process >> it. While processing it would be more efficient (you don't need to >> retransmit the whole ClientHello), it means the plaintext epoch must >> support this hybrid state where 1.3 ACKs are processed but never sent! Or >> perhaps receiving that ACK transitions you to the 1.3 state machine even >> though you don't know the version yet. That all sounds like a mess, so I >> would advocate you simply drop it on the floor. >> > >> > 5. If a 1.3-capable client receives half of the server's first message >> (HRR or ServerHello), does it send an ACK? Again, because of (1), I believe >> the answer must be "no". If you don't know the server's selected version, >> the server may not be 1.3-capable and may not be compatible with the ACK. >> > >> > 6. What does a 1.3-capable server do if it receives an ACK prior to >> picking the TLS version? Unlike (4), I believe this is impossible. If the >> client has something to ACK, the server must have sent something, which the >> server will only do once it's received the full ClientHello and thus picked >> the version. However, given (4), I suspect an implementation will naturally >> just drop that ACK. In this state error vs drop is kinda academic. >> > >> > From what I can tell, RFC 9147 is silent on all of this. I think it >> should say something. I believe these are the plausible options: >> > >> > OPTION A -- There are no ACKs in epoch 0. >> > >> > We avoid this ridiculous transition point and say that ACKs only exist >> starting epoch 1. Epoch 0 uses the old DTLS 1.2 state machine. This is very >> attractive from a simplicity perspective, but since RFC 9147 was already >> published with this ambiguity, I think we need to, at minimum, say that >> DTLS 1.3 implementations drop epoch 0 ACKs on the floor. It also means that >> packet loss in HelloRetryRequest flows may be less efficient. That said, if >> your HelloRetryRequest is stateless (not applicable to all DTLS uses), >> you're probably not doing anything with ACKs anyway. Saying those ACKs >> avoids having to think about that case, at the cost of a worse transport >> for stateful HelloRetryRequest. >> > >> > OPTION B -- Epoch 0 enables ACKing once the version is learned. >> > >> > Once you know the version, you start sending and processing ACKs. >> Before you know the version, you drop ACKs on the floor and never send >> them. This requires convincing ourselves that the transition point works >> out, notably when one side is still ACK-less and the other side is still >> ACK-ful, but I believe it works out. >> > >> > OPTION C -- Epoch 0 always receives and acts on ACKs, but it doesn't >> send ACKs until the version is learned. >> > >> > This is the same as above, but instead of dropping ACKs, you go ahead >> and let that drive your state machine. But you don't send them. This makes >> reasoning about the protocol even more complicated because there are even >> more states you can be in w.r.t. your known version vs the state of your >> transport. It does improve behavior around packet loss, but I think it only >> helps this edge case in question (4) above, which is already a case where >> servers aren't expected to send ACKs anyway. >> > >> > I think I lean towards Option A for simplicity, even though it >> decidedly contradicts a lot of text in the RFC right now. That will be hard >> to encode in an erratum as a few things need to change. But I also have 7 >> other eratta open against this document, so maybe it's time for rfc9147bis. >> > >> > David >> > _______________________________________________ >> > TLS mailing list -- tls@ietf.org >> > To unsubscribe send an email to tls-le...@ietf.org >> >>
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