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
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