Code change:
Implementations MUST either abandon an association or re-key prior to allowing the sequence number to wrap. <del> Implementations MUST NOT allow the epoch to wrap, but instead MUST establish a new association, terminating the old association. </del> 4.2.2. When receiving protected DTLS records, the recipient does not have a full epoch or sequence number value in the record and so there is <del> some opportunity for ambiguity.  Because the full epoch and sequence number are used to compute the per-record nonce, failure to reconstruct these values leads to failure to deprotect the record, and so implementations MAY use a mechanism of their choice to determine the full values.  This section provides an algorithm which is comparatively simple and which implementations are RECOMMENDED to follow. </del> <ins> some opportunity for ambiguity.  Because the full sequence number is used to compute the per-record nonce and the epoch determines the keys, failure to reconstruct these values leads to failure to deprotect the record, and so implementations MAY use a mechanism of their choice to determine the full values.  This section provides an algorithm which is comparatively simple and which implementations are RECOMMENDED to follow. </ins> If the epoch bits match those of the current epoch, then implementations SHOULD reconstruct the sequence number by computing

Related discussion:
text: Yet another attempt to This builds on Chris's PR but just omits the epoch entirely from the AEAD nonce calculation. This is more consistent with TLS and doesn't require special case reasoning about why we only need to build in the bottom 16 bits. This relies entirely on the keys being different. Needs analysis.
This seems promising.
NAME this LGTM, but there's a conflict. Can you resolve prior to merging? NAME are you OK with the new rationale, and text that allows it to be relaxed in the future?
If I have understood and calculated correctly DTLS 1.3 limits the number of packets in a AES-GCM connection to 2^40.5 compared to 2^64 in DTLS 1.2. This is quite a severe limitation that is not mentioned explicitly. The AEAD limits in themselves are not a problem, but the combination of AEAD limits (2^24.5) and a small epoch (2^16) is a problem. I think this is major problem in some use cases of DTLS. E.g. most 3GPP 5G use cases where DTLS or DTLS/SCTP is used for semi-permanent connections (lasting years) and where the terminating the connection cause severe disturbances. I did not find any discussion about this problem (but I could easily have missed it) It is known that the limit of 2^48 packets in SRTP can be reached and 2^40.5 is significantly smaller than this... If the document was not in such a late stage, I would have suggested to increase the size of the epoch to 32 bits..... I think this and the other problems DTLS 1.3 has with semi-permanent connections has to be solved sooner than later.
NAME thanks for filing this. I hear you about the late stage, but IIRC we never represent the entire epoch on the wire in any case, so it seems like just changing it to 32 bits would be pretty easy, so why not do it now?
How do the chacha20 numbers compare to the AES-GCM ones?