Code change:
peer will retransmit in any case and therefore the ACK just allows for selective retransmission, as opposed to the whole flight retransmission in previous versions of DTLS.  For instance in the <del> flow shown in Figure 11 if the client does not send the ACK message when it received record 1 indicating loss of record 0, the entire flight would be retransmitted.  When DTLS 1.3 is used in deployments with lossy networks, such as low-power, long range radio networks as well as low-power mesh networks, the use of ACKs is recommended. </del> <ins> flow shown in dtls-key-update if the client does not send the ACK message when it received record 1 indicating loss of record 0, the entire flight would be retransmitted.  When DTLS 1.3 is used in deployments with lossy networks, such as low-power, long range radio networks as well as low-power mesh networks, the use of ACKs is recommended. </ins> The use of the ACK for the second case is mandatory for the proper functioning of the protocol.  For instance, the ACK message sent by the client in Figure 13, acknowledges receipt and processing of record 4 (containing the NewSessionTicket message) and if it is not sent the server will continue retransmission of the NewSessionTicket <del> indefinitely until its transmission cap is reached. </del> <ins> indefinitely until its maximum retransmission timemout value is reached. </ins> 8.

Related discussion:
text: However, a DTLS implementation which would ordinarily issue an alert SHOULD generate a new alert message if the offending record is received again (e.g., as a retransmitted handshake message). Implementations SHOULD detect when a peer is persistently sending bad messages and terminate the local connection state after such misbehavior is detected. Note that alerts are not reliably transmitted; implementation SHOULD NOT depend on receiving alerts in order to signal errors or connection closure. [BA] For the fatal alert case, it does seem like retransmission would be a good idea; otherwise the peer can be left hanging. Section 7.1 "Disruptions" such as reordering do not affect timers, correct? ACKs SHOULD NOT be sent for these flights unless generating the responding flight takes significant time. What is "significant time"? Editorial Comments (NITs) Section 2 The reader is also as to be familiar with [BA] "as" -> "assumed" Section 3 The basic design philosophy of DTLS is to construct "TLS over datagram transport". Datagram transport does not require nor provide reliable or in-order delivery of data. The DTLS protocol preserves this property for application data. Applications such as media streaming, Internet telephony, and online gaming use datagram transport for communication due to the delay-sensitive nature of transported data. The behavior of such applications is unchanged when the DTLS protocol is used to secure communication, since the DTLS protocol does not compensate for lost or reordered data traffic. [BA] While low-latency streaming and gaming does use DTLS to protect data (e.g. for protection of WebRTC data channel), telephony and RTC Audio/Video uses DTLS/SRTP for key derivation only, and SRTP for protection of data. So you might want to make a distinction. Section 3.1 Note that timeout and retransmission do not apply to the HelloRetryRequest since this would require creating state on the server. The HelloRetryRequest is designed to be small enough that it will not itself be fragmented, thus avoiding concerns about interleaving multiple HelloRetryRequests. [BA] I would add "For more detail on timeouts and retransmission, see Section 5.8." Transport Layer Mapping DTLS messages MAY be fragmented into multiple DTLS records. Each DTLS record MUST fit within a single datagram.