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: This document has been reviewed as part of the transport area review team's ongoing effort to review key IETF documents. These comments were written primarily for the transport area directors, but are copied to the document's authors and WG to allow them to address any issues raised and also to the IETF discussion list for information. When done at the time of IETF Last Call, the authors should consider this review as part of the last-call comments they receive. Please always CC EMAIL if you reply to or forward this review. TSV-ART Review of draft-ietf-tls-dtls13-41 Reviewer: Bernard Aboba Summary: The timeout and retransmission scheme looks workable for common cases, but could use some refinement to make it more robust. Technical Comments Handling Invalid Records Unlike TLS, DTLS is resilient in the face of invalid records (e.g., invalid formatting, length, MAC, etc.). In general, invalid records SHOULD be silently discarded, thus preserving the association; however, an error MAY be logged for diagnostic purposes. [BA] How does silent discard of invalid records interact with retransmission timers? Implementations which choose to generate an alert instead, MUST generate error alerts to avoid attacks where the attacker repeatedly probes the implementation to see how it responds to various types of error. Note that if DTLS is run over UDP, then any implementation which does this will be extremely susceptible to denial-of-service (DoS) attacks because UDP forgery is so easy. Thus, this practice is NOT RECOMMENDED for such transports, both to increase the reliability of DTLS service and to avoid the risk of spoofing attacks sending traffic to unrelated third parties. [BA] "this practice" refers to "generate an alert instead", correct? Timer Values Though timer values are the choice of the implementation, mishandling of the timer can lead to serious congestion problems, for example if [BA] Saying "timer values are the choice of the implementation" seems odd, because it is followed by normative language. I would delete this and start the sentence with "Mishandling...". many instances of a DTLS time out early and retransmit too quickly on a congested link.