Code change:
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 <del> (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. </del> <ins> (DoS) attacks because UDP forgery is so easy.  Thus, the practice of generating fatal alerts 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. </ins> If DTLS is being carried over a transport that is resistant to forgery (e.g., SCTP with SCTP-AUTH), then it is safer to send alerts

Related discussion:
text: In order to avoid IP fragmentation, clients of the DTLS record layer SHOULD attempt to size records so that they fit within any PMTU estimates obtained from the record layer. [BA] You might reference PMTU considerations described in Section 4.4. Post-handshake client authentication Messages of each category can be sent independently, and reliability is established via independent state machines each of which behaves as described in Section 5.8.1. For example, if a server sends a NewSessionTicket and a CertificateRequest message, two independent state machines will be created. As explained in the corresponding sections, sending multiple instances of messages of a given category without having completed earlier transmissions is allowed for some categories, but not for others. Specifically, a server MAY send multiple NewSessionTicket messages at once without awaiting ACKs for earlier NewSessionTicket first. Likewise, a server MAY send multiple CertificateRequest messages at once without having completed earlier client authentication requests before. In contrast, implementations MUST NOT have send KeyUpdate, NewConnectionId or RequestConnectionId [BA] "send" -> "sent" Example of Handshake with Timeout and Retransmission The following is an example of a handshake with lost packets and retransmissions. Note that the client sends an empty ACK message because it can only acknowledge Record 1 sent by the server once it has processed messages in Record 0 needed to establish epoch 2 keys, which are needed to encrypt to decrypt messages found in Record 1. [BA] "encrypt to decrypt" -> "encrypt or decrypt"? Section 7.3 In the first case the use of the ACK message is optional because the 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 flow shown in Figure 11 if the client does not send the ACK message [BA] Figure 11 is the DTLS State Machine. Are you referring to another figure? 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 indefinitely until its transmission cap is reached.