3.1. Packet Loss

3.1. Packet Loss

DTLS uses a simple retransmission timer to handle packet loss. Figure 1 demonstrates the basic concept, using the first phase of the DTLS handshake:

         Client                                   Server
         ------                                   ------
         ClientHello           ------>

                                 X<-- HelloRetryRequest
                                                  (lost)

         [Timer Expires]

         ClientHello           ------>
         (retransmit)

                Figure 1: DTLS Retransmission Example

Once the client has transmitted the ClientHello message, it expects to see a HelloRetryRequest or a ServerHello from the server. However, if the timer expires, the client knows that either the ClientHello or the response from the server has been lost, which causes the client to retransmit the ClientHello. When the server receives the retransmission, it knows to retransmit its HelloRetryRequest or ServerHello.

The server also maintains a retransmission timer for messages it sends (other than HelloRetryRequest) and retransmits when that timer expires. Not applying retransmissions to the HelloRetryRequest avoids the need to create 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.

For more detail on timeouts and retransmission, see Section 5.8.