The CAPWAP protocol operates as a reliable transport. For each request message, a response message is defined, which acknowledges receipt of the request message. In addition, the control header Sequence Number field is used, which is an identifier value used to match request and response packets. When a CAPWAP packet with a Request Message Type Value is received, the value of the Sequence Number field is copied into the corresponding response message.
Response messages are not explicitly acknowledged; therefore, if a response message is not received, the original request message is retransmitted until "MaxRetransmit" has been reached. After this level has been reached, the link layer considers the peer dead and the AP goes to CAPWAP State DTLS Teardown, which means that the AP needs to reestablish the DTLS session from the beginning. Also refer to the CAPWAP protocol state machine, shown in Figure 4-5.
The CAPWAP data channel uses a Keep-Alive packet to bind the CAPWAP control channel with the data channel and to maintain freshness of the data channel, ensuring that the channel is still functioning.
The CAPWAP data channel Keep-Alive packet is transmitted by the AP when the DataChannelKeepAlive timer expires. (The default is 30 seconds.)
The AP retransmits the CAPWAP data channel Keep-Alive packet in the same manner as the CAPWAP control messages. If the DataChannelDeadlnterval timer expires, the AP tears down the control DTLS session and the data DTLS session if one existed.
