3.2.1.1.6.2 LastPacketSentTimestamp
The protocol MUST maintain a timestamp indicating the last time a packet was sent on this SendingChannel.
PingTimer
The SendingChannel MUST maintain a timer on expiration indicates a PING PDU must be sent to the receiving channel. The PING PDU is sent to the receiving channel when no data has been sent within half of the time value of the KeepAliveInterval.