3.2.1.2.1 PingTimer
If the SendingChannel is part of a Virtual Connection in the Outbound Proxy or Client roles, the SendingChannel maintains a PingTimer that 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 value of the KeepAliveInterval.
The PingTimer expiration time is set to half of the value of the KeepAlive Interval. When the PingTimer expires, the protocol MUST determine if the time since the LastPacketSentTimestamp is greater than half of the value of the KeepAliveInterval. If so, the protoclol MUST send a PING PDU to the receiving channel.