The HbAfterRto SCTP agent extends the current retransmission policy. In addition to SCTP's current policy of retransmitting to an alternate destination on a timeout, a heartbeat is sent immediately to the destination on which a timeout occurred. Extra heartbeats provide a mechanism for a sender to update an alternate destination's RTT estimate more frequently, thus resulting in a better RTT estimate on which to base the RTO value.
For example, suppose a packet is lost in transit to the primary destination, and later gets retransmitted to an alternate destination. Also suppose that the retransmission times out. The lost packet is retransmitted again to yet another alternate destination (if one exists; otherwise, the primary). More importantly, a heartbeat is also sent to the alternate destination which timed out. If the heartbeat is successfully acked, that destination acquires an additional RTT measurement to help reduce its recently doubled RTO .