RTT is the time taken by the icmp pkt to travel from src to dst & then back to src. Though I have never come across any specific values that has been defined as good, slow or bad connectivity. But from experience I can say that for LAN it is <1ms, for longer distances over ethernet <10ms, for WAN/serial links it is 30-60ms, for MPLS networks it is ~300ms. FOr internet, <350ms is a good value.Pls note that these are neither the ideal values nor are defined in any books.
also note that at times ISP has a SLA with customer based on the RTT values. for example, over MPLS a customer may have contracted an SLA of 500ms with ISP, so it may vary based on the ISP n/w, congestion on the link, errors, distance, type of media etc.
hope that clarifies.
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