In this paper we describe a passive technique for round trip time (RTT) estimation called Synthetic Packet-Pairs (SPP). Regular and frequent measurement of round trip time (RTT) between points on the Internet is becoming increasingly important for a range of highly interactive real-time applications. Active probing techniques are possible but problematic. The extra packet traffic imposed by active probes along a network path can modify the behaviour of the network under test. In addition, estimated RTT results may be misleading if the network handles active probe packets differently to regular IP packets. In contrast, SPP provides frequently updated RTT estimates using IP traffic already present in the network. SPP estimates the RTT between two measurement points without requiring precise time synchronisation between each point. SPP accurately estimates the RTT experienced by any application’s traffic without needing modifications to the application itself or the routers along the path. In addition, SPP works with applications that do not exhibit symmetric client-server packet exchanges (such as many online multiplayer games) and applications generating IP multicast traffic. Given the popularity of 802.11 Wireless LANs, and their sensitivity to the load imposed by active probing schemes, we experimentally demonstrate the advantages of SPP in a small 802.11b test bed.