Many user applications (and the transport protocols that make them possible) require two-way communications. To assess this capability@ and to achieve test system simplicity@ round-trip loss measurements are frequently conducted in practice. The Two-Way Active Measurement Protocol specified in RFC 5357 establishes a round-trip loss measurement capability for the Internet. However@ there is currently no round-trip packet loss metric specified according to the RFC 2330 framework. This memo adds round-trip loss to the set of IP Performance Metrics (IPPM).