"Introduction This memo defines a metric for round-trip delay of packets across Internet paths. It builds on notions introduced and discussed in the IPPM Framework document@ RFC 2330 [1]@ and follows closely the corresponding metric for One-way Delay (""A One-way Delay Metric for IPPM"") [2]; the reader is assumed to be familiar with those documents. The memo was largely written by copying material from the One-way Delay metric. The intention is that@ where the two metrics are similar@ they will be described with similar or identical text@ and that where the two metrics differ@ new or modified text will be used. This memo is intended to be parallel in structure to a future companion document for Packet Loss."