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Pragmatic general multicast (PGM) is a reliable multicast transport protocol that runs over a best effort datagram service, such as IP multicast. PGM obtains scalability via hierarchy, forward error correction, NAK elimination, and NAK suppression. It employs a novel polling scheme for NAK delay tuning to facilitate scaling up and down. This article describes the architecture of PGM, and discusses performance and security issues. We show that PGM supports asymmetric networks, achieves high network utilization, and is capable of high-speed (> 100 Mb/s) operation. PGM is currently an IETF experimental RFC that has been implemented in both commercial and academic settings.