Pragmatic General Multicast (PGM) is a

reliable multicast A reliable multicast is any computer networking protocol that provides a '' reliable'' sequence of packets to multiple recipients simultaneously, making it suitable for applications such as multi-receiver file transfer. Overview Multicast is a net ...

computer network transport protocol. PGM provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file-transfer. ''

Multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused wit ...

'' is a

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

addressing method for the delivery of

information Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random, ...

to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the multiple destinations split (typically

network switch A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A netw ...

es and routers). However, like the

User Datagram Protocol In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages (transported as datagrams in packets) to other hosts on an Internet Protocol (IP) networ ...

, multicast does not guarantee the delivery of a message stream. Messages may be dropped, delivered multiple times, or delivered out of order. A

protocol, like PGM, adds the ability for receivers to detect lost and/or out-of-order messages and take corrective action (similar in principle to TCP), resulting in a gap-free, in-order message stream. While TCP uses ACKs to acknowledge groups of packets sent (something that would be uneconomical over multicast), PGM uses the concept of ''negative acknowledgements'' (NAKs). A NAK is sent

unicast Unicast is data transmission from a single sender (red) to a single receiver (green). Other devices on the network (yellow) do not participate in the communication. In computer networking, unicast is a one-to-one transmission from one point in ...

back to the host via a defined network-layer hop-by-hop procedure whenever there is a detection of data loss of a specific sequence. As PGM is heavily reliant on NAKs for integrity, when a NAK is sent, a ''NAK confirmation'' (NCF) is sent via multicast for every hop back. ''Repair data'' (RDATA) is then sent back either from the source or from a Designated Local Repairer (DLR) at some point closer to the destination. PGM is an

IETF The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and ...

experimental protocol. It is not yet a standard, but has been implemented in some networking devices and

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s, including

Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was release to manufacturing, released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Wind ...

and later versions of Microsoft Windows, as well as in third-party libraries for

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...

, Windows and Solaris.

External links

*https://tools.ietf.org/html/rfc3208 *https://github.com/steve-o/openpgm/ *https://web.archive.org/web/20110111200232/http://www.cisco.com/en/US/docs/ios/12_0t/12_0t5/feature/guide/pgmscale.html Communications protocols {{Compu-network-stub