Pragmatic General Multicast
Encyclopedia
Pragmatic General Multicast (PGM) is a reliable multicast
transport protocol. PGM provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file-transfer
.
is a network addressing method for the delivery of information
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
es and routers). However, like the User Datagram Protocol
, multicast does not guarantee the delivery of a message stream. Messages may be dropped, delivered multiple times, or delivered out of order. A reliable multicast
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 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).
PGM is an IETF experimental protocol. It is not yet a standard, but has been implemented in some networking devices and operating system
s, including Windows XP
and later versions of Microsoft Windows
, as well as in third-party libraries for Linux
, Windows and Solaris.
Reliable multicast
A reliable multicast protocol is a computer networking protocol that provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file transfer or streaming media.-Overview:...
transport protocol. PGM provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file-transfer
File transfer
File transfer is a generic term for the act of transmitting files over a computer network or the Internet. There are numerous ways and protocols to transfer files over a network. Computers which provide a file transfer service are often called file servers. Depending on the client's perspective the...
.
Overview
MulticastMulticast
In computer networking, multicast is the delivery of a message or information to a group of destination computers simultaneously in a single transmission from the source creating copies automatically in other network elements, such as routers, only when the topology of the network requires...
is a network addressing method for the delivery of information
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
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
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
es and routers). However, like the User Datagram Protocol
User Datagram Protocol
The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
, multicast does not guarantee the delivery of a message stream. Messages may be dropped, delivered multiple times, or delivered out of order. A reliable multicast
Reliable multicast
A reliable multicast protocol is a computer networking protocol that provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file transfer or streaming media.-Overview:...
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
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
), 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 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).
PGM is an IETF experimental protocol. It is not yet a standard, but has been implemented in some networking devices and operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s, including Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
and later versions of Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
, as well as in third-party libraries for Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, Windows and Solaris.
External links
- RFC 3208
- http://openpgm.googlecode.com/
- http://msdn.microsoft.com/library/default.asp?url=/library/en-us/randz/protocol/pgm_protocol.asp
- http://www.cisco.com/en/US/docs/ios/12_0t/12_0t5/feature/guide/pgmscale.html
- http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_1-2/reliable_multicast.html
- http://www.juniper.net/techpubs/software/junos/junos85/swconfig85-multicast/id-11463816.html#id-11463816