Reliability (computer networking)
Encyclopedia
In computer networking, a reliable protocol is one that provides reliability properties with respect to the delivery of data
to the intended recipient(s), as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.
A reliable multicast
protocol may ensure reliability on a per-recipient basis, as well as provide properties that relate the delivery of data to different recipients, such as e.g. total order
, atomicity
, or virtual synchrony
.
Reliable protocols typically incur more overhead than unreliable protocols, and as a result, are slower and less scalable. This often is not an issue for unicast
protocols, but it may be a problem for multicast
protocols.
TCP
, the main protocol used in the Internet
today, is a reliable unicast protocol.
UDP
, often used in computer games
or other situations where speed is an issue and the loss of a little data is not as important because of the transitory nature of the data, is an unreliable protocol.
Often, a reliable unicast protocol is also connection-oriented
. For example, the TCP/IP protocol is connection-oriented, with the virtual circuit
ID consisting of source and destination IP address
es and port numbers. Some unreliable protocols are connection-oriented as well. These include ATM
and Frame Relay
, on which a substantial part of all Internet traffic is passed.
An example of a reliability property for a unicast
protocol is "at least once", i.e.. at least one copy of the message is guaranteed to be delivered to the recipient.
Reliability properties for multicast
protocols can be expressed on a per-recipient basis (simple reliability properties), or they may relate the fact of delivery or the order of delivery among the different recipients (strong reliability properties).
In the context of multicast protocols, strong reliability properties express the guarantees that the protocol provides with respect to the delivery of messages to different recipients.
An example of a strong reliability property is last copy recall, meaning that as long as at least a single copy of a message remains available at any of the recipients, every other recipient that does not fail eventually also receives a copy. Strong reliability properties such as this one typically require that messages are retransmitted or forwarded among the recipients.
An example of a reliability property stronger than last copy recall is atomicity
. The property states that if at least a single copy of a message has been delivered to a recipient, all other recipients will eventually receive a copy of the message. In other words, each message is always delivered to either all or none of the recipients.
One of the most complex strong reliability properties is virtual synchrony
.
Strong reliability properties are offered by group communication system
s (GCS) such as ISIS
, Appia framework
, Spread
, JGroups
or QuickSilver Scalable Multicast. The QuickSilver Properties Framework is a flexible platform that allows strong reliability properties to be expressed in a purely declarative manner, using a simple rule-based language, and automatically translated into a hierarchical protocol.
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
to the intended recipient(s), as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.
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 may ensure reliability on a per-recipient basis, as well as provide properties that relate the delivery of data to different recipients, such as e.g. total order
Total order
In set theory, a total order, linear order, simple order, or ordering is a binary relation on some set X. The relation is transitive, antisymmetric, and total...
, atomicity
Atomicity
In database systems, atomicity is one of the ACID transaction properties. In an atomic transaction, a series of database operations either all occur, or nothing occurs...
, or virtual synchrony
Virtual synchrony
Virtual synchrony is an interprocess messaging passing technology. Virtual synchrony systems allow programs running in a network to organize themselves into process groups, and to send messages to groups...
.
Reliable protocols typically incur more overhead than unreliable protocols, and as a result, are slower and less scalable. This often is not an issue for unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
protocols, but it may be a problem for multicast
Multicast
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...
protocols.
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...
, the main protocol used in the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
today, is a reliable unicast protocol.
UDP
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...
, often used in computer games
Computer Games
"Computer Games" is a single by New Zealand group, Mi-Sex released in 1979 in Australia and New Zealand and in 1981 throughout Europe. It was the single that launched the band, and was hugely popular, particularly in Australia and New Zealand...
or other situations where speed is an issue and the loss of a little data is not as important because of the transitory nature of the data, is an unreliable protocol.
Often, a reliable unicast protocol is also connection-oriented
Connection-oriented
Connection-oriented communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or...
. For example, the TCP/IP protocol is connection-oriented, with the virtual circuit
Virtual circuit
In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...
ID consisting of source and destination IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
es and port numbers. Some unreliable protocols are connection-oriented as well. These include ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
and Frame Relay
Frame relay
Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
, on which a substantial part of all Internet traffic is passed.
Reliability properties
In the context of distributed protocols, reliability properties specify the guarantees that the protocol provides with respect to the delivery of messages to the intended recipient(s).An example of a reliability property for a unicast
Unicast
right|200pxIn computer networking, unicast transmission is the sending of messages to a single network destination identified by a unique address.-Addressing methodologies:...
protocol is "at least once", i.e.. at least one copy of the message is guaranteed to be delivered to the recipient.
Reliability properties for multicast
Multicast
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...
protocols can be expressed on a per-recipient basis (simple reliability properties), or they may relate the fact of delivery or the order of delivery among the different recipients (strong reliability properties).
In the context of multicast protocols, strong reliability properties express the guarantees that the protocol provides with respect to the delivery of messages to different recipients.
An example of a strong reliability property is last copy recall, meaning that as long as at least a single copy of a message remains available at any of the recipients, every other recipient that does not fail eventually also receives a copy. Strong reliability properties such as this one typically require that messages are retransmitted or forwarded among the recipients.
An example of a reliability property stronger than last copy recall is atomicity
Atomicity
In database systems, atomicity is one of the ACID transaction properties. In an atomic transaction, a series of database operations either all occur, or nothing occurs...
. The property states that if at least a single copy of a message has been delivered to a recipient, all other recipients will eventually receive a copy of the message. In other words, each message is always delivered to either all or none of the recipients.
One of the most complex strong reliability properties is virtual synchrony
Virtual synchrony
Virtual synchrony is an interprocess messaging passing technology. Virtual synchrony systems allow programs running in a network to organize themselves into process groups, and to send messages to groups...
.
Strong reliability properties are offered by group communication system
Group communication system
The term Group Communication System refers to a software platform that implements some form of group communication. Examples of group communication systems include IS-IS, JGroups, Spread Toolkit, Appia framework, QuickSilver, and the group services component of IBM's RSCT...
s (GCS) such as ISIS
ISIS
ISIS is an industry standard interface for image scanning technologies, developed by Pixel Translations in 1990 ....
, Appia framework
Appia framework
Appia is an open source layered communication toolkit implemented in Java, and licensed under the Apache License, version 2.0. It was born in the University of Lisbon, Portugal, by the DIALNP research group that is hosted in the LaSIGE research unit...
, Spread
Spread
Spread may refer to:*Statistical dispersion*Spread , an edible paste put on other foods*the score difference being wagered on in spread betting*the measure of line inclination in rational trigonometry...
, JGroups
JGroups
JGroups is a reliable multicast system that's written in the Java language.JGroups adds a "grouping" layer over a transport protocol, internally keeping a list of participants...
or QuickSilver Scalable Multicast. The QuickSilver Properties Framework is a flexible platform that allows strong reliability properties to be expressed in a purely declarative manner, using a simple rule-based language, and automatically translated into a hierarchical protocol.