In computer networking, a reliable protocol is a communication protocol that notifies the sender whether or not the delivery of data to intended recipients was successful. Reliability is a synonym for assurance, which is the term used by the ITU and ATM Forum, and leads to fault-tolerant messaging. Reliable protocols typically incur more overhead than unreliable protocols, and as a result, function more slowly and with less scalability. This often is not an issue for unicast protocols, but it may become a problem for reliable multicast protocols. Transmission Control Protocol (TCP), the main protocol used on the Internet, is a reliable unicast protocol; it provides the abstraction of a reliable byte stream to applications. UDP is an unreliable protocol and is often used in computer games, streaming media or in other situations where speed is an issue and some data loss may be tolerated because of the transitory nature of the data. Often, a reliable unicast protocol is also connection oriented. For example, TCP is connection oriented, with the virtual-circuit ID consisting of source and destination IP addresses and port numbers. However, some unreliable protocols are connection oriented, such as Asynchronous Transfer Mode and Frame Relay. In addition, some connectionless protocols, such as IEEE 802.11, are reliable.
History Building on the packet switching concepts proposed by Donald Davies, the first communication protocol on the ARPANET was a reliable packet delivery procedure to connect its hosts via the 1822 interface. A host computer simply arranged the data in the correct packet format, inserted the address of the destination host computer, and sent the message across the interface to its connected Interface Message Processor (IMP). Once the message was delivered to the destination host, an acknowledgment was delivered to the sending host. If the network could not deliver the message, the IMP would send an error message back to the sending host. Meanwhile, the developers of CYCLADES and of ALOHAnet demonstrated that it was possible to build an effective computer network without providing reliable packet transmission. This lesson was later embraced by the designers of Ethernet. If a network does not guarantee packet delivery, then it becomes the host's responsibility to provide reliability by detecting and retransmitting lost packets. Subsequent experience on the ARPANET indicated that the network itself could not reliably detect all packet delivery failures, and this pushed responsibility for error detection onto the sending host in any case. This led to the development of the end-to-end principle, which is one of the Internet's fundamental design principles.
Reliability properties A reliable service is one that notifies the user if delivery fails, while an unreliable one does not notify the user if delivery fails. For example, Internet Protocol (IP) provides an unreliable service. Together, Transmission Control Protocol (TCP) and IP provide a reliable service, whereas User Datagram Protocol (UDP) and IP provide an unreliable one. 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 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. Reliable messaging is the concept of message passing across an unreliable infrastructure whilst being able to make certain guarantees about the successful transmission of the messages. For example, that if the message is delivered, it is delivered at most once, or that all messages successfully delivered arrive in a particular order. Reliable delivery can be contrasted with best-effort delivery, where there is no guarantee that messages will be delivered quickly, in order, or at all.
Implementations A reliable delivery protocol can be built on an unreliable protocol. An extremely common example is the layering of Transmission Control Protocol on the Internet Protocol, a combination known as TCP/IP. Strong reliability properties are offered by group communication systems (GCSs) such as IS-IS, Appia framework, 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. One protocol that implements reliable messaging is WS-ReliableMessaging, which handles reliable delivery of SOAP messages. The ATM Service-Specific Coordination Function provides for transparent assured delivery with AAL5. IEEE 802.11 attempts to provide reliable service for all traffic. The sending station will resend a frame if it does not receive an ACK frame within a predetermined period of time.
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