Reliability (computer networking)

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In computer networking, a reliable protocol 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. The term "reliable" is a synonym for assured, which is the term used by the ITU and ATM Forum in the context of the ATM Service-Specific Coordination Function, for example for transparent assured delivery with AAL5.[1][2][3]

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.

TCP, the main protocol used on the Internet, is a reliable unicast protocol. UDP, often used in computer games or in 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, TCP is connection-oriented, with the virtual-circuit ID consisting of source and destination IP addresses and port numbers. Some unreliable protocols are connection-oriented as well. These include ATM and frame relay. There are also reliable connectionless protocols, such as AX.25 when it passes data in I-frames. But this combination occurs rarely: reliable-connectionless is uncommon in commercial and academic networks.


When the ARPANET pioneered packet switching, it provided a reliable packet delivery procedure to its connected hosts via its 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. Once the message was delivered to the destination host, an acknowledgement was delivered to the sending host. If the network could not deliver the message, it 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 assumptions.

Reliability properties[edit]

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, IP provides an unreliable service. Together, TCP and IP provide a reliable service, whereas UDP and IP provide an unreliable one. All these protocols use packets, but UDP packets are generally called datagrams.[4]

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.

Strong reliability properties are offered by group communication systems (GCS) such as IS-IS, 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.


  1. ^ Young-ki Hwang, et al., Service Specific Coordination Function for Transparent Assured Delivery with AAL5 (SSCF-TADAS), Military Communications Conference Proceedings, 1999. MILCOM 1999, vol.2, pages 878 - 882, DOI: 10.1109/MILCOM.1999.821329
  2. ^ ATM Forum, The User Network Interface (UNI), v. 3.1, ISBN 0-13-393828-X, Prentice Hall PTR, 1995.
  3. ^ ITU-T, B-ISDN ATM Adaptation Layer specification: Type 5 AAL, Recommendation I.363.5, International Telecommunication Union, 1998.
  4. ^ Kurose, James F. & Ross, Keith W. (2007), "Computer Networking: A Top-Down Approach" ISBN 0-321-49770-8