Connectionless communication, often referred to as CL-mode communication,Information Processing Systems - Open Systems Interconnection, "Transport Service Definition - Addendum 1: Connectionless-mode Transmission", International Organization for Standardization, International Standard 8072/AD 6, December 1986. is a data transmission method used in

packet switching In telecommunications, packet switching is a method of grouping data into '' packets'' that are transmitted over a digital network. Packets are made of a header and a payload. Data in the header is used by networking hardware to direct the p ...

networks in which each data unit is individually addressed and routed based on information carried in each unit, rather than in the setup information of a prearranged, fixed data channel as in

connection-oriented communication Connection-oriented communication is a network communication mode in telecommunications and computer networking, where a communication session or a semi-permanent connection is established before any useful data can be transferred. The establishe ...

Attributes

Under connectionless communication between two network end points, a message can be sent from one end point to another without prior arrangement. The device at one end of the communication transmits data addressed to the other, without first ensuring that the recipient is available and ready to receive the data. Some protocols allow for error correction by requesting retransmission.

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. ...

(IP) and

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) network ...

(UDP) are connectionless protocols. A packet transmitted in a connectionless mode is frequently called a

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The del ...

. Connectionless protocols are usually described as

stateless protocol A stateless protocol is a communication protocol in which the receiver must not retain session state from previous requests. The sender transfers relevant session state to the receiver in such a way that every request can be understood in isolatio ...

s because the end points have no protocol-defined way to remember where they are in a "conversation" of message exchanges. Connectionless communication has lower overhead than

because in

the communicating peers must first establish a logical or physical data channel or ''connection'' in a dialog preceding the exchange of user data. Connectionless communication allows for

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 with ...

and broadcast operations in which the same data are transmitted to several recipients in a single transmission. In connectionless transmissions the service provider usually cannot guarantee that there will be no loss,

error An error (from the Latin ''error'', meaning "wandering") is an action which is inaccurate or incorrect. In some usages, an error is synonymous with a mistake. The etymology derives from the Latin term 'errare', meaning 'to stray'. In statistics ...

insertion, misdelivery, duplication, or out-of-

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is calle ...

delivery of the packet. However, the effect of errors may be reduced by implementing error correction within an application protocol. In connectionless mode there is less opportunity for optimization possible when sending several data units between the same two peers. By establishing a connection at the beginning of such a data exchange the components ( routers, bridges) along the network path would be able to pre-compute (and hence

cache Cache, caching, or caché may refer to: Places United States * Cache, Idaho, an unincorporated community * Cache, Illinois, an unincorporated community * Cache, Oklahoma, a city in Comanche County * Cache, Utah, Cache County, Utah * Cache County ...

) routing-related information, avoiding re-computation for every packet. In connection-oriented communication, network components can also reserve capacity for the transfer of the subsequent data units of a video download, for example.

Architecture and implementations

Distinction between connectionless and connection-oriented transmission may take place at several layers of the OSI Reference Model: *

Transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end ...

: TCP is a connection-oriented transport protocol. UDP is connectionless. *

Network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of tran ...

. *

Data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...

: The

IEEE 802.2 IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. The original standard developed by the Institute of Electrical and Electronics En ...

protocol at the

Logical Link Control In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface ...

sublayer of the data link layer may provide both connectionless and connection-oriented services. In fact, some

network protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

s (such as SNA's Path Control in its early stages) require a connection-oriented data link layer. Others (like IP) do not. (After the appearance of APPN, SNA could operate on a connectionless data link service as well.) Notable connectionless protocols are:

(IP),

(UDP), Internet Control Message Protocol (ICMP), Internetwork Packet Exchange (IPX), Transparent Inter-process Communication,

NetBIOS NetBIOS () is an acronym for Network Basic Input/Output System. It provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network. As strictly an API, Ne ...

, and Fast and Secure Protocol (FASP).

References

*{{FS1037C Network protocols Computer networking Internet architecture Internet protocols