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Best-effort delivery describes a network service in which the network does not provide any guarantees that data is delivered or that a user is given a guaranteed quality of service level or a certain priority. In a best-effort network all users obtain best-effort service, meaning that they obtain unspecified variable bit rate and delivery time, depending on the current traffic load. It can be contrasted with reliable delivery, which can be built on top of best-effort delivery (possibly without latency and throughput guarantees), or with circuit switching schemes which maintain a defined, continuous quality of service.
The postal service delivers letters using a best-effort delivery approach. The delivery of a certain letter is not scheduled in advance – no resources are preallocated in the post office. The service will make their "best effort" to try to deliver a message, but the delivery may be delayed if too many letters arrive to a postal office suddenly. The sender is not informed if a letter has been delivered successfully.
Conventional telephone networks are not based on best-effort communication, but on circuit switching. During the connection phase of a new call, resources are reserved in the telephone exchanges, or a busy signal informs the user that the call is blocked due to lack of free capacity. An ongoing phone call can never be interrupted due to overloading of the network, and is guaranteed constant bandwidth.
The Internet protocol offers a best-effort service of delivering datagrams between hosts. IPv4 is a connectionless internet protocol that depends on the best-effort delivery approach. IPv4 datagrams may be lost, arbitrarily delayed, corrupted, or duplicated. The applications built on top of it implement the additional services they require on an end-to-end basis. Transmission control protocol (TCP) provides a guaranteed delivery of an octet stream between a pair of hosts to the above layer, internally splitting the stream into packets and resending these when lost or corrupted. User datagram protocol (UDP) provides a thinner abstraction layer which only error-checks the datagrams. Both are transport-layer protocols and provide multiplexing between processes on the same host implemented with port numbers.