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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. By removing features such as recovery of lost or corrupted data and preallocation of resources, the network operates more efficiently, and the network nodes are inexpensive.

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 mailman will make his "best effort" to try to deliver a message, but the delivery may be delayed if too many letters all of a sudden arrives to a postal office. The sender is not informed if a letter has been delivered successfully. However, the sender can pay extra for a delivery confirmation receipt, which requires that the carrier get a signature from the recipient to prove the successful delivery.

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 the user is informed 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, but is guaranteed constant bandwidth.

Conventional IP routers only provide best-effort service. The simplicity of routers is a key factor why IP has been much more successful than more complex protocols such as X.25 and ATM. In X.25, data was retransmitted on a node-to-node basis in case of detected errors. An ATM network can offer service categories with guaranteed bandwidth or latency. The Available Bit Rate (ABR) ATM service category gives best effort delivery, without Quality of Service guarantees.

However, the best-effort paradigm is to some extent abandoned on the Internet. Modern IP routers also provide mechanisms for differentiated or guaranteed quality of service to certain data flows, based on for example the IntServ or DiffServ protocols. This may be utilized in networks with capacity problems for the reservation of resources to delay-sensitive services and services that require constant bit rate, for example real time multimedia communication such as IP telephony and IP-TV.

Albeit Internet in its primary form is based on best-effort communication, guaranteed delivery can be provided by higher layer protocols, handled by host computers rather than the network nodes. The TCP transport protocol provides guaranteed services while UDP transport protocol provides best effort delivery. TCP verifies that all information transmitted is received fully on the other end. UDP does its best to deliver packets to the destination, but takes no steps to recover packets that are lost or misdirected. The more stable TCP protocol is often used to deliver data like web pages and email, while UDP is often used for media streaming or network gaming.

Note that TCP does not reserve any resources in advance, and does not provide any guarantees regarding quality of service, for example bit rate. In that sense, it can be considered as best-effort communication. For end-to-end transport layer reservation of resources, the resource reservation protocol (RSVP) may be used.

Although UDP is used, higher layer application protocols may provide guaranteed delivery of messages and files. Examples are protocols for shared disk access, which in the early days were based on UDP.

• Encyclopedia of Networking & Telecommunications ISBN 0-07-212005-3