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Best effort delivery describes a network service in which the does not provide any guarantees that data is deliveried or that a user achieves a certain quality of service or priority. 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 about 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. 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 bitrate, for example realtime multimedia communicaiton 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 reserved for media streaming or network gaming. Allthough UDP is used, higher layer application protocols may provide guaranteed delivery of messages and files. Examples are protocols for shared disk access.
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