Editing End-to-end principle (section)

===File transfer===
An example of the end-to-end principle is that of an arbitrarily reliable file transfer between two endpoints in a distributed network of a varying, nontrivial size:<ref name="SRC1984" />  The only way two endpoints can obtain a completely reliable transfer is by transmitting and acknowledging a [[checksum]] for the entire data stream; in such a setting, lesser checksum and acknowledgment ([[acknowledgement (data networks)|ACK]]/NACK) protocols are justified only for the purpose of optimizing performance{{snd}}they are useful to the vast majority of clients, but are not enough to fulfill the reliability requirement of this particular application. A thorough checksum is hence best done at the endpoints, and the network maintains a relatively low level of complexity and reasonable performance for all clients.<ref name="SRC1984" />