摘要: Just about all communication systems attempt to ensure that the data gets to the other end of the link without errors. Since it is impossible to build an error-free physical layer (although some short links can get very close to e... 展开 Just about all communication systems attempt to ensure that the data gets to the other end of the link without errors. Since it is impossible to build an error-free physical layer (although some short links can get very close to error-free operation) this means there is a requirement for any packets that do arrive with errors in them to be re-transmitted. What the receiver and transmitter then do about it is the subject of this one. Protocols that guarantee to get data to the destination correctly (or inform the layer above them that the attempt has failed) are known as reliable protocols. All reliable protocols need a bi-directional communication link, as the receiver has to be able to transmit short packets (called acknowledgements) back to the transmitter, informing the transmitter whether the information has arrived correctly, or not. Since these acknowledgements (or ACKs) can be efficiently combined with those required for flow control, this function is often done at the same layers: usually the transport layer and/or the data link layer. If a packet fails to arrive, or arrives in error, the receiver can send back a negative acknowledgement (or NAK), which asks the transmitter to resend the information. This process is known as ARQ (Automatic Repeat request). 收起