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Retransmission (data networks)
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<p>Retransmission, essentially identical with <a href="/facts/Automatic_repeat_request/1bR2urmF">automatic repeat request</a> (ARQ), is the resending of <a href="/facts/Packet_(information_technology)/xCYf8SJu">packets</a> which have been either damaged or lost. Retransmission is one of the basic mechanisms used by <a href="/facts/Network_protocol/5uady7cB">protocols</a> operating over a <a href="/facts/Packet_switched/olDlDP1R">packet switched</a> <a href="/facts/Computer_network/3w5RM99p">computer network</a> to provide reliable communication (such as that provided by a <a href="/facts/Reliable_byte_stream/YbSblvM5">reliable byte stream</a>, for example <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">TCP</a>). </p><p>Such networks are usually "unreliable", meaning they offer no guarantees that they will not delay, damage, or lose <a href="/facts/Packet_(information_technology)/xCYf8SJu">packets</a>, or deliver them out of order. Protocols which provide reliable communication over such networks use a combination of <a href="/facts/Acknowledgement_(data_networks)/NCc7vbwE">acknowledgments</a> (i.e., an explicit <a href="/facts/Receipt/JN7TNeL6">receipt</a> from the destination of the <a href="/facts/Data/AUBFN97o">data</a>), retransmission of missing or damaged packets (usually initiated by a <a href="/facts/Timeout_(telecommunication)/va8CczEQ">time-out</a>), and <a href="/facts/Checksums/AqxQ0I8g">checksums</a> to provide that reliability. </p>