Retransmission (data networks)

Acknowledgment

• Positive Acknowledgement: the receiver explicitly notifies the sender which packets, messages, or segments were received correctly. Positive Acknowledgement therefore also implicitly informs the sender which packets were not received and provides detail on packets which need to be retransmitted.
• Negative Acknowledgment : the receiver explicitly notifies the sender which packets, messages, or segments were received incorrectly and thus may need to be retransmitted.
• Selective Acknowledgment : the receiver explicitly lists which packets, messages, or segments in a stream are acknowledged. Positive selective acknowledgment is an option in TCP that is useful in Satellite Internet access.
• Cumulative Acknowledgment: the receiver acknowledges that it correctly received a packet, message, or segment in a stream which implicitly informs the sender that the previous packets were received correctly. TCP uses cumulative acknowledgment with its TCP sliding window.

  Retransmission

• if no such acknowledgment is forthcoming within a reasonable time, the time-out
• the sender discovers, often through some out of band means, that the transmission was unsuccessful
• if the receiver knows that expected data has not arrived, and so notifies the sender
• if the receiver knows that the data has arrived, but in a damaged condition, and indicates that to the sender