Retransmission (data networks)


Retransmission, essentially identical with automatic repeat request, is the resending of packets which have been either damaged or lost. Retransmission is one of the basic mechanisms used by protocols operating over a packet switched computer network to provide reliable communication.
Such networks are usually "unreliable", meaning they offer no guarantees that they will not delay, damage, or lose packets, or deliver them out of order. Protocols which provide reliable communication over such networks use a combination of acknowledgments, retransmission of missing or damaged packets, and checksums to provide that reliability.

Acknowledgment

There are several forms of acknowledgement which can be used alone or together in networking protocols:
  • 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

Retransmission is a very simple concept. Whenever one party sends something to the other party, it retains a copy of the data it sent until the recipient has acknowledged that it received it. In a variety of circumstances the sender automatically retransmits the data using the retained copy. Reasons for resending include:
  • 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