Retransmission, essentially identical with
automatic repeat request
Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a message) and timeout ...
(ARQ), is the resending of
packets which have been either damaged or lost. Retransmission is one of the basic mechanisms used by
protocols
Protocol may refer to:
Sociology and politics
* Protocol (politics), a formal agreement between nation states
* Protocol (diplomacy), the etiquette of diplomacy and affairs of state
* Etiquette, a code of personal behavior
Science and technology
...
operating over a
packet switched
In telecommunications, packet switching is a method of grouping data into short messages in fixed format, i.e. '' packets,'' that are transmitted over a digital network. Packets consist of a header and a payload. Data in the header is used b ...
computer network
A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...
to provide reliable communication (such as that provided by a
reliable byte stream
A reliable byte stream is a common service paradigm in computer networking; it refers to a byte stream in which the bytes which emerge from the communication channel at the recipient are exactly the same, and in exactly the same order, as they we ...
, for example
TCP).
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 (i.e., an explicit
receipt
A receipt (also known as a packing list, packing slip, packaging slip, (delivery) docket, shipping list, delivery list, bill of the parcel, Manifest (transportation), manifest, or customer receipt) is a document acknowledging that something h ...
from the destination of the
data
Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...
), retransmission of missing or damaged packets (usually initiated by a
time-out), 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 (NACK): the receiver explicitly notifies the sender which packets, messages, or segments were received incorrectly and thus may need to be retransmitted (RFC 4077).
* Selective Acknowledgment (SACK): the receiver explicitly lists which packets, messages, or segments in a stream are acknowledged (either negatively or positively). Positive selective acknowledgment is an option in TCP (RFC 2018) that is useful in
Satellite Internet access
Satellite Internet access is Internet access provided through communication satellites; if it can sustain high-speed Internet, high speeds, it is termed satellite broadband. Modern consumer grade satellite Internet service is typically provide ...
(RFC 2488).
* 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
A sliding window protocol is a feature of packet-based data transmission Protocol (computing), protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the data link layer (OSI model#Laye ...
.
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
See also
*
Error control
In information theory and coding theory with applications in computer science and telecommunications, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communi ...
*
Reliable system design
Reliability, reliable, or unreliable may refer to:
Science, technology, and mathematics Computing
* Data reliability (disambiguation), a property of some disk arrays in computer storage
* Reliability (computer networking), a category used to des ...
*
Truncated binary exponential backoff
*
TCP congestion avoidance algorithm
Transmission Control Protocol (TCP) uses a congestion control algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and a congestion window (CWND) ...
*
Development of TCP
*
QSL card
References
{{reflist
Network protocols