Automatic repeat request (ARQ) is a feedback-based error-control method: when a receiver or sender detects that a data frame may have been lost or corrupted, the sender retransmits it. The exact trigger, retry limit, and delivery behavior depend on the protocol using ARQ.
How automatic repeat request works
A sender transmits a frame and uses feedback from the receiver to determine whether it arrived successfully. If an acknowledgement does not arrive when expected, or the receiver explicitly reports an error, the sender can transmit the frame again. A protocol may also use receiver-status polling to identify what still needs to be sent.
ARQ improves delivery reliability by combining error or loss detection with retransmission. It does not guarantee that every frame will eventually arrive: individual protocols can limit retries or stop a transaction after repeated failures.
Common ARQ methods
Stop-and-wait ARQ
The sender transmits one frame and waits for its acknowledgement before sending the next. If the frame is lost or corrupted, it retransmits it. This approach is simple, but waiting after each frame can leave a link underused when its bandwidth-delay product is high. RFC 3366 describes stop-and-wait as a better fit for links with low bandwidth-delay products.
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Go-Back-N ARQ
Go-Back-N uses a sliding window, allowing multiple numbered frames to be in flight before acknowledgements arrive. If a frame is lost or corrupted, the sender may retransmit that frame and the later outstanding frames as well.
Selective Repeat ARQ
Selective Repeat, also called Selective Reject, also uses a sliding window, but retransmits selected frames instead of resending every outstanding frame from the error onward. It requires careful design of sequence numbers and window size; RFC 3366 specifies that the sequence space must be larger than twice the link window for selective repeat.
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What differs between ARQ implementations
ARQ is a family of error-control techniques, not one universal set of retry rules. When assessing a particular implementation, check its link conditions and requirements alongside these design trade-offs:
- Utilization and throughput: Stop-and-wait can spend substantial time waiting on links with a high bandwidth-delay product; window-based methods can keep more data in flight.
- Retransmission overhead: Go-Back-N may resend later outstanding frames after an error, while Selective Repeat targets selected frames.
- State and implementation: Sliding-window designs require sequence-number management; Selective Repeat adds window and buffering considerations.
- Delivery behavior: Ordering, latency, and how long the sender continues retrying depend on the protocol and its deployment.
No one method is best for every link. The right choice depends on the link’s bandwidth-delay characteristics, error conditions, implementation constraints, and application needs. RFC 3366 offers guidance for link designers rather than a rule that every ARQ system must follow.
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Example: retries in ITU-T G.9903
ITU-T Recommendation G.9903 provides one specific example of ARQ behavior. In that specification, if an originator does not receive an acknowledgement within a waiting period, it assumes the transmission failed and retries during the specified contention period. After several unacknowledged retries, it can terminate the transaction or try again. Those actions describe that standard’s implementation; they are not general ARQ rules.
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Sources
- RFC 3366: Advice to link designers on link Automatic Repeat reQuest (ARQ), an IETF Best Current Practice published in August 2002.
- ITU-T Recommendation G.9903 (2017), Amendment 2 (March 2023), describing ARQ behavior in a particular standard.
- NPTEL / IIT Kharagpur course material: “Error Control Techniques”; a publication date is not stated in the source excerpt.
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