backoffalgoritme

Backoff algorithm (Norwegian: backoffalgoritme) is a strategy used to coordinate access to a shared resource by introducing deliberate delays between retry attempts after a failure. The core idea is to randomize wait times to avoid repeated collisions or contention when multiple agents attempt access simultaneously. Backoff algorithms are used in network media access control, distributed systems, and protocol software to balance throughput and latency.

In Ethernet networks using CSMA/CD, after a collision a node chooses a random backoff period from a contention window CW that grows after each collision (exponential backoff). The backoff is computed as a random number of time slots in 0..CW-1, where CW starts at CWmin and doubles after each collision up to a maximum CWmax. This reduces the chance that two nodes pick the same slot again.

In wireless networks with CSMA/CA (as in Wi-Fi), a backoff counter is decremented while the medium is free. When it reaches zero, the station transmits. The contention window is adjusted similarly, and additional DIFS and SIFS timings influence the effective wait.

In software and distributed systems, backoff is used for retrying operations such as network calls or resource acquisition after failures. Exponential backoff gradually increases the timeout or wait interval, often with jitter to avoid synchronized retries. TCP uses exponential backoff for retransmission timeouts, while other protocols may use linear or customized schemes. Benefits include reduced congestion and improved stability; drawbacks include higher latency and potential starvation if misconfigured.

Key parameters include initial window size, maximum window size, and maximum retry count. A well-chosen backoff strategy depends on network characteristics, traffic patterns, and latency requirements.