- Floyd, S., and Jacobson, V., Random Early Detection gateways for Congestion Avoidance (compressed postscript for A4 paper, compressed postscript for Letter paper, pdf one-column, pdf two-column). early.twocolumn.ps.Z Or in parts: [Part 1][Part 2] [Part 3] [Part 4] [Part 5]. IEEE/ACM Transactions on Networking, V.1 N.4, August 1993, p. 397-413. Abstract.
In the conventional tail drop algorithm, a router or other network component buffers as many packets as it can, and simply drops the ones it cannot buffer. If buffers are constantly full, the network is congested. Tail drop distributes buffer space unfairly among traffic flows. Tail drop can also lead to TCP global synchronization as all TCP connections "hold back" simultaneously, and then step forward simultaneously. Networks become under-utilized and flooded by turns. RED addresses these issues.
RED monitors the average queue size and drops (or marks when used in conjunction with ECN) packets based on statistical probabilities. If the buffer is almost empty, all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buffer is full, the probability has reached 1 and all incoming packets are dropped.
RED is more fair than tail drop, in the sense that it does not possess a bias against bursty traffic that uses only a small portion of the bandwidth. The more a host transmits, the more likely it is that its packets are dropped as the probability of a host's packet being dropped is proportional to the amount of data it has in a queue. Early detection helps avoid TCP global synchronization.