{ "mode": "man", "parameter": "RED", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/RED/8/json", "generated": "2026-06-15T14:36:21Z", "synopsis": "tc qdisc ... red limit bytes [ min bytes ] [ max bytes ] avpkt bytes [ burst packets ] [ ecn\n] [ harddrop ] [ nodrop ] [ bandwidth rate ] [ probability chance ] [ adaptive ] [ qevent\nearlydrop block index ] [ qevent mark block index ]", "sections": { "NAME": { "content": "red - Random Early Detection\n", "subsections": [] }, "SYNOPSIS": { "content": "tc qdisc ... red limit bytes [ min bytes ] [ max bytes ] avpkt bytes [ burst packets ] [ ecn\n] [ harddrop ] [ nodrop ] [ bandwidth rate ] [ probability chance ] [ adaptive ] [ qevent\nearlydrop block index ] [ qevent mark block index ]\n\n", "subsections": [] }, "DESCRIPTION": { "content": "Random Early Detection is a classless qdisc which manages its queue size smartly. Regular\nqueues simply drop packets from the tail when they are full, which may not be the optimal be‐\nhaviour. RED also performs tail drop, but does so in a more gradual way.\n\nOnce the queue hits a certain average length, packets enqueued have a configurable chance of\nbeing marked (which may mean dropped). This chance increases linearly up to a point called\nthe max average queue length, although the queue might get bigger.\n\nThis has a host of benefits over simple taildrop, while not being processor intensive. It\nprevents synchronous retransmits after a burst in traffic, which cause further retransmits,\netc.\n\nThe goal is to have a small queue size, which is good for interactivity while not disturbing\nTCP/IP traffic with too many sudden drops after a burst of traffic.\n\nDepending on if ECN is configured, marking either means dropping or purely marking a packet\nas overlimit.\n", "subsections": [] }, "ALGORITHM": { "content": "The average queue size is used for determining the marking probability. This is calculated\nusing an Exponential Weighted Moving Average, which can be more or less sensitive to bursts.\n\nWhen the average queue size is below min bytes, no packet will ever be marked. When it ex‐\nceeds min, the probability of doing so climbs linearly up to probability, until the average\nqueue size hits max bytes. Because probability is normally not set to 100%, the queue size\nmight conceivably rise above max bytes, so the limit parameter is provided to set a hard max‐\nimum for the size of the queue.\n\n", "subsections": [] }, "PARAMETERS": { "content": "min Average queue size at which marking becomes a possibility. Defaults to max /3\n\n\nmax At this average queue size, the marking probability is maximal. Should be at least\ntwice min to prevent synchronous retransmits, higher for low min. Default to limit /4\n\nprobability\nMaximum probability for marking, specified as a floating point number from 0.0 to 1.0.\nSuggested values are 0.01 or 0.02 (1 or 2%, respectively). Default : 0.02\n\nlimit Hard limit on the real (not average) queue size in bytes. Further packets are dropped.\nShould be set higher than max+burst. It is advised to set this a few times higher than\nmax.\n\nburst Used for determining how fast the average queue size is influenced by the real queue\nsize. Larger values make the calculation more sluggish, allowing longer bursts of\ntraffic before marking starts. Real life experiments support the following guideline:\n(min+min+max)/(3*avpkt).\n\navpkt Specified in bytes. Used with burst to determine the time constant for average queue\nsize calculations. 1000 is a good value.\n\nbandwidth\nThis rate is used for calculating the average queue size after some idle time. Should\nbe set to the bandwidth of your interface. Does not mean that RED will shape for you!\nOptional. Default : 10Mbit\n\necn As mentioned before, RED can either 'mark' or 'drop'. Explicit Congestion Notification\nallows RED to notify remote hosts that their rate exceeds the amount of bandwidth\navailable. Non-ECN capable hosts can only be notified by dropping a packet. If this\nparameter is specified, packets which indicate that their hosts honor ECN will only be\nmarked and not dropped, unless the queue size hits limit bytes. Recommended.\n\nharddrop\nIf average flow queue size is above max bytes, this parameter forces a drop instead of\necn marking.\n\nnodrop With this parameter, traffic that should be marked, but is not ECN-capable, is en‐\nqueued. Without the parameter it is early-dropped.\n\nadaptive\n(Added in linux-3.3) Sets RED in adaptive mode as described in\nhttp://icir.org/floyd/papers/adaptiveRed.pdf\nGoal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :\n(max - min) / 2\n\n", "subsections": [] }, "QEVENTS": { "content": "See tc (8) for some general notes about qevents. The RED qdisc supports the following\nqevents:\n\n\nearlydrop\nThe associated block is executed when packets are early-dropped. This includes non-ECT\npackets in ECN mode.\n\nmark The associated block is executed when packets are marked in ECN mode.\n\n", "subsections": [] }, "EXAMPLE": { "content": "# tc qdisc add dev eth0 parent 1:1 handle 10: red\nlimit 400000 min 30000 max 90000 avpkt 1000\nburst 55 ecn adaptive bandwidth 10Mbit\n\n", "subsections": [] }, "SEE ALSO": { "content": "tc(8), tc-choke(8)\n\n", "subsections": [] }, "SOURCES": { "content": "o Floyd, S., and Jacobson, V., Random Early Detection gateways for Congestion Avoidance.\nhttp://www.aciri.org/floyd/papers/red/red.html\n\no Some changes to the algorithm by Alexey N. Kuznetsov.\n\no Adaptive RED : http://icir.org/floyd/papers/adaptiveRed.pdf\n\n", "subsections": [] }, "AUTHORS": { "content": "Alexey N. Kuznetsov, , Alexey Makarenko , J\nHadi Salim , Eric Dumazet . This manpage\nmaintained by bert hubert \n\n\n\niproute2 13 December 2001 RED(8)", "subsections": [] } }, "summary": "red - Random Early Detection", "flags": [], "examples": [ "# tc qdisc add dev eth0 parent 1:1 handle 10: red", "limit 400000 min 30000 max 90000 avpkt 1000", "burst 55 ecn adaptive bandwidth 10Mbit" ], "see_also": [ { "name": "tc", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/tc/8/json" }, { "name": "tc-choke", "section": "8", "url": "https://www.chedong.com/phpMan.php/man/tc-choke/8/json" } ] }