1RED(8) Linux RED(8)
2
6 red - Random Early Detection
7
9 tc qdisc ... red limit bytes [ min bytes ] [ max bytes ] avpkt bytes [
10 burst packets ] [ ecn ] [ harddrop] [ bandwidth rate ] [ probability
11 chance ] [ adaptive ]
12
15 Random Early Detection is a classless qdisc which manages its queue
16 size smartly. Regular queues simply drop packets from the tail when
17 they are full, which may not be the optimal behaviour. RED also per‐
18 forms tail drop, but does so in a more gradual way.
19 Once the queue hits a certain average length, packets enqueued have a
21 configurable chance of being marked (which may mean dropped). This
22 chance increases linearly up to a point called the max average queue
23 length, although the queue might get bigger.
24 This has a host of benefits over simple taildrop, while not being pro‐
26 cessor intensive. It prevents synchronous retransmits after a burst in
27 traffic, which cause further retransmits, etc.
28 The goal is to have a small queue size, which is good for interactivity
30 while not disturbing TCP/IP traffic with too many sudden drops after a
31 burst of traffic.
32 Depending on if ECN is configured, marking either means dropping or
34 purely marking a packet as overlimit.
35
37 The average queue size is used for determining the marking probability.
38 This is calculated using an Exponential Weighted Moving Average, which
39 can be more or less sensitive to bursts.
40 When the average queue size is below min bytes, no packet will ever be
42 marked. When it exceeds min, the probability of doing so climbs lin‐
43 early up to probability, until the average queue size hits max bytes.
44 Because probability is normally not set to 100%, the queue size might
45 conceivably rise above max bytes, so the limit parameter is provided to
46 set a hard maximum for the size of the queue.
47
50 min Average queue size at which marking becomes a possibility.
51 Defaults to max /3
52 max At this average queue size, the marking probability is maximal.
55 Should be at least twice min to prevent synchronous retransmits,
56 higher for low min. Default to limit /4
57 probability
59 Maximum probability for marking, specified as a floating point
60 number from 0.0 to 1.0. Suggested values are 0.01 or 0.02 (1 or
61 2%, respectively). Default : 0.02
62 limit Hard limit on the real (not average) queue size in bytes. Fur‐
64 ther packets are dropped. Should be set higher than max+burst.
65 It is advised to set this a few times higher than max.
66 burst Used for determining how fast the average queue size is influ‐
68 enced by the real queue size. Larger values make the calculation
69 more sluggish, allowing longer bursts of traffic before marking
70 starts. Real life experiments support the following guideline:
71 (min+min+max)/(3*avpkt).
72 avpkt Specified in bytes. Used with burst to determine the time con‐
74 stant for average queue size calculations. 1000 is a good value.
75 bandwidth
77 This rate is used for calculating the average queue size after
78 some idle time. Should be set to the bandwidth of your inter‐
79 face. Does not mean that RED will shape for you! Optional.
80 Default : 10Mbit
81 ecn As mentioned before, RED can either 'mark' or 'drop'. Explicit
83 Congestion Notification allows RED to notify remote hosts that
84 their rate exceeds the amount of bandwidth available. Non-ECN
85 capable hosts can only be notified by dropping a packet. If this
86 parameter is specified, packets which indicate that their hosts
87 honor ECN will only be marked and not dropped, unless the queue
88 size hits limit bytes. Recommended.
89 harddrop
91 If average flow queue size is above max bytes, this parameter
92 forces a drop instead of ecn marking.
93 adaptive
95 (Added in linux-3.3) Sets RED in adaptive mode as described in
96 http://icir.org/floyd/papers/adaptiveRed.pdf
97 Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
98 (max - min) / 2
99
102 # tc qdisc add dev eth0 parent 1:1 handle 10: red
103 limit 400000 min 30000 max 90000 avpkt 1000
104 burst 55 ecn adaptive bandwidth 10Mbit
105
108 tc(8), tc-choke(8)
109
112 o Floyd, S., and Jacobson, V., Random Early Detection gateways for
113 Congestion Avoidance.
114 http://www.aciri.org/floyd/papers/red/red.html
115 o Some changes to the algorithm by Alexey N. Kuznetsov.
117 o Adaptive RED : http://icir.org/floyd/papers/adaptiveRed.pdf
119
122 Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, Alexey Makarenko
123 <makar@phoenix.kharkov.ua>, J Hadi Salim <hadi@nortelnetworks.com>,
124 Eric Dumazet <eric.dumazet@gmail.com>. This manpage maintained by bert
125 hubert <ahu@ds9a.nl>
126iproute2 13 December 2001 RED(8)