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 ] [ nodrop ] [ bandwidth rate ] [
11 probability chance ] [ adaptive ] [ qevent early_drop block index ] [
12 qevent mark block index ]
13
16 Random Early Detection is a classless qdisc which manages its queue
17 size smartly. Regular queues simply drop packets from the tail when
18 they are full, which may not be the optimal behaviour. RED also per‐
19 forms tail drop, but does so in a more gradual way.
20 Once the queue hits a certain average length, packets enqueued have a
22 configurable chance of being marked (which may mean dropped). This
23 chance increases linearly up to a point called the max average queue
24 length, although the queue might get bigger.
25 This has a host of benefits over simple taildrop, while not being pro‐
27 cessor intensive. It prevents synchronous retransmits after a burst in
28 traffic, which cause further retransmits, etc.
29 The goal is to have a small queue size, which is good for interactivity
31 while not disturbing TCP/IP traffic with too many sudden drops after a
32 burst of traffic.
33 Depending on if ECN is configured, marking either means dropping or
35 purely marking a packet as overlimit.
36
38 The average queue size is used for determining the marking probability.
39 This is calculated using an Exponential Weighted Moving Average, which
40 can be more or less sensitive to bursts.
41 When the average queue size is below min bytes, no packet will ever be
43 marked. When it exceeds min, the probability of doing so climbs lin‐
44 early up to probability, until the average queue size hits max bytes.
45 Because probability is normally not set to 100%, the queue size might
46 conceivably rise above max bytes, so the limit parameter is provided to
47 set a hard maximum for the size of the queue.
48
51 min Average queue size at which marking becomes a possibility.
52 Defaults to max /3
53 max At this average queue size, the marking probability is maximal.
56 Should be at least twice min to prevent synchronous retransmits,
57 higher for low min. Default to limit /4
58 probability
60 Maximum probability for marking, specified as a floating point
61 number from 0.0 to 1.0. Suggested values are 0.01 or 0.02 (1 or
62 2%, respectively). Default : 0.02
63 limit Hard limit on the real (not average) queue size in bytes. Fur‐
65 ther packets are dropped. Should be set higher than max+burst.
66 It is advised to set this a few times higher than max.
67 burst Used for determining how fast the average queue size is influ‐
69 enced by the real queue size. Larger values make the calculation
70 more sluggish, allowing longer bursts of traffic before marking
71 starts. Real life experiments support the following guideline:
72 (min+min+max)/(3*avpkt).
73 avpkt Specified in bytes. Used with burst to determine the time con‐
75 stant for average queue size calculations. 1000 is a good value.
76 bandwidth
78 This rate is used for calculating the average queue size after
79 some idle time. Should be set to the bandwidth of your inter‐
80 face. Does not mean that RED will shape for you! Optional.
81 Default : 10Mbit
82 ecn As mentioned before, RED can either 'mark' or 'drop'. Explicit
84 Congestion Notification allows RED to notify remote hosts that
85 their rate exceeds the amount of bandwidth available. Non-ECN
86 capable hosts can only be notified by dropping a packet. If this
87 parameter is specified, packets which indicate that their hosts
88 honor ECN will only be marked and not dropped, unless the queue
89 size hits limit bytes. Recommended.
90 harddrop
92 If average flow queue size is above max bytes, this parameter
93 forces a drop instead of ecn marking.
94 nodrop With this parameter, traffic that should be marked, but is not
96 ECN-capable, is enqueued. Without the parameter it is early-
97 dropped.
98 adaptive
100 (Added in linux-3.3) Sets RED in adaptive mode as described in
101 http://icir.org/floyd/papers/adaptiveRed.pdf
102 Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
103 (max - min) / 2
104
107 See tc (8) for some general notes about qevents. The RED qdisc supports
108 the following qevents:
109 early_drop
112 The associated block is executed when packets are early-dropped.
113 This includes non-ECT packets in ECN mode.
114 mark The associated block is executed when packets are marked in ECN
116 mode.
117
120 # tc qdisc add dev eth0 parent 1:1 handle 10: red
121 limit 400000 min 30000 max 90000 avpkt 1000
122 burst 55 ecn adaptive bandwidth 10Mbit
123
126 tc(8), tc-choke(8)
127
130 o Floyd, S., and Jacobson, V., Random Early Detection gateways for
131 Congestion Avoidance.
132 http://www.aciri.org/floyd/papers/red/red.html
133 o Some changes to the algorithm by Alexey N. Kuznetsov.
135 o Adaptive RED : http://icir.org/floyd/papers/adaptiveRed.pdf
137
140 Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, Alexey Makarenko
141 <makar@phoenix.kharkov.ua>, J Hadi Salim <hadi@nortelnetworks.com>,
142 Eric Dumazet <eric.dumazet@gmail.com>. This manpage maintained by bert
143 hubert <ahu@ds9a.nl>
144iproute2 13 December 2001 RED(8)