1TC(8) Linux TC(8)
2
6 ETS - Enhanced Transmission Selection scheduler
7
9 tc qdisc ... ets [ bands number ] [ strict number ] [ quanta bytes
10 bytes bytes... ] [ priomap band band band... ]
11 tc class ... ets [ quantum bytes ]
13
16 The Enhanced Transmission Selection scheduler is a classful queuing
17 discipline that merges functionality of PRIO and DRR qdiscs in one
18 scheduler. ETS makes it easy to configure a set of strict and band‐
19 width-sharing bands to implement the transmission selection described
20 in 802.1Qaz.
21 On creation with 'tc qdisc add', a fixed number of bands is created.
23 Each band is a class, although it is not possible to directly add and
24 remove bands with 'tc class' commands. The number of bands to be cre‐
25 ated must instead be specified on the command line as the qdisc is
26 added.
27 The minor number of classid to use when referring to a band is the band
29 number increased by one. Thus band 0 will have classid of major:1, band
30 1 that of major:2, etc.
31 ETS bands are of two types: some number may be in strict mode, the
33 remaining ones are in bandwidth-sharing mode.
34
37 When dequeuing, strict bands are tried first, if there are any. Band 0
38 is tried first. If it did not deliver a packet, band 1 is tried next,
39 and so on until one of the bands delivers a packet, or the strict bands
40 are exhausted.
41 If no packet has been dequeued from any of the strict bands, if there
43 are any bandwidth-sharing bands, the dequeuing proceeds according to
44 the DRR algorithm. Each bandwidth-sharing band is assigned a deficit
45 counter, initialized to quantum assigned by a quanta element. ETS main‐
46 tains an (internal) ''active'' list of bandwidth-sharing bands whose
47 qdiscs are non-empty. This list is used for dequeuing. A packet is
48 dequeued from the band at the head of the list if the packet size is
49 smaller or equal to the deficit counter. If the counter is too small,
50 it is increased by quantum and the scheduler moves on to the next band
51 in the active list.
52 Only qdiscs that own their queue should be added below the bandwidth-
54 sharing bands. Attaching to them non-work-conserving qdiscs like TBF
55 does not make sense -- other qdiscs in the active list will be skipped
56 until the dequeue operation succeeds. This limitation does not exist
57 with the strict bands.
58
61 The ETS qdisc allows three ways to decide which band to enqueue a
62 packet to:
63 - Packet priority can be directly set to a class handle, in which case
65 that
66 is the queue where the packet will be put. For example, band number 2
67 of
68 a qdisc with handle of 11: will have classid 11:3. To mark a packet
69 for
70 queuing to this band, the packet priority should be set to 0x110003.
71 - A tc filter attached to the qdisc can put the packet to a band by
73 using
74 the flowid keyword.
75 - As a last resort, the ETS qdisc consults its priomap (see below),
77 which
78 maps packets to bands based on packet priority.
79
82 strict The number of bands that should be created in strict mode. If
83 not given, this value is 0.
84 quanta Each bandwidth-sharing band needs to know its quantum, which is
87 the amount of bytes a band is allowed to dequeue before the
88 scheduler moves to the next bandwidth-sharing band. The quanta
89 argument lists quanta for the individual bandwidth-sharing
90 bands. The minimum value of each quantum is 1. If quanta is not
91 given, the default is no bandwidth-sharing bands, but note that
92 when specifying a large number of bands, the extra ones are in
93 bandwidth-sharing mode by default.
94 bands Number of bands given explicitly. This value has to be at least
97 large enough to cover the strict bands specified through the
98 strict keyword and bandwidth-sharing bands specified in quanta.
99 If a larger value is given, any extra bands are in bandwidth-
100 sharing mode, and their quanta are deduced from the interface
101 MTU. If no value is given, as many bands are created as neces‐
102 sary to cover all bands implied by the strict and quanta key‐
103 words.
104 priomap
107 The priomap maps the priority of a packet to a band. The argu‐
108 ment is a list of numbers. The first number indicates which band
109 the packets with priority 0 should be put to, the second is for
110 priority 1, and so on.
111 There can be up to 16 numbers in the list. If there are fewer,
113 the default band that traffic with one of the unmentioned prior‐
114 ities goes to is the last one.
115
118 Add a qdisc with 8 bandwidth-sharing bands, using the interface MTU as
119 their quanta. Since all quanta are the same, this will lead to equal
120 distribution of bandwidth between the bands, each will get about 12.5%
121 of the link. The low 8 priorities go to individual bands in a reverse
122 1:1 fashion (such that the highest priority goes to the first band).
123 # tc qdisc add dev eth0 root handle 1: ets bands 8 priomap 7 6 5 4 3 2
126 1 0
127 # tc qdisc show dev eth0
128 qdisc ets 1: root refcnt 2 bands 8 quanta 1514 1514 1514 1514 1514 1514
129 1514 1514 priomap 7 6 5 4 3 2 1 0 7 7 7 7 7 7 7 7
130 Tweak the first band of the above qdisc to give it a quantum of 2650,
133 which will give it about 20% of the link (and about 11.5% to the
134 remaining bands):
135 # tc class change dev eth0 classid 1:1 ets quantum 2650
138 # tc qdisc show dev eth0
139 qdisc ets 1: root refcnt 2 bands 8 quanta 2650 1514 1514 1514 1514 1514
140 1514 1514 priomap 7 6 5 4 3 2 1 0 7 7 7 7 7 7 7 7
141 Create a purely strict Qdisc with reverse 1:1 mapping between priori‐
144 ties and bands:
145 # tc qdisc add dev eth0 root handle 1: ets strict 8 priomap 7 6 5 4 3 2
148 1 0
149 # tc qdisc sh dev eth0
150 qdisc ets 1: root refcnt 2 bands 8 strict 8 priomap 7 6 5 4 3 2 1 0 7 7
151 7 7 7 7 7 7
152 Add a Qdisc with 6 bands, 3 strict and 3 ETS with 35%-30%-25% weights:
155 # tc qdisc add dev eth0 root handle 1: ets strict 3 quanta 3500 3000
157 2500 priomap 0 1 1 1 2 3 4 5
158 # tc qdisc sh dev eth0
159 qdisc ets 1: root refcnt 2 bands 6 strict 3 quanta 3500 3000 2500 pri‐
160 omap 0 1 1 1 2 3 4 5 5 5 5 5 5 5 5 5
161 Create a Qdisc such that traffic with priorities 2, 3 and 4 are
164 strictly prioritized over other traffic, and the rest goes into band‐
165 width-sharing classes with equal weights:
166 # tc qdisc add dev eth0 root handle 1: ets bands 8 strict 3 priomap 3 4
168 0 1 2 5 6 7
169 # tc qdisc sh dev eth0
170 qdisc ets 1: root refcnt 2 bands 8 strict 3 quanta 1514 1514 1514 1514
171 1514 priomap 3 4 0 1 2 5 6 7 7 7 7 7 7 7 7 7
172
175 tc(8), tc-prio(8), tc-drr(8)
176
179 Parts of both this manual page and the code itself are taken from PRIO
180 and DRR qdiscs.
181 ETS qdisc itself was written by Petr Machata.
182iproute2 December 2019 TC(8)