Manual: tc-ets

TC(8)							    Linux						       TC(8)

NAME
       ETS - Enhanced Transmission Selection scheduler

SYNOPSIS
       tc qdisc ... ets [ bands number ] [ strict number ] [ quanta bytes bytes bytes...  ] [ priomap band band band...	 ]

       tc class ... ets [ quantum bytes ]

DESCRIPTION
       The  Enhanced Transmission Selection scheduler is a classful queuing discipline that merges functionality of PRIO and
       DRR qdiscs in one scheduler. ETS makes it easy to configure a set of strict and bandwidth-sharing bands to  implement
       the transmission selection described in 802.1Qaz.

       On  creation with 'tc qdisc add', a fixed number of bands is created. Each band is a class, although it is not possi‐
       ble to directly add and remove bands with 'tc class' commands. The number of bands to  be  created  must	 instead  be
       specified on the command line as the qdisc is added.

       The  minor  number  of  classid to use when referring to a band is the band number increased by one. Thus band 0 will
       have classid of major:1, band 1 that of major:2, etc.

       ETS bands are of two types: some number may be in strict mode, the remaining ones are in bandwidth-sharing mode.

ALGORITHM
       When dequeuing, strict bands are tried first, if there are any. Band 0 is tried	first.	If  it	did  not  deliver  a
       packet, band 1 is tried next, and so on until one of the bands delivers a packet, or the strict bands are exhausted.

       If  no packet has been dequeued from any of the strict bands, if there are any bandwidth-sharing bands, the dequeuing
       proceeds according to the DRR algorithm. Each bandwidth-sharing band is assigned a deficit  counter,  initialized  to
       quantum	assigned  by  a quanta element. ETS maintains an (internal) ''active'' list of bandwidth-sharing bands whose
       qdiscs are non-empty. This list is used for dequeuing. A packet is dequeued from the band at the head of the list  if
       the  packet  size is smaller or equal to the deficit counter. If the counter is too small, it is increased by quantum
       and the scheduler moves on to the next band in the active list.

       Only qdiscs that own their queue should be added below the bandwidth-sharing bands. Attaching to	 them  non-work-con‐
       serving	qdiscs like TBF does not make sense -- other qdiscs in the active list will be skipped until the dequeue op‐
       eration succeeds. This limitation does not exist with the strict bands.

CLASSIFICATION
       The ETS qdisc allows three ways to decide which band to enqueue a packet to:

       - Packet priority can be directly set to a class handle, in which case that
	 is the queue where the packet will be put. For example, band number 2 of
	 a qdisc with handle of 11: will have classid 11:3. To mark a packet for
	 queuing to this band, the packet priority should be set to 0x110003.

       - A tc filter attached to the qdisc can put the packet to a band by using
	 the flowid keyword.

       - As a last resort, the ETS qdisc consults its priomap (see below), which
	 maps packets to bands based on packet priority.

PARAMETERS
       strict The number of bands that should be created in strict mode. If not given, this value is 0.

       quanta Each bandwidth-sharing band needs to know its quantum, which is the amount of bytes a band is allowed  to	 de‐
	      queue  before the scheduler moves to the next bandwidth-sharing band. The quanta argument lists quanta for the
	      individual bandwidth-sharing bands.  The minimum value of each quantum is 1. If quanta is not given,  the	 de‐
	      fault is no bandwidth-sharing bands, but note that when specifying a large number of bands, the extra ones are
	      in bandwidth-sharing mode by default.

       bands  Number  of bands given explicitly. This value has to be at least large enough to cover the strict bands speci‐
	      fied through the strict keyword and bandwidth-sharing bands specified in quanta.	If a larger value is  given,
	      any  extra  bands	 are  in  bandwidth-sharing mode, and their quanta are deduced from the interface MTU. If no
	      value is given, as many bands are created as necessary to cover all bands implied by  the	 strict	 and  quanta
	      keywords.

       priomap
	      The priomap maps the priority of a packet to a band. The argument is a list of numbers. The first number indi‐
	      cates which band the packets with priority 0 should be put to, the second is for priority 1, and so on.

	      There  can  be up to 16 numbers in the list. If there are fewer, the default band that traffic with one of the
	      unmentioned priorities goes to is the last one.

EXAMPLE & USAGE
       Add a qdisc with 8 bandwidth-sharing bands, using the interface MTU as their quanta. Since all quanta are  the  same,
       this will lead to equal distribution of bandwidth between the bands, each will get about 12.5% of the link. The low 8
       priorities go to individual bands in a reverse 1:1 fashion (such that the highest priority goes to the first band).

       # tc qdisc add dev eth0 root handle 1: ets bands 8 priomap 7 6 5 4 3 2 1 0
       # tc qdisc show dev eth0
       qdisc ets 1: root refcnt 2 bands 8 quanta 1514 1514 1514 1514 1514 1514 1514 1514 priomap 7 6 5 4 3 2 1 0 7 7 7 7 7 7
       7 7

       Tweak  the  first band of the above qdisc to give it a quantum of 2650, which will give it about 20% of the link (and
       about 11.5% to the remaining bands):

       # tc class change dev eth0 classid 1:1 ets quantum 2650
       # tc qdisc show dev eth0
       qdisc ets 1: root refcnt 2 bands 8 quanta 2650 1514 1514 1514 1514 1514 1514 1514 priomap 7 6 5 4 3 2 1 0 7 7 7 7 7 7
       7 7

       Create a purely strict Qdisc with reverse 1:1 mapping between priorities and bands:

       # tc qdisc add dev eth0 root handle 1: ets strict 8 priomap 7 6 5 4 3 2 1 0
       # tc qdisc sh dev eth0
       qdisc ets 1: root refcnt 2 bands 8 strict 8 priomap 7 6 5 4 3 2 1 0 7 7 7 7 7 7 7 7

       Add a Qdisc with 6 bands, 3 strict and 3 ETS with 35%-30%-25% weights:

       # tc qdisc add dev eth0 root handle 1: ets strict 3 quanta 3500 3000 2500 priomap 0 1 1 1 2 3 4 5
       # tc qdisc sh dev eth0
       qdisc ets 1: root refcnt 2 bands 6 strict 3 quanta 3500 3000 2500 priomap 0 1 1 1 2 3 4 5 5 5 5 5 5 5 5 5

       Create a Qdisc such that traffic with priorities 2, 3 and 4 are strictly prioritized over other traffic, and the rest
       goes into bandwidth-sharing classes with equal weights:

       # tc qdisc add dev eth0 root handle 1: ets bands 8 strict 3 priomap 3 4 0 1 2 5 6 7
       # tc qdisc sh dev eth0
       qdisc ets 1: root refcnt 2 bands 8 strict 3 quanta 1514 1514 1514 1514 1514 priomap 3 4 0 1 2 5 6 7 7 7 7 7 7 7 7 7

SEE ALSO
       tc(8), tc-prio(8), tc-drr(8)

AUTHOR
       Parts of both this manual page and the code itself are taken from PRIO and DRR qdiscs.
       ETS qdisc itself was written by Petr Machata.

iproute2						December 2019						       TC(8)