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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 possible 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-conserving qdiscs like TBF does not make sense -- other  qdiscs
       in  the  active list will be skipped until the dequeue operation 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 dequeue 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 default
              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 specified 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 indicates 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.