Provided by: iproute2_5.2.0-1ubuntu2_amd64 bug


       ETF - Earliest TxTime First (ETF) Qdisc


       tc  qdisc  ...  dev  dev  parent  classid  [  handle  major: ] etf clockid clockid [ delta
       delta_nsecs ] [ deadline_mode ] [ offload ]


       The ETF (Earliest TxTime First) qdisc allows applications to control the  instant  when  a
       packet should be dequeued from the traffic control layer into the netdevice. If offload is
       configured and supported by the network interface card, the  it  will  also  control  when
       packets leave the network controller.

       ETF achieves that by buffering packets until a configurable time before their transmission
       time (i.e. txtime, or deadline), which can be configured through the delta option.

       The qdisc uses a rb-tree internally so packets are always 'ordered' by  their  txtime  and
       will be dequeued following the (next) earliest txtime first.

       It  relies  on the SO_TXTIME socket option and the SCM_TXTIME CMSG in each packet field to
       configure the behavior of time dependent sockets: the clockid to be used as  a  reference,
       if  the  expected mode of txtime for that socket is deadline or strict mode, and if packet
       drops should be reported on the socket's error queue. See socket(7) for more information.

       The etf qdisc will drop any packets with a txtime in the past,  or  if  a  packet  expires
       while waiting for being dequeued.

       This  queueing  discipline  is  intended  to  be  used  by TSN (Time Sensitive Networking)
       applications, and it exposes a traffic shaping functionality that is  commonly  documented
       as  "Launch  Time"  or "Time-Based Scheduling" by vendors and the documentation of network
       interface controllers.

       ETF is meant to be installed under  another  qdisc  that  maps  packet  flows  to  traffic
       classes, one example is mqprio(8).


              Specifies  the  clock  to  be used by qdisc's internal timer for measuring time and
              scheduling events. The qdisc expects that packets passing through it  to  be  using
              this same clockid as the reference of their txtime timestamps. It will drop packets
              coming from sockets that do not comply with that.

              For more information about time and clocks on Linux, please refer  to  time(7)  and

              After  enqueueing  or dequeueing a packet, the qdisc will schedule its next wake-up
              time for the next txtime minus this delta value.  This means delta can be used as a
              fudge  factor  for the scheduler latency of a system.  This value must be specified
              in nanoseconds.  The default value is 0 nanoseconds.

              When deadline_mode is set, the qdisc will handle txtime with a different semantics,
              changed  from  a 'strict' transmission time to a deadline.  In practice, this means
              during the dequeue flow etf(8) will set the txtime of the packet being dequeued  to
              'now'.  The default is for this option to be disabled.

              When  offload  is  set, etf(8) will try to configure the network interface so time-
              based transmission arbitration is  enabled  in  the  controller.  This  feature  is
              commonly   referred   to  as  "Launch  Time"  or  "Time-Based  Scheduling"  by  the
              documentation of network interface controllers.  The default is for this option  to
              be disabled.


       ETF  is  used  to  enforce  a  Quality of Service. It controls when each packets should be
       dequeued and transmitted, and can be used for limiting the data rate of a  traffic  class.
       To  separate  packets into traffic classes the user may choose mqprio(8), and configure it
       like this:

       # tc qdisc add dev eth0 handle 100: parent root mqprio num_tc 3 \
            map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 \
            queues 1@0 1@1 2@2 \
            hw 0

       To replace the current queueing discipline by ETF in traffic class number 0, issue:

       # tc qdisc replace dev eth0 parent 100:1 etf \
            clockid CLOCK_TAI delta 300000 offload

       With the options above, etf will be configured to use CLOCK_TAI  as  its  clockid_t,  will
       schedule packets for 300 us before their txtime, and will enable the functionality on that
       in the network interface card. Deadline mode will not be configured for this mode.


       Jesus Sanchez-Palencia <>
       Vinicius Costa Gomes <>