tc-etf(8) — Linux manual page


ETF(8)                            Linux                           ETF(8)

NAME         top

       ETF - Earliest TxTime First (ETF) Qdisc

SYNOPSIS         top

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

DESCRIPTION         top

       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).

PARAMETERS         top

              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 clock_gettime(3).

              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

              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(8) currently drops any packet which does not have a
              socket associated with it or if the socket does not have
              SO_TXTIME socket option set. But, this will not work if
              the launchtime is set by another entity inside the kernel
              (e.g. some other Qdisc). Setting the skip_sock_check will
              skip checking for a socket associated with the packet.

EXAMPLES         top

       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

AUTHORS         top

       Jesus Sanchez-Palencia <>
       Vinicius Costa Gomes <>

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling
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iproute2                       05 Jul 2018                        ETF(8)