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
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
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 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 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(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.
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
# 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 \
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.
This page is part of the iproute2 (utilities for controlling TCP/IP
networking and traffic) project. Information about the project can
be found at
If you have a bug report for this manual page, send it to
firstname.lastname@example.org, email@example.com. This page was obtained
from the project's upstream Git repository
2020-06-09. (At that time, the date of the most recent commit that
was found in the repository was 2020-06-05.) If you discover any
rendering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
iproute2 05 Jul 2018 ETF(8)