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 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.
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 \
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
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
2021-08-27. (At that time, the date of the most recent commit
that was found in the repository was 2021-08-18.) If you
discover any rendering problems in this HTML version of the page,
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iproute2 05 Jul 2018 ETF(8)