.TH ETF 8 "05 Jul 2018" "iproute2" "Linux"

.SH NAME

ETF \- Earliest TxTime First (ETF) Qdisc

.SH SYNOPSIS

.B tc qdisc ... dev

dev

.B parent

classid

.B [ handle

major:

.B ] etf clockid

clockid

.B [ delta

delta_nsecs

.B ] [ deadline_mode ]

.B [ offload ]

.SH DESCRIPTION

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

.B 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

.B 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

.BR 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

.BR mqprio(8).

.SH PARAMETERS

.TP

clockid

.br

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

.B 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

.BR time(7)

and

.BR clock_gettime(3).

.TP

delta

.br

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

.B 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.

.TP

deadline_mode

.br

When

.B 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

.BR etf(8)

will set the txtime of the packet being dequeued to 'now'.

The default is for this option to be disabled.

.TP

offload

.br

When

.B offload

is set,

.BR 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.

.TP

skip_sock_check

.br

.BR 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.

.SH EXAMPLES

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

.BR mqprio(8),

and configure it like this:

.EX

# 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

.EE

.P

To replace the current queueing discipline by ETF in traffic class

number 0, issue:

.P

.EX

# tc qdisc replace dev eth0 parent 100:1 etf \\

	clockid CLOCK_TAI delta 300000 offload

.EE

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.

.SH AUTHORS

Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>

.br

Vinicius Costa Gomes <vinicius.gomes@intel.com>