сетевой эмулятор (Network Emulator)
Имя (Name)
NetEm - Network Emulator
Синопсис (Synopsis)
tc qdisc ... dev DEVICE ] add netem OPTIONS
OPTIONS := [ LIMIT ] [ DELAY ] [ LOSS ] [ CORRUPT ] [ DUPLICATION
] [ REORDERING ] [ RATE ] [ SLOT ]
LIMIT := limit packets
DELAY := delay TIME [ JITTER [ CORRELATION ]]]
[ distribution { uniform | normal | pareto | paretonormal
} ]
LOSS := loss { random PERCENT [ CORRELATION ] |
state p13 [ p31 [ p32 [ p23 [ p14]]]] |
gemodel p [ r [ 1-h [ 1-k ]]] } [ ecn ]
CORRUPT := corrupt PERCENT [ CORRELATION ]]
DUPLICATION := duplicate PERCENT [ CORRELATION ]]
REORDERING := reorder PERCENT [ CORRELATION ] [ gap DISTANCE ]
RATE := rate RATE [ PACKETOVERHEAD [ CELLSIZE [ CELLOVERHEAD ]]]]
SLOT := slot { MIN_DELAY [ MAX_DELAY ] |
distribution { uniform | normal | pareto |
paretonormal | FILE } DELAY JITTER }
[ packets PACKETS ] [ bytes BYTES ]
Описание (Description)
NetEm is an enhancement of the Linux traffic control facilities
that allow to add delay, packet loss, duplication and more other
characteristics to packets outgoing from a selected network
interface. NetEm is built using the existing Quality Of Service
(QOS) and Differentiated Services (diffserv) facilities in the
Linux kernel.
netem OPTIONS
netem has the following options:
limit packets
maximum number of packets the qdisc may hold queued at a time.
delay
adds the chosen delay to the packets outgoing to chosen network
interface. The optional parameters allows to introduce a delay
variation and a correlation. Delay and jitter values are
expressed in ms while correlation is percentage.
distribution
allow the user to choose the delay distribution. If not
specified, the default distribution is Normal. Additional
parameters allow to consider situations in which network has
variable delays depending on traffic flows concurring on the same
path, that causes several delay peaks and a tail.
loss random
adds an independent loss probability to the packets outgoing from
the chosen network interface. It is also possible to add a
correlation, but this option is now deprecated due to the noticed
bad behavior.
loss state
adds packet losses according to the 4-state Markov using the
transition probabilities as input parameters. The parameter p13
is mandatory and if used alone corresponds to the Bernoulli
model. The optional parameters allows to extend the model to
2-state (p31), 3-state (p23 and p32) and 4-state (p14). State 1
corresponds to good reception, State 4 to independent losses,
State 3 to burst losses and State 2 to good reception within a
burst.
loss gemodel
adds packet losses according to the Gilbert-Elliot loss model or
its special cases (Gilbert, Simple Gilbert and Bernoulli). To use
the Bernoulli model, the only needed parameter is p while the
others will be set to the default values r=1-p, 1-h=1 and 1-k=0.
The parameters needed for the Simple Gilbert model are two (p and
r), while three parameters (p, r, 1-h) are needed for the Gilbert
model and four (p, r, 1-h and 1-k) are needed for the Gilbert-
Elliot model. As known, p and r are the transition probabilities
between the bad and the good states, 1-h is the loss probability
in the bad state and 1-k is the loss probability in the good
state.
ecn
can be used optionally to mark packets instead of dropping them.
A loss model has to be used for this to be enabled.
corrupt
allows the emulation of random noise introducing an error in a
random position for a chosen percent of packets. It is also
possible to add a correlation through the proper parameter.
duplicate
using this option the chosen percent of packets is duplicated
before queuing them. It is also possible to add a correlation
through the proper parameter.
reorder
to use reordering, a delay option must be specified. There are
two ways to use this option (assuming 'delay 10ms' in the options
list).
reorder 25% 50% gap 5
in this first example, the first 4 (gap - 1) packets are delayed
by 10ms and subsequent packets are sent immediately with a
probability of 0.25 (with correlation of 50% ) or delayed with a
probability of 0.75. After a packet is reordered, the process
restarts i.e. the next 4 packets are delayed and subsequent
packets are sent immediately or delayed based on reordering
probability. To cause a repeatable pattern where every 5th packet
is reordered reliably, a reorder probability of 100% can be used.
reorder 25% 50%
in this second example 25% of packets are sent immediately (with
correlation of 50%) while the others are delayed by 10 ms.
rate
delay packets based on packet size and is a replacement for TBF.
Rate can be specified in common units (e.g. 100kbit). Optional
PACKETOVERHEAD (in bytes) specify an per packet overhead and can
be negative. A positive value can be used to simulate additional
link layer headers. A negative value can be used to artificial
strip the Ethernet header (e.g. -14) and/or simulate a link layer
header compression scheme. The third parameter - an unsigned
value - specify the cellsize. Cellsize can be used to simulate
link layer schemes. ATM for example has an payload cellsize of 48
bytes and 5 byte per cell header. If a packet is 50 byte then ATM
must use two cells: 2 * 48 bytes payload including 2 * 5 byte
header, thus consume 106 byte on the wire. The last optional
value CELLOVERHEAD can be used to specify per cell overhead - for
our ATM example 5. CELLOVERHEAD can be negative, but use
negative values with caution.
Note that rate throttling is limited by several factors: the
kernel clock granularity avoid a perfect shaping at a specific
level. This will show up in an artificial packet compression
(bursts). Another influence factor are network adapter buffers
which can also add artificial delay.
slot
defer delivering accumulated packets to within a slot. Each
available slot can be configured with a minimum delay to acquire,
and an optional maximum delay. Alternatively it can be
configured with the distribution similar to distribution for
delay option. Slot delays can be specified in nanoseconds,
microseconds, milliseconds or seconds (e.g. 800us). Values for
the optional parameters BYTES will limit the number of bytes
delivered per slot, and/or PACKETS will limit the number of
packets delivered per slot.
These slot options can provide a crude approximation of bursty
MACs such as DOCSIS, WiFi, and LTE.
Note that slotting is limited by several factors: the kernel
clock granularity, as with a rate, and attempts to deliver many
packets within a slot will be smeared by the timer resolution,
and by the underlying native bandwidth also.
It is possible to combine slotting with a rate, in which case
complex behaviors where either the rate, or the slot limits on
bytes or packets per slot, govern the actual delivered rate.
Ограничения (Limitations)
The main known limitation of Netem are related to timer
granularity, since Linux is not a real-time operating system.
Примеры (Examples)
tc qdisc add dev eth0 root netem rate 5kbit 20 100 5
delay all outgoing packets on device eth0 with a rate of
5kbit, a per packet overhead of 20 byte, a cellsize of 100
byte and a per celloverhead of 5 byte:
Источники (Sources)
1. Hemminger S. , "Network Emulation with NetEm", Open Source
Development Lab, April 2005 (http://devresources.linux-
foundation.org/shemminger/netem/LCA2005_paper.pdf)
2. Netem page from Linux foundation,
(https://wiki.linuxfoundation.org/networking/netem)
3. Salsano S., Ludovici F., Ordine A., "Definition of a general
and intuitive loss model for packet networks and its
implementation in the Netem module in the Linux kernel",
available at http://netgroup.uniroma2.it/NetemCLG
Смотри также (See also)
tc(8), tc-tbf(8)
