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CAKE(8) Linux CAKE(8)
CAKE - Common Applications Kept Enhanced (CAKE)
tc qdisc ... cake
[ bandwidth RATE | unlimited* | autorate-ingress ]
[ rtt TIME | datacentre | lan | metro | regional | internet* |
oceanic | satellite | interplanetary ]
[ besteffort | diffserv8 | diffserv4 | diffserv3* ]
[ flowblind | srchost | dsthost | hosts | flows | dual-srchost |
dual-dsthost | triple-isolate* ]
[ nat | nonat* ]
[ wash | nowash* ]
[ split-gso* | no-split-gso ]
[ ack-filter | ack-filter-aggressive | no-ack-filter* ]
[ memlimit LIMIT ]
[ fwmark MASK ]
[ ptm | atm | noatm* ]
[ overhead N | conservative | raw* ]
[ mpu N ]
[ ingress | egress* ]
(* marks defaults)
CAKE (Common Applications Kept Enhanced) is a shaping-capable
queue discipline which uses both AQM and FQ. It combines COBALT,
which is an AQM algorithm combining Codel and BLUE, a shaper
which operates in deficit mode, and a variant of DRR++ for flow
isolation. 8-way set-associative hashing is used to virtually
eliminate hash collisions. Priority queuing is available through
a simplified diffserv implementation. Overhead compensation for
various encapsulation schemes is tightly integrated.
All settings are optional; the default settings are chosen to be
sensible in most common deployments. Most people will only need
to set the bandwidth parameter to get useful results, but reading
the Overhead Compensation and Round Trip Time sections is
strongly encouraged.
CAKE uses a deficit-mode shaper, which does not exhibit the
initial burst typical of token-bucket shapers. It will
automatically burst precisely as much as required to maintain the
configured throughput. As such, it is very straightforward to
configure.
unlimited (default)
No limit on the bandwidth.
bandwidth RATE
Set the shaper bandwidth. See tc(8) or examples below for
details of the RATE value.
autorate-ingress
Automatic capacity estimation based on traffic arriving at
this qdisc. This is most likely to be useful with cellular
links, which tend to change quality randomly. A bandwidth
parameter can be used in conjunction to specify an initial
estimate. The shaper will periodically be set to a bandwidth
slightly below the estimated rate. This estimator cannot
estimate the bandwidth of links downstream of itself.
The size of each packet on the wire may differ from that seen by
Linux. The following parameters allow CAKE to compensate for
this difference by internally considering each packet to be
bigger than Linux informs it. To assist users who are not expert
network engineers, keywords have been provided to represent a
number of common link technologies.
Manual Overhead Specification
overhead BYTES
Adds BYTES to the size of each packet. BYTES may be
negative; values between -64 and 256 (inclusive) are accepted.
mpu BYTES
Rounds each packet (including overhead) up to a minimum
length BYTES. BYTES may not be negative; values between 0 and 256
(inclusive) are accepted.
atm
Compensates for ATM cell framing, which is normally found on
ADSL links. This is performed after the overhead parameter
above. ATM uses fixed 53-byte cells, each of which can carry 48
bytes payload.
ptm
Compensates for PTM encoding, which is normally found on
VDSL2 links and uses a 64b/65b encoding scheme. It is even more
efficient to simply derate the specified shaper bandwidth by a
factor of 64/65 or 0.984. See ITU G.992.3 Annex N and IEEE 802.3
Section 61.3 for details.
noatm
Disables ATM and PTM compensation.
Failsafe Overhead Keywords
These two keywords are provided for quick-and-dirty setup. Use
them if you can't be bothered to read the rest of this section.
raw (default)
Turns off all overhead compensation in CAKE. The packet
size reported by Linux will be used directly.
Other overhead keywords may be added after "raw". The
effect of this is to make the overhead compensation operate
relative to the reported packet size, not the underlying IP
packet size.
conservative
Compensates for more overhead than is likely to occur on any
widely-deployed link technology.
Equivalent to overhead 48 atm.
ADSL Overhead Keywords
Most ADSL modems have a way to check which framing scheme is in
use. Often this is also specified in the settings document
provided by the ISP. The keywords in this section are intended
to correspond with these sources of information. All of them
implicitly set the atm flag.
pppoa-vcmux
Equivalent to overhead 10 atm
pppoa-llc
Equivalent to overhead 14 atm
pppoe-vcmux
Equivalent to overhead 32 atm
pppoe-llcsnap
Equivalent to overhead 40 atm
bridged-vcmux
Equivalent to overhead 24 atm
bridged-llcsnap
Equivalent to overhead 32 atm
ipoa-vcmux
Equivalent to overhead 8 atm
ipoa-llcsnap
Equivalent to overhead 16 atm
See also the Ethernet Correction Factors section below.
VDSL2 Overhead Keywords
ATM was dropped from VDSL2 in favour of PTM, which is a much more
straightforward framing scheme. Some ISPs retained PPPoE for
compatibility with their existing back-end systems.
pppoe-ptm
Equivalent to overhead 30 ptm
PPPoE: 2B PPP + 6B PPPoE +
ETHERNET: 6B dest MAC + 6B src MAC + 2B ethertype + 4B Frame
Check Sequence +
PTM: 1B Start of Frame (S) + 1B End of Frame (Ck) + 2B TC-
CRC (PTM-FCS)
bridged-ptm
Equivalent to overhead 22 ptm
ETHERNET: 6B dest MAC + 6B src MAC + 2B ethertype + 4B Frame
Check Sequence +
PTM: 1B Start of Frame (S) + 1B End of Frame (Ck) + 2B TC-
CRC (PTM-FCS)
See also the Ethernet Correction Factors section below.
DOCSIS Cable Overhead Keyword
DOCSIS is the universal standard for providing Internet service
over cable-TV infrastructure.
In this case, the actual on-wire overhead is less important than
the packet size the head-end equipment uses for shaping and
metering. This is specified to be an Ethernet frame including
the CRC (aka FCS).
docsis
Equivalent to overhead 18 mpu 64 noatm
Ethernet Overhead Keywords
ethernet
Accounts for Ethernet's preamble, inter-frame gap, and Frame
Check Sequence. Use this keyword when the bottleneck being
shaped for is an actual Ethernet cable.
Equivalent to overhead 38 mpu 84 noatm
ether-vlan
Adds 4 bytes to the overhead compensation, accounting for an
IEEE 802.1Q VLAN header appended to the Ethernet frame header.
NB: Some ISPs use one or even two of these within PPPoE; this
keyword may be repeated as necessary to express this.
Active Queue Management (AQM) consists of embedding congestion
signals in the packet flow, which receivers use to instruct
senders to slow down when the queue is persistently occupied.
CAKE uses ECN signalling when available, and packet drops
otherwise, according to a combination of the Codel and BLUE AQM
algorithms called COBALT.
Very short latencies require a very rapid AQM response to
adequately control latency. However, such a rapid response tends
to impair throughput when the actual RTT is relatively long.
CAKE allows specifying the RTT it assumes for tuning various
parameters. Actual RTTs within an order of magnitude of this
will generally work well for both throughput and latency
management.
At the 'lan' setting and below, the time constants are similar in
magnitude to the jitter in the Linux kernel itself, so congestion
might be signalled prematurely. The flows will then become sparse
and total throughput reduced, leaving little or no back-pressure
for the fairness logic to work against. Use the "metro" setting
for local lans unless you have a custom kernel.
rtt TIME
Manually specify an RTT.
datacentre
For extremely high-performance 10GigE+ networks only.
Equivalent to rtt 100us.
lan
For pure Ethernet (not Wi-Fi) networks, at home or in the
office. Don't use this when shaping for an Internet access link.
Equivalent to rtt 1ms.
metro
For traffic mostly within a single city. Equivalent to rtt
10ms.
regional
For traffic mostly within a European-sized country.
Equivalent to rtt 30ms.
internet (default)
This is suitable for most Internet traffic. Equivalent to
rtt 100ms.
oceanic
For Internet traffic with generally above-average latency,
such as that suffered by Australasian residents. Equivalent to
rtt 300ms.
satellite
For traffic via geostationary satellites. Equivalent to rtt
1000ms.
interplanetary
So named because Jupiter is about 1 light-hour from Earth.
Use this to (almost) completely disable AQM actions. Equivalent
to rtt 3600s.
With flow isolation enabled, CAKE places packets from different
flows into different queues, each of which carries its own AQM
state. Packets from each queue are then delivered fairly,
according to a DRR++ algorithm which minimizes latency for
"sparse" flows. CAKE uses a set-associative hashing algorithm to
minimize flow collisions.
These keywords specify whether fairness based on source address,
destination address, individual flows, or any combination of
those is desired.
flowblind
Disables flow isolation; all traffic passes through a single
queue for each tin.
srchost
Flows are defined only by source address. Could be useful
on the egress path of an ISP backhaul.
dsthost
Flows are defined only by destination address. Could be
useful on the ingress path of an ISP backhaul.
hosts
Flows are defined by source-destination host pairs. This is
host isolation, rather than flow isolation.
flows
Flows are defined by the entire 5-tuple of source address,
destination address, transport protocol, source port and
destination port. This is the type of flow isolation performed
by SFQ and fq_codel.
dual-srchost
Flows are defined by the 5-tuple, and fairness is applied
first over source addresses, then over individual flows. Good
for use on egress traffic from a LAN to the internet, where it'll
prevent any one LAN host from monopolising the uplink, regardless
of the number of flows they use.
dual-dsthost
Flows are defined by the 5-tuple, and fairness is applied
first over destination addresses, then over individual flows.
Good for use on ingress traffic to a LAN from the internet, where
it'll prevent any one LAN host from monopolising the downlink,
regardless of the number of flows they use.
triple-isolate (default)
Flows are defined by the 5-tuple, and fairness is applied
over source *and* destination addresses intelligently (ie. not
merely by host-pairs), and also over individual flows. Use this
if you're not certain whether to use dual-srchost or dual-
dsthost; it'll do both jobs at once, preventing any one host on
*either* side of the link from monopolising it with a large
number of flows.
nat
Instructs Cake to perform a NAT lookup before applying flow-
isolation rules, to determine the true addresses and port numbers
of the packet, to improve fairness between hosts "inside" the
NAT. This has no practical effect in "flowblind" or "flows"
modes, or if NAT is performed on a different host.
nonat (default)
Cake will not perform a NAT lookup. Flow isolation will be
performed using the addresses and port numbers directly visible
to the interface Cake is attached to.
CAKE can divide traffic into "tins" based on the Diffserv field.
Each tin has its own independent set of flow-isolation queues,
and is serviced based on a WRR algorithm. To avoid perverse
Diffserv marking incentives, tin weights have a "priority
sharing" value when bandwidth used by that tin is below a
threshold, and a lower "bandwidth sharing" value when above.
Bandwidth is compared against the threshold using the same
algorithm as the deficit-mode shaper.
Detailed customisation of tin parameters is not provided. The
following presets perform all necessary tuning, relative to the
current shaper bandwidth and RTT settings.
besteffort
Disables priority queuing by placing all traffic in one tin.
precedence
Enables legacy interpretation of TOS "Precedence" field.
Use of this preset on the modern Internet is firmly discouraged.
diffserv4
Provides a general-purpose Diffserv implementation with four
tins:
Bulk (CS1, LE in kernel v5.9+), 6.25% threshold,
generally low priority.
Best Effort (general), 100% threshold.
Video (AF4x, AF3x, CS3, AF2x, CS2, TOS4, TOS1), 50%
threshold.
Voice (CS7, CS6, EF, VA, CS5, CS4), 25% threshold.
diffserv3 (default)
Provides a simple, general-purpose Diffserv implementation
with three tins:
Bulk (CS1, LE in kernel v5.9+), 6.25% threshold,
generally low priority.
Best Effort (general), 100% threshold.
Voice (CS7, CS6, EF, VA, TOS4), 25% threshold, reduced
Codel interval.
fwmark MASK
This options turns on fwmark-based overriding of CAKE's tin
selection. If set, the option specifies a bitmask that will be
applied to the fwmark associated with each packet. If the result
of this masking is non-zero, the result will be right-shifted by
the number of least-significant unset bits in the mask value, and
the result will be used as a the tin number for that packet.
This can be used to set policies in a firewall script that will
override CAKE's built-in tin selection.
memlimit LIMIT
Limit the memory consumed by Cake to LIMIT bytes. Note that
this does not translate directly to queue size (so do not size
this based on bandwidth delay product considerations, but rather
on worst case acceptable memory consumption), as there is some
overhead in the data structures containing the packets,
especially for small packets.
By default, the limit is calculated based on the bandwidth
and RTT settings.
wash
Traffic entering your diffserv domain is frequently mis-
marked in transit from the perspective of your network, and
traffic exiting yours may be mis-marked from the perspective of
the transiting provider.
Apply the wash option to clear all extra diffserv (but not ECN
bits), after priority queuing has taken place.
If you are shaping inbound, and cannot trust the diffserv
markings (as is the case for Comcast Cable, among others), it is
best to use a single queue "besteffort" mode with wash.
split-gso
This option controls whether CAKE will split General
Segmentation Offload (GSO) super-packets into their on-the-wire
components and dequeue them individually.
Super-packets are created by the networking stack to improve
efficiency. However, because they are larger they take longer to
dequeue, which translates to higher latency for competing flows,
especially at lower bandwidths. CAKE defaults to splitting GSO
packets to achieve the lowest possible latency. At link speeds
higher than 10 Gbps, setting the no-split-gso parameter can
increase the maximum achievable throughput by retaining the full
GSO packets.
CAKE supports overriding of its internal classification of
packets through the tc filter mechanism. Packets can be assigned
to different priority tins by setting the priority field on the
skb, and the flow hashing can be overridden by setting the
classid parameter.
Tin override
To assign a priority tin, the major number of the
priority field needs to match the qdisc handle of the cake
instance; if it does, the minor number will be interpreted as the
tin index. For example, to classify all ICMP packets as 'bulk',
the following filter can be used:
# tc qdisc replace dev eth0 handle 1: root cake diffserv3
# tc filter add dev eth0 parent 1: protocol ip prio 1 \
u32 match icmp type 0 0 action skbedit priority 1:1
Flow hash override
To override flow hashing, the classid can be set. CAKE
will interpret the major number of the classid as the host hash
used in host isolation mode, and the minor number as the flow
hash used for flow-based queueing. One or both of those can be
set, and will be used if the relevant flow isolation parameter is
set (i.e., the major number will be ignored if CAKE is not
configured in hosts mode, and the minor number will be ignored if
CAKE is not configured in flows mode).
This example will assign all ICMP packets to the first queue:
# tc qdisc replace dev eth0 handle 1: root cake
# tc filter add dev eth0 parent 1: protocol ip prio 1 \
u32 match icmp type 0 0 classid 0:1
If only one of the host and flow overrides is set, CAKE will
compute the other hash from the packet as normal. Note, however,
that the host isolation mode works by assigning a host ID to the
flow queue; so if overriding both host and flow, the same flow
cannot have more than one host assigned. In addition, it is not
possible to assign different source and destination host IDs
through the override mechanism; if a host ID is assigned, it will
be used as both source and destination host.
# tc qdisc delete root dev eth0
# tc qdisc add root dev eth0 cake bandwidth 100Mbit ethernet
# tc -s qdisc show dev eth0
qdisc cake 1: root refcnt 2 bandwidth 100Mbit diffserv3 triple-
isolate rtt 100.0ms noatm overhead 38 mpu 84
Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
backlog 0b 0p requeues 0
memory used: 0b of 5000000b
capacity estimate: 100Mbit
min/max network layer size: 65535 / 0
min/max overhead-adjusted size: 65535 / 0
average network hdr offset: 0
Bulk Best Effort Voice
thresh 6250Kbit 100Mbit 25Mbit
target 5.0ms 5.0ms 5.0ms
interval 100.0ms 100.0ms 100.0ms
pk_delay 0us 0us 0us
av_delay 0us 0us 0us
sp_delay 0us 0us 0us
pkts 0 0 0
bytes 0 0 0
way_inds 0 0 0
way_miss 0 0 0
way_cols 0 0 0
drops 0 0 0
marks 0 0 0
ack_drop 0 0 0
sp_flows 0 0 0
bk_flows 0 0 0
un_flows 0 0 0
max_len 0 0 0
quantum 300 1514 762
After some use:
# tc -s qdisc show dev eth0
qdisc cake 1: root refcnt 2 bandwidth 100Mbit diffserv3 triple-
isolate rtt 100.0ms noatm overhead 38 mpu 84
Sent 44709231 bytes 31931 pkt (dropped 45, overlimits 93782
requeues 0)
backlog 33308b 22p requeues 0
memory used: 292352b of 5000000b
capacity estimate: 100Mbit
min/max network layer size: 28 / 1500
min/max overhead-adjusted size: 84 / 1538
average network hdr offset: 14
Bulk Best Effort Voice
thresh 6250Kbit 100Mbit 25Mbit
target 5.0ms 5.0ms 5.0ms
interval 100.0ms 100.0ms 100.0ms
pk_delay 8.7ms 6.9ms 5.0ms
av_delay 4.9ms 5.3ms 3.8ms
sp_delay 727us 1.4ms 511us
pkts 2590 21271 8137
bytes 3081804 30302659 11426206
way_inds 0 46 0
way_miss 3 17 4
way_cols 0 0 0
drops 20 15 10
marks 0 0 0
ack_drop 0 0 0
sp_flows 2 4 1
bk_flows 1 2 1
un_flows 0 0 0
max_len 1514 1514 1514
quantum 300 1514 762
tc(8), tc-codel(8), tc-fq_codel(8), tc-htb(8)
Cake's principal author is Jonathan Morton, with contributions
from Tony Ambardar, Kevin Darbyshire-Bryant, Toke Høiland-
Jørgensen, Sebastian Moeller, Ryan Mounce, Dean Scarff, Nils
Andreas Svee, and Dave Täht.
This manual page was written by Loganaden Velvindron. Please
report corrections to the Linux Networking mailing list
<[email protected]>.
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
[email protected], [email protected]. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-11.) 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 [email protected]
iproute2 19 July 2018 CAKE(8)
Pages that refer to this page: systemd.network(5), tc(8), tc-ctinfo(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|