CAKE(8) Linux CAKE(8)
NAME
CAKE - Common Applications Kept Enhanced (CAKE)
SYNOPSIS
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)
DESCRIPTION
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 implementa-
tion. Overhead compensation for various encapsulation schemes is
tightly integrated.
All settings are optional; the default settings are chosen to be sensi-
ble 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.
SHAPER PARAMETERS
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 periodi-
cally be set to a bandwidth slightly below the estimated rate. This
estimator cannot estimate the bandwidth of links downstream of itself.
OVERHEAD COMPENSATION PARAMETERS
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 compati-
bility 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.
ROUND TRIP TIME PARAMETERS
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 speci-
fying 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 mag-
nitude 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.
FLOW ISOLATION PARAMETERS
With flow isolation enabled, CAKE places packets from different flows
into different queues, each of which carries its own AQM state. Pack-
ets 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, desti-
nation 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, desti-
nation 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-isola-
tion 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 per-
formed on a different host.
nonat (default)
Cake will not perform a NAT lookup. Flow isolation will be per-
formed using the addresses and port numbers directly visible to the
interface Cake is attached to.
PRIORITY QUEUE PARAMETERS
CAKE can divide traffic into "tins" based on the Diffserv field. Each
tin has its own independent set of flow-isolation queues, and is ser-
viced 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 follow-
ing 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% thresh-
old.
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 selec-
tion. 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.
OTHER PARAMETERS
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 sin-
gle 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 effi-
ciency. 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 through-
put by retaining the full GSO packets.
OVERRIDING CLASSIFICATION WITH TC FILTERS
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.
EXAMPLES
# 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
SEE ALSO
tc(8), tc-codel(8), tc-fq_codel(8), tc-htb(8)
AUTHORS
Cake's principal author is Jonathan Morton, with contributions from
Tony Ambardar, Kevin Darbyshire-Bryant, Toke Hoiland-Jorgensen, Sebas-
tian Moeller, Ryan Mounce, Dean Scarff, Nils Andreas Svee, and Dave
Taht.
This manual page was written by Loganaden Velvindron. Please report
corrections to the Linux Networking mailing list <netdev AT vger.ker-
nel.org>.
iproute2 19 July 2018 CAKE(8)
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