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Manual: tc-cake

CAKE(8) Linux CAKE(8)

NAME
CAKE - Common Applications Kept Enhanced (CAKE)

DESCRIPTION
CAKE (Common Applications Kept Enhanced) is a shaping-capable queue discipline which uses both AQM and FQ. It com‐
bines 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 encap‐
sulation 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.

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 periodically 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 com‐
pensate 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 tech‐
nologies.

Manual Overhead Specification
overhead BYTES
Adds BYTES to the size of each packet. BYTES may be negative; values between -64 and 256 (inclusive) are ac‐
cepted.

mpu BYTES
Rounds each packet (including overhead) up to a minimum length BYTES. BYTES may not be negative; values be‐
tween 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 op‐
erate 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 informa‐
tion. 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 bot‐
tleneck 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 neces‐
sary to express this.

ROUND TRIP TIME PARAMETERS
Active Queue Management (AQM) consists of embedding congestion signals in the packet flow, which receivers use to in‐
struct 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 ac‐
tions.
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. 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 combi‐
nation 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 individ‐
ual 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 intelli‐
gently (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 ef‐
fect 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 di‐
rectly 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-isola‐
tion 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, rela‐
tive 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.

OTHER PARAMETERS
ingress
Indicates that CAKE is running in ingress mode (i.e. running on the downlink of a connection). This changes
the shaper to also count dropped packets as data transferred, as these will have already traversed the link
before CAKE can choose what to do with them.

In addition, the AQM will be tuned to always keep at least two packets queued per flow. The reason for this is
that retransmits are more expensive in ingress mode, since dropped packets have to traverse the link again;
thus, keeping a minimum number of packets queued will improve throughput in cases where the number of active
flows are so large that they saturate the link even at their minimum window size.

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 net‐
work, 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 band‐
widths. 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.

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 in‐
stance; 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
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
.

iproute2 19 July 2018 CAKE(8)

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Manual: tc-cake

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