tc-cake(8) — Linux manual page


CAKE(8)                             Linux                            CAKE(8)

NAME         top

       CAKE - Common Applications Kept Enhanced (CAKE)

SYNOPSIS         top

       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         top

       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


       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.

            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


       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.

            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

            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

            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.

            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

            Equivalent to overhead 10 atm

            Equivalent to overhead 14 atm

            Equivalent to overhead 32 atm

            Equivalent to overhead 40 atm

            Equivalent to overhead 24 atm

            Equivalent to overhead 32 atm

            Equivalent to overhead 8 atm

            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.

            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

            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

       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

            Equivalent to overhead 18 mpu 64 noatm

   Ethernet Overhead Keywords
            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

            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.

            For extremely high-performance 10GigE+ networks only.
       Equivalent to rtt 100us.

            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.

            For traffic mostly within a single city.  Equivalent to rtt

            For traffic mostly within a European-sized country.  Equivalent
       to rtt 30ms.

       internet (default)
            This is suitable for most Internet traffic.  Equivalent to rtt

            For Internet traffic with generally above-average latency, such
       as that suffered by Australasian residents.  Equivalent to rtt 300ms.

            For traffic via geostationary satellites.  Equivalent to rtt

            So named because Jupiter is about 1 light-hour from Earth.  Use
       this to (almost) completely disable AQM actions.  Equivalent to rtt


       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 minimises latency for "sparse" flows.  CAKE
       uses a set-associative hashing algorithm to minimise flow collisions.

       These keywords specify whether fairness based on source address,
       destination address, individual flows, or any combination of those is

            Disables flow isolation; all traffic passes through a single
       queue for each tin.

            Flows are defined only by source address.  Could be useful on
       the egress path of an ISP backhaul.

            Flows are defined only by destination address.  Could be useful
       on the ingress path of an ISP backhaul.

            Flows are defined by source-destination host pairs.  This is
       host isolation, rather than flow isolation.

            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

            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.

            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.

            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.

            Disables priority queuing by placing all traffic in one tin.

            Enables legacy interpretation of TOS "Precedence" field.  Use of
       this preset on the modern Internet is firmly discouraged.

            Provides a general-purpose Diffserv implementation with four
                 Bulk (CS1), 6.25% threshold, generally low priority.
                 Best Effort (general), 100% threshold.
                 Video (AF4x, AF3x, CS3, AF2x, CS2, TOS4, TOS1), 50%
                 Voice (CS7, CS6, EF, VA, CS5, CS4), 25% threshold.

       diffserv3 (default)
            Provides a simple, general-purpose Diffserv implementation with
       three tins:
                 Bulk (CS1), 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.


            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

       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.


            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

       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         top

       # 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
        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         top

       tc(8), tc-codel(8), tc-fq_codel(8), tc-htb(8)

AUTHORS         top

       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

COLOPHON         top

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iproute2                        19 July 2018                         CAKE(8)

Pages that refer to this page: ctinfo(8)tc(8)tc-ctinfo(8)