TC(8)

💖 TC(8) 💖

okay so bestie, this is TC(8) from Linux — the manual page for tc, aka the traffic control tool 🫶 if you've ever wanted to tell your kernel "slow this down", "prioritize this", or "honestly? drop that", you are in the right place. let's go ✨

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🌸 NAME 🌸

• tc — show / manipulate traffic control settings 💅

🎀 SYNOPSIS 🎀

tc [ OPTIONS ] qdisc [ add | change | replace | link | delete ] dev DEV
   [ parent qdisc-id | root ] [ handle qdisc-id ] [ ingress_block BLOCK_INDEX ]
   [ egress_block BLOCK_INDEX ] qdisc [ qdisc specific parameters ]

tc [ OPTIONS ] class [ add | change | replace | delete | show ] dev DEV
   parent qdisc-id [ classid class-id ] qdisc [ qdisc specific parameters ]

tc [ OPTIONS ] filter [ add | change | replace | delete | get ] dev DEV
   [ parent qdisc-id | root ] [ handle filter-id ] protocol protocol prio priority
   filtertype [ filtertype specific parameters ] flowid flow-id

tc [ OPTIONS ] filter [ add | change | replace | delete | get ] block BLOCK_INDEX
   [ handle filter-id ] protocol protocol prio priority filtertype
   [ filtertype specific parameters ] flowid flow-id

tc [ OPTIONS ] chain [ add | delete | get ] dev DEV [ parent qdisc-id | root ]
   filtertype [ filtertype specific parameters ]

tc [ OPTIONS ] chain [ add | delete | get ] block BLOCK_INDEX filtertype
   [ filtertype specific parameters ]

tc [ OPTIONS ] [ FORMAT ] qdisc { show | list } [ dev DEV ]
   [ root | ingress | handle QHANDLE | parent CLASSID ] [ invisible ]

tc [ OPTIONS ] [ FORMAT ] class show dev DEV

tc [ OPTIONS ] filter show dev DEV

tc [ OPTIONS ] filter show block BLOCK_INDEX

tc [ OPTIONS ] chain show dev DEV

tc [ OPTIONS ] chain show block BLOCK_INDEX

tc [ OPTIONS ] monitor [ file FILENAME ]

OPTIONS := { [ -force ] -b[atch] [ filename ] | [ -n[etns] name ] | [ -N[umeric] ] |
   [ -nm | -nam[es] ] | [ { -cf | -c[onf] } [ filename ] ] [ -t[imestamp] ] |
   [ -t[short] | [ -o[neline] ] | [ -echo ] }

FORMAT := { -s[tatistics] | -d[etails] | -r[aw] | -i[ec] | -g[raph] | -j[son] |
   -p[retty] | -col[or] }

so yeah, OPTIONS and FORMAT are both little bundles of choices, and the bracket situation is giving "pick whichever ones you want". 🎀

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✨ DESCRIPTION ✨

Tc is used to configure Traffic Control in the Linux kernel. and Traffic Control, bestie, consists of the following four iconic behaviours:

🦋 SHAPING

when traffic is shaped, its rate of transmission is under control. and shaping is highkey more than just lowering the available bandwidth — it's also used to smooth out bursts in traffic for better network behaviour. shaping happens on egress. 📤

⏰ SCHEDULING

by scheduling the transmission of packets, it's possible to improve interactivity for the traffic that needs it, while still guaranteeing bandwidth to bulk transfers. reordering is also called prioritizing, and it happens only on egress.

👮 POLICING

whereas shaping deals with transmission of traffic, policing pertains to traffic arriving. so policing occurs on ingress. 📥

💀 DROPPING

traffic exceeding a set bandwidth may also be dropped forthwith — both on ingress and on egress. no thoughts, just gone.

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so the processing of traffic is controlled by three kinds of objects: qdiscs, classes and filters. that's the whole cast. 🫶

👑 QDISCS 👑

qdisc is short for 'queueing discipline' and it is elementary to understanding traffic control. whenever the kernel needs to send a packet to an interface, it gets enqueued to the qdisc configured for that interface. immediately afterwards, the kernel tries to get as many packets as possible from the qdisc, so it can hand them over to the network adaptor driver. it's giving "inbox zero, but for packets". 📬

a simple QDISC is the pfifo one, which does no processing at all and is a pure First In, First Out queue. it does however store traffic when the network interface can't handle it momentarily. it's the chill one. 😌

🧺 CLASSES 🧺

some qdiscs can contain classes, which contain further qdiscs — traffic may then be enqueued in any of the inner qdiscs, which are within the classes. when the kernel tries to dequeue a packet from such a classful qdisc, it can come from any of the classes. a qdisc may for example prioritize certain kinds of traffic by trying to dequeue from certain classes before others. so some classes are simply the main characters. 👑

🔍 FILTERS 🔍

a filter is used by a classful qdisc to determine in which class a packet will be enqueued. whenever traffic arrives at a class with subclasses, it needs to be classified. various methods may be employed to do so, and one of these are the filters. all filters attached to the class are called, until one of them returns with a verdict. if no verdict was made, other criteria may be available — and this differs per qdisc.

⚠️ it is important to notice that filters reside within qdiscs — they are not masters of what happens. filters are the vibe consultants, not the CEO. 💅

the available filters are:

• basic — filter packets based on an ematch expression. see tc-ematch(8) for details. 🍓
• bpf — filter packets using (e)BPF, see tc-bpf(8) for details.
• cgroup — filter packets based on the control group of their process. see tc-cgroup(8) for details.
• flow, flower — flow-based classifiers, filtering packets based on their flow (identified by selectable keys). see tc-flow(8) and tc-flower(8) for details. 🌸
• fw — filter based on fwmark. directly maps fwmark value to traffic class. see tc-fw(8).
• route — filter packets based on routing table. see tc-route(8) for details.
• u32 — generic filtering on arbitrary packet data, assisted by syntax to abstract common operations. see tc-u32(8) for details.
• matchall — traffic control filter that matches every packet. see tc-matchall(8) for details. it's the "yes, all of you, come in" of filters. 🫶

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⚡ QEVENTS ⚡

qdiscs may invoke user-configured actions when certain interesting events take place in the qdisc. each qevent can either be unused, or can have a block attached to it. to this block are then attached filters using the "tc block BLOCK_IDX" syntax. the block is executed when the qevent associated with the attachment point takes place. for example, a packet could be dropped, or delayed, etc., depending on the qdisc and the qevent in question. it's the qdisc's way of going "not me watching this happen 👀" and then doing something about it.

for example:

tc qdisc add dev eth0 root handle 1: red limit 500K avpkt 1K \
   qevent early_drop block 10
tc filter add block 10 matchall action mirred egress mirror dev eth1

🌈 CLASSLESS QDISCS 🌈

the classless qdiscs are:

• choke — CHOKe (CHOose and Keep for responsive flows, CHOose and Kill for unresponsive flows) is a classless qdisc designed to both identify and penalize flows that monopolize the queue. CHOKe is a variation of RED, and the configuration is similar to RED.
• codel — CoDel (pronounced "coddle") is an adaptive "no-knobs" active queue management algorithm (AQM) scheme that was developed to address the shortcomings of RED and its variants.
• [p|b]fifo — simplest usable qdisc, pure First In, First Out behaviour. limited in packets or in bytes.
• fq — Fair Queue Scheduler realises TCP pacing and scales to millions of concurrent flows per qdisc. 🔥
• fq_codel — Fair Queuing Controlled Delay is a queuing discipline that combines Fair Queuing with the CoDel AQM scheme. FQ_Codel uses a stochastic model to classify incoming packets into different flows and is used to provide a fair share of the bandwidth to all the flows using the queue. each such flow is managed by the CoDel queuing discipline. reordering within a flow is avoided since Codel internally uses a FIFO queue.
• fq_pie — FQ-PIE (Flow Queuing with Proportional Integral controller Enhanced) is a queuing discipline that combines Flow Queuing with the PIE AQM scheme. FQ-PIE uses a Jenkins hash function to classify incoming packets into different flows and is used to provide a fair share of the bandwidth to all the flows using the qdisc. each such flow is managed by the PIE algorithm.
• gred — Generalized Random Early Detection combines multiple RED queues in order to achieve multiple drop priorities. this is required to realize Assured Forwarding (RFC 2597).
• hhf — Heavy-Hitter Filter differentiates between small flows and the opposite, heavy-hitters. the goal is to catch the heavy-hitters and move them to a separate queue with less priority, so that bulk traffic does not affect the latency of critical traffic. goodbye to the people hogging the aux cord 🎧
• ingress — this is a special qdisc as it applies to incoming traffic on an interface, allowing for it to be filtered and policed.
• mqprio — the Multiqueue Priority Qdisc is a simple queuing discipline that allows mapping traffic flows to hardware queue ranges using priorities and a configurable priority to traffic class mapping. a traffic class in this context is a set of contiguous qdisc classes which map 1:1 to a set of hardware exposed queues.
• multiq — Multiqueue is a qdisc optimized for devices with multiple Tx queues. it has been added for hardware that wishes to avoid head-of-line blocking. it will cycle through the bands and verify that the hardware queue associated with the band is not stopped prior to dequeuing a packet.
• netem — Network Emulator is an enhancement of the Linux traffic control facilities that allow one to add delay, packet loss, duplication and more other characteristics to packets outgoing from a selected network interface. chaos mode: enabled 🎭
• pfifo_fast — standard qdisc for 'Advanced Router' enabled kernels. consists of a three-band queue which honors Type of Service flags, as well as the priority that may be assigned to a packet.
• pie — Proportional Integral controller-Enhanced (PIE) is a control theoretic active queue management scheme. it is based on the proportional integral controller but aims to control delay.
• red — Random Early Detection simulates physical congestion by randomly dropping packets when nearing configured bandwidth allocation. well suited to very large bandwidth applications.
• sfb — Stochastic Fair Blue is a classless qdisc to manage congestion based on packet loss and link utilization history while trying to prevent non-responsive flows (i.e. flows that do not react to congestion marking or dropped packets) from impacting performance of responsive flows. unlike RED, where the marking probability has to be configured, BLUE tries to determine the ideal marking probability automatically. it's the self-sufficient one. ✨
• sfq — Stochastic Fairness Queueing reorders queued traffic so each 'session' gets to send a packet in turn. take turns bestie, it's not that hard 💅
• tbf — the Token Bucket Filter is suited for slowing traffic down to a precisely configured rate. scales well to large bandwidths.

🛠️ CONFIGURING CLASSLESS QDISCS 🛠️

in the absence of classful qdiscs, classless qdiscs can only be attached at the root of a device. full syntax:

tc qdisc add dev DEV root QDISC QDISC-PARAMETERS

to remove, issue:

tc qdisc del dev DEV root

💡 the pfifo_fast qdisc is the automatic default in the absence of a configured qdisc. so if you never touched anything, congrats, you've been running pfifo_fast this whole time.

👑 CLASSFUL QDISCS 👑

bestie the classful qdiscs are the ones with actual classes inside, and here's the whole lineup:

• ATM — maps flows onto virtual circuits of an underlying asynchronous transfer mode device. it's giving vintage networking, but okay 💅
• DRR — the Deficit Round Robin Scheduler, aka a way more flexible glow-up of Stochastic Fairness Queuing. and here's the tea: unlike SFQ, there are no built-in queues — you have to add classes yourself and then set up filters so packets get classified accordingly. this is genuinely useful, like, e.g. for using RED qdiscs with different settings for particular traffic. also there's no default class — if a packet can't be classified, it gets dropped. no home, no hope 😭
• ETS — merges the functionality of PRIO and DRR qdiscs into one single scheduler. it makes it easy to configure a set of strict and bandwidth-sharing bands to implement the transmission selection described in 802.1Qaz. one scheduler, two vibes, iconic ✨
• HFSC — Hierarchical Fair Service Curve. guarantees precise bandwidth and delay allocation for leaf classes, and allocates excess bandwidth fairly. unlike HTB, it makes use of packet dropping to achieve low delays, which interactive sessions benefit from. lowkey the people-pleaser of schedulers 🫶
• HTB — the Hierarchy Token Bucket, implementing a rich linksharing hierarchy of classes with an emphasis on conforming to existing practices. it facilitates guaranteeing bandwidth to classes, while also allowing you to specify upper limits to inter-class sharing. it contains shaping elements based on TBF, and it can prioritize classes. main character energy fr fr 👑
• PRIO — a non-shaping container for a configurable number of classes which are dequeued in order. this allows for easy prioritization of traffic, where lower classes are only able to send if higher ones have no packets available. and to make configuration easier, Type Of Service bits are honored by default. the way it just… works 💖
• QFQ — Quick Fair Queueing, an O(1) scheduler that provides near-optimal guarantees, and is the first to achieve that goal with a constant cost also with respect to the number of groups and the packet length. the QFQ algorithm has no loops, and uses very simple instructions and data structures that lend themselves very well to a hardware implementation. no loops, all vibes 🦋

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🧠 THEORY OF OPERATION 🧠

okay so classes form a tree, where every class has a single parent. but a class may have multiple children. some qdiscs let you add classes at runtime (HTB), while others (PRIO) are created with a static number of children. no take-backsies on that one.

Qdiscs which allow dynamic addition of classes can have zero or more subclasses to which traffic may be enqueued.

Furthermore, each class contains a leaf qdisc which by default has pfifo behaviour, although another qdisc can be attached in place. This qdisc may again contain classes, but each class can have only one leaf qdisc. one leaf per class, that's the rule 🍓

When a packet enters a classful qdisc it can be classified to one of the classes within. Three criteria are available, although not all qdiscs will use all three:

• tc filters — if tc filters are attached to a class, they are consulted first for relevant instructions. Filters can match on all fields of a packet header, as well as on the firewall mark applied by iptables.
• Type of Service — some qdiscs have built in rules for classifying packets based on the TOS field.
• skb->priority — userspace programs can encode a class-id in the skb->priority field using the SO_PRIORITY option.

Each node within the tree can have its own filters, but higher level filters may also point directly to lower classes. it's giving shortcuts, and we love that 💅

If classification did not succeed, packets are enqueued to the leaf qdisc attached to that class. Check qdisc specific manpages for details, however. not me giving you homework 🙈

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🏷️ NAMING 🏷️

All qdiscs, classes and filters have IDs, which can either be specified or be automatically assigned.

IDs consist of a major number and a minor number, separated by a colon — major:minor. Both major and minor are hexadecimal numbers and are limited to 16 bits. There are two special values: root is signified by major and minor of all ones, and unspecified is all zeros.

• QDISCS — a qdisc, which potentially can have children, gets assigned a major number, called a 'handle', leaving the minor number namespace available for classes. The handle is expressed as '10:'. It is customary to explicitly assign a handle to qdiscs expected to have children.
• CLASSES — classes residing under a qdisc share their qdisc major number, but each have a separate minor number called a 'classid' that has no relation to their parent classes, only to their parent qdisc. The same naming custom as for qdiscs applies.
• FILTERS — filters have a three part ID, which is only needed when using a hashed filter hierarchy.

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⚙️ PARAMETERS ⚙️

The following parameters are widely used in TC. For other parameters, see the man pages for individual qdiscs. iykyk.

💸 RATES

Bandwidths or rates. These parameters accept a floating point number, possibly followed by either a unit (both SI and IEC units supported), or a float followed by a '%' character to specify the rate as a percentage of the device's speed (e.g. 5%, 99.5%).

⚠️ Warning: specifying the rate as a percentage means a fraction of the current speed; if the speed changes, the value will not be recalculated. not it being stale 😭

• bit or a bare number — Bits per second
• kbit — Kilobits per second
• mbit — Megabits per second
• gbit — Gigabits per second
• tbit — Terabits per second
• bps — Bytes per second
• kbps — Kilobytes per second
• mbps — Megabytes per second
• gbps — Gigabytes per second
• tbps — Terabytes per second

To specify in IEC units, replace the SI prefix (k-, m-, g-, t-) with IEC prefix (ki-, mi-, gi- and ti-) respectively. glow up achieved ✨

TC store rates as a 32-bit unsigned integer in bps internally, so we can specify a max rate of 4294967295 bps.

⏱️ TIMES

Length of time. Can be specified as a floating point number followed by an optional unit:

• s, sec or secs — Whole seconds
• ms, msec or msecs — Milliseconds
• us, usec, usecs or a bare number — Microseconds.

TC defined its own time unit (equal to microsecond) and stores time values as 32-bit unsigned integer, thus we can specify a max time value of 4294967295 usecs.

📦 SIZES

Amounts of data. Can be specified as a floating point number followed by an optional unit:

• b or a bare number — Bytes.
• kbit — Kilobits
• kb or k — Kilobytes
• mbit — Megabits
• mb or m — Megabytes
• gbit — Gigabits
• gb or g — Gigabytes

TC stores sizes internally as 32-bit unsigned integer in byte, so we can specify a max size of 4294967295 bytes. no cap, that's the ceiling 📦

🔢 VALUES

Other values without a unit. These parameters are interpreted as decimal by default, but you can indicate TC to interpret them as octal and hexadecimal by adding a '0' or '0x' prefix respectively. cute little number dialects 🎀

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🛠️ TC COMMANDS 🛠️

The following commands are available for qdiscs, classes and filter:

• add — Add a qdisc, class or filter to a node. For all entities, a parent must be passed, either by passing its ID or by attaching directly to the root of a device. When creating a qdisc or a filter, it can be named with the handle parameter. A class is named with the classid parameter.
• delete — A qdisc can be deleted by specifying its handle, which may also be 'root'. All subclasses and their leaf qdiscs are automatically deleted, as well as any filters attached to them. it takes everyone down with it 💀
• change — Some entities can be modified 'in place'. Shares the syntax of 'add', with the exception that the handle cannot be changed and neither can the parent. In other words, change cannot move a node. no relocating bestie 🚫
• replace — Performs a nearly atomic remove/add on an existing node id. If the node does not exist yet it is created.
• get — Displays a single filter given the interface DEV, qdisc-id, priority, protocol and filter-id.
• show — Displays all filters attached to the given interface. A valid parent ID must be passed.
• link — Only available for qdiscs and performs a replace where the node must exist already.

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👀 MONITOR 👀

The tc utility can monitor events generated by the kernel such as adding/deleting qdiscs, filters or actions, or modifying existing ones. it's literally just watching, lowkey peaceful 🦋

The following command is available for monitor :

• file — If the file option is given, the tc does not listen to kernel events, but opens the given file and dumps its contents. The file has to be in binary format and contain netlink messages.

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🌈 OPTIONS 🌈

• -b, -b filename, -batch, -batch filename — read commands from provided file or standard input and invoke them. First failure will cause termination of tc.
• -force — don't terminate tc on errors in batch mode. If there were any errors during execution of the commands, the application return code will be non zero. it's giving "keep going, we'll deal later" 🫶
• -o, -oneline — output each record on a single line, replacing line feeds with the '\' character. This is convenient when you want to count records with wc(1) or to grep(1) the output.
• -n, -net, -netns <NETNS> — switches tc to the specified network namespace NETNS. Actually it just simplifies executing of:

ip netns exec NETNS tc [ OPTIONS ] OBJECT { COMMAND | help }

to

tc -n[etns] NETNS [ OPTIONS ] OBJECT { COMMAND | help }

• -N, -Numeric — Print the number of protocol, scope, dsfield, etc directly instead of converting it to human readable name.
• -cf, -conf <FILENAME> — specifies path to the config file. This option is used in conjunction with other options (e.g. -nm).
• -t, -timestamp — When tc monitor runs, print timestamp before the event message in format:

Timestamp: <Day> <Month> <DD> <hh:mm:ss> <YYYY> <usecs> usec

• -ts, -tshort — When tc monitor runs, prints short timestamp before the event message in format:

[<YYYY>-<MM>-<DD>T<hh:mm:ss>.<ms>]

• -echo — Request the kernel to send the applied configuration back. it's giving receipts, and honestly? respect 💅

✨ FORMAT ✨

okay so the show command has extra formatting options, and they're lowkey iconic 💅

• -s, -stats, -statistics — spits out more stats about packet usage 📊
• -d, -details — gives you more detailed info about rates and cell sizes 🔍
• -r, -raw — outputs raw hex values for handles, no decoding bestie ✨
• -p, -pretty — for the u32 filter, it decodes offset and mask values into equivalent filter commands based on TCP/IP. and in JSON output, it adds whitespace so it's actually readable 🎀
• -iec — prints rates in IEC units (ie. 1K = 1024)
• -g, -graph — shows classes as an ASCII graph. if the -s option was specified it also prints generic stats info under each class. heads up tho: classes can be filtered only by the dev option 👑
• -c[color][={always|auto|never} — configures color output. if the parameter is omitted or set to always, color output is enabled no matter what stdout's doing. if it's auto, stdout gets checked to be a terminal before color output turns on. if it's never, color output is off, periodt. if you specify it multiple times, the last one wins. also this flag is just ignored if -json is also given 💀
• -j, -json — displays results in JSON format 🌈
• -nm, -name — resolves the class name from /etc/iproute2/tc_cls, or from whatever file the -cf option points at. this file is literally just a mapping of classid to class name:

# Here is comment
1:40   voip # Here is another comment
1:50   web
1:60   ftp
1:2    home

and listen, tc will not fail if -nm was specified without the -cf option but /etc/iproute2/tc_cls doesn't exist — which is what makes it possible to pass the -nm option for creating a tc alias 🫶

• -br, -brief — prints only the essential data needed to identify the filter and action (handle, cookie, etc.) plus stats. ngl this one is currently only supported by tc filter show and tc actions ls commands ✨

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🌸 EXAMPLES 🌸

tc -g class show dev eth0

Shows classes as an ASCII graph on the eth0 interface. iconic ✨

tc -g -s class show dev eth0

Shows classes as an ASCII graph with stats info under each class. the upgrade we deserved 💖

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📜 HISTORY 📜

tc was written by Alexey N. Kuznetsov and added in Linux 2.2. a whole legend, fr fr 👑

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🦋 SEE ALSO 🦋

tc-basic(8), tc-bfifo(8), tc-bpf(8), tc-cake(8), tc-cgroup(8), tc-choke(8), tc-codel(8), tc-drr(8), tc-ematch(8), tc-ets(8), tc-flow(8), tc-flower(8), tc-fq(8), tc-fq_codel(8), tc-fq_pie(8), tc-fw(8), tc-gact(8), tc-hfsc(7), tc-hfsc(8), tc-htb(8), tc-mqprio(8), tc-pfifo(8), tc-pfifo_fast(8), tc-pie(8), tc-red(8), tc-route(8), tc-sfb(8), tc-sfq(8), tc-stab(8), tc-tbf(8), tc-u32(8)

User documentation at http://lartc.org/, but please direct bugreports and patches to: <netdev@vger.kernel.org> 📬

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💜 AUTHOR 💜

Manpage maintained by bert hubert (ahu@ds9a.nl)

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and that's a wrap bestie!! 🎀 this page came from the iproute2 docs, dated 16 December 2001 — yes it's been serving main character energy since 2001 and it still eats 💅 go configure something gorgeous, you got this 💖✨