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Chapter 31. Linux traffic control

Linux Traffic Control (TC) helps in policing, classifying, shaping, and scheduling network traffic. It uses queuing disciplines (qdisc) and filters to manage and manipulate packet transmission, including mangling packet content.

The scheduling mechanism arranges or rearranges the packets before they enter or exit different queues. The most common scheduler is the First-In-First-Out (FIFO) scheduler. You can do the qdiscs operations temporarily using the tc utility or permanently using NetworkManager.

In Red Hat Enterprise Linux, you can configure default queueing disciplines in various ways to manage the traffic on a network interface.

31.1. Overview of queuing disciplines
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Queuing disciplines (qdiscs) help with queuing up and, later, scheduling of traffic transmission by a network interface.

A qdisc has two operations;

  • enqueue requests so that a packet can be queued up for later transmission and
  • dequeue requests so that one of the queued-up packets can be chosen for immediate transmission.

Every qdisc has a 16-bit hexadecimal identification number called a handle, with an attached colon, such as 1: or abcd:. This number is called the qdisc major number. If a qdisc has classes, then the identifiers are formed as a pair of two numbers with the major number before the minor, <major>:<minor>, for example abcd:1. The numbering scheme for the minor numbers depends on the qdisc type. Sometimes the numbering is systematic, where the first-class has the ID <major>:1, the second one <major>:2, and so on. Some qdiscs allow the user to set class minor numbers arbitrarily when creating the class.

Classful qdiscs

Different types of qdiscs exist and help in the transfer of packets to and from a networking interface. You can configure qdiscs with root, parent, or child classes. The point where children can be attached are called classes. Classes in qdisc are flexible and can always contain either multiple children classes or a single child, qdisc. There is no prohibition against a class containing a classful qdisc itself, this facilitates complex traffic control scenarios.

Classful qdiscs do not store any packets themselves. Instead, they enqueue and dequeue requests down to one of their children according to criteria specific to the qdisc. Eventually, this recursive packet passing ends up where the packets are stored (or picked up from in the case of dequeuing).

Classless qdiscs
Some qdiscs contain no child classes and they are called classless qdiscs. Classless qdiscs require less customization compared to classful qdiscs. It is usually enough to attach them to an interface.

31.2. Inspecting qdiscs of a network interface using the tc utility
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By default, Red Hat Enterprise Linux systems use fq_codel qdisc. You can inspect the qdisc counters using the tc utility.

Procedure

  1. Optional: View your current qdisc: 

    # tc qdisc show dev enp0s1
    Copy to Clipboard Toggle word wrap

  2. Inspect the current qdisc counters: 

    # tc -s qdisc show dev enp0s1
qdisc fq_codel 0: root refcnt 2 limit 10240p flows 1024 quantum 1514 target 5.0ms interval 100.0ms memory_limit 32Mb ecn
Sent 1008193 bytes 5559 pkt (dropped 233, overlimits 55 requeues 77)
backlog 0b 0p requeues 0
    Copy to Clipboard Toggle word wrap
    • dropped - the number of times a packet is dropped because all queues are full
    • overlimits - the number of times the configured link capacity is filled
    • sent - the number of dequeues

31.3. Updating the default qdisc
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If you observe networking packet losses with the current qdisc, you can change the qdisc based on your network-requirements.

Procedure

  1. View the current default qdisc: 

    # sysctl -a | grep qdisc
net.core.default_qdisc = fq_codel
    Copy to Clipboard Toggle word wrap

  2. View the qdisc of current Ethernet connection: 

    # tc -s qdisc show dev enp0s1
qdisc fq_codel 0: root refcnt 2 limit 10240p flows 1024 quantum 1514 target 5.0ms interval 100.0ms memory_limit 32Mb ecn
Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
backlog 0b 0p requeues 0
maxpacket 0 drop_overlimit 0 new_flow_count 0 ecn_mark 0
new_flows_len 0 old_flows_len 0
    Copy to Clipboard Toggle word wrap

  3. Update the existing qdisc: 

    # sysctl -w net.core.default_qdisc=pfifo_fast
    Copy to Clipboard Toggle word wrap

  4. To apply the changes, reload the network driver: 

    # modprobe -r NETWORKDRIVERNAME
# modprobe NETWORKDRIVERNAME
    Copy to Clipboard Toggle word wrap

  5. Start the network interface: 

    # ip link set enp0s1 up
    Copy to Clipboard Toggle word wrap

Verification

  • View the qdisc of the Ethernet connection: 

    # tc -s qdisc show dev enp0s1
qdisc pfifo_fast 0: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 373186 bytes 5333 pkt (dropped 0, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
....
    Copy to Clipboard Toggle word wrap

Changing the current qdisc can optimize how the kernel handles packets as they pass through a network interface. For testing and benchmarking purposes, you can change the qdisc temporarily.

Procedure

  1. Optional: View the current qdisc: 

    # tc -s qdisc show dev enp0s1
    Copy to Clipboard Toggle word wrap

  2. Update the current qdisc: 

    # tc qdisc replace dev enp0s1 root htb
    Copy to Clipboard Toggle word wrap

Verification

  • View the updated current qdisc: 

    # tc -s qdisc show dev enp0s1
qdisc htb 8001: root refcnt 2 r2q 10 default 0 direct_packets_stat 0 direct_qlen 1000
Sent 0 bytes 0 pkt (dropped 0, overlimits 0 requeues 0)
backlog 0b 0p requeues 0
    Copy to Clipboard Toggle word wrap

Changing the current qdisc can optimize how the kernel handles packets as they pass through a network interface. If you identified the optimal qdisc for your scenario, set it permanently in the NetworkManager connection profile.

Procedure

  1. Optional: View the current qdisc: 

    # tc qdisc show dev enp0s1
  qdisc fq_codel 0: root refcnt 2
    Copy to Clipboard Toggle word wrap

  2. Update the current qdisc: 

    # nmcli connection modify enp0s1 tc.qdiscs 'root pfifo_fast'
    Copy to Clipboard Toggle word wrap

  3. Optional: To add another qdisc over the existing qdisc, use the +tc.qdisc option: 

    # nmcli connection modify enp0s1 +tc.qdisc 'ingress handle ffff:'
    Copy to Clipboard Toggle word wrap

  4. Activate the changes: 

    # nmcli connection up enp0s1
    Copy to Clipboard Toggle word wrap

Verification

  • View current qdisc the network interface: 

    # tc qdisc show dev enp0s1
qdisc pfifo_fast 8001: root refcnt 2 bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
qdisc ingress ffff: parent ffff:fff1 ----------------
    Copy to Clipboard Toggle word wrap

31.6. Available qdiscs in RHEL
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Each qdisc addresses unique networking-related issues. You can use any of them to shape network traffic based on your networking requirements.

These qdiscs are available in RHEL:

Expand
Table 31.1. Available schedulers in RHEL
qdisc nameIncluded inOffload support

Asynchronous Transfer Mode (ATM)

kernel-modules-extra

 

Credit-Based Shaper

kernel-modules-extra

Yes

CHOose and Keep for responsive flows, CHOose and Kill for unresponsive flows (CHOKE)

kernel-modules-extra

 

Controlled Delay (CoDel)

kernel-core

 

Enhanced Transmission Selection (ETS)

kernel-modules-extra

Yes

Fair Queue (FQ)

kernel-core

 

Fair Queuing Controlled Delay (FQ_CODel)

kernel-core

 

Generalized Random Early Detection (GRED)

kernel-modules-extra

 

Hierarchical Fair Service Curve (HSFC)

kernel-core

 

Heavy-Hitter Filter (HHF)

kernel-core

 

Hierarchy Token Bucket (HTB)

kernel-core

Yes

INGRESS

kernel-core

Yes

Multi Queue Priority (MQPRIO)

kernel-modules-extra

Yes

Multiqueue (MULTIQ)

kernel-modules-extra

Yes

Network Emulator (NETEM)

kernel-modules-extra

 

Proportional Integral-controller Enhanced (PIE)

kernel-core

 

PLUG

kernel-core

 

Quick Fair Queueing (QFQ)

kernel-modules-extra

 

Random Early Detection (RED)

kernel-modules-extra

Yes

Stochastic Fair Blue (SFB)

kernel-modules-extra

 

Stochastic Fairness Queueing (SFQ)

kernel-core

 

Token Bucket Filter (TBF)

kernel-core

Yes

Trivial Link Equalizer (TEQL)

kernel-modules-extra

 
Important

The qdisc offload requires hardware and driver support on NIC.

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