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Class-based queuing (CBQ) is a packet-scheduling and link-sharing method that sorts network traffic into classes and allocates bandwidth through a hierarchy. A classifier puts packets into classes; schedulers choose which class’s queue to serve. In hierarchical CBQ, classes may also use spare capacity elsewhere in the hierarchy, so an allocation is not necessarily a hard cap.
How CBQ handles a packet
CBQ combines several distinct functions in a traffic-control system. The packet path is easiest to understand in this order:
- Classification: A classifier applies rules to packets and directs matches to the appropriate traffic class. Classification determines where packets go; it does not decide which class gets served next. See the IETF’s RFC 3290 for the classifier’s role in a router datapath.
- Queuing: Packets wait in the queue associated with their class until they can be transmitted.
- Scheduling: A scheduler selects a queue to serve according to its configured service discipline. CBQ can also shape traffic and prioritize classes in implementations such as Linux’s
cbqqdisc.
How the hierarchy shares bandwidth
A CBQ hierarchy represents link capacity at a top level and divides allocations among lower-level schedulers and queues. This lets an administrator express how traffic categories should share capacity at more than one level. The defining feature of hierarchical CBQ is that an under-provisioned class can look beyond its own branch: higher-level schedulers may find spare capacity in another part of the tree. The IETF describes this link-sharing model in RFC 3670.
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What CBQ does not do by itself
Scheduling determines which queue is served and can be configured to prioritize traffic, limit rates, or isolate flows. It does not, by itself, control how large the queues grow. Large queues can still add delay even when the scheduler allocates service as intended.
Active queue management (AQM) addresses queue size and delay, so it complements rather than replaces a scheduler such as CBQ. RFC 7567 explains that scheduling and queue management have different roles and names CBQ alongside other per-flow or hierarchical scheduling approaches, including WFQ, SFQ, DRR, HTB, and HFSC. The behavior of any such approach depends on its implementation and configuration. See RFC 7567.
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CBQ on Linux
Linux exposes CBQ as the classful cbq queuing discipline, configured through the tc traffic-control utility. The tc-cbq(8) manual describes it as a hierarchical link-sharing model with shaping and prioritization capabilities.
The documented Linux implementation estimates link idle time using dequeue timing and the configured underlying bandwidth. That estimate can be difficult to establish on paths such as PPPoE or PPTP, where actual bandwidth may be uncertain. The manual also warns that coarse kernel timing can make shaping imprecise over short intervals; tolerating substantial errors in the bandwidth estimate comes with coarser shaping. These are caveats about Linux’s documented implementation, not universal properties of every system called CBQ. Check the manual for the version installed on the target system before relying on specific behavior.
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