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Deficit weighted round robin is a way to share network service among queues by giving each one a byte allowance, called a quantum, and tracking unused allowance with a deficit counter. Queues with larger quanta are intended to receive a larger share of service when they remain backlogged. The established name in the standards material is Deficit Round Robin (DRR); “deficit weighted round robin” describes its weighted use, not a universally standardized name or acronym.
How deficit weighted round robin works
A scheduler keeps a queue for each flow or traffic class and visits active queues in turn. Each queue has a quantum measured in bytes. On a visit, the scheduler adds that quantum to the queue’s deficit counter, then transmits packets whose sizes fit within the available credit, subtracting each packet’s size from the counter. When no further packet fits, it moves on; remaining credit is handled according to the particular DRR variant.
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- Set a byte quantum for each queue. To express relative weights, choose quanta in proportion to the intended service shares.
- Add credit when visiting a queue. The scheduler increases its deficit counter by that queue’s quantum.
- Send packets while credit allows. Each packet consumes credit equal to its size in bytes.
- Move to the next queue. Any remaining credit is carried or handled according to the implementation’s rules.
RFC 7806 describes DRR as using byte-based quanta to handle variable-length packets and discusses credit carried when a dequeue opportunity is not fully used. RFC 8290 describes FQ-CoDel’s variant: it tracks byte credits, subtracts packet sizes, and adds a quantum when the credits reach zero or less. Because details such as what happens when a queue empties can vary, exact behavior depends on the implementation. RFC 7806 · RFC 8290
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Why DRR tracks bytes instead of packet counts
With equal packet-count turns, a queue whose packets are large can receive substantially more bandwidth than a queue whose packets are small. DRR charges each queue for the bytes it sends instead. For example, if one queue’s packets are each one-third of its quantum, it can send three in a visit; a queue with packets the size of its quantum can send one. The allowance is therefore based on data volume, not simply how many packets happen to fit in a turn. RFC 8290
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How weights relate to quantum size
A larger quantum adds more byte credit each round. If two queues stay backlogged, assigning them quanta in a 2:1 ratio is a straightforward way to aim for byte service in approximately that ratio. It is an intended relative allocation, not a guarantee of exact throughput: queue activity, packet sizes, configuration, and implementation details affect actual service. The foundational DRR paper, published by M. Shreedhar and G. Varghese in June 1996, describes assigning a corresponding quantum to a flow seeking a larger relative bandwidth allocation. RFC 7806 likewise describes a per-round quantum as the intended bytes dequeued in a round. Shreedhar and Varghese, “Efficient Fair Queuing Using Deficit Round Robin” · RFC 7806
DRR is not the same thing as FQ-CoDel
DRR is a scheduling mechanism. FQ-CoDel combines a modified DRR scheduler with CoDel, an active queue management algorithm that addresses queue delay. RFC 8290 also describes FQ-CoDel’s distinction between “new” and “old” queues. Its flow classification and delay management are features of that combined implementation, not part of the general definition of DRR. RFC 8290
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RFC 8290 specifies a default FQ-CoDel quantum of 1514 bytes, corresponding in that design to an Ethernet MTU plus a 14-byte hardware header. That value is specific to FQ-CoDel’s documented defaults; it is not a universal DRR quantum. The RFC notes that quantum size affects scheduling granularity, while a quantum that is too small can add scheduling overhead. RFC 8290
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