Abstract
In most systems, fair-queueing packet schedulers are the algorithms of choice for providing bandwidth and delay guarantees. These guarantees are computed assuming that the scheduler is directly attached to the transmit unit with no interposed buffering, and, for timestamp-based schedulers, that the exact number of bits transmitted is known when timestamps need to be updated.Unfortunately, both assumptions are unrealistic. In particular, real communication devices normally include FIFO queues (possibly very deep ones) between the scheduler and the transmit unit. And the presence of these queues does invalidate the proofs of the service guarantees of existing timestamp-based fair-queueing schedulers.In this paper we address these issues with the following two contributions. First, we show how to modify timestamp-based, worst-case optimal and quasi-optimal fair-queueing schedulers so as to comply with the presence of FIFO queues, and with uncertainty on the number of bits transmitted. Second, we provide analytical bounds of the actual guarantees provided, in these real-world conditions, both by modified timestamp-based fair-queueing schedulers and by basic round-robin schedulers. These results should help designers to make informed decisions and sound tradeoffs when building systems.
Highlights
Packet schedulers play a critical role in providing bandwidth and delay guarantees on nonoverprovisioned transmission links
Given a packet p arriving at time ta, we prove the thesis in two steps: first we compute an upper bound to the time that elapses from ta to when p is dequeued from Approximated variants of WF2Q+ (AFQ), say time tc, we add to this upper bound the maximum time that may elapse from time tc to the time instant tc at which p is transmitted
Instantiating the general bounds proved in the previous section, we show and discuss the service guarantees provided by most AFQ schedulers, Scheduler WF2Q+ S-KPS GFQ QFQ Deficit Round Robin (DRR)
Summary
Packet schedulers play a critical role in providing bandwidth and delay guarantees on nonoverprovisioned transmission links. We provide general worst-case bounds on bandwidth, packet (queueing) delay and jitter, for both the resulting family of modified schedulers and basic round-robin schedulers. These bounds take into account exactly the effects of FIFOs and uncertainty on the number of bits transmitted. We prove that FIFOs cause the minimum possible service degradation While partially reassuring, this result means that, in any case, sizing the output queues is critical to avoid that guarantees of sophisticated schedulers degrade to those of, e.g., DRR, or, vice versa, it means that resources should not be wasted on complex scheduling algorithms when short queues are not available.
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