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All networking subsystems work by manipulating packets in one way or another Doing anything with a packet involves going through a CPU cycle While it's true that smart engineering can pack more instructions into a cycle, the fact remains that every QoS action requires additional tasks that come at a cost The trick is to get the most service-quality effect at the least processing-overhead cost Routers, switches, and other network devices don't need disks because their function is to forward traffic Packets are only stored temporarily, where DRAM is used to set up and manage one or more packet queues while the CPU figures out how to forward them Indeed, queuing is so important to router operations that most routers have separate DRAM called packet memory dedicated specifically to that purpose One of the primary tools used by QoS to influence service levels is queue manipulation Because virtually all routers use first-in/first-out (FIFO) ordering by default, queue manipulation mechanisms are often referred to as non-FIFO queuing The most common name for the practice, though, is class-based queuing (CBQ) CBQ works by somehow defining packets to a class by flow If, for example, a router is instructed to favor one type of traffic over another, it must intervene and influence the queue's default FIFO order to bump the favored class of traffic toward the front This process is shown in Figure 15-4, in which packet C is bumped to the front of the queue
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Figure 15-4: Queue manipulation is the most basic of QoS tools There's no avoiding the fact that queue intervention comes at a cost in processing overhead The router must spend extra CPU cycles to inspect packets for class type and then to resequence the queue As discussed, packets can be differentiated either by tuple or marking, but the favored CBQ method is marking because it costs less in processing and administrative overhead It takes more CPU effort to match a four-tuple rule than to simply recognize the rule represented in a predetermined bit value No matter the recognition method used, however, overhead costs rise as queue manipulation schemes get more sophisticated CBQ is the best example of how network administrators must weigh a cost-benefit analysis for each QoS policy They need to balance improved network predictability against any unintended latency caused by packet inspection and queue manipulation
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Network engineering literature describes a classical three-layer hierarchy design model Illustrated in Figure 15-5, the three-layer model enforces segmentation in order to separate unrelated traffic The access layer connects devices to the local network and contains such local traffic as database lookups and print jobs The distribution layer is responsible for most routing, security, and other intelligent functions all of which require substantial packet manipulation The core layer does minimal packet manipulation; it moves largely preconditioned flows at very high speeds, usually over switched Ethernet or ATM links
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Figure 15-5: The classical three-layer hierarchy design model is significant to QoS Hierarchical topology is almost a requirement nowadays, and for reasons other than just traffic isolation A second reason is that a rational topology layout facilitates route aggregation a tactic aggressively pursued in large intranets using the OSPF routing protocol and on the open Internet using BGP because it greatly reduces the size of routing tables and thereby increases overall network speed Hierarchical layering also helps QoS, which defines gateway routers between layers as ingress points (a natural place to differentiate service policy) Certainly, QoS ingress routers, too, are defined at gateways between autonomous systems, and even among declared multilayer QoS zones within an autonomous system But it makes greatest sense to trigger QoS rules at ingress points between hierarchical layers For example, an FTP packet might be accorded a Bronze CBQ service level entering the distribution layer, but be bumped up to Gold as it passes through the core layer's ingress router because the network manager has an SLA that favors long packet streams due to the way the ISP has tuned its ATM QoS parameters, for example
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