IP networks, in general, and the Internet, as a particularly prominent example, are inherently nondeterministic in their predictability regarding operational parameters such as latency, delay, round-trip time (RTT), jitter (delay variation), and packet loss. The default service offering associated with the Internet is characterized as a best-effort variable-service response (RFC 2990, "Next Steps for the IP QoS Architecture").
To improve the situation for certain traffic classes, policy routing and quality of service (QoS) measures were introduced. Prioritizing one class of service always works at the cost of regular best-effort traffic. There is no free lunch, and statistical overbooking is the foundation of affordable Internet service provider (ISP) offerings. One strong driving force for QoS is expedited transport of real-time delay-sensitive traffic such as voice, video, and delicate data (such as storage traffic or dedicated business applications).
Note that queuing and scheduling on UNIX systems is often an integral part of packet filtering and Network Address Translation (NAT) implementations. This chapter covers neither firewall features nor NAT. (See Chapter 15, "Network Address Translation.") However, this chapter does cover packet-filtering architectures as they relate to queuing and scheduling.