Quality of Service (QoS) refers to the real time performance of networking protocols and systems. Under ideal QoS, application traffic arrives quickly, consistently, and reliably. Poor QoS is characterized by slow performance, variable delay, or network errors. Technical terms for three negative conditions QoS must overcome are:
Several protocols have features or options to improve QoS. IPv4 includes an 8-bit field originally called Type of Service (ToS). This field has since been renamed Differentiated Services (DiffServ). In either case, these bits can be configured to request priority service for more urgent classes of traffic. IPv6 likewise contains an 8-bit Traffic Class field. At layer two, the IEEE's 802.1p working groups allocated three bits from the 802.1q VLAN header to indicate Class of Service.
With respect to transport layer protocols, the decision to use TCP or UDP is often influenced by QoS considerations. TCP minimizes packet loss, at the cost of increased latency. UPD minimizes latency, at the expensive of potential packet loss. Real time streamer applications (like VoIP or video) that can tolerate some packet loss tend to favor UDP because of its better latency. However, during call setup and tear down, these same applications will mostly use TCP because of its better assurances that all packets will arrive intact.
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