class of service: 1. A mechanism for managing traffic in a network by specifying message or packet priority. 2. The characteristics and guarantees of the transport layer of a Fibre Channel circuit. Fibre Channel classes of service include: connection services (Classes 1), guaranteed frame delivery with end-to-end flow control (Class 2), packetized frame datagrams (Class 3), quality-of-service sub-channels (e.g., constant sub-rate or constant latency) (Class 4). Different classes of service may simultaneously exist in a fabric. The form and reliability of delivery in Class 3 circuits may vary with the topology. Different classes of service may simultaneously exist in a fabric. 3. The identification and grouping of data packets based on a priority label (in the packet header) or via other mechanisms (such as "per hop behavior", defined by the IETF's Differentiated Services).
See also: acknowledged class, Class 6