This document discusses quality of service (QoS) models and mechanisms at different layers of the network. It provides information on common QoS markings like IP Precedence, DSCP, and 802.1p. It also describes different queuing mechanisms like priority queuing, weighted fair queuing, class-based weighted fair queuing, and low latency queuing. Key terms related to QoS like per-hop behavior, traffic policing and shaping are also defined.