"Audio and video applications currently used in packet networks implement client buffers (e.g.@ de-jitter buffers and play-out buffers) at the receiver side. The behaviour of these buffers is essential to the end-to-end performance of these applications. This Recommendation defines models for these client buffers for audio/video applications currently used in actual networks@ in order to estimate the client state based on the dynamic network behaviour. In this Recommendation@ different buffer models are derived depending on the different applications taken into account. However@ these buffer models may be based on common intrinsic characteristics: i.e.@ buffer size@ service rate@ initial delay and re-buffering delay. Example buffer performance metrics include peak/average/instantaneous buffer occupation. In this Recommendation@ buffer models may accept as input@ either live traffic arriving on measurement points@ packet capture (pcap) files@ or post-processed files. Buffer models are expected to provide client states (e.g.@ codec play-out begin@ buffer overflow@ buffer dry out@ play-out resume) associated with relevant timing information. As part of future work@ these buffer models could be used to derive new performance parameters and associated statistics in order to complement the IP-layer metrics defined in Recommendation Y.1540. It is out of scope for this Recommendation to define specific de-jitter buffers for use in actual client media stream delivery. In this Recommendation the following applications@ rate and buffer characteristics@ and associated protocol stacks are considered: a) Audio applications: i) Ideally constant fill@ constant play-out rate and relatively low bandwidth. b) Video ""low bitrate mode"" quarter common intermediate format-quarter video graphics array (QCIF-QVGA) resolutions@ mostly for mobile TV and streaming with the sub application areas: i) Linear mobile TV over real-time transport protocol (RTP) (including mobile TV over a 3G mobile network with multimedia broadcast/multicast services (MBMS) and with unicast transport over RTP/UDP/IP). ii) Multimedia streaming (including 3GPP packet-switched streaming service (PSS) with transport over RTP/UDP/IP). iii) [IETF RFC 3984] variable bit rate@ up to 2 Mbps@ with separate buffers for audio and video@ and separate play-out rates for each buffer. c) Video ""high bitrate mode"" standard-definition (SD) and high-definition (HD) television@ mostly for Internet Protocol television (IPTV) with the following sub-application areas: i) Linear broadcast TV (including transmission over MPEG2-TS/RTP/UDP/IP@ MPEG2-TS/UDP/IP and RTP/UDP/IP transport). ii) Video on-demand (including transmission over MPEG2-TS/RTP/UDP/IP@ MPEG2-TS/UDP/IP and RTP/UDP/IP transport). iii) [IETF RFC 2250]@ constant bit rate (CBR) or variable bit rate (VBR) up to 50 Mbps@ with separate buffers for audio and video@ and separate play-out rates for each buffer. d) Over-the-top video applications: i) Non-adaptive streaming (MPEG4_fileformat/HTTP/TCP@ HTTP progressive download@ HTTP partial download@ chunks): ?C Progressive download: TCP transport@ TCP flow control (65Mbps super-peak rates@ peak rate depends on network conditions and path)@ classic single buffer@ leaky bucket at constant frame rate. ii) Adaptive streaming dynamic adaptive streaming over HTTP (DASH/TCP): ?C Chunk Delivery: (e.g.@ DASH) bursts of packets@ like sequential FTP file transfers."