In the TCP/IP protocol suite, a mechanism for providing endto-end network transport functions suitable for applications transmitting real-time data, such as audio, video, or simulation data, over multicast or unicast network services. Defined in IETF RFC 1889 (1996), RTP provides end-to-end delivery services including payload type identification, sequence numbering, and timestamping. In combination, the sequence numbering and timestamping provide the receiving node with sufficient information to resequence them as necessary. RTP does not address resource reservation and does not guarantee quality of service (QoS) for real-time services. RTP does not either guarantee delivery through the network or prevent out-of-order delivery, and it does not assume that the underlying network is reliable and delivers datagrams in sequence to the receiving machine. RTP does, however, prevent out-of-order delivery to the application. Applications such as voice over Internet Protocol (VoIP) generally run RTP on top of the User Datagram Protocol (UDP), which provides multiplexing and checksum services. In the context of the OSI Reference Model, RTP falls into both the Session Layer (Layer 5) and the Presentation Layer (Layer 6). RTP Control Protocol (RTCP) is an upper-layer companion protocol that allows monitoring of the data delivery. See also application, checksum, datagram, IETF, multicast, multiplex, payload, Presentation Layer, protocol, protocol suite, QoS, real-time, RTCP, Session Layer, TCP/IP, transport, UDP, unicast, and VoIP.