1.4 Relationship to Other Protocols
RTPME sessions are usually initiated through an application layer control protocol such as Session Initiation Protocol (SIP) [MS-SIP] or H.323 [H323]. RTP transport parameters (protocol, IP, port) for sessions established through SIP are usually communicated through a multimedia session description protocol such as Session Description Protocol (SDP) Extensions [MS-SDP] or H.245 protocol [MS-H245]. Hosts communicate using UDP. RTP and RTCP packets can be encrypted and/or authenticated through the default algorithm Data Encryption Standard (DES) in cipher block chaining (CBC) mode as specified in [RFC3550]. For audio communications, RTP supports a redundancy mechanism for forward error correction (FEC) [MS-RTPRAD], as well as a mechanism for communicating Dual Tone Multiple Frequency (DTMF) [MS-RTPDT] events. Negotiation for these and other payload properties (including supported codecs, sampling rates, and dynamic payload type mappings) can also be done through SDP. For video communications, because data for a single frame can sometimes span more than one RTP packet, various video encapsulation methods can be used, such as H.261 and H.263 [MS-H26XPF].
The following figure illustrates this hierarchy between protocols. SIP, H.323, and SDP are not represented in this figure because they are parallel to RTP.

Figure 1: Hierarchy of the RTP protocol