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20.4 LOWER LAYER PROTOCOLS FOR VoIP 663Figure 20.4-4 Association setup and shutdown sequence.An association is shut down with the shutdown procedure. Congestion and flowcontrol work the same way as for TCP, with the window value (in bytes) initiallyestablished by the sender in the INIT chunk. Windows apply to associations andnot to individual streams.SCTP Users. An application using SCTP is called an upper layer protocol (ULP).ULP is a generic term, which reflects the fact that SCTP can be used as a generalpurposetransport layer protocol.20.4.3 The Session Description Protocol (SDP)The SDP protocol [24] is used to carry descriptions of multimedia sessions, whichmay involve voice, video, and whiteboard streams. The protocol covers onlydescriptions and not the negotiation of the session characteristics, which is theresponsibility of the signaling application. SDP must work in concert with the signalingprotocols used for session setup, such as SIP (Chapter 21).

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