14.1. The offer
An entity wanting to create a session or currently in a session can generate an SDP session description that constitutes the offer. This offer contains the set of media streams and the set of codecs the offerer wishes to use, as well as the Internet Protocol (IP) addresses and ports the offerer would like to use to receive the media.
The session line, or s-line, is always set to "-" or a single space in the offer/answer. Since signalling protocols, such as SIP, are used to create unicast sessions, the start and stop times in an SDP offer/answer exchange is set to 0 and 0, respectively.
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