Session Descriptions
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... session
description it receives as the answer, and MUST ignore any
session descriptions in subsequent responses to the initial
INVITE.
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The same offer-answer model that applies to session descriptions in
INVITEs (Section 13.2.1) applies to re-INVITEs. As a result, a UAC
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... header field is used in requests to indicate the
preferred languages for reason phrases, session descriptions, or
status responses carried as message bodies in the response. If no
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... ACK. Provisional (1xx)
responses MAY contain message bodies, including session descriptions.
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