SIP Precondition

This came up in practice material also..

SIP Precondition:

RFC 3312 [3] defines the framework for SIP [2] preconditions, which
   is a generic framework that allows SIP UAs (User Agents) to suspend
   the establishment of a session until a set of preconditions are met.
   Although only Quality of Service (QoS) preconditions have been
   defined so far, this framework supports different types of
   preconditions.  (QoS preconditions are defined by RFC 3312 as well).
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