P-Early-Media header field is used within SIP [1] messages in certain SIP networks to authorize the cut-through of backward and/or forward early media when permitted by the early media policies of the networks involved. The P-Early-Media header field is intended for use in a SIP network, such as a 3GPP IMS [13][14] that has the following characteristics: its early media policy prohibits the exchange of early media between end users; it is interconnected with other SIP networks that have unknown, untrusted, or different policies regarding early media; and it has the capability to "gate" (enable/disable) the flow of early media to/from user equipment.
Within an isolated SIP network, it is possible to gate early media associated with all endpoints within the network to enforce a desired early media policy among network endpoints. However, when a SIP network is interconnected with other SIP networks, only the boundary node connected to the external network can determine which early media policy to apply to a session established between endpoints on different sides of the boundary. The P-Early-Media header field provides a means for this boundary node to communicate this early media policy decision to other nodes within the network.
References:
https://tools.ietf.org/html/rfc5009
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