P-Early-Media SIP Header

The P-Early-Media SIP header is defined in RFC 5009, Private Header (P-Header) Extension to the Session Initiation Protocol (SIP) for Authorization of Early Media. RFC 5009 defines the use of the P-Early-Media header within SIP messages in certain SIP networks to authorize the cut-through of backward (server-to-client) and/or forward (client-to-server) 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 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.

A SIP network containing both PSTN gateways and SIP end devices, for example, can maintain such an early media policy by gating "off" any early media with a SIP end device acting as UAS, gating "on" early media with a SIP end device acting as UAC, and gating "on" early media at each PSTN gateway. This is what we have been doing for years with the SIP early media suppression feature, which allows determining who can send early media and in what direction.

Unfortunately, in SIP interconnection scenarios there is no means of assuring that the interconnected network is implementing a compatible early media policy, thus allowing the exchange of user data within early media under some circumstances. For example, if a network "A" allows all early media with user equipment as UAC and an interconnected network "B" allows all early media with user equipment as UAS, any session established between user equipment as UAC in "A" and user equipment as UAS in "B" will allow bidirectional user data exchange as early media.

The P-Early-Media header is used for the purpose of requesting and authorizing requests for backward and/or forward early media. It’s sent from UAS to UAC to indicate authorization for early media.