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3.17.5.2 Server Behavior

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The server identifies an emergency call when it detects a Priority header with value "emergency" in the INVITE. The server retrieves the location policy based on the LocationPolicyTagID sent within the Presence Information Data Format Location Object (PIDF-LO) embedded as a MIME part inside the message body of the INVITE. The PIDF-LO format is specified in [RFC4119].The server ignores the geolocation header and picks the last MIME part that has a PIDF-LO embedded in it. The geolocation  header is defined in [RFC6442]. Upon receiving the emergency call, in addition to routing the call to E911 Service providers or public switched telephone network (PSTN), the server MUST send an IM message on behalf of the client endpoint making the E911 call to each target in the NotificationUri specified in the location policy. The IM INVITE request MUST be constructed as follows:

  1. The request MUST contain a Priority header with the value "emergency".

  2. The request MUST contain a Call-Info header with the SIP URI of the user making the emergency call. The Call-Info header MUST have a purpose parameter with the value "ms-emergency-notification". The ABNF, as defined in [RFC5234], for the Call-Info header is defined in [RFC3261], section 25.1.

  3. The body of the message MUST be plain text containing all the descendants of the civicAddress and method elements in the PIDF-LO as name-value pairs. The civicAddress and method element schema are defined in [RFC4119].

The server MUST continue to route the emergency call regardless of any errors encountered while generating or routing the IM message.