MLPP Preemption Events in SIP Reason Header

The device sends the SIP Reason header (as defined in RFC 4411) to indicate the reason and type of a preemption event. The device sends a SIP BYE or CANCEL request, or SIP 480, 486, 488 response (as appropriate) with a Reason header whose Reason-params can includes one of the following preemption cause classes:

■

Reason: preemption ;cause=1 ;text=”UA Preemption”

■

Reason: preemption ;cause=2 ;text=”Reserved Resources Preempted”

■

Reason: preemption ;cause=3 ;text=”Generic Preemption”

■

Reason: preemption ;cause=4 ;text=”Non-IP Preemption”

This Reason cause code indicates that the session preemption has occurred in a non-IP portion of the infrastructure. The device sends this code in the following scenarios:

●

The device performs a network preemption of a busy call (when a high priority call is received), the device sends a SIP BYE or CANCEL request with this Reason cause code.

●

The device performs a preemption of a B-channel for a Tel-to-IP outbound call request from the softswitch for which it has not received an answer response (e.g., Connect), and the following sequence of events occurs:

The device sends a Q.931 DISCONNECT over the ISDN MLPP to the partner switch to preempt the remote end instrument.

ii.

The device sends a 488 (Not Acceptable Here) response with this Reason cause code.

■

Reason: preemption; cause=5; text=”Network Preemption”

This Reason cause code indicates preempted events in the network. Within the Defense Switched Network (DSN) network, the following SIP request messages and response codes for specific call scenarios have been identified for signaling this preemption cause:

●

SIP:BYE - If an active call is being preempted by another call

●

CANCEL - If an outgoing call is being preempted by another call

●

480 (Temporarily Unavailable), 486 (User Busy), 488 (Not Acceptable Here) - Due to incoming calls being preempted by another call.

The device receives SIP requests with preemption reason cause=5 in the following cases:

●

The softswitch performs a network preemption of an active call - the following sequence of events occurs:

The softswitch sends the device a SIP BYE request with this Reason cause code.

ii.

The device initiates the release procedures for the B-channel associated with the call request and maps the preemption cause to ISDN Cause = #8 ‘Preemption’. This value indicates that the call is being preempted. For ISDN, it also indicates that the B-channel is not reserved for reuse.

iii.

The device sends a SIP 200 OK in response to the received BYE, before the SIP end instrument can proceed with the higher precedence call.

●

The softswitch performs a network preemption of an outbound call request for the device that has not received a SIP 2xx response - the following sequence of events occur:

The softswitch sends the device a SIP 488 (Not Acceptable Here) response code with this Reason cause code. The device initiates the release procedures for the B-channel associated with the call request and maps the preemption cause to ISDN Cause = #8 ‘Preemption’.

ii.

The device deactivates any user signaling (e.g., ringback tone) and when the call is terminated, it sends a SIP ACK message to the softswitch.