This feature optionally strips preconditions information from initial INVITEs .

Feature cheat sheet

B2BUA Instance SAS Support Originating / Terminating Point in Session Plan Network Operator Data Subscriber Data Stateful or Stateless POJO Feature or SBB Feature

Any and All

No

Terminating

MMTel_Post_SipAccess_SubscriberCheck, MMTel_Post_SipAccess_PartyResponse, SCC_Post_SipAccess_SubscriberCheck, SCC_Post_SipAccess_PartyResponse

Yes

No

Stateless

POJO

Behaviour

When SIP forking occurs, this may cause issues with upstream devices that cannot properly handle preconditions negotiation on multiple early dialogs. If this is a problem, then this can be worked around by stripping preconditions from the initial INVITE, if requested by another feature that is performing SIP forking. The StripPreconditions feature detects whether any other features have requested this behaviour, and removes preconditions from the outbound initial INVITE.

The StripPreconditions feature works as follows.

Features that execute prior to the StripPreconditions feature add legs to a session state variable variable called StripPreconditionsLegNames. Currently only MMTelParallelFA and SCCTADSParallelRouting features add legs to the collection, although any custom feature is permitted to add legs. When the StripPreconditions feature runs, it checks the StripPreconditionsLegNames session state variable and performs the following.

  • check if feature’s EnabledFlag is true. If not selected the feature ends

  • strip precondition attributes a=curr, a=des, and a=conf from the SDP of the INVITE. If there are no attributes to strip the feature ends.

  • strip the "preconditions" option tag in the Supported header from the INVITE

The feature supports the removal of preconditions from initial INVITEs and retarget INVITEs.

Prerequisite features

The feature must run before SDPMonitor mode "post" and DiameterPerLegInfo features.

Session state input variables

Variable name Variable type Description 
StripPreconditionsLegNames

Set<String>

Collection of leg names that will have preconditions information stripped from initial INVITEs.

Session state output variables

None.

Network operator data

This feature reads the EnabledFlag in the feature configuration profile table StripPreconditions to determine if the feature will strip preconditions.

Parameter Type Description Default 
EnabledFlag

boolean

true or false

false

Statistics

StripPreconditions statistics are tracked by the sentinel.volte.sip SBB and can be found under the following parameter set in REM:
SLEE-Usage → sentinel.volte.sip service → sentinel.volte.sip SBB → feature → StripPreconditions
or with rhino-stats:
"SLEE-Usage.Services.ServiceID[name=sentinel.volte.sip,vendor=OpenCloud,version=4.1].SbbID[name=sentinel.volte.sip,vendor=OpenCloud,version=4.1].feature.StripPreconditions"

Name Type Description 
Started

Counter

Incremented each time the feature runs

 
FailedToStart

Counter

Incremented when a fatal error occurs before feature execution

 
IssuedWarning

Counter

Incremented when a non-fatal error occurs during feature execution

 
FailedDuringExecution

Counter

Incremented when a fatal error occurs during feature execution

 
TimedOut

Counter

Incremented when feature execution does not complete within a reasonable time frame

 
RemovePreconditionsFields

Counter

Incremented when the feature strips preconditions attributes from INVITE SDP

 
InviteNotFoundInQueue

Counter

Incremented when the feature is unable to find the INVITE from an internal system queue

 
MessageQueueEmpty

Counter

Incremented when the feature is unable to find any SIP messages from an internal system queue

 
InvalidSDP

Counter

Incremented when the feature fails to parse the SDP

Values examined

Value Name Source Notes

StripPreconditionsLegNames

Session State

Collection of leg names that will have preconditions stripped. The collection can be updated by any feature that has access to the session state variable StripPreconditionsLegNames. Currently the MMTelParallelFA and SCCTADSParallelRouting features add legs to the collection.

SDP

Outgoing INVITE request

Check if precondition attributes a=curr, a=des, and a=conf are present in the SDP of the INVITE. If yes strip, if no then don’t execute any further.

Supported Header

Outgoing INVITE request

Remove 'preconditions' option tag from the Supported header unless there are no preconditions attributes present in the SDP.
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Sentinel VoLTE Version 4.1