What is SCC Determine Session Type?

This feature classifies incoming INVITE and MESSAGE requests to determine if they are invoking SCC services.

An INVITE request may be classified as:

  • a Reorigination INVITE request

  • an Access Transfer Request

    • Which will be further classified by the type of Access Transfer

  • a standard INVITE request

A MESSAGE request may be classified as:

  • an OpenCloud internal Access Transfer directive

  • not of interest to the SCC AS

Once classified, it rejects those that the implementation is known not to handle, by sending a 403 Forbidden error response. If an incoming request appears to be invoking an SCC service but is missing critical information, it will be rejected with a 400 Bad Request error response.

Feature Cheat Sheet

B2BUA Instance Originating / Terminating Point(s) in Session Plan Network Operator Data Subscriber Data Stateful or Stateless POJO Feature or SBB Feature Other notes

SCC

Both Originating and Terminating

SipAccess_SessionAccept, SipTransaction_Accept

Yes

None

Stateless

POJO

Classifies an incoming Request, sets a session state field

This feature is part of a group of features that implement SCC-AS behaviour. This feature is a prerequisite for SCCSendRequestToAnchor and for DetermineVoltePlanId (on an SCC-AS only).

Network Operator Data

VoLTE Shared Config

Configuration from the JSLEE profile table VoLTESharedConfigProfileTable is used to define the per node URI The following values are used:

  • SCCASURI - When running as an SCC AS this value is used for the ASURI in Session State

Conferencing

Conferencing handover detection relies on configuration data that is retrieved from the SCCConfHandlingConfigProfileTable JSLEE profile table.

The following values are used:

  • EnableSCCConfHandling - Used to determine whether conferencing handover is enabled

  • ConfFactoryPSI - Used to determine if the request is from the conference factory

Reorigination

Reorigination detection relies on configuration data that is retrieved from the JSLEE profile table named SCCCamelToIMSReOriginationConfigProfileTable.

The following values are used:

  • UsePrefix - Used to determine if a network prefix is used to detect reorigination

  • NetworkPrefix - The prefix that should be used to detect reorigination

Access Transfer

Access Transfer detection relies on the platform having three values configured:

  • the Access Transfer Update - Session Transfer Identifier (ATU-STI)

  • the Session Transfer Number - Single Radio (STN-SR)

  • the SCC AS’s own URI

The ATU-STI and STN-SR are configured in a JSLEE profile table RegistrarConfigurationTable, and the SCC AS URI is configured in a second JSLEE profile table VoLTESharedConfigProfileTable.

Notes on Config Profiles

The profile tables used by this feature are RegistrarConfigurationTable, VoLTESharedConfigProfileTable, SCCCamelToIMSReoriginationConfigProfileTable, and SCCConfHandlingConfigProfileTable.

The Profile Name within the tables is scoped according to the Sentinel Selection Key.

An example profile name is "OpenCloud:::::". This means:

The platform name is "OpenCloud". There is no Network Operator, Session Type, Plan Id, or Subscription ID scoping. The values may also be configured through the Sentinel REM screen:

  • Sentinel ▶ Feature Configuration ▶ SCC Determine Session Type (SIP)

  • Sentinel ▶ Feature Configuration ▶ SCC Camel To IMS Reorigination (SIP)

  • Sentinel ▶ Feature Configuration ▶ VoLTE Shared Config Profile (SIP)

Session Output Variables

Session State Variable Name Java Type Valid Values

SCCSessionType

A Java enum with type name com.opencloud.volte.sentinel.common.sessionstate.types.SCCSessionType

AccessTransfer, Reorigination, Standard

AccessTransferType

A Java enum with type name com.opencloud.volte.sentinel.common.sessionstate.types.AccessTransferType

eSRVCC_Anchored, eSRVCC_Not_Anchored, Additional_Session_Transfer, SRVCC, Internal_Directive, Bad_Request_STNSR, Bad_Request_ATUSTI, Bad_Request_SCCURI, None

ASURI

String

A SIP URI

Statistics

SCCDetermineSessionType statistics are tracked by the volte.sentinel.sip SBB and can be found under the following parameter set:
SLEE-Usage → [volte.sentinel.sip service name] → [volte.sentinel.sip SBB name] → feature.SCCDetermineSessionType

Statistic

Increments when…​

 
Started

The feature is started.

 
FailedToStart

Sentinel VoLTE encounters an error while attempting to start the feature.

 
IssuedWarning

A non-fatal problem is encountered and the feature and issues a warning.

 
FailedDuringExecution

A fatal problem is encountered and the feature cannot execute correctly.

 
TimedOut

The feature takes too long to complete and Sentinel VoLTE aborts execution.

 
SelectedStandardScc

The feature determines that the incoming INVITE request is for a standard SCC session.

 
SelectedReorigination

The feature determines that the incoming INVITE request is for reorigination.

 
SelectedESRVCCAnchored

The feature determines that the incoming INVITE request is for rel-10 access transfer with anchored media.

 
SelectedESRVCCNotAnchored

The feature determines that the incoming INVITE request is for rel-10 access transfer with non-anchored media.

 
SelectedSRVCC

The feature determines that the incoming INVITE request is for rel-8 access transfer.

 
SelectedAdditionalSessionTransfer

The feature determines that the incoming INVITE request is for an additional session transfer.

 
SelectedInternalAccessTransferDirective

The feature determines that the incoming MESSAGE request is for an OpenCloud internal access transfer directive.

 
InvokedProxyAndDoNotRecordRouteForRequest

The feature determines that the incoming MESSAGE request should be ignored and proxied by the SCC AS.

 
ReceivedBadRequestToSTNSR

The feature receives a malformed request for rel-8 access transfer.

 
ReceivedBadRequestToATUSTI

The feature receives a malformed request for rel-10 access transfer.

 
ReceivedBadRequestToSCCURI

The feature receives a malformed request for an additional session transfer.

Behaviour

The feature begins by checking that it was triggered by an initial SIP INVITE or MESSAGE request.

  • If the trigger is not an initial SIP request at all, the feature will raise an unsupported trigger event error and complete processing.

  • If the trigger is an initial SIP request but is not an INVITE or MESSAGE, then no further action is required and the feature will complete processing.

  • If the trigger is an initial INVITE or MESSAGE request, the feature will move on to load its configuration

If any configuration data is missing, then the feature will raise an invalid configuration error and complete processing.

Once configuration is loaded, the feature will analyse the incoming request to determine what type of session the request is for. It populates the SCCSessionType and AccessTransferType session state fields based on what it determines.

For an INVITE request the feature will check if it is one of:

  • A "Reorigination" request

  • A "Rel-8 Access Transfer (SRVCC)" request

  • A "Rel-10 Access Transfer (eSRVCC)" request

  • An "Additional Session Transfer" request

If none of the above is true, then the INVITE will be treated as a "Standard SCC Session".

For a MESSAGE request the feature will check if it is an "Internal Access Transfer Directive", if it is not then the request will be treated as a "Proxy and Do Not Record Route" case.

The following sections detail the conditions and feature behaviour for each possible session type.

Reorigination

Reorigination is determined by looking at the Request-URI of the incoming INVITE. If it is a tel: URI that begins with the configured reorigination prefix number, then the request is considered a reorigination request and the feature will do the following:

  • set the SCCSessionType to Reorigination.

  • set the AccessTransferType to None.

  • increment the SelectedReorigination statistic.

Rel-8 Access Transfer (SRVCC)

Rel-8 Access Transfer is determined by looking at the Request-URI of the incoming INVITE. If it begins with the configured STN-SR prefix number, then the request is considered a rel-8 access transfer request. In 3GPP specifications this is known as a "SIP INVITE request due to STN-SR".

The feature will then do an additional check for the presence of a P-Asserted-Identity header on the INVITE request. If the header is present, then the request is considered to be a rel-8 access transfer request and the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to SRVCC.

  • increment the SelectedSRVCC statistic.

If the P-Asserted-Identity header is absent, then the request is malformed and the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to Bad_Request_STNSR.

  • increment the ReceivedBadRequestToSTNSR statistic.

  • terminate the session by replying to the INVITE with a 400 Bad Request response.

Rel-10 Access Transfer (eSRVCC)

Rel-10 Access Transfer is determined by looking at the Request-URI of the incoming INVITE. If it matches the configured value for the ATU-STI, then the request is considered to be a rel-10 access transfer request. In 3GPP specifications this is known as a "SIP INVITE request due to ATU-STI for PS to CS SRVCC".

The feature will then do an additional check for the presence of a Target-Dialog or Replaces header on the INVITE request. If one of those headers is present, then the request is considered to be a rel-10 access transfer request for the case where media is anchored in the ATCF and the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to eSRVCC_Anchored.

  • increment the SelectedESRVCCAnchored statistic.

If both the Target-Dialog and Replaces headers are absent, the feature will move on to check for the presence of a P-Asserted-Identity header. If that header is present, then the request is considered to be a rel-10 access transfer request for the case where media is not anchored in the ATCF, and the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to eSRVCC_Not_Anchored.

  • increment the SelectedESRVCCNotAnchored statistic.

If both the P-Asserted-Identity is also absent, then the request is malformed and the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to Bad_Request_ATUSTI.

  • increment the ReceivedBadRequestToATUSTI statistic.

  • terminate the session by replying to the INVITE with a 400 Bad Request response.

Additional Session Transfer

Additional session transfer is determined by looking at the Request-URI of the incoming INVITE. If it matches the configured value for the URI of the SCC-AS, then the request is considered to be for an additional session transfer.

The feature will then do an additional check for the presence of a Target-Dialog header on the INVITE request. If the header is present, then the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to Additional_Session_Transfer.

  • increment the SelectedAdditionalSessionTransfer statistic.

  • terminate the session by replying to the INVITE with a 403 Forbidden response, as this type of access transfer is not supported by the SCC-AS.

If the Target-Dialog header is absent, then the request is malformed and the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to Bad_Request_SCCURI.

  • increment the ReceivedBadRequestToSCCURI statistic.

  • terminate the session by replying to the INVITE with a 400 Bad Request response.

Standard SCC Session

If an INVITE meets none of the above conditions, then the request is considered to be for a standard SCC session and the feature will do the following:

  • set the SCCSessionType to Standard.

  • set the AccessTransferType to None.

  • increment the SelectedStandardScc statistic.

Internal Access Transfer Directive

An Internal Access Transfer Directive is a MESSAGE request sent from a remote SCC AS node that is handling an access transfer request. Such a request is sent when the remote node determines that the node receiving the request is responsible for handling a session associated with the access transfer that it is processing. The request instructs the receiving node on what to do with that associated session.

To be considered an Internal Access Transfer Directive, a MESSAGE request must meet two conditions:

If those conditions are met, then the feature will do the following:

  • set the SCCSessionType to AccessTransfer.

  • set the AccessTransferType to Internal_Directive.

  • increment the SelectedInternalAccessTransferDirective statistic.

Proxy And Do Not Record Route

If an incoming MESSAGE request is not an Internal Access Transfer Directive, then the SCC AS does not need to handle it. The feature will do the following

  • set the SCCSessionType to Standard.

  • set the AccessTransferType to None.

  • increment the InvokedProxyAndDoNotRecordRouteForRequest statistic.

  • Instruct Sentinel to proxy the message without recording itself on the route.

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