The SDP Monitor system feature keeps track of all Session Description Protocol (SDP) offers and answers on a leg.

It works like this:

  1. When an SDP offer is received, the feature updates the leg’s latestReceivedSDP, previousReceivedSDP and latestReceivedSDPSequenceNumber and waits for the corresponding answer.

  2. When the corresponding SDP answer is sent, the feature updates the leg’s latestSentSDP, previousSentSDP and latestSentSDPSequenceNumber.

  3. If the offer/answer exchange is successful the current committedSDP becomes the previousCommittedSDP and the leg’s committedSDP is set to the latest sent SDP on the leg. The committedSDPSequenceNumber is set to the latest SDP sequence number.

  4. If the offer/answer exchange is unsuccessful, the previous committedSDP and committedSDPSequenceNumber value(s) (if any) are restored. At which point the feature waits for a new SDP offer and the offer/answer exchange continues.

The feature also maintains an SDP field specifying the exchange initiator. The field is used by the Diameter Service Info Feature feature to set the Media-Initiator-Flag AVP in charging requests. Other features can access the SDP information using the Sentinel SIP Leg API.

Details

Feature script name

SDPMonitor

Applicable contexts

SIP service

Prerequisite features

None

Feature execution points
Pre

SipAccess_SessionStart SipAccess_PartyRequest SipAccess_PartyResponse SipMidSession_PartyRequest SipMidSession_PartyResponse SipLegEnd SipInstructionExecutionFailure

Post

All execution points (to examine messages queued by other features).
Note
Pre- and Post- Execution Points

The SDP Monitor feature must run as a pre-system feature on the execution points given above, so that it sees all incoming SIP messages.

It must also run as a post-system feature on all execution points, to ensure that messages sent by other features can be inspected before transmission.

A feature parameter must be specified when invoking the feature in execution scripts. This will be either pre in Pre system feature execution points or post in all Post system feature execution points.

Leg SDP fields

These fields on the leg interface are available to other Sentinel SIP features.

Field What it specifies

LatestReceivedSDP

The latest SDP received on the leg.

LatestSentSDP

The latest SDP sent on the leg.

CommittedSDP

The SDP that was committed after the last successful offer/answer exchange.

LatestSentSDPSequenceNumber

The SDP origin sequence number in the last SDP sent on the leg.

LatestReceivedSDPSequenceNumber

The SDP origin sequence number in the last SDP received on the leg.

CommittedSDPSequenceNumber

The SDP origin sequence number in this leg’s committed SDP.

PreviousReceivedSDP

The previous SDP received on the leg.

PreviousSentSDP

The previous SDP sent on the leg.

PreviousSentSDPSequenceNumber

The previous origin sequence number in the previous SDP sent on the leg.

PreviousReceivedSDPSequenceNumber

The previous origin sequence number in the previous SDP received on the leg.

PreviousCommittedSDP

The SDP that was committed by the previous offer/answer exchange.

Session state inputs and outputs

This feature does not read from or write to session state.

Feature responses

Response Reason

featureHasFinished

Feature has finished.

featureCannotStart

Feature parameter error and was unable to start.

Configuration

This feature is not configurable.

Statistics

Statistic What it counts

SDPRollback

The number of times an offer/answer exchange failed and the latest SDP was rolled back. Caused by rejection or failure to send.

SDPOfferReceived

The number of SDP offers that have been received by the feature.

SDPOfferSent

The number of SDP offers that have been sent by the feature.

SDPAnswerReceived

The number of SDP answers that have been received by the feature.

SDPRejectionReceived

The number of sent SDP offers rejected by receiving final error responses.

SDPAnswerSent

The number of SDP answers the feature has sent.

SDPRejectionSent

The number of received SDP offers rejected by sending final error responses.

SDPCommitted

The number of times a successful offer/answer exchange has resulted in a committed SDP.
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Sentinel Express Version 2.7.0