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Support for SIP INFO Message

Overview

Prior to this feature the CSP SIP stack could not generate an INFO message nor could it report an inbound INFO message.

With this feature, the SIP stack can generate the INFO message by the host and report the receipt of an INFO message to the host.

The INFO message also supports the Subject header with read and write access.

Pertinent Specification

RFC 2976

Description

The INFO method carries the session related control information that is generated during a session. ISUP and ISDN signaling messages used to control telephony call services are examples of session control information.

By default, inbound SIP INFO messages are not reported to the host.

This feature is supported by call agent and non-call agent calls.

Reporting the Inbound INFO Message

When the CSP receives an INFO message, the CSP sends the host a PPL Event Indication message. This message contains the Subject Header TLV 0x295B if the inbound INFO message has a Subject header and its length is less than 250 bytes.

The PPL Event 0x002C in SIP component 0x00A7 reports the message.

The following two TLVs are supported in this Event Indication:

• Subject Header (0x295B)

• Message Body (0x295C)

• Content Type (0x295D)

The PPL Event Indication message contains the Subject Header TLV 0x295B if the Subject header is present in the inbound INFO message.

The PPL Event Indication message contains the Message Body TLV 0x295C if the inbound INFO has a message body and its length is less than 500 bytes.

If the SIP stack receives an INFO message for any call in a non-answered state, the CSP responds with error response 488 - Not Acceptable Here.

Generating the Outbound INFO Message

The CSP SIP stack allows the host to generate an outbound INFO message using the PPL Event Request (0x0025) in SIP UA component (0x00A7).

The following three TLVs are supported in this Event Request:

• Subject Header (0x295B)

• Message Body (0x295C)

• Content Type (0x295D)

These TLVs are optional - there are no mandatory TLVs.

Since the INFO method is used for communicating mid-session signaling information along the signaling path for the call, the CSP accepts this Event Request only when the call is in the answered state. In other states, the Event Request is NACKed with the value 0x1303 (Invalid PPL Event) or 0x1308 (Invalid State).

Reporting the INFO Message Response

If a User Agent Server receives an INFO request it must send a final response. The response to the INFO message is reported to the host in a PPL Event Indication message.

The PPL Event 0x002D for SIP component 0x00A7 reports the message.

The PPL Event Indication message contains the SIP Response Code TLV 0x2915. If no response is received from the endpoint, the INFO message is not re-transmitted and the CSP does not report and type of response.

API Messages Used

• PPL Event Indication (0x0043)

• PPL Event Request (0x0044)

PPL Information

PPL Component 0x00A7:

• 0x0025 - Generate INFO Request

• 0x002D - Response for INFO Send Indication

• 0x002C - Report INFO Received

Configuring

By default, inbound SIP INFO messages are not reported to the host. Set bit 6 in the PPL Event Notification Mask TLV (0x0282) in the VoIP Protocol Configure message to enable this feature.

Call Flow

The following call flow shows messages involved with the Reporting the Inbound INFO message.

Session-Initiation Protocol. A signaling protocol used for establishing sessions in an IP network. A session could be a simple, two-way telephone call, or it could be a collaborative multi-media conference session.
See Integrated Services Digital Network User Part.
See Integrated Services Digital Network.
See Programmable Protocol Language.
Tag-Length-Value block. A standardized block that allows data to inserted in a standard way into API messages.
A condition that, when met, drives a state machine from one state to another. A condition that drives a Component out of its current state by invoking an associated primitive.
The condition of a component at a given instant. The current "context" of a PPL object, such as Idle, Seizing, Answered. In PPL files, a State is represented by a circle.
A programmable software component that uses PPL state machines that can be modified with the PPL Tool.
Voice over Internet Protocol. The delivery of voice information in digital form in discrete packets rather than in the traditional circuit-committed protocols of the public-switched telephone network (PSTN).