SIP Subject Header in REFER
Currently the RFC 3515 does not support Subject Header in the REFER message. With this feature, the Subject Header field is now an optional header in the REFER message. This feature allows read and write access for the Subject Header field in the REFER message.
Read access allows the host to receive the content of the Subject Header field, if it is present in the inbound REFER message. You can configure whether or not to report the Subject Header field.
Write access allows the host to fill into the Subject Header field of the outbound REFER message. This functionality is available for Call Agent and non-Call Agent calls.
Subject Header Field Read/Write Access
The reporting of the Subject header field is within the context of the SIP REFER message. The Subject header field is reported using the SIP Subject TLV - 0295B in the PPL Event Indication for Blind Refer (PPL Component 0x00A7, Event 0x0027).
The SIP Subject TLV is used within the NPDI Universal ICB 0x0033. With the Subject Header field reporting enabled, when the CSP receives a SIP REFER that contains the Subject Header field, it will send a Host TLV (SIP Subject TLV - 0x295B) to the host application in the PPL Event Indication message. If the Subject Header field value is greater than 250 characters, then it will not be reported to the host.
The write access is within the context of the SIP REFER message. The SIP Stack allows the host to fill the Subject Header field using the SIP Subject TLV - 0x295B in the PPL Event Request to generate the REFER message (PPL Component 0x00A7, Event 0x0021).
The SIP Subject TLV is used within the NPDI Universal ICB 0x0033. If the SIP Subject TLV exceeds 250, the PPL Event Request is NACKed with 0x1304.
The following messages are used by this feature. Refer to the API Reference for the formats.
• VoIP Protocol Configure (0x00EE)
• PPL Event Indication (0x0043)
• Request for Service with Data (0x002D)
PPL Component 0x00A7:
• PPL Event Request 0x0027 - Generate outbound REFER message.
You can configure the SIP to report the Subject Header field from the REFER message. It is disabled by default. You enable it by setting a bit mask during the SIP stack configuration.
Use the SIP Message Information Mask TLV (0x027F) in the VoIP Protocol Configure (0x00EE) message. Set bit 11 to enable the feature.
This call flow assumes that the Referrer and Referee are in conversation. The Referee will contact another SIP Endpoint which is omitted in the figure below to keep it simple. This call flow also assumes that reporting Subject Header field is enabled.
The call flow below show both read and write access of Subject Header field for a REFER message. The PPL Event Indication (0x0021) reports the Subject Header field in the SIP Subject TLV. The PPL Event Request (0x0027) uses the SIP Subject TLV to generate a REFER message with the Subject Header field.