HP-UX Java SIP Stack Programmer's Guide (February 2008)

The REFER Method
The SIP stack enables the implementation of transfer services using the REFER method,
as defined in RFC 3515 (The Session Initiation Protocol (SIP) Refer Method). The REFER
method can be used in call transfer applications. It indicates that the recipient must
contact a resource using the contact information provided in the request. The protocol
for emitting and responding to a REFER request is identical to that of a BYE request.
A user agent accepting a REFER request must request approval from the user that is
sending the request. If the user grants approval, the user agent must contact the resource
identified by the URI in the Refer-To header field in the request. The NOTIFY
mechanism is used to inform the user that is sending the REFER method about the
status of the reference.
SIP for Telephones
SIP for Telephones (SIP-T), as defined in RFC 3372 (Session Initiation Protocol for
Telephones (SIP-T): Context and Architectures), is a specification that defines how to
interwork SIP with PSTN. The SIP-T specification does not define any new SIP extension,
but uses the existing extension (such as PRACK, INFO, and so on) and other advanced
SIP features (such as Multipart MIME and 183 response request) to translate ISDN user
part (ISUP) and SIP messages, and in some cases, the tunnel ISUP in SIP.
The SIP stack provides all the necessary building blocks required by an application to
be compliant with SIP-T, for example, soft switches and PSTN gateways.
Provisional Response Acknowledgement in SIP
The SIP product contains two types of responses, namely provisional and final. Final
responses convey the result of the request processing. These responses are sent reliably.
Provisional responses provide information about the progress of the request processing.
These responses are not sent reliably. Provisional response acknowledgement (PRACK
) is an IETF SIP extension that enables reliable transmission of provisional responses,
as specified in RFC 3262 (Reliability of Provisional Responses in the Session Initiation Protocol
(SIP)). The PRACK message is used in opening one-way media sessions before a call is
established and for quality of service (QoS) negotiation before completing an INVITE
transaction.
Each provisional response contains a sequence number that is stored in the RSeq header
field in the provisional response. The RSeq header is used to transmit the provisional
responses reliably. The PRACK messages contain an RAck header field, which indicates
the sequence number of the provisional response that is being acknowledged. This
header field is sent in a PRACK request to support reliable transmission of provisional
responses.
HP-UX JSR32 Features 43