HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

REFER 271
13
REFER
INTRODUCTION REFER is a SIP method defined by RFC 3515, “The Session Initiation Protocol
(SIP) Refer Method”. The REFER method indicates that the recipient of the
REFER request should contact a third party using the contact information
provided in the REFER request. RFC 3515 provides a mechanism allowing the
party that is sending the REFER to be notified of the outcome of the referenced
request. Once it is known whether the reference succeeded or failed, the agent
receiving the REFER request notifies the agent that sent the REFER request
about this result using the NOTIFY request.
The REFER mechanism utilizes the Subscribe-Notify mechanism and therefore
all REFER functionality is handled by the Subscription layer of the SIP Stack. It
is recommended to read the Event Notification chapter in this guide before
reading this chapter.
REFER-
SUBSCRIPTION
According to RFC 3515, a REFER request implicitly establishes a subscription
to the refer event. The SIP Stack handles both incoming and outgoing REFER
requests using the subscription. A subscription that handles a REFER request is
called a refer-subscription.
The REFER method can be used in the following two ways, which are handled
by the REFER Subscription API:
 A REFER sent within the scope of an existing dialog—UA1
that is in a session with UA2 can create a refer-subscription
within this session, and send the REFER to UA2 to establish a
session with UA3. This can be done to achieve call transfer.
You can send the REFER on both early and confirmed dialogs.