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

24 HP-UX C SIP Stack Programmer’s Guide
Feature Support
REFER The SIP Stack allows the implementation of transfer services through the use of
the REFER extension method, as defined in RFC 3515. This includes the usage
of NOTIFY messages. REFER processing is fully controllable by the
application via the Subscription API.
SIP-T SIP-T (SIP for Telephony, RFC 3372) is an umbrella specification defining how
to interwork SIP with PSTN (SS7/ISUP) networks. The SIP-T specification
does not define any new SIP extensions, but rather uses existing extensions
(PRACK, INFO) and other SIP advanced features, such as Multipart MIME and
183 response request, in order to translate between ISUP and SIP messages and,
in some cases, tunnel ISUP in SIP.
The SIP Stack provides all the necessary building blocks needed by a SIP-T
compliant application (for example, Softswitches and PSTN GWs).
PRACK PRACK (RFC 3262) is an IETF SIP extension that allows for the sending of
provisional responses (1xx class) in a reliable way. One of the new elements
introduced in this extension is the PRACK (PRovisional ACKnowledgment)
method, which is used to indicate the reception of the provisional response, and
RSeq and RAck message headers to identify the provisional response. PRACK
is useful for such things as opening of one-way media sessions before call
establishment, and for QoS negotiation before completing the INVITE
transaction.
The SIP Stack implements PRACK, providing fully automated behavior with
optional application control.
ENUM In VoIP networks, an endpoint has an IP address, and may have a telephone
number. When the telephone number is known, the corresponding IP address
needs to be found. ENUM is a standard that enables translating telephone
numbers to SIP addresses. Translation is done using the existing internet DNS
(NAPTR) query mechanism. The ENUM mechanism is a requirement in cellular
IMS networks, and many fixed and mobile operators also choose to implement
ENUM.
The SIP Stack version 4.0 provides users with a fast implementation of the
ENUM mechanism. Both client and server developers can use this feature.
CONNECTION REUSE When SIP entities use a connection-oriented protocol to send a request, they
typically originate their connections from an ephemeral port. The SIP Protocol
includes mechanisms that insure that responses to a request, and new requests
sent in the original direction, reuse an existing connection. However, new
requests sent in the opposite direction are unlikely to reuse the existing