Thursday, 08 November 2007 04:31
Initiating a Session in SIP When a user agent client desires to initiate a session (for example,
audio, video, or a game), it formulates an INVITE request. The
INVITE request asks a server to establish a session. This request
may be forwarded by proxies, eventually arriving at one or more UAS
that can potentially accept the invitation. These UASs will
frequently need to query the user about whether to accept the
invitation. After some time, those UASs can accept the invitation
(meaning the session is to be established) by sending a 2xx response.
If the invitation is not accepted, a 3xx, 4xx, 5xx or 6xx response is
sent, depending on the reason for the rejection. Before sending a
final response, the UAS can also send provisional responses (1xx) to
advise the UAC of progress in contacting the called user.
Read more...
Thursday, 08 November 2007 04:25
Dialogs in SIP A key concept for a user agent is that of a dialog. A dialog
represents a peer-to-peer SIP relationship between two user agents
that persists for some time. The dialog facilitates sequencing of
messages between the user agents and proper routing of requests
between both of them. The dialog represents a context in which to
interpret SIP messages. Section 8 discussed method independent UA
processing for requests and responses outside of a dialog. This
section discusses how those requests and responses are used to
construct a dialog, and then how subsequent requests and responses
are sent within a dialog.
Read more...
Thursday, 08 November 2007 04:21
Registrations in SIP SIP offers a discovery capability. If a user wants to initiate a
session with another user, SIP must discover the current host(s) at
which the destination user is reachable. This discovery process is
frequently accomplished by SIP network elements such as proxy servers
and redirect servers which are responsible for receiving a request,
determining where to send it based on knowledge of the location of
the user, and then sending it there. To do this, SIP network
elements consult an abstract service known as a location service,
which provides address bindings for a particular domain. These
address bindings map an incoming SIP or SIPS URI, sip:
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
,
for example, to one or more URIs that are somehow "closer" to the
desired user, sip:
This e-mail address is being protected from spambots. You need JavaScript enabled to view it
, for example.
Ultimately, a proxy will consult a location service that maps a
received URI to the user agent(s) at which the desired recipient is
currently residing.
Read more...
Thursday, 08 November 2007 04:17
Canceling a Request in SIP The previous section has discussed general UA behavior for generating
requests and processing responses for requests of all methods. In
this section, we discuss a general purpose method, called CANCEL.
The CANCEL request, as the name implies, is used to cancel a previous
request sent by a client. Specifically, it asks the UAS to cease
processing the request and to generate an error response to that
request. CANCEL has no effect on a request to which a UAS has
already given a final response. Because of this, it is most useful
to CANCEL requests to which it can take a server long time to
respond. For this reason, CANCEL is best for INVITE requests, which
can take a long time to generate a response. In that usage, a UAS
that receives a CANCEL request for an INVITE, but has not yet sent a
final response, would "stop ringing", and then respond to the INVITE
with a specific error response (a 487).
Read more...
Thursday, 08 November 2007 04:12
General User Agent Behavior in SIP A user agent represents an end system. It contains a user agent
client (UAC), which generates requests, and a user agent server
(UAS), which responds to them. A UAC is capable of generating a
request based on some external stimulus (the user clicking a button,
or a signal on a PSTN line) and processing a response. A UAS is
capable of receiving a request and generating a response based on
user input, external stimulus, the result of a program execution, or
some other mechanism.
Read more...
|
|
