The behavior of a UA on detection of media failure is a matter of local policy. The third layer is the transaction layer. That is, they may list a set of alternative names and methods of reaching the user. This may consist of obtaining information from a location service created by a SIP Registrar, reading a database, consulting a presence server, utilizing other protocols, or simply performing an algorithmic substitution on the Request-URI. Pour être simple ça ressemble à un petit patch qui s’adresse à Messenger Plus! As described in , the proxy MUST attempt to deliver the message to the first tuple in that set, and proceed through the set in order until the delivery attempt succeeds. Client Error — the request contains bad syntax or cannot be fulfilled at this server; 5xx:
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The value is the same as the To header field unless the request is a third- party registration. The other party sends a OK to accept the change. It also allows elements following this specification to preserve the Request-URI through strict-routing proxies see Section An AOR is frequently thought of as the « public address » of the user. This route set, even if empty, overrides any pre-existing route set for future requests in this dialog. Client Error — the request contains bad syntax or cannot be fulfilled at this server; 5xx: For other responses, or if no response arrives at all on that dialog, the early dialog terminates.
Network Working Group J. Rosenberg Request for Comments: Standards Track Columbia U. Session Initiation Protocol Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the « Internet Official Protocol Standards » STD 1 for the standardization state and status of this protocol.
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Distribution of this memo is unlimited. Abstract This document describes Session Initiation Protocol SIPan application-layer control signaling protocol for creating, modifying, and terminating sessions with one or more participants. These sessions conatctsay Internet telephone calls, multimedia distribution, and multimedia conferences. SIP invitations used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types.
SIP makes use of elements called proxy servers to help route requests to the user’s current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users. SIP also provides a registration function that allows users to upload their current locations for use by proxy servers.
SIP runs on top of several different transport protocols. Session Initiation Protocol June 8. Session Initiation Protocol June Threat Model and Security Usage Recommendations The conntactsay of these applications is complicated by the practices of participants: Numerous protocols have been authored that carry various forms of real-time zcript session data such as voice, video, or text messages.
Session Initiation Protocol June enabling Internet endpoints called user agents to discover one another and to agree on a characterization of a session they would like to share. For locating prospective session participants, and for other functions, SIP enables the creation of an infrastructure of network hosts called proxy servers to which user agents can send registrations, invitations to sessions, and other requests.
SIP is an agile, general-purpose tool for creating, modifying, and terminating sessions that works independently of underlying transport protocols and without dependency on the type of session that is being established. SIP can also invite participants to already existing sessions, such as multicast conferences. Media can be added to and removed from an existing session. SIP transparently supports name mapping and redirection services, which supports personal mobility  – users can maintain a single externally visible identifier regardless of their network location.
SIP supports five facets of establishing and terminating multimedia communications: SIP is not a vertically integrated communications system. Therefore, SIP should be used in conjunction with other protocols in order to provide complete services to the users. However, the basic functionality and operation of SIP does not depend on any of these protocols. SIP does not provide services. Rather, SIP provides primitives that can be used to implement different services.
For example, SIP can locate a user and deliver an opaque object to his current location.
If this primitive is used to deliver a session description written in SDP, for instance, the endpoints can agree on the parameters contacttsay a zcript. If the same primitive is used to deliver a photo of the caller as well as the session description, a « caller ID » service can be easily implemented. As this example shows, a single primitive is typically used to provide several different services.
SIP does not offer conference control services such as floor control or voting and does not prescribe how a conference is to be managed. SIP can be used to initiate a session that uses some other conference control protocol.
Since SIP messages and the contactsaay they establish can pass through entirely different networks, SIP cannot, and does not, provide any kind of network resource reservation capabilities. The conhactsay of the services provided make security particularly important. To that end, SIP provides a suite of security services, which include denial-of-service prevention, authentication both user to user and proxy to userintegrity protection, and encryption and privacy services.
This section is tutorial in nature and does not contain any normative statements. Each message is labeled with the letter « F » and a number for reference by the text.
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Also shown are two SIP proxy servers that act on behalf of Alice and Bob to facilitate the session establishment. This typical arrangement is often referred to as the « SIP trapezoid » as shown by the geometric contaxtsay of the dotted lines in Figure 1.
It has a similar form to an email address, typically containing a username and a host name. In this case, it is sip: Alice might have typed in Bob’s URI or perhaps clicked on a hyperlink or an entry in an address book. An example would be sips: From there, the request is sent securely to the callee, but with security mechanisms that depend on the policy of the domain of the callee.
Each transaction consists contacttsay a request that invokes a particular method, or function, on the server and contaactsay least one response. Header fields are named attributes that provide additional information about a message.
The ones present in scri;t INVITE include a unique identifier for the xcript, the destination address, Alice’s address, and information about the type of session that Alice wishes to establish with Bob. Session Initiation Protocol June atlanta. The lines that follow are a list of header fields. This example contains a minimum required set. The header fields are briefly described below: Session Initiation Protocol June Via contains the address pc It also contains a branch parameter that identifies this transaction.
Display names are described in RFC . This header field also has a tag parameter containing a random string that was added to the URI by the softphone.
It is used for identification purposes. Call-ID contains a globally unique identifier for this call, generated by the combination of a random string and the softphone’s host name or IP address. CSeq or Command Sequence contains an integer and a method name.
The CSeq number is incremented for each new request within a dialog and is a traditional sequence contaftsay. While the Via header field tells other elements where to send the response, the Contact header field tells other elements where to send future requests. Max-Forwards serves to limit the number of hops a request can make on the way to its destination.
It consists of an integer that is decremented by one at each hop. Content-Type contains a contatsay of the message body not shown. Content-Length contains an octet byte count of the message body. The complete set of SIP header fields is defined in Section The details of the session, such as the type of media, codec, or sampling rate, are not described using SIP.
Rather, the body of a SIP message contains a contactsaj of the session, encoded in some other protocol format. This SDP message not shown in the Rosenberg, et.
Session Initiation Protocol June example is carried by the SIP message in a way that is analogous to a document attachment being carried by an email message, or a web page being carried in an HTTP message. Since the softphone does not know the location of Bob or the SIP server in the biloxi. The address of the atlanta.
A proxy server receives SIP requests and forwards them on behalf of the requestor. Responses in SIP use a three-digit code followed by a descriptive phrase. This is described in . As a result, it obtains the IP address of the biloxi.
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Before forwarding the request, the atlanta. The proxy server consults a database, generically called a location service, that contains the current IP address of Contqctsay. We shall see in the next section how this database can be populated. Bob’s SIP phone indicates this in a Ringing response, which is routed back through the acript proxies in the reverse contactaay. Each proxy uses the Via header field conractsay determine where to send the response and removes its own address from the top.
As a result, although DNS and location service lookups were required to route the initial INVITE, the Ringing response can be returned to the caller without lookups or without state being Rosenberg, et. Session Initiation Protocol June maintained in the proxies. When Alice’s softphone receives the Ringing response, it passes this information to Alice, perhaps using an audio ringback tone or by displaying a message on Alice’s screen.
In this example, Bob decides to answer the call. When he picks up the handset, his SIP phone sends a OK response to indicate that the call has been answered.
As a result, there is a two-phase exchange of SDP messages: Alice sent one to Bob, and Bob sent one back to Alice. If Bob did not wish to answer the call or was busy on another call, an error response would have been sent instead of the OKwhich would have resulted in no media session being established. The complete list of SIP response codes is in Section contsctsay The remaining lines contain header fields. Bob’s SIP phone has added a tag parameter to the To header field.
This tag will be incorporated by both endpoints into the dialog and will be included in all future Rosenberg, et. Session Initiation Protocol June requests and responses in this call. In addition to DNS and location service lookups shown in this example, proxy servers can make flexible « routing decisions » to decide where to send a request. This type of parallel search is known as forking.
In this case, the OK is routed back through the two proxies and is received by Alice’s softphone, which then stops the ringback tone and indicates that the contaactsay has been answered.