One of the most unique parts of SIP is the concept of presence. The public switched telephone network PSTN can provide basic presence information—whether a phone is on- or off- hook—when a call is initiated.
However, SIP takes that further. It can provide information on the willingness of the other party to receive calls, not just the ability, before the call is attempted. This is similar in concept to instant messaging applications—you can choose which users appear on your list, and they can choose to display different status types, such as offline, busy, and so on. Users who subscribe to that instant messaging service know the availability of those on their list before they try to contact them.
SIP presence information is available only to subscribers. SIP is already influencing the marketplace. Cellular phone providers use SIP to offer additional services in their 3G networks. The Microsoft real-time communications platform—including instant messaging, voice, video, and application-sharing—is based on SIP. Some hospitals are implementing SIP to allow heart monitors and other devices to send an instant message to nurses.
You can expect to see its use increase as more applications and extensions are created for SIP. UAs can act as either clients or servers. The user agent client UAC is the device that is initiating a call, and the user agent server UAS is the device that is receiving the call. The SIP protocol defines several other functional components. These functional entities can be implemented as separate devices, or the same device can perform multiple functions.
All these functions work together to accomplish the goal of establishing and managing a session between two UAs. SIP servers can also interact with other application servers to provide services, such as authentication or billing. You can configure Cisco routers as SIP gateways. SRST is not on by default; you must configure it. SIP uses plain-text messages, following the format of standard Internet text messages. This helps in troubleshooting, because it is easy to read SIP messages.
However, you must understand the types of messages and their formats to successfully troubleshoot them. This section helps you with that understanding. SIP messages are either requests or responses to a request; the function that the request invokes on a server is called a method. Several types of SIP methods exist. The original SIP specification included the following six methods. Cisco gateways can both send and receive these methods, except where noted.
Cisco gateways also support the following additional methods, but they only respond to them. They do not generate them. SIP entities can send additional messages in response to a method; these responses are listed in Table Responses to SIP methods fall into six categories. The Series designates informational or provisional responses, such as for Trying, and for Alerting. A Series response means that the request was successful; it includes for OK, and for Accepted.
The Series redirects the user to a different location for the called endpoint. Examples include for Moved Permanently and for Moved Temporarily. A Series response is received due to a server failure, such as for Server Internal Error or for Service Unavailable. The Series is used for a global failure, including when the call is declined. Unsupported URI 1 Scheme. The SDP fields have the following meanings:. Continuing the call, the called side the UAS returns a provisional response Trying, shown in Example Now the call is established.
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Each transaction consists of a request that invokes a particular method, or function on the server and at least one response. Also shown are two SIP proxy servers that act on behalf of Tan and Bob to facilitate the session establishment. It has a similar form to an email address, typically containing a username and a host name. In this case, it is sip:bob tokyo. Tan 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:bob tokyo. 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.
The ones present in an INVITE include a unique identifier for the call, the destination address, Tan's address, and information about the type of session that Tan wishes to establish with Bob. The penang. The tokyo. The proxy server consults a database that contains the current IP address of Bob. Bob's SIP phone indicates this in a Ringing response, which is routed back through the two proxies in the reverse direction.
In this example, Bob decides to answer the call. When he click answer, his SIP client sends a OK response to indicate that the call has been answered. 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 OK , which would have resulted in no media session being established.
The lookups performed by the two proxies are no longer needed, so the proxies drop out of the call flow. Tan and Bob's media session has now begun, and they send media packets using the format to which they agreed in the exchange of SDP. In general, the end-to-end media packets take a different path from the SIP signaling messages. During the session, either Tan or Bob may decide to change the characteristics of the media session. At the end of the call, Bob disconnects hangs up first and generates a BYE message.
Registration is another common operation in SIP. Registration is one way that the tokyo. The registrar writes this association, also called a binding, to a database, called the location service, where it can be used by the proxy. The lowest layer of SIP is its syntax and encoding.
The second layer is the transport layer. It defines how a client sends requests and receives responses and how a server receives requests and sends responses over the network. All SIP elements contain a transport layer.
The third layer is the transaction layer. Transactions are a fundamental component of SIP. A transaction is a request sent by a client transaction using the transport layer to a server transaction, along with all responses to that request sent from the server transaction back to the client. The transaction layer handles application-layer retransmissions, matching of responses to requests, and application-layer timeouts. Any task that a user agent client UAC accomplishes takes place using a series of transactions.
The transaction layer has a client component referred to as a client transaction and a server component referred to as a server transaction , each of which are represented by a finite state machine that is constructed to process a particular request. The layer above the transaction layer is called the transaction user TU. Each of the SIP entities, except the stateless proxy, is a transaction user.
When a TU wishes to send a request, it creates a client transaction instance and passes it the request along with the destination IP address, port, and transport to which to send the request. A TU that creates a client transaction can also cancel it.
When a client cancels a transaction, it requests that the server stop further processing, revert to the state that existed before the transaction was initiated, and generate a specific error response to that transaction. This is done with a CANCEL request, which constitutes its own transaction, but references the transaction to be cancelled.
The SIP elements, that is, user agent clients and servers, stateless and stateful proxies and registrars, contain a core that distinguishes them from each other [5].
Cores, except for the stateless proxy, are transaction users. For a UAC, these rules govern the construction of a request; for a UAS, they govern the processing of a request and generating a response. Certain other requests are sent within a dialog.
A dialog is a peer-to-peer SIP relationship between two user agents that persists for some time. The dialog facilitates sequencing of messages and proper routing of requests between the user agents.
The PRACK method applies to all provisional responses except the Trying response, which is never reliably transported. It is used to send an instant message using SIP. An IM usually consists of short messages exchanged in real time by participants engaged in text conversation. A OK response is normally received to indicate that the message has been delivered at its destination.
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