these extensions are described in section 4. · RFC A Message Summary and Message Waiting Indication Event Package . Find the most up-to-date version of IETF – RFC at Engineering Find the most up-to-date version of RFC at Engineering
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Several SIP extensions have been added to the basic protocol specification in order to extend its functionality. These include an efficient use of the radio interface by minimizing the exchange of signaling messages between the mobile terminal and the network, a minimum session setup time by performing tasks prior to session establishment instead of during session establishment, a minimum support required in the terminal, the support for roaming and non-roaming scenarios with terminal mobility management supported by the access network, not SIPand support for IPv6 addressing.
Other requirements involve protocol extensions, such as SIP header fields to exchange user or server information, and SIP methods to support new network functionality: Other specific requirements are: There is a mechanism  in SIP for extension negotiation between user agents UA or servers, consisting of three header fields: When a client initiates a SIP dialog with a server, it states the extensions it requires to be used and also other extensions that are understood supportedand the server will then send a response with a list of extensions that it requires.
If these extensions are not listed in the client’s message, the response from the server will be an error response. Likewise, if the server does not support any of the client’s required extensions, it will send an error response with a list of its unsupported extensions.
This kind of extensions are called option tagsbut SIP can also be extended with new methods. In that case, user agents or servers use the Allow header to state which methods they support. To require the use of a rffc method in a particular dialog, they must use an option tag associated to that method. These two extensions allow users to ieetf their preferences about the service the IMS provides. With the caller preferences extension, 325 the calling party is able to indicate the kind of user agent they want to reach e.
Accept-Contact to describe the desired destination user agents, Reject-Contact to state the user agents to avoid, and Request-Disposition to specify how the request should be handled by servers in the network i.
By using the user agent capabilities extension,  user ieyf terminals iftf describe themselves when they register so that others can search for eitf according to their caller preferences extension headers.
The aim of event notification is iett obtain the status of a rgc resource e. Event notification is necessary in the IMS framework to inform about the presence of a user i. Moreover, event notification can be used to provide additional services such as voicemail i.
To this end, the specific event notification extension  defines a framework for event notification in SIP, with two new methods: Each kind of event a subscriber can subscribe to is defined in a tfc event package.
There is also an allow-events header to indicate event notification capabilities, and the accepted and bad event response codes to indicate if a subscription request has been preliminary accepted or has been turned down because the notifier does not understand the kind of event requested. In order to make an efficient use of the signaling messages, it is also possible to establish a limited notification rate not real-time notifications through a mechanism called event throttling.
Moreover, there is also rrfc mechanism for conditional event notification that allows the notifier to decide whether or not to send the complete NOTIFY message efc on if there is something new to notify rf last subscription or there is not.
SIP and Open Standards
The event notification framework defines how a user agent can subscribe to events about the state of a resource, but it does not specify how that state can be published. Ietff SIP extension for event state publication  was defined to allow user agents to publish the state of an event to the entity notifier that is responsible for composing the event state and distributing it to the subscribers.
The state publication framework defines a new ieft PUBLISH, which is used to request the publication of the state of the resource specified in the request-URI, with reference to the event stated in the Event headerand with ieyf information carried in the message body.
The functionality of sending instant messages to provide a service similar to text messaging is defined in the instant messaging extension.
This functionality is supported rcf the new MESSAGE method, that can be used to send an instant message to the resource stated in the request-URI, with the content carried in the message body. A typical use of this mechanism is call transfer: The REFER message also implies 32655 event subscription to the result of the operation, so that the sender will know whether or not the recipient could contact the third person.
In the basic SIP specification,  only requests and final responses i. This mechanism is necessary since SIP can run not only over reliable transport protocols TCP that assure that the message is delivered, but also over unreliable ones UDP that offer no delivery guarantees, and it is even possible that both kinds of protocols are present in different parts of the transport network.
However, in such an scenario as the IMS framework, it is necessary to extend this reliability to provisional responses to INVITE requests for session establishment, this is, to start a call. The reliability of provisional responses extension  provides a mechanism to confirm that provisional responses such as the Ringing response codethat lets the caller know that the callee is being alerted, are successfully received.
To do so, this extension defines a new method: PRACK, which is the request 2365 used to tell the sender of a provisional response that his or her iietf has rff received. To indicate that the user agent requests or supports reliable provisional responses, the rel option tag will be used. This can be used to negotiate and allocate the call resources before the called party is alerted.
In the IMS framework, it is required that once the callee is alerted, the chances of a session failure are minimum.
An important source of failure is the inability to reserve network resources to iwtf the session, so these resources should be allocated before the phone rings. In basic SIP, this exchange eventually rffc the callee to be alerted. To solve this problem, the concept of preconditions  was introduced.
In this concept rvc caller states a set of constraints about the session i. This establishment will occur if and only if both the caller and the callee agree that the preconditions istf met. The new SDP attributes are meant to describe the current status of the resource reservation, the desired status of the reservation to proceed with session establishment, and the confirmation statusto indicate when the rvc status should be confirmed. In the IMS, the initial session parameter negotiation can be done by using the provisional responses and session description updating extensions, along with SDP in the body of the messages.
This request will be answered by the provisional reliable response code Session Progressthat will carry the SDP list of supported codecs by both the caller and the callee.
Once this response has been sent, the called party has selected the codec too, and starts resource reservation on its side.
It is then, when all the resources for the call are in place, when the caller is alerted. In the IMS framework it is fundamental to handle user identities for authentication, authorization and accounting purposes.
The IMS is meant to provide multimedia services over IP networks, but also needs a mechanism to charge users for it. All this functionality is supported by new special header fields. The Private Header Extensions to SIP,  also known as Ieft, are special header fields whose applicability is limited to private networks with a certain topology and characteristics of lower layers ‘ protocols.
They were designed specifically to meet the 3GPP 365 because a more general solution was not available.
ITU-T work programme
These header fields are used for a variety of purposes including charging and information about the networks a call traverses:. The private extensions for asserted identity within trusted networks  are designed to enable a network of trusted SIP servers to assert the identity of authenticated users, only within an administrative domain with previously agreed policies for generation, transport and usage of this identification information.
These extensions also allow users to request privacy so that their identities are not spread outside the trust domain. To indicate so, they must insert the privacy token id into the Privacy header field. The main functionality is supported by the P-Asserted-Identity extension header.
When a proxy server receives a request from an untrusted entity and authenticates the user i. The P-Preferred-Identity extension header is also defined, so that a user with several public identities is able to tell the proxy which public identity should be included in the P-Asserted-Identity header when the user is authenticated.
Finally, when privacy is requested, proxies must withhold asserted identity information outside the trusted domain by removing P-Asserted-Identity headers before forwarding user requests to untrusted identities outside the Trust Domain. There exist analogous extension headers for handling the identification of services of users,  instead of the users themselves.
In this case, Uniform Resource Names are used to identify a service e. There are several mechanisms to achieve this, such as:. The security mechanisms agreement extension for SIP  was then introduced to provide a secure mechanism for negotiating the security algorithms and parameters to be used by the P-CSCF and the terminal.
This extension uses three new header fields to support the negotiation process:. The necessity in the IMS of reserving resources to provide quality of service QoS leads to another security issue: To obtain transmission resources, the user agent must present an authorization token to the network i. This token will be obtained from its P-CSCF, which may be in charge of QoS policy control or have an interface with the policy control entity in the network i.
The private extensions for media authorization  link session signaling to the QoS mechanisms applied to media in the network, by defining the mechanisms for obtaining authorization tokens and the P-Media-Authorization header field to carry these tokens from the P-CSCF to the user agent.
This extension is only applicable within administrative domains with trust relationships. Source routing is the mechanism that allows the sender of a message to specify partially or completely the route the message traverses.
ITU-T A.5 reference justification
In SIP, the route header field, filled by the sender, supports this functionality by listing a set of proxies the message will visit. In the IMS context, there are certain network entities i. To allow the sender to discover such entities and populate the route header field, there are mainly two extension header fields: This way, the registrar is able to discover and record the sequence of proxies that must uetf transited to get back to the user agent.
In the IMS every rcc agent is served by its P-CSCF, which is discovered by using the Dynamic Host Configuration Protocol or an equivalent mechanism when the user enters the IMS network, and all requests and responses from or to the user agent must traverse this proxy.
The extension for service route discovery during registration  consists of a Service-Route header field that is used by the registrar in a 2XX response to a REGISTER request to inform the registering user of the entity that must forward every request originated by him or her. In the IMS it is possible for a user to have multiple terminals e. Therefore, a mechanism is rgc in order to route requests to the desired device or application. They are commonly obtained during the registration process: The efficient use of network resources, which may include a radio interface or other low-bandwidth access, is essential in the IMS in order to provide the user with an acceptable experience in rtc of latency.
To achieve this goal, SIP messages can be compressed using the mechanism known as SigComp  signaling compression. Compression algorithms perform this operation by substituting repeated words in the message 365 its position in a dictionary where all these words only appear once. In a first approach, this dictionary may be built for each message by the compressor and sent to the decompressor along with the message itself. However, eitf many words are repeated in different messages, the extended jetf for SigComp  define a way to use a shared dictionary among subsequent messages.
There is a mechanism  to indicate that a SIP message is desired to be compressed.
When used in request-URIs, it indicates that the request is to be compressed, while in Via header fields it signals that the subsequent response is to be compressed. This way the recipient of the message can decide whether or not to follow the reference to fetch the resource, depending on the bandwidth available. Network address translation NAT makes it impossible for a terminal to be reached from outside its private networksince it uses a private address that is mapped to a public eitf when packets originated by the terminal cross the NAT.
Therefore, NAT traversal mechanisms are needed for both the signaling plane and the media plane. Apart from all the explained extensions to SIP that make it possible for the IMS to work successfully, it is also necessary that the IMS framework interworks and exchanges services with existing network infrastructures, mainly the Public switched telephone network PSTN.
There are several standards that address this requirements, such as efc following two for services interworking between the PSTN and the Internet i. Moreover, the SIP INFO method extension is designed to carry user information between terminals without affecting the signaling dialog and can be used to transport the dual-tone multi-frequency signaling to provide telephone keypad function for users.
From Wikipedia, the free encyclopedia. Retrieved November 29, Merging the Internet and the Cellular Worlds 3 ed.