In this post I will start to list some types of SIP interactions that are possible in an IMS architecture between service-related entities, i.e. devices and application servers in the network.
First, you need to know that there exist two types of identities in IMS:
- Public User Identities (IMPUs) are SIP URIs (sip:user@domain) or TEL URIs (tel:+3312345) that identify users.
- Public Service Identities (PSIs) are SIP URIs (sip:service@domain) or TEL URIs (tel:+3354321) that identity services, service features or user groups.
Now, based on standard 3GPP and SIP routing procedures, as well as the IMS service architecture based on service profiles stored in the HSS (see previous post), the following use cases are possible.
#1 John accesses a network-based IMS service he is subscribed to
The service may be explicitly identified via a PSI, or may implicitly be identified by the content of the SIP message (e.g. the message is a PUBLISH, it has a header called "event:" with the value "presence"). In this latter case, the SIP request is likely to be addressed to John himself (self-addressing).
John is a subscriber of the operator offering the service. He may be roaming or not.
Routing to the application server hosting the service is based on John's service profile in the HSS, and the fact that John originated the SIP service request.
This service routing mechanism ensures that John is authorized to access the service. If not, the service profile would not route the request to the application server. Other SIP routing mechanisms would then be used, which would either route the request to an Application Server handling non-authorized requests, or would reject the request if the target address cannot be resolved to an IP address.
It also ensures that the routing to the Application Server can be specific to John and different than the one for another user.
Service examples:
- John publishes new presence information (SIP method is PUBLISH)
- John discovers the services he is subscribed to (SIP method is SUBSCRIBE)
- John records a greetings message for his multimedia mailbox (SIP method is INVITE)
- John starts a Video on Demand session (SIP method is INVITE)
In all cases, John must be authorized to access the service. In the first three cases, the service is personal to John (John's presence, John's service registry, John's mailbox). In the last case, the service can be shared with others (VoD).
#2 John accesses a public network-based IMS service
The service must be explicitly identified via a PSI.
John might be a subscriber of the operator offering the service. He may alternatively be the subscriber of another operator, or simply a user accessing the service from the Internet.
The routing of the SIP request is based on the resolution of the PSI to an application server hosting the service. Either the PSI has a DNS entry, or the PSI is associated to a service profile in the HSS which determines that some SIP messages addressed to it have to be routed to this AS.
Service examples:
- Public news service (SIP method might be SUBSCRIBE or INVITE)
- Pay per view Video on Demand (SIP method is INVITE)
- Discussion Forum (SIP method is INVITE for messaging session)
#3 John accesses a network-based IMS service his girlfriend Mary is subscribed to
John might be a subscriber of the operator offering the service. He may alternatively be the subscriber of another operator, or simply a user accessing the service from the Internet.
The service is implicitly identified by the content of the SIP message, which is addressed to Mary. For instance, the SIP message might be a SUBSCRIBE addressed to Mary, which has a header called "event:" whose value is "presence".
The routing of the SIP message is based on the service profile associated to Mary in Mary's operator network and the fact that the request is terminating to Mary.
Service examples:
- Access to Mary's presence (SIP method is SUBSCRIBE)
- Access to Mary's user groups definitions (SIP method is SUBSCRIBE)
- Update of John's information in Mary's phone book (SIP method is PUBLISH)
These service requests are typically subject to authorization from Mary.
#4 John accesses a client-based IMS service from Mary
As the service is client-based, it might either be offered by the operator(s) of John and Mary, or by other service providers.
The service is implicitly identified by the content of the SIP message, which is addressed to Mary. For instance, the SIP message is an INVITE, and its session descriptor and/or a specific feature tag in the SIP message identifies a game.
Both John and Mary might be roaming.
The routing of the SIP message to Mary's devices is based on standard SIP routing procedures, possibly with forking and more sophisticated target determination mechanisms, in case Mary is reachable on multiple devices.
Service examples:
- Voice call (SIP method is INVITE)
- Multimedia session (SIP method is INVITE)
- Gaming session (SIP method is INVITE)
- Paul's checks if Mary is currently in a session (SIP method is SUBSCRIBE)
#5 John accesses a service located in one of his devices/servers connected to IMS
The service is located in a device or a server that belongs to John and is connected to the IMS network like an IMS user. It could also be accessible through the Internet. It might be offered by John's operator or by another service provider (possibly John himself).
The service is addressed through an identity perceived as a user identity from the IMS network. In a typical case the device or server is registered through John's identity. John therefore issues a SIP request addressed to himself, which will be routed to the device/server through normal SIP routing procedures. In order to route the request to a specific client (and avoid forking procedures), the request is likely to be addressed to the Globally Routable User Agent URI (GRUU) associated to the device or server. The GRUU permits to explicitly identify a device associated to the user.
Service examples:
- John retrieves a secured HTTP link to access private information on his PC (SIP method is SUBSCRIBE, uses the fact that IMS is a secured network and that John's identities are asserted both on the client and the server)
- John programs Windows Media Server to record a specific program (SIP method is PUBLISH)
#6 John accesses a service located in the Internet
The service might be explicitly identified by the SIP URI (kind of PSI) or might be implicitly identified in the SIP message, which is addressed to an Internet user.
The routing of the SIP message to the Internet is based on standard SIP routing procedures applied by the IMS core network.
Service examples:
- Access to presence located in the Internet (SIP method is SUBSCRIBE)
- VoIP session with Internet client
- Chat/IM with internet client
More use cases in the next post...
Christophe
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