Friday, June 22, 2007

IMS Public Service Identities (PSIs)

After IMS Public User Identities (IMPUs), here is a first post on the second type of identity specified in 3GPP IMS specifications. I will start with the basics.

The concept of IMS Public Service Identity (PSI) was introduced in 3GPP R6. Like an IMPU, a PSI might be defined as a SIP URI (e.g. sip:service@operator) or a TEL URI (e.g. tel:1-234-567890).

To read about PSIs as defined in 3GPP, check chapters 4.3.6 and 5.4.12 of TS 23.228.

Definition (Sort of)

According to TS 23.228, PSIs identify "services which are hosted by application servers". The specification then states that PSIs are used to identify user groups (i.e. sets of users). Is a "user group" a "service"? I do not think so, and this makes the definition of a PSI in 3GPP specifications a little bit ambiguous or short to say the least.

For me, a PSI identifies an IMS service related resource. This might be the service itself (e.g. PSI to change session from 2-party to multiparty, PSI used to retrieve service configuration information), a service feature (e.g. PSI used for starting an ad-hoc push to talk session), a service instance (e.g. PSI identifying a pre-defined conference, PSI identifying a specific content, PSI identifying a specific chat room), a user group (e.g. request to PSI should be exploded to every user in the group identified by the PSI). A PSI could also identify a specific service parameter, a bundling of individual services...

My terminology might not be very accurate, and my own definition is certainly not complete, but basically a PSI identifies anything you might want to identify as the initiator or recipient of a SIP request, which is not an IMS user (for which an IMPU will be used instead). The definition for PSI is therefore directly linked to how SIP and IMS will be used for the delivery of future services. The more IMS and SIP are used to deliver innovative services in an innovative manner, the more the definition for a PSI might be extended.

Prior to its introduction in 3GPP specifications, the concept was not formalized but already existed in practice. For instance, 3GPP already mentioned in R5 the possibility to name a conference through a specific SIP or TEL URI. Similarly, pre-OMA specifications of Push To Talk Over Cellular also made use of a specific SIP URI to start an ad-hoc push to talk session. Concerning the IETF, though it has not been conceptualized as in 3GPP, the possibility to use a SIP URI for something else than a user is common practice (as I once read in an IETF SIP-related RFC: "In the Internet nobody knows you're a dog").

"Private" or "Restricted Access" PSIs

The terms "Private PSI" and "Restricted Access PSI" do not exist in 3GPP specifications, so do not look for them elsewhere than on this blog! Note that the association of the term "private" to an acronym whose first letter means "public" might be a hint for one of two things 1) the author of this blog writes nonsense, 2) 3GPP might not have selected the best name for the concept. You chose.

Private and Restricted Access PSIs are only usable by subscribers of the operator that owns the PSIs. As you will see below (*spoiler*) this is because their routing is based on service profiles associated to the users initiating SIP requests to them.

A Restricted Access PSI can be used by users who are explicitly authorized to access the resource identified by the PSI, i.e. their subscription permits access to the resource. For instance, a specific video streaming content is accessible to a set of users who have paid for it.

A Private PSI has a specific meaning to a limited set of users, possibly a single one. For instance, a user group consisting of John's family members and identified by a PSI like sip:MyFamily@operator, may be accessible by John alone, or John and the members of his family. Other IMS users (e.g. Mary) might have a user group with the same name, but for them this name will point at another group definition (e.g. another family).

Routing of SIP requests addressed to a Private or Restricted Access PSI is determined by the service profiles associated to the users who are authorized to use this PSI. More precisely, their service profile stored in the HSS includes one or several initial filter criteria(s) which determine(s) how a SIP request originating from this user and addressed to this PSI must be routed.

"Really Public" PSIs

"Really Public" PSIs are usable by anybody, including subscribers of other operators than the one that owns the PSI.

As they must be globally routable across various IMS domains (e.g. the request addressed to the PSI may originate from another IMS nettork or from the Internet), 3GPP had to define specific means to route SIP requests addressed to these PSIs. This is actually the reason why the concept was introduced in 3GPP specifications, and the reason for the name of the "public" in PSI.

3GPP was very generous with Really Public PSIs, as it standardized no less than 3 alternative ways to route them (4 if you count 2 sub-variants for subdomain based routing). Considering the complexity and cost induced by the standardization of options and alternative solutions (that need to be implemented and operated afterwards), I never understood why 3GPP (more especially operators) permitted this to happen. But who am I to complain?

Two of these alternatives lead to a very similar result as they route all SIP requests addressed to one PSI to a single application server (i.e. the PSI is the only criterion used to route the SIP request).

The first alternative is subdomain based routing. The PSI will look like sip:anything@service.operator. The "service.operator" subdomain will be resolved through DNS to the application server serving all the PSIs constructed with this subdomain. Sub-variants include the case where the subdomain is resolved to the AS in the network originating the request, and the one where this is the network serving the PSI that resolves the subdomain into the AS address.

The second alternative relies on an address stored in the HSS. When the request addressed to the PSI reaches a signalling entry point to the network serving the PSI (which is an I-CSCF), this I-CSCF matches the PSI with the address of an AS retrieved from the HSS.

The third mechanism is for me by far the most powerful from a service perspective. It consists in treating the PSI as an IMPU (IMS Public User Identity). This is, a service profile is associated to the "PSI user" (this term is used in the specifications) as it would be to an IMPU.

When a request addressed to the PSI is received by the I-CSCF, this one routes the request as if it was addressed to an IMPU (i.e. the HSS provides the I-CSCF with the address of an S-CSCF serving the PSI user). Routing of the PSI by the S-CSCF is then based on a set of initial filter criteria forming the service profile of the PSI.

This approach is interesting as it permits to make PSIs benefit from the service architecture defined for IMS users. More especially, it permits:
- To discriminate service routing of requests addressed to the PSI according to details of the SIP request. For instance, an INVITE for a voice session and an INVITE for a video session addressed to the same PSI may be routed to different specialized media servers. As another example, a subscription to presence (SUBSCRIBE for presence event package) and an instant message (MESSAGE) addressed to the same PSI could be routed either to a presence server or a messaging server.
- To chain several ASs on the signalling path of the SIP request, instead of routing it to a unique AS. For instance, a SIP MESSAGE addressed to the sip:The_IMS_Lantern_ChatRoom@operator PSI, a chat forum for this blog, could be routed through an archiving server, an anti-virus server, and an anti-spam server, prior to reaching its final destination: the chat room server that will dispatch it to all the users currently in the chat room.

Distinct vs. Wildcarded PSIs

Distinct PSIs are PSIs that are fully defined by the operator and provisionned as such in the HSS (e.g. sip:This_Service@operator, provisionned in the HSS with a matching application server address or a service profile).

Wildcarded PSIs permit an operator to provision PSI routing information in the HSS according to a naming pattern. All the PSIs sharing the same naming pattern will be routed the same way. For instance, sip:*_ChatRoom@operator is a wildcarded PSI that corresponds to the IMS Lantern chat room PSI introduced above.

Wildcarded PSIs permit to factorize service routing information between multiple PSIs. They also permit PSIs to be (partly) created by end users, and to be managed fully by an application server without involvement of the operator's provisioning system. For instance, I could create myself the name of the IMS Lantern chat room in the chat room server. No need to change any routing information in the HSS. Every request to the IMS Lantern chat room would be routed to the chat room server according to the wilcarded PSI it is based on.

Activation / De-activation of PSIs

An application server may activate or de-activate a PSI in the HSS through the Sh reference point (interface between an AS and the HSS based on Diameter).

This permits to have a greater application control on PSI routing information. You could imagine the case where a service is available only during certain hours of the day, or days of the week. A PSI could also be usable during a limited period (e.g. a sports event). There are certainly many other usages for the dynamic activation/de-activation of PSIs.

SIP Requests May Originate From a PSI

The IMS service architecture permits application servers to generate SIP requests towards users or other application servers.

It is possible for an AS to use a PSI as the originating address of a SIP request. This permits a service to identify itself as the originator of a service interaction towards a user or another application.

Future post(s) will show potential service patterns making use of PSIs (i.e. ideas on how to use the PSI ingredient to cook some IMS services).



sofiene said...

Hey Christophe,
I remember one of your posts where you criticise the 3GPP for trying to standardize what they call "IMS Communication Service Identities" (ICSI). The PSI play in some way the same role as (ICSI) as they are defined to "identify services, which are hosted by Application Servers". We find here the same idea of locating a service in an Application Server with little care of the interactions with other services.
However, I think that PSI can be useful in two cases (sure there are many others that you can explain to us):
1- It allows to non IMS users to access IMS services without need to register in the IMS network.
2- It allows Application servers to generate requests which can be useful for some services like Automatic Call Back.


Christophe Gourraud said...

Hi Sofiene,

Though the terminology is very ambiguous in specifications, communication services identified through an ICSI and services (or service resources)identified through a PSI are supposed to be different.

PSIs are used when the SIP request is addressed directly to the service or resource (implemented in an IMS application server). For instance, you set up a SIP session for a specific conference, or a specific content, and you therefore address the INVITE to a URI that identifies the conference or the content.

ICSIs are supposed to be used to identify a communication service used while a SIP session is typically set up between two endpoints. For instance, A sends an INVITE addressed to B, and as A intends to set up a voice session, the client inserts a voice telephony ICSI in the INVITE. Usually, the examples given for an ICSI (voice, messaging, push to talk) relate to the media used in the end-to-end session.

In the case of a pre-defined voice conference call, user A would issue an INVITE addressed to the conference TEL or SIP URI (a PSI) with an ICSI identifying that this is for a voice-only call. For setting up a session with a video content, the client would issue the INVITE to a PSI identifying the content, with an ICSI identifying the "video call" intention.

What I do not like with ICSIs, is that they seem to legitimate the fact that IMS sessions would be limited to specific media components, thus replicating pre-IMS service silos. Moreover, it was proposed to identify ICSIs through an IETF construct that has another semantic, thus risking interoperability issues between IMS and non-IMS networks. Finally, some constraints possibly put on IMS clients and on end-to-end applications could have a devastating impact on service innovation.

There is nothing like that with PSIs because, as I wrote, the concept exists in IETF SIP, even if it does not have a name. Basically, a SIP URI may virtually identify anything, even if the original intention was to identify human beings for communication purposes.

This said, and risking to kill the attempt at clarification I just made, you could use PSIs instead of ICSIs if you wanted.

However, this would be quite special...

Let's imagine that for setting up a voice call with user B, the client of user A would issue an INVITE addressed to a PSI like sip:voice_call@operator_for_A. It would then include the URI for user B in another header of the INVITE. An application server serving user A could replace the PSI to sip:voice_call@operator_for_B, in order to permit the routing of the INVITE to the right destination network. Then, an AS serving the PSI could replace it with the URI for user B (taken from the other header in the INVITE), so that the request reaches a device associated to user B.


Anonymous said...
This comment has been removed by a blog administrator.
Anonymous said...

do you see PSI could be usable for supporting N11 services by operator? how?

AC said...


According to the above article, if I develop an application (in SIP AS)that can activate and deactivate a PSI, the information I should provide to HSS includes: AS address(mandatory) and service profile for the PSI(optional). In addition, the application can be accessible only to the service administrators.

Is the reasoning correct?


Jack Chrysler said...
This comment has been removed by the author.
rajdeepa said...

Hi Christophe,
In relation to PSIs how is the S-CSCF failures taken care of prior to IMS restoration procedures.
Say an S-CSCF has been assigned for a PSI and that S-CSCF goes down. How does another S-CSCF gets allocated for the PSI.
I believe in the specifications prior to IMS restoration procedures there is no solution provided for such a situation.
I way out is to manually de-register the PSI from the HSS. But this doesn't sound to be a practical solution. Is there any way around this problem.

Rajdeep Ahluwalia

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