IMS Service Routing: ISC for S-CSCF

This is the third (but not last) post in a series that tries to describe the service-related and IMS-specific mechanisms used to route SIP requests in an IMS network.

After a first post that set the scene, and positioned IMS service routing into a bigger end-to-end context (big picture), after a second post that described the concept of IMS service profile, this post will describe how the IMS core network entity called S-CSCF makes use of the service profile to route SIP requests to application servers over the ISC reference point in the phases 2 and 5 of the big picture.

However, it takes two to tango. While this post concentrates on ISC from an S-CSCF perspective, the next one will address it from the point of view of an IMS application server.

Where to Look in 3GPP Specifications

The ISC procedures at the S-CSCF are not described in a dedicated document or in a dedicated chapter of a 3GPP specification. They are embedded in the detailed description of the S-CSCF procedures in TS 24.229.

Though it definitely makes sense to read other parts of the specification in order to fully understand the procedures, the following sections are the most important:

- Section 5.4.3.2 describes the procedures at the S-CSCF when it receives a request initiated by the user it serves. This corresponds to the phase 2 in the big picture diagram.

- Section 5.4.3.3 describes the procedures at the S-CSCF when it receives a request that is terminated at the user it serves. This corresponds to phase 5.

- Subsections in 5.4.1 address IMS registration procedures, which imply a specific ISC behavior.

As always, I recommend to read the specifications, while on my side I will try to provide a different description of the procedures, that can help the reader to more easily decrypt the occasionally obscure wording and the untold stories from the specifications.

Overview of the Procedure

When it receives an initial or standalone SIP request that is originated by or terminated to a "user" (IMPU, PSI) it serves, the S-CSCF processes the initial filter criterias in the service profile associated to the IMPU or PSI, according to their priority. If the trigger point in a filter criteria is true, the S-CSCF forwards the SIP request to the corresponding application server.

If the application server sends a related SIP request back to the S-CSCF, it proceeds with the next iFC, and so on until a terminating condition is met.

This may lead to the so-called chaining of several application servers in the signalling path that originated from a SIP endpoint (client or application server) and will terminate at another one (client or application server). This chaining may take place on both sides of the end-to-end signalling path (i.e. phases 2 and 5).

When a terminating condition is met and there is still a SIP request to be processed, the S-CSCF then proceeds with normal SIP routing procedures in the IMS (phase 3 or 6).

iFCs Are Only Applied on Standalone or Initial Requests

A SIP standalone request is an isolated transaction requiring a single final reponse. An example of standalone request is MESSAGE.

A SIP initial request starts a dialogue, which will include a set of related transactions. a SIP session started with an INVITE is an example of SIP dialogue, but there are others, like dialogues initiated by SUBSCRIBE or REFER.

The S-CSCF applies service profile based routing only on standalone or initial requests. If an AS is contacted and decides to act as a SIP proxy or Back-to-Back User Agent (B2BUA), for an initial request starting a dialogue, it determines if it wants to remain in the signalling path for the subsequent requests of the same dialogue. This decision is recorded in a specific header called Record-Route.

The S-CSCF then processes the subsequent requests in the same dialogue according to normal SIP routing procedures. These subsequent requests may either be routed to the AS or not, depending on its initial decision.

The Original Dialogue Identifier

The original dialogue identifier is described in section 5.4.3.4 of TS 24.229.

You can read the technical details by yourself, but this token inserted by the S-CSCF in the SIP request before it is forwarded to an AS is a kind of secret word, that the S-CSCF will recover from the SIP request (or a derived one) when it comes back.

This secret word, both created and consumed by the S-CSCF serves two purposes.

This is first a convenient way for the S-CSCF to discriminate between the SIP requests it receives from the core network and those that are coming back from the application layer. When the SIP request includes an original dialogue identifier, it can be associated to an ongoing ISC procedure (i.e. ongoing processing of iFCs).

However, the original dialogue identifier was introduced for another reason.

When an application server receives a request originated from A and addressed to B, as it is shown in the big picture, it may decide to insert itself in the signalling path between the two endpoints. One way to do it is to just proxy the initial request back to the S-CSCF. However, a proxy behavior is very constraining, and does not permit the AS to have much control on the future SIP dialogue. For instance, the AS cannot change the SIP request as it wants, or temper with its body (e.g. change a codec in the SDP). Moreover, in the case of a SIP session, if the AS acts as a SIP proxy, it cannot decide during the session to e.g. play an announcement or transfer the session to another user.

In order to do this, the AS has to act as a Routeing Back-to-Back User Agent (B2BUA), which makes it act as an endpoint towards A and as another endpoint towards B. In the process, it may decide to make itself visible (i.e. place a PSI in the SIP requests towards A and B) or transparent (i.e. A and B cannot see there is a B2BUA, i.e. a service, inserted between them).

The problem is that when it acts as a routeing B2BUA, the AS creates a brand new SIP dialogue on the B-side and the S-CSCF has no standard SIP way to relate this new dialogue to the one on the A-side. The original dialogue identifier serves this purpose.

The procedure at the AS when it acts as a routeing B2BUA mandates it to copy the header in which the original dialogue identifier was placed (the Route header) in the request starting a new dialogue downstream (the second half of the B2BUA).

When it matches the original dialogue identifier from an incoming request with one it inserted in an outgoing request to the AS, the S-CSCF knows that the two requests relate to the same logical dialogue. This is important for charging, but also for being able to proceed with the evaluation of iFCs and associated service routing until the end.

iFC Processing Termination Condition

When does the S-CSCF stop processing iFCs?

There can be three conditions for the S-CSCF to stop its processing of iFCs:

1) An application server has decided to serve as an endpoint for the SIP request (not a routeing B2BUA, a real endpoint). The AS provides the responses to the request and does not proxy it further. Routing of the SIP request in the IMS stops there (phase 2 or 5).

2) This is a terminating case (phase 5) and an application server has modified the request-uri (i.e. intended destination) of the SIP request. The S-CSCF then processes the request according to this new request-uri, without any attempt to apply other iFCs that related to the previous request-uri.

3) All the iFCs in the service profile have been processed, and there is still a SIP request to be routed further (phase 3 or 6).

Requests Initiated by an Application Server

An AS can decide at any time to initiate a new SIP request towards the IMS core network. This SIP request may be generated totally out of the blue, or be the side effect of an interaction with a user or a 3rd party through e.g. a web page or web services. It may also be the side effect of an incoming SIP request. For instance, when receiving an INVITE from A to B, the AS may decide to send an instant message to A, B, or C.

In some cases the request is initiated on behalf of a user. In that case, the AS inserts the IMPU of the user (e.g. A) as the initiator of the request. As it acts on behalf of a user, its request must be processed as if it was really originated by the user. It must therefore be routed to an S-CSCF serving the user so that it applies originating procedures (phase 2), which include the processing of the service profile associated to the IMPU.

Similarly, if the AS uses a PSI as the originator identity for the SIP request, it may need to route it to an S-CSCF, so that originating procedures apply as well, this time for the PSI.

An S-CSCF uses different ports to process incoming and terminating SIP requests. However, in the standards, an AS has no means to know which port an S-CSCF uses for originating requests, it only knows the port for terminating requests. This is the reason why the specifications mandate that when an AS issues a SIP request on behalf of an IMPU (PSI) towards an S-CSCF serving this IMPU (PSI), it includes a specific parameter called "orig", which permits the S-CSCF to detect that the request that is coming on the terminating port should actually be processed as if it was coming on the originating one (see section 5.4.3.1 in TS 24.229).

Support of IMS Registration Notification

For an application server, knowing when an IMPU registers and de-registers from the IMS core network might be an essential information. For instance, a message store might want to be notified when a user registers, and is therefore able to receive a message that is anxiously waiting for her.

However, SIP REGISTER is the only SIP request that cannot be forwarded by the S-CSCF for the simple reason that the S-CSCF, acting as a SIP registrar, is itself the endpoint for it.

Consequently, in order to notify application servers about registration events as requested in the service profile (i.e. registration, re-registration, de-registration) , the S-CSCF issues a 3rd party REGISTER towards them. This is, the S-CSCF registers the IMPU of the user in the application server on behalf of the user.

The information in this 3rd party register is more limited than the in the original REGISTER issued by the IMS client towards the S-CSCF. For instance, 3rd party registration does not permit the application server to discover the capabilities of the IMS client (which can be very important for some services), and it only provides information about a single IMPU being registered, while the user might register several IMPUs at once.

For accessing more information about the registration, the application server can subscribe to the registration event package (using SIP SUBSCRIBE). It will receive the registration information in the body of the NOTIFY(s) issued by the S-CSCF. The registration event package is also an alternative to 3rd party registration for the AS to be notified of de-registration and re-registration events.

Note that, if 3rd party registration (a standard IETF mechanism) is specific to the ISC reference point in the IMS architecture, the usage of the registration event package is not: the I-CSCF and standard IMS clients are supposed to subscribe to this event package in order to be notified about de-registration when it has been decided by the IMS core network (e.g. the operator has decided to de-register the IMPU).

Details of SIP on ISC

SIP messages over ISC include IMS-specific headers, some of which I already listed in a previous post.

However, it is important to note that none of these headers is specific to ISC. These are IMS SIP headers that are also used on other reference points.

Therefore, there is no such a thing as a specific SIP that would be used only on the ISC reference point. What makes ISC special from a core network perspective is essentially the usage of a service profile by the S-CSCF for deciding on how to route an incoming SIP request to possibly multiple IMS application servers.

Christophe

More on the SCIM!

Today is a "lazy post" day. I think I might do it from time to time.

In April, I made a series of three posts on the infamous IMS Service Capability Interaction Manager (SCIM).

In the first one, I described the history of the concept in 3GPP. In the second one, I reviewed the features usually associated to the SCIM, mainly to say that I do not like them. Finally, I presented some of my ideas on what a SCIM could be, would it try to add value on top of the intrinsic capabilities of IMS.

As the SCIM is a topic that seems to interest a lot of people, I decided to add a little bit more meat on this subject.

You can find below three sides I made two years ago, when I first tried to formulate my vision of the SCIM. I think I still stand behind most of what is written there, and normally (hopefully) this should be consistant with my previous post on the topic.





Christophe

IMS Service Routing: Service Profile

In the previous post, I showed that, in a typical end-to-end SIP interaction between two IMS clients, there is a succession of routing phases. Two of them (phases 2 and 5) make use of an IMS-specific mechanism, which does not exist as a standard in any other SIP network: service profile -based SIP routing.

In this post I will describe what an IMS service profile is. In the next one, I will detail how a service profile is used in the interactions between the IMS core network and the IMS application layer.

Service Profiles in Short

IMS service profiles can be seen as SIP routing information stored in a network database called the Home Subscriber Server (HSS).

This SIP routing information can be associated to the two types of public identities that exist in the IMS: IMPUs (for users) and PSIs (for services and service-related resources). An IMS service profile therefore influences the routing of SIP requests that are either originated or addressed to a particular IMPU or PSI.

The IMS core network entity that utilizes service profiles for SIP routing is the S-CSCF, which retrieves them from the HSS over the Cx (Diameter-based) reference point. A service profile is transferred over Cx in a standardized XML format.

A service profile is composed of a list of initial filter criterias (iFC), which are processed one after the other by the S-CSCF. An iFC essentially consists of a condition to be met by the SIP request and the address of an application server the SIP request should be routed in that case.

The processing of a service profile by the S-CSCF may lead to the routing of a SIP request to several application servers over the IMS Service Control (ISC) reference point. If no application server decides to serve as an endpoint to the SIP request, the S-CSCF then proceeds with normal SIP routing procedures (i.e. phase 3 or phase 6).

Service Profile in 3GPP Specifications

While this post provides a description of IMS service profiles, nothing can replace a direct access to the sources.

Section 5.2 in TS 23.218 provides a quick overview of the service profile, as well as the associated procedures for the S-CSCF.

However, the best information can be found in the annexes of TS 29.228, which specifies the Cx reference point:

Annex B provides a UML model for the HSS user profile, which essentially consists of the service profile. The figures in this post are taken from this annex.

Annex C describes the conjunctive and disjunctive normal forms that can be used to define initial filter criterias. This is also the place where you can find an XML sample of a service profile.

Annex D provides the specification of the XML schema used to transfer the service profile over Cx.

Service Profiles in More Details

The figure at the top of this post (click to enlarge) shows that a service profile is associated to possibly several IMPUs or PSIs. In the case of IMPUs, they are part of the same IMS subscription (i.e. they correspond to the same subscriber, typically a user in a mobile context).

Core Network Service Authorization identifies a policy (through an integer) which is supposed to be used by the S-CSCF to decide which types of media are authorized inside a SIP session associated to the IMPU or PSI. In the latest version of the specification, Core Network Service Authorization also includes a list of communication service identifiers that are authorized for the IMPU/PSI. However, the recent decision to let the IETF specify the support of communication services in IMS may possibly lead to future changes to this part. As of now, most IMS implementations I know do not support Core Network Service Authorization yet.

The most important part of the service profile is the set of initial filter criterias that constitute it.

In order to optimize the provisioning and storage of service profiles, the concept of Shared iFC sets was introduced. These are sets of initial filter criterias that are shared among multiple subscribers. Instead of being integrally defined in the service profile, they are referenced through an integer. According to the specification, this integer points at iFC sets that are locally stored in the S-CSCF.

Initial Filter Criterias (iFCs)

An iFC is composed of the following elements.

A Priority, which is defined as an integer. The term might be misleading, as each iFC in a service profile must have a different priority, permitting the S-CSCF to process the iFCs in a deterministic order.

A Trigger Point, which consists of a set of criterias to be met by the SIP request to be routed to an AS. These criterias are defined as a set of Service Point Triggers (SPT) that are linked through logical boolean operators (AND, OR, NOT).

The SIP URI of an Application Server, to which the request should be routed if the Trigger Point is true.

A Default Handling element, which indicates what the S-CSCF should do in case the AS does not respond (continue with processing or reject the SIP request).

An optional Service Information element, which is a string provisionned in the service profile, that the S-CSCF should place in the body of the SIP request before it is sent to the AS (the AS is supposed to know what to do with it). Originally, it was assumed that a Service Information element could be added in every SIP request that the S-CSCF would forward (proxy) to an AS. This could have been interesting, for instance, to provide a kind of identification of the iFC that led to the forwarding (based on this ID, the AS could determine what services need to be executed). However, it soon appeared that tempering with the body of a SIP request is incompatible with the behavior of a SIP proxy. The possibility to use Service Information is therefore limited to the only SIP request issued to an AS that does not result from a proxy behavior: REGISTER (see next post). Consequently, the usage of Service Information should remain very limited in the future.

Trigger Point & Service Point Trigger

A Trigger Point consists of a set of Service Point Triggers, that are linked through boolean operators: AND, OR, NOT.

The characteristics of a SIP request that SPTs permit to check are as follows.

The Session Case permits to define if the request was originated by the public identity while it was registered, if it was originated by the public identity while it was not registered, if it terminated to the public identity while it was registered, or if it terminated to the public identity while it was not registered.

Originating cases correspond to the phase 2 described in the previous post, while terminating cases correspond to the phase 5.

The terminating unregistered case corresponds to the situation where a request is addressed to a public identity while no IMS client is currently reachable with this identity. The request may then be routed to an application server that supports service logic that handles this situation (e.g. call forwarding, message store). Alternatively, though addressed to an IMPU, the request may not be targeted at an IMS client, but to an IMS application server hosting logic for this IMPU, and which should always be reachable. This is for instance the case for presence (presence requests are addressed to an IMPU but should reach a presence server) or for the automatic service discovery and configuration example I gave in a past post.

The originating unregistered case corresponds to the situation where an application server issues a request on behalf of an IMPU that is not currently registered with IMS. For instance, the service logic sends an instant message on behalf of a user, even if this user is not currently registered. This case is also required for Voice Call Continuity (VCC).

The SIP Method element permits to check if the SIP request is an INVITE, a SUBSCRIBE, a MESSAGE, a PUBLISH or... any method that may be created in the future (the type is a string, not an enumeration).

The Request-URI element permits to check the URI the SIP request is addressed to. This is typically used for iFCs that relate to private or restricted access to a PSI (the request-URI is the PSI).

The SIP Header element permits to check if a particular header exists in the SIP request, as well as the content of this header. It is possible to use a wildcard in the value of the content. Note that once again, there is no pre-conception about the header, which can be any standard, non-standard or future standard header.

Finally, the Session Description element permits to check the Session Description Prototocol (SDP) body that may be attached to the SIP request it it is an INVITE. The SDP permits to describe the details of the content of a session (e.g. media, application, codec).

With these elements, it is possible to determine the routing to an IMS application server of any existing and future SIP message, based on any combination of criteria met by this SIP request.

In addition, a specific element, Registration Type, permits to inform the S-CSCF that it should notify an AS of one or several IMS registration events associated to an IMPU: initial registration, de-registration and re-registration (renewal of a registration before the registration timer expires).

An Example

John has a service profile associated to the sip:John@operator IMPU whose translation in plain English is:

(Priority 10): every registered originating INVITE for a voice session should be routed to sip:vcc_server@operator

(Priority 20): every registered originating INVITE should be routed to sip:multimedia_session_control_orig@operator

(Priority 25): every terminating INVITE should be routed to sip:multimedia_session_control_term@operator

(Priority 30): every terminating INVITE for voice should be routed to sip:vcc_server@operator

(Priority 32): every originating MESSAGE to request-URI sip:My_Family@operator should be routed to sip:message_exploder@operator

(Priority 40): every originating INVITE to request-URI sip:My_Family@operator should be routed to sip:multimedia_conference_server@operator

(Priority 50): every terminating SUBSCRIBE with a header called "event" whose value is "presence" should be routed to sip:presence_server@operator

(Priority 60): every originating PUBLISH or SUBSCRIBE to Request-URI sip:John@operator with a header called "event" whose value is "presence" should be routed to sip:presence_server@operator

The overall sequencing permits to prioritize the processing of some requests (e.g. session set up) over others (e.g. presence requests).

There are cases where the iFCs are mutually exclusive (e.g. 10 and 25, 30 and 32). It does not really matter if one is processed before the other.

There are cases where several iFCs may be true for the same SIP request. It is very important to define a coherent sequencing between them. For instance, if John issues an INVITE for a voice session addressed to a user group representing his family, it is important that the following order is respected: voice call continuity server (in case there is the need to switch the call between IMS and circuit-switched), multimedia call features server for originating calls (e.g. call blocking), and then a conferencing server that will start a multiparty call between John and all the members of the family group. In a typical case, the INVITE will chain the VCC server, the multimedia call features server and the conferencing server together. It will terminate at the conferencing server which serves as a bridge between all the participants in the conference, including John.

iFCs 10 and 20 hint at the possibility to indicate the direction of the request (originating, terminating) in the SIP URI identifying the IMS application server (an alternative would be to add a parameter in the URI). The two SIP URIs may point at the same AS, but permit the AS to determine the direction of the request and may help it determine which services should be invoked.

iFCs 32 and 40 show that a service profile may combine a Request-URI (the usual information used to route a SIP request) with othercharacteristics of the SIP message to decide on its routing.

iFCs 50 and 60 could be combined into a single iFC.

Christophe