This post is the first in a series that will describe how the routing of SIP service-related requests is performed in the IMS.
Before going into the details of service profiles, initial filter criteria and the ISC (IMS Service Control) interface, I will start with an attempt at placing IMS service routing in the larger context of end-to-end SIP routing within IMS.
I think this is something missing in most representations of the IMS service architecture, which tend to fragment the overall perception of this architecture, and make it more difficult to comprehend for the novice.
The figure you can see at the top (click to enlarge) shows the end-to-end interaction between two IMS users. However, there exist variants related to which entity initiates the SIP interaction (IMS client or IMS AS), which entity terminates the SIP interaction (IMS client or IMS AS), and whether the end-to-end SIP interaction fully takes place between IMS entities or involves entities in an external network like the circuit-switched network or the Internet. I will address these variants below using specific colors.
Before describing what IMS SIP routing permits to achieve, let's start with one of its apparent limitations compared with how SIP can be used in a non-IMS network.
Restriction to IETF SIP Routing
The routing of a SIP request to its destination is based on the resolution of the request-uri address (IMPU or PSI in IMS) to one or several targets. However, before reaching its destination, the SIP request may traverse several servers, and the originator of the request may itself define in a specific Route header (part of) the path to be followed .
This may permit the client to decide the routing of the SIP request through a set of servers that will deliver added-value services.
This is not possible in the IMS. More precisely, whatever SIP route an IMS client may define in a SIP request it originates will be discarded by the IMS core network (the P-CSCF for that matter) and replaced by another route which ensures the phase 1 of IMS SIP routing described below.
Is this a limitation to the power of IMS, compared to what is possible in, say, the Internet? I don't think so.
I do not think that relying on a SIP client to determine a set of application servers supposed to add value to the end-to-end SIP interaction is a very pragmatic and agile approach to service delivery. The mechanism is limited by the knowledge and sophistication available in the SIP client, which may be put under pressure as soon as more than one application server needs to be inserted in the signalling route (which application servers? In which order?).
Then, you have to compare what is lost (the possibility for an IMS client to define a service route) with what is gained with the IMS architecture: service profile based routing.
The IMS routing of SIP requests based on service profiles (phases 2 and 5 below), is sophisticated, very agile and permits to keep IMS clients very simple, as they do not have to care about service-related routing. It also accurately reflects the business relationship between an IMS user paying an operator, and the operator providing value for money in return.
While some may look at the IMS architecture as a way to deprive users from their freedom to access the services they want to access (note anyway that the limitation is only in the route to access a destination, not the destination itself), others can see the same architecture as a way to simplify the life of users (and their devices) and to concentrate service-related complexity in the hands of those who are asking money for value.
Service Routing for Each Party (See Figure)
An important feature of the IMS service architecture, which is not apparent in figures taken from IMS specifications, is the fact that each party in a two-party or multiparty SIP interaction benefits from its own service routing, and therefore its own services.
As shown in the figure above, an end-to-end SIP interaction between John and Mary will first lead to the invocation of services associated to John (phase 2), and then to services associated to Mary (phase 5).
This feature is very natural for a telecommunications service architecture (it is similar to IN), but it is not to most of the non-IMS SIP implementations, more especially those supporting enterprise VoIP services. Typically, these non-IMS implementations will combine the execution of features that apply to all parties of a call. This difference is usually the most important one to overcome when porting a non-IMS application server on the IMS.
In the IMS architecture, a specific party in an end-to-end SIP interaction is either identified by an IMPU or a PSI. While the figure shows an example of IMPU to IMPU interaction, the following alternatives could also exist:
- IMPU to PSI: an IMS client or an AS-based application acting on behalf of a user interacts with an AS-based application identified by a PSI.
- PSI to IMPU: an AS-based application interacts with an IMS client.
- PSI to PSI: an AS-based application interacts with another AS-based application.
Here is a practical advice for when you define or analyze an IMS message flow: S-CSCFs do not hang just like that in the architecture; they always serve someone or something, and it certainly helps to make it explicit (see for instance the figures in this or the previous post).
Preliminary to the Description of End-To-End IMS Signalling
Here follows the decomposition of an end-to-end SIP interaction into 6 distinct phases.
But first a couple of things...
There is of course a pre-requisite: John is registered with IMS. This implies that his client discovered the P-CSCF it will be associated to (according to 3GPP or TISPAN procedures), and performed the IMS registration procedure leading to John's authentication, the association of an S-CSCF to him, and the setting up of a security association between its client and the P-CSCF.
The figure shows direct interfaces between different operators' networks. However, the GSM Association (GSMA) defined the possibility for 3rd party transit networks to be used in-between, in order to reduce the need for bilateral operator agreements.
Phase 1: routing of the SIP request to the S-CSCF serving the originating party
As described above, the P-CSCF removes any potential route defined by the IMS client and replaces it with a route to the S-CSCF serving the user in its home network. This route was defined during the IMS registration of the user, and may traverse an I-CSCF in case the home network wants to hide its network topology to the potential visited network (note that in practice a border gateway might be used instead).
Phase 2: service profile -based routing for originating user
The S-CSCF serving the originating party applies the service profile for the originating IMPU. This service profile consists of a list of initial filter criteria. It corresponds to the set of services the originating party is associated to. In practice, this means that the service route for the SIP request originated by John is defined by the operator and implemented by the S-CSCF.
In this phase, the S-CSCF may route the SIP request to 0, 1 or several application servers (one after the other in a signalling chain or pipe).
It is possible that one of the application servers terminates the SIP request (i.e. acts as a SIP User Agent). This may happen in three cases:
1) The request was addressed to a PSI hosted by the application server (e.g. user tries to access content)
2) The originating party issued a request addressed to itself, which was actually targeted at one of its services in the network. For instance, SIP requests that update the presence information of a user (PUBLISH for presence event package) are addressed to the user's own IMPU. Another example can be seen in the automatic service discovery and configuration example I described in an earlier post.
3) The application server terminates the request on behalf of the B-party. This may happen, for instance, with services that block traffic to unauthorized destinations. Another example could be the case where a service, potentially after having accessed the presence of the B-party, decides to transform an IMS instant message (SIP MESSAGE) into an email, an SMS, or an Internet IM. In this case, IMS SIP routing is terminated to be superseded by an alternative protocol outside of IMS.
Signalling path termination: the SIP interaction may terminate at phase 2 if one of the application servers decides to act as an endpoint for the SIP request.
Signalling path termination: the SIP interaction may terminate at phase 2 if one of the application servers decides to act as an endpoint for the SIP request.
IMS exit point: the SIP interaction may exit IMS at phase 2 if an application server serves as a gateway towards a different protocol/domain (e.g. email, SMS, MMS, Internet IM protocol). Note that phase 2 takes place before the IMS core network tries to route the SIP request based on the request-uri. This means that the request-uri might not be an IMS routable URI: it could be a URI specific to the protocol and/or domain to which the SIP request is actually targeted.
In case no application server terminates the SIP request, the S-CSCF proceeds with the next phase.
Phase 3: routing of the SIP request to the destination network
The S-CSCF routes the SIP request according to its request-uri.
The S-CSCF routes the SIP request according to its request-uri.
In case the request-uri is a TEL URI, the S-CSCF may route the request to the circuit-switched network (see previous post - the request will be routed to the circuit-switched network when the TEL URI does not resolve into a SIP URI through ENUM).
Through DNS, the S-CSCF may also determine that the domain of the request-uri is resolved into a non-IMS network like the Internet.
IMS exit point: the SIP request is routed outside of IMS by the IMS core network if it is a TEL URI that does not resolve into a SIP URI, or if it is a SIP URI whose domain is outside of the IMS (e.g. in the Internet).
Side comment: to come back to the possibility for an IETF SIP client to define by itself a route towards application servers... If it is not possible for an IMS client to define such a route, it is possible for an IMS application server to do it on behalf of the client. This route could binclude application servers in the Internet and could be provided by the IMS client to the IMS AS through signalling or self-provisioning.
In case the B-party is an IMS user (same or different operator), DNS resolves the request-uri to an I-CSCF of the target operator, that acts as an entry point to its network. The request may traverse an I-CSCF of the originating network, in order to hide the network topology to the destination network (note that in practice a border gateway might be used instead).
Phase 4: routing of the SIP request to the S-CSCF serving the destination user
Either the B-party is registered with IMS or not.
If it is, the I-CSCF retrieves the location from the HSS and routes the request to the S-CSCF serving it.
If it is not, there is a procedure to dynamically allocate an S-CSCF to the B-party and to route the request to this S-CSCF. The rationale is that the B-party's IMS services in the network might need to be reachable even when the B-party is not available (e.g. presence, a voice call continuity server that will route the call to the circuit-switched network, a call forwarding service).
Alternative entry points to phase 4: instead of originating from the IMS and having followed phases 1 to 3, the request received by the I-CSCF may originate from the circuit-switched network (request is received from MGCF, i.e. signalling gateway with CS), or from a non-IMS network like the Internet.
IMS entry point: instead of an S-CSCF in the originating network, the request may come from the circuit-switched network (through an MGCF), or from a non-IMS SIP network (e.g. the Internet).
IMS entry point: instead of an S-CSCF in the originating network, the request may come from the circuit-switched network (through an MGCF), or from a non-IMS SIP network (e.g. the Internet).
Phase 5: service profile -based routing for the terminating user
The S-CSCF serving the terminating party applies the service profile associated to the IMPU (or PSI) corresponding to the request-uri. This service profile consists of a list of initial filter criteria. It corresponds to a set of services associated to the terminating party.
In this phase, the S-CSCF may route the SIP request to 0, 1 or several application servers (one after the other).
It is possible that one of the application servers terminates the SIP request (i.e. acts as a SIP User Agent). This may happen in two cases:
1) The request was addressed to a PSI hosted by the application server (e.g. user tries to access content, user posts message in chat room). This is the case where an operator offers services to the world. Note that there exist alternative mechanisms to route a request to a PSI.
2) The application server terminates the request on behalf of the B-party. This might be the case for presence (the request was addressed to the B-party but was actually aimed at its presence server) and any other service acting on behalf of the terminating party (e.g. voice call continuity, call or message blocking or forwarding services).
Phase 6: delivery to the terminating party
In case there is still a SIP request to be delivered to a B-party, the S-CSCF routes the request to the P-CSCF(s) serving the B-party (there might be several in case several endpoints are registered with the same IMPU), which in turn contacts the device of the B-party.
SIP requests issued by application servers
This post essentially addressed the situation where the SIP request is initiated by an IMS client corresponding to an IMPU. However, to be complete, you also have to consider that IMS application servers are able to generate SIP requests, using either an IMPU or PSI as the originating party, and an IMPU or PSI as the request-uri.
When an AS issues a request with an IMPU as originating address, it is supposed to route it to an S-CSCF serving the IMPU. Starting with 3GPP R7, it is possible to allocate an S-CSCF to the IMPU even if it is not registered with IMS (i.e. the IMS AS issues a request on behalf of an IMPU which is not registered with IMS). IMS routing then proceeds with phase 2.
When an AS issues a request using a PSI as the originating party, it may either reach a S-CSCF serving the PSI, or route the request directly to an I-CSCF for the terminating party. Depending on the case, IMS routing then proceeds with phase 2 (the service profile is associated to the PSI) or directly with phase 4.
A specific case for having an IMS AS issuing a SIP request to the IMS is when it acts as a gateway towards another protocol and/or domain. This is the symetric case to what was described in purple for phases 2 and 5 above.
A specific case for having an IMS AS issuing a SIP request to the IMS is when it acts as a gateway towards another protocol and/or domain. This is the symetric case to what was described in purple for phases 2 and 5 above.
Christophe
There is no direct relationship to this post, but I just added the initial architecture proposed in 3GPP for the SCIM in the first post I wrote about it in May.
There is no direct relationship to this post, but I just added the initial architecture proposed in 3GPP for the SCIM in the first post I wrote about it in May.
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42 comments:
Hi Christophe
Allow me to add my first thoughts when reading your title "The big picture"; for me one of the BIGGEST inventions of the IMS architecture was to add the "two-level" approach to SIP.
With "two-level" I mean P-CSCF and S-CSCF; in former pre-IMS or NGN times, there was only the notion of the "one does it all" SIP server.
By introducing the P-CSCF and S-CSCF - and allowing a roaming network in between them (!) - you mimic the well adopted business model of mobile telephony to (fixnet) SIP communication.
The P-CSCF can/should be owned by the ISP (close to the access), the S-CSCF is owned by the service provider (close to the communcation service). With this two-level approach you basically allow the ISP to be part of the game. This is more important than one might think...
Hi, Christophe,
You mentioned that in Phase 1, the request may traverse an I-CSCF in case the home network wants to hide its network topology. What is the mechanism to do this? Since the registration procedure defined in the 24.229 only requires S-CSCF to insert service-route. Where the I-CSCF route information is send back to the UE?
Thanks,
John
Hi Joerg,
The signalling route to be used for post-registration SIP messages is documented in the Path header in the REGISTER. The P-CSCF does it, the I-CSCF may or may not do it.
Then, the S-CSCF constructs the service-route header based on the Path header. The service-route header is included in the response to the REGISTER.
Therefore: Path in REGISTER to document the route along the way, Service-route in answer to report the route back, with or without I-CSCF.
Christophe
Concerning the two levels between P-CSCF and S-CSCF...
I agree this is a great finding, more especially when you consider that the media in a SIP session can follow a more efficient path directly between the "visited" networks of the parties involved in a session.
However, just note that in the mobile world, the location of the P-CSCF in a visited network is still hypothetical, as currently there are no GPRS roaming agreements permitting it. The P-CSCF is always in the home network.
You can also make this architecture 3-level, including the other fundamental separation between SIP entities performing core functionalities (CSCFs) and SIP entities performing more advanced or agile functionalities (SIP AS).
Christophe
Hi Christophe
In your response to Jorge you mentioned that the P-CSCF inserts a Path header as part of the signaling route. I do not understand why the P-CSCF needs to do this. RFC 3327 says that the UA normally ignores the Path header and I thought that P-CSCF discovery determined what P-CSCF the UA uses. So why is it needed? I have been working with the OpenIMS core and I see the Path header inserted by the P-CSCF but my User Agents do not use it to do a DNS query and the INVITE messages they send to not include the information either.
hello Christophe,
Could you help me with an example or some more clarity on this non-IMS case?.
"Typically, these non-IMS implementations will combine the execution of features that apply to all parties of a call."
-thanks
harish
Hi Harish,
The IMS takes place into a multi-operator context, in which it cannot be assumed that the different parties involved in a SIP interaction, for instance a VoIP call, are subscribed to the same operator. It applies a clear segmentation of the services applying for each party, which is classical in the telecommunications domain. If you consider a call between users A and B, each will have its own S-CSCF, providing access to its own services.
On the other hand, most typical SIP network implementations take place in a closed context (an enterprise, a single Internet service provider), in which it is assumed that in most cases users A and B belong to the same domain. The typical architecture is to have a single SIP server supporting services for both A and B, and the logic on this server is usually not segmented user per user. Instead of reaching separate service logic for A and B, SIP signaling will reach a single service logic applying whatever has to be applied on both A and B.
Of course, some IMS-based interactions can take place between two users belonging in the same domain, while non IMS-based interactions can take place between users in two domains. However, the architectural difference remains:
- Even in the same operator domain, the IMS architecture discriminates between the A and B parties and will execute their services separately.
- In an implementation which is not architected in the half call model, the logic will detect that one party does not belong in the domain and will simply execute services related to the one in the domain.
Christophe
Hi Cristophe, hi all,
first of all thank you for this great blog.
I have a very basic question. I have been through a number of documents describing SIP routing and service invocation and I have not been able to figure out how the S-CSCF distinguishes between session cases and specifically between originating and not-originating (terminating) call cases.
I know that the S-CSCF and the AS assist each other in this manner by adding the "orig" parameter in the route header. But as far as I know, no such parameter is added by the UAC in the originating call case or by the S-CSCF when forwarding to another IMS core (terminating call case).
Thank you in advance
niko
I have the following question related to IMS routing.
Assuming that an IMS mobile (A) is based in the U.S, but has roamed from the U.S to Spain. As part of the registration procedure, the I-CSCF in Spain will query A's HSS to get A's S-CSCF. A's HSS returns the S-CSCF in the U.S in the Diameter UAA message.
Now, A makes a call to a PSTN user (B) in Spain. The SIP INVITE will go from A to P-CSCF in Spain to S-CSCF in the U.S. and breaks out to MGCF in the U.S, goes back to the PSTN in Spain and terminates to B. This kind of routing is not very efficient.
Is there a way to assign the S-CSCF of the visited network in Spain to A?
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thank you so much for posting this, I was seeking this information, because I have had some probhlems with IMS service, thanks again...
That is very good comment you shared.Thank you so much that for you shared those things with us.Im wishing you to carry on with ur achivments.All the best.
I am trying to implement this scenario using Open IMS Core. I am having problem configuring the I-CSCF. I'll be grateful if anyone can help me with the correct configurations.
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This diagram is incorrect. Alice's SIP messaging would not leave her network through her I-CSCF. It would leave through a SBC on the edge, and enter into Bob's network through is I-CSCF.
The I-CSCF is only used when locating an S-CSCF (which is necessary when locating Bob's S-CSCF)
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