Third revision of IMS signaling course. The lecture was part of the communication protocols class 2014 delivered to students from FIIT STU Bratislava, Slovakia and University Zilina, Slovakia.

1. IMS Signaling
SEBASTIAN SCHUMANN
SLOVAK TELEKOM
12 MARCH 2014
BRATISLAVA, SLOVAKIA
Revision 3

2. Outline
 Summary “SIP & IMS Basics”
 IMS and its Services
 Protocols
 Service Triggering
 Q/A

3. Note!
 These slides only summarize the lecture. Take notes.
 Fixed agenda: IMS Signaling
 Besides that:
 Ask questions (how is it done in real-world, how did Slovak
Telekom do it)
 Interrupt (I don’t understand, can you provide samples, can
we skip that)
 Contribute (I’ve heard/read that…, I’m interested in…)
 Discuss…

4. Summary “SIP & IMS Basics”
Reference to lecture.

5. SIP Overview
 Protocol overview
 Basic architecture
 Types of servers
 Separated by basic functionality
 Request methods, responses, call flows
 Session Description Protocol (SDP)

6. IP Multimedia Subsystem (IMS)
 Basic overview
 Architecture
 Horizontally layered approach
 Functional elements
 Flow details
 Registration
 Basic call setup

7. IMS and its Services

8. Technology Trends
Services
Data/IPNetworks
MobileNetworks
PSTN/ISDN
CATV
Access Transport & Switching Networks
Wireless
Access
Wireline
Access
IP Backbone
Existing and newly
emerging services
Service & Network Control
(QoS, Security, IP Mobility)
Too costly, per-service network
architecture
Single/simple/cost-effective network
infrastructure for existing & new services

9. Access & Transport
Plane
Core
Network
Session
Control Plane
Service Architecture
Applications/Services
Plane
HSSCSCF
Access
Networ
k
Other
Networks
Web Portal
Application
Servers
Session
Control
Centralized
Databases
Media
Control &
Gateways
Media
Server
IMS: Simplified Concept

10. Recapitulation
 IMS is an open architecture for mobile and fixed services
 The core and its services are independent from the access
 Layered architecture
 Transport, session control, applications
 Transparency through standard interfaces
 Session Control Layer
 End point registration, authentication
 Session establishment, routing, interconnect
 Application Layer
 Service Logic

11. Recapitulation ctd.
 Service Control Layer
 SIP: P/I/S-CSCF, (BGCF, I-BCF, MRFC, AS)
 Diameter: HSS, (RACS/NASS, PCRF)
 Application Layer
 SIP/Diameter interface towards service control layer
 SIP/XCAP interface (based on HTTP) towards UE
 Call related application logic
 IMS service (e.g. Presence, PoC)
 Service Creation Environment
 Northbound integration through service APIs

12. IMS entities
(Wiley, The IMS Concepts and Services)
 Session management and routing family (CSCFs)
 Databases (HSS, SLF)
 Services (e.g. AS)
 Support functions (PDF, SEG, THIG)
 Charging
 Interworking functions (BGCF, MGCF, IMS-MGW, SGW)

13. Core nodes
 CSCF components separate logical functionality
 P-CSCF eq. SIP Proxy. It acts as access point for UE towards the
IMS core.
 I-CSCF eq. Inbound Proxy. It is placed on the borders of two
IMS domains. Entry point for served home users from visited
networks.
 S-CSCF eq. SIP Registrar/Location Server. It also acts as an
anchor point for IMS service control (ISC) and service
invocation (based on iFCs).
 HSS contains all subscriber and service related data
 No special entity in basic SIP (simply: user database)

14. Session Management & Routing
 Proxy-CSCF – User contact point with the IM CN
 SIP compression, IPSec association, PDF interaction
 Interrogating-CSCF – Subscriber contact point
 Next-hop lookup from HSS, S-CSCF assignment and
routing, THIG functionality
 Serving-CSCF – Service profile internal procedures
 Handling registration, challenging UE, routing decisions
 Responsible for Registration and Session Establishment,
Charging Data Generation, Media content check

15. Databases
 HSS
 Data storage for all subscriber and service-related data
 SLF
 Find HSS address for multiple HSS environment

16.  SIP Application Server
 Stand-alone AS
 Northbound integration using various protocols possible,
e.g., HTTP REST, Parlay X
 Open Service Access (OSA) gateway
 Connect northbound to OSA Parlay based AS
 IM Service Switching Function (SSF)
 Connect northbound the AS layer to legacy services using
IN protocols (e.g. INAP, CAMEL)
Application Server

17. Other real-world components
 Resource and Admission Control Subsystem (RACS)
 Mechanisms for applications to request and reserve the
resources from access network (Session Admission Control,
resource reservation)
 Network Attachment Subsystem (NASS)
 Registration and initialization of CPE for access to IMS services
(IP addresses and configuration, user authentication)
 Policy and Charging Control (PCC) framework
 Policy and Charging Rules Function (PCRF)
 Policy and Charging Enforcement Function (PCEF)

18.  Signaling
 SIP (signaling protocol)
 SDP (embedded in SIP, describes the session, negotiation)
 Media
 RTP (end-to-end media delivery (audio, video))
 MSRP (messaging, file transfer)
 DNS
 Diameter (AAA)
 IPSec (secure communication)
 MEGACO (media gateway control)
Protocols (extract)

19. Protocols

20. User Identities
 User identities
 Private User Identity (user@realm)
 Authentication and Subscription identification
 Not used for routing
 Public User Identity (sip:user@domain.tld or tel:+1234567890)
 Contact to be reached by others
 SIP URI or tel URI
 Implicit set of public user identities for grouping
registration
 Services and other network entities can be addressed
using a SIP URI
 User identities are part of the user profile

21. Relations
between
identities IMS
Subscriber
Private
UID 2
Private
UID 1
Public
UID 1
Public
UID 3
Public
UID 2
Public
UID n
.
.
.
}Implicit Set

22. IMS Registration
 Required before a user can access services or perform
calls
 Precondition: UE has IP address & knows IMS entry point
 All CSCF are used
 P-CSCF (home/visited): Entry point, determines I-CSCF
 I-CSCF (home): Determines S-CSCF
 S-CSCF (home): Authenticates the subscriber, registers IMS
subscriber, interacts with service layer
 User assigned to one S-CSCF after successful registration
 Knows user profile until de-registration

23. Registration
(simplified)
P-CSCF
DNS
UE

24. Domain Name Service
 Link IP addresses with domain names
 Support in locating SIP servers (NAPTR, SRV, A/AAAA)
 NAPTR resolves the preferred protocol and the DNS
string to locate the service
 ngnlab.eu. 7200 IN NAPTR 10 50 "s“ "SIP+D2T“ _sip._udp.ngnlab.eu.
 SRV look-up for a NAPTR given address indicates the
domain and port the service listens on
 _sip._udp.ngnlab.eu. 7200 IN SRV 0 0 5060 icscf.ngnlab.eu.
 A/AAAA to find the IP address of the domain name
 icscf.ngnlab.eu. 7200 IN A 147.175.103.213

25. Registration
(simplified)
P-CSCF I-CSCF S-CSCF
DNS
UE
HSS

26. Important SIP “additions”
 P-Access-Network-Info includes port location/cell
 From/To eq. IMPU
 Path informs S-CSCF about routing destination for terminating requests
 Collected during registration using INVITE (e.g. P-CSCF)
 Populated to Route headers in in-registration terminating requests on S-CSCF
 Authorization contains IMPI and other values
 200 OK Service-Route to populate S-CSCF address to P-CSCF for
originating requests
 Populated to Route headers in in-registration originating requests on UE

27. Important SIP “additions” – ctd.
 Choose a registered IMPU for session establishment
 UE – P-CSCF: P-Preferred-Identity
 P-CSCF – I/S-CSCF : P-Asserted-Identity
 P-Associated-URI informs client about registered
IMPUs
 Event: reg after registration to inform UE about
events on S-CSCF (e.g. HSS-initiated deregistration)

28.  After registration, subscriber is reachable through
public user identity for communication
 IMS subscriber can access services now or perform
calls
 P-CSCF (home or local)
 Proxy, contacts assigned S-CSCF for the calling subscriber
 S-CSCF (home)
 Service control and logic
 Contacts application or other party
 I-CSCF
 Entry point for communication from other domain
IMS Session Establishment

29. Session
establishment
outgoing
(simplified)
P-CSCF I-CSCFS-CSCF
DNS
UE

30. Session
establishment
incoming
(simplified)
P-CSCFI-CSCF S-CSCF
HSS
UE
To simplify matters, DNS is omitted in these slides.

31.  Message content within a SIP session (similar to RTP)
 Rendezvous mechanism mandatory (e.g. SDP)
 MSRP URI’s
 Accepted content
 SDP
c=IN IP4 atlanta.example.com
m=message 7654 TCP/MSRP *
a=accept-types:text/plain
a=path:msrp://atlanta.example.com:7654/jshA7weztas;tcp
Message Session Relay Protocol
(MSRP)

32. MSRP ctd.
 MSRP exchange
MSRP a786hjs2 SEND
To-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
From-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
Message-ID: 87652491
Byte-Range: 1-25/25
Content-Type: text/plain
Hey Bob, are you there?
-------a786hjs2$
MSRP a786hjs2 200 OK
To-Path: msrp://atlanta.example.com:7654/jshA7weztas;tcp
From-Path: msrp://biloxi.example.com:12763/kjhd37s2s20w2a;tcp
-------a786hjs2$

33. MSRP ctd.
 Key concepts
 Framing/message chunking (+ vs $)
 MSRP Addressing (URIs for send/recv, lists for relays)
 Scheme: msrp/msrps for TLS. TCP transport.
 Methods (e.g. SEND) and response codes (e.g. 200 OK)
 MSRP relays in the path
 More in RFC 4975 (protocol), RFC 4976 (relays)

34. Messaging/Presence
 SIP MESSAGE
 SIP SIMPLE
 SIP SUBSCRIBE/NOTIFY
 SIP PUBLISH
 Many extensions
 Standard bodies: IETF, OMA

35. Service Triggering

36. Application Layer Interaction
 User profile contains service profile
 Service Profile
 Public Identification (assigned subscribers)
 Initial Filter Criteria (triggering AS interaction)
 Initial Filter Criteria (iFC)
 Trigger points with service point triggers
(conditions when to interact)
 Application server (SIP URI for interaction)

37. Service Profile

38. Service Profile
ctd.

39. Triggering

40. Triggering
ctd.

41. Filtering
 Only initial SIP requests
 Initial filter criteria (iFC) retrieved from HSS during
registration
 Subsequent filter criteria (sFC) provided by
application server (beyond 3GPP R8)
 Allows dynamic definition of trigger points during
application runtime

42.  I/S-CSCF are interaction points with the service layer
 I-CSCF for public service identities (PSI)  explicit access
 S-CSCF for services (of served users)  implicit access
 Applications have interface towards HSS
 User profile information
 Location information, service information
 Complexity of security, authorization, access
interaction etc. all handled by the core
Application Routing

43. Application Routing ctd.
 Application server (AS) can have different functions
 Terminating AS (e.g., acting as user agent)
 Originating AS (e.g., wake up service, click to dial)
 SIP Proxy server (e.g., for SIP header manipulation)
 Back-to-back user agent (e.g., for deeper modifications in
SIP dialog as supplementary service enabler)

44. Q/A
Your Questions!

45. Is anything still unanswered?
 How do IMS services work?
 Why is the IMS needed for some communications services?
Is it?
 But I have heard of service X, why don’t they use the IMS?
 Will we build all future services on top of IMS?
 Are IMS services only those inherited from the Telco past?
 Will Telco’s deploy multiple IMS? IMS in the cloud? Share an
IMS?
 Will IMS bring in new revenues? Is it cheaper to deploy
services on the IMS compared to stand-alone deployments?

46.  Deepen understanding of CSCF/HSS roles
 Function of components
 Routing within control layer and towards applications
 Understanding IMS user identities
 Service control, routing, application layer interaction
 Many things omitted in this presentation
 Network access layer, IMS reference points names
 S-CSCF assignments in detail, SLF/HSS separation
 QoS resource reservation
 PSTN Breakout
 Charging
 Hopefully covered all open questions (last chance )
Summary

47. Thank you.
SEBASTIAN SCHUMANN
MAIL@S-SCHUMANN.COM
@S_SCHUMANN
SSCHUMANN

48. This lecture is available at the following link:
http://bit.ly/fiit-ims-signaling-3
Download

49. References
 All references are stated on the respective slides.
 If you feel that content where you hold the
copyright is displayed within these slides and you
do not like it, miss a link/reference, or want me to
remove it altogether please let me know.