IMS Signaling (Rev. 3)

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Information about IMS Signaling (Rev. 3)

Published on March 11, 2014

Author: sschumann



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.


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

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…

Summary “SIP & IMS Basics” Reference to lecture.

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

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

IMS and its Services

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

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

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

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

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)

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)

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

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

 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

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)

 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)


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

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

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

Registration (simplified) P-CSCF DNS UE

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  7200 IN NAPTR 10 50 "s“ "SIP+D2T“  SRV look-up for a NAPTR given address indicates the domain and port the service listens on  7200 IN SRV 0 0 5060  A/AAAA to find the IP address of the domain name  7200 IN A

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

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

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)

 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

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

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

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

MSRP ctd.  MSRP exchange MSRP a786hjs2 SEND To-Path: msrp://;tcp From-Path: msrp://;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://;tcp From-Path: msrp://;tcp -------a786hjs2$

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)

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

Service Triggering

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)

Service Profile

Service Profile ctd.


Triggering ctd.

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

 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

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)

Q/A Your Questions!

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?

 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


This lecture is available at the following link: Download

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.

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