add IMS slides
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High-Level intro IMS, VoLTE, VoWFi
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==================================
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:author: Harald Welte <laforge@gnumonks.org>
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:copyright: 2021 by Harald Welte (License: CC-BY-SA)
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:backend: slidy
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:max-width: 45em
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== Overview
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What this talk is about
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* IMS basics
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* How IMS is used in VoLTE
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* How IMS is used in VoWiFi
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== Evolution of telephone signaling
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* circuit-switched
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** PSTN
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** ISDN Call Control (Q.931 + ISUP)
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** GSM L3 CC Call Control (derived from Q.931)
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** BICC in core networks
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* packet-switched
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** H.323
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** IETF SIP
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** ITU SIP-I as ISUP replacement in telco networks
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** 3GPP IMS (based on SIP)
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== IMS: IP Multimedia System
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* predates LTE
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* specified at 2G + 3G time
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* idea was to
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** do voice over packet switched GPRS/UMTS services
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** use one shared telephony system for both cellular and wired (DSL, Cable) access
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== IMS and VoLTE / VoWiFi
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* VoLTE is a marketing term for IMS over a LTE access network
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* VoWiFi is a marketing term for IMS accesss over untrusted networks
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** *untrusted* basically means not operated by a commercial telecom operator
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== Basic IMS Core Network
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* primarily a set of SIP proxies / switches (P-CSCF, I-CSCF, S-CSCF)
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* subscriber configuratoin stored in HSS (like in LTE)
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* optionally augmented by media gateways
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** for interworking with PSTN / ISDN circuit switched interfaces
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** for interworking with 2G/3G circuit switched domain
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.IMS Core Network
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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Sub_A [label="Subscriber\nA"];
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Sub_B [label="Subscriber\nB"];
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Sub_C [label="Subscriber\nC"];
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Sub_D [label="Subscriber\nD"];
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subgraph cluster_A {
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label = "IMS core";
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PCSCF [label="P-CSCF",shape="square"];
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ICSCF [label="I-CSCF",shape="square"];
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SCSCF [label="S-CSCF",shape="square"];
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}
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HSS [label="HSS",shape="square"];
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Sub_A -> PCSCF [dir=none,label="SIP"];
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Sub_B -> PCSCF [dir=none,label="SIP"];
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Sub_C -> PCSCF [dir=none,label="SIP"];
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Sub_D -> PCSCF [dir=none,label="SIP"];
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PCSCF -> ICSCF [dir=none,label="SIP"];
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ICSCF -> SCSCF [dir=none,label="SIP"];
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ICSCF -> HSS [dir=none,label="DIAMETER"];
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SCSCF -> HSS [dir=none,label="DIAMETER"];
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}
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----
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== P-CSCF: Proxy Call Session Control Function
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* IP address to which IMS clients sent SIP traffic
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* also serves as IPsec endpoint in case of VoLTE
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* forwards SIP to I-CSCF
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== I-CSCF: Interrogating Call Session Control Function
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* resolve which S-CSCF shall be used to serve the subscriber
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** involves interrogation of HSS via DIAMETER
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* forwards SIP to S-CSCF it has resolved
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== S-CSCF: Service Call Session Control Function
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* anchor to which the subscribers actually register
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* confirms subscriber authorization with HSS (via DIAMETER)
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* in charge of actual call control
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== SIP dialect used in IMS
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* don't assume it is just plain old SIP like you know it from VoIP
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* various 3GPP specific additions and extensions
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* for example, introduction of EAP-AKA for authentication via ISIM card
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== IMS in VoWiFi
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* Introduction of the `ePDG` network element
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* resembles a bit the P-GW/GGSN in 3GPP use case, but with the public internet as "RAN"
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* acts as IPsec tunnel endpoint from/to clients
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* IPsec IKE has been extended by 3GPP
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** to use USIM/ISIM (UMTS AKA) based authentication
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** to derive IPsec encryption keys from UMTS AKA
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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Sub_A [label="Subscriber\nA"];
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ePDG [label="ePDG",shape="square"];
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subgraph cluster_A {
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label = "IMS core";
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PCSCF [label="P-CSCF",shape="square"];
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ICSCF [label="I-CSCF",shape="square"];
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SCSCF [label="S-CSCF",shape="square"];
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}
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HSS [label="HSS",shape="square"];
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Sub_A -> ePDG [dir=none,label="SIP/TCP/IP/IPSEC"];
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ePDG -> PCSCF [dir=none,label="SIP/TCP/IP"];
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ePDG -> HSS [dir=none,label="DIAMETER"];
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PCSCF -> ICSCF [dir=none,label="SIP"];
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ICSCF -> SCSCF [dir=none,label="SIP"];
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ICSCF -> HSS [dir=none,label="DIAMETER"];
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SCSCF -> HSS [dir=none,label="DIAMETER"];
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}
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----
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== IMS in VoWiFi
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* ePDG IP address discovery
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** hardcoded in UE, or
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** configurable in UE, or
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** DNS based resolution via epdg.epc.mncXXX.mccYYY.pub.3gppnetwork.org
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Once ePDG is discovered, IKE Phase 1 is started to set up IPsec SA
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* typically NAT-T (UDP encapsulated ESP) is used
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* typically both encryption and integrity protection used
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== IMS in VoLTE
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* you would assume it's simply accessing the IMS core via E-UTRAN + EPC
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** but...you underestimate the mind-boggling complexity that is required for 3GPP specs
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* EPC supports QoS, so we must make use of that
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** dedicated bearers wit different QoS for
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*** default bearer (background IP, QCI=8/9)
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*** IMS signaling (high reliability, QCI=5)
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*** IMS user plane (low latency, QCI=1)
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* IMS registration establishes QCI=5 bearer
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* call setup establishes QCI=1 bearer
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== dedicated bearers in LTE
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* in 2G/3G, we know PDP contexts
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** PDP context resembles IP tunnel between MS/UE and GGSN
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** you can establish multiple concurrent PDP contexts to different GGSNs, each with its own IP
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* LTE extends this with the notion of EPS bearers and TFTs
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** think of those like sub-flows within a PDP context
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** you connect to one APN but have multiple concurrent EPS bearers to that APN
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** each EPS bearer comes with a set of packet filter rules that determie to which bearer a given packet gets mapped. Those packet filter rules are called TFTs.
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== IMS in VoLTE
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[graphviz]
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----
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digraph G {
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rankdir=LR;
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Sub_A [label="Subscriber\nA"];
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EUTRAN [shape="square"];
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PGW [label="P-GW for IMS APN",shape="square"];
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PCRF [label="PCRF",shape="square"];
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subgraph cluster_A {
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label = "IMS core";
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PCSCF [label="P-CSCF",shape="square"];
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ICSCF [label="I-CSCF",shape="square"];
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SCSCF [label="S-CSCF",shape="square"];
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}
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HSS [label="HSS",shape="square"];
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Sub_A -> PGW [dir=none,label="EPS bearers for QCI=1/QCI=5"];
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PGW -> PCSCF [dir=none,label="SIP/TCP/IP (opt. IPsec)"];
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PGW -> HSS [dir=none,label="DIAMETER"];
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PCRF -> HSS [dir=none,label="DIAMETER"];
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PGW -> PCRF [dir=none,label="DIAMETER"];
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PCSCF-> PCRF [dir=none,label="DIAMETER"];
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PCSCF -> ICSCF [dir=none,label="SIP"];
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ICSCF -> SCSCF [dir=none,label="SIP"];
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ICSCF -> HSS [dir=none,label="DIAMETER"];
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SCSCF -> HSS [dir=none,label="DIAMETER"];
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}
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----
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== IMS Open Source Software
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In case you want to play with this:
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* https://github.com/fasferraz/SWu-IKEv2
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* FHoSS: Fraunhofer HSS with IMS related features (Java)
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* Kamailio: SIP softswitch that can serve as P/I/S-CSCF
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* doubango: IMS library / framework for client side
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* imsdroid: Android stand-alone IMS client application using doubango framework
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== EOF
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End of File
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