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