Virtual IMS Core

Cross Project Spec - None

User Story Tracker - None

Problem description

Problem Definition

This use case is about deploying a virtual IMS core as an NFV function in OpenStack. It replaces the version previously uploaded to the TelcoWG repository [1].

An IMS core [2] is a key element of Telco infrastructure, handling VoIP device registration and call routing. Specifically, it provides SIP-based call control for voice and video as well as SIP based messaging apps.

An IMS core is mainly a compute application with modest demands on storage and network - it provides the control plane, not the media plane (packets typically travel point-to-point between the clients) so does not require high packet throughput rates and is reasonably resilient to jitter and latency.

As a core Telco service, the IMS core must be deployable as an HA service capable of meeting strict Service Level Agreements (SLA) with users. Here HA refers to the availability of the service for completing new call attempts, not for continuity of existing calls. As a control plane rather than media plane service the user experience of an IMS core failure is typically that audio continues uninterrupted but any actions requiring signalling (e.g. conferencing in a 3rd party) fail. However, it is not unusual for client to send periodic SIP “keep-alive” SIP pings during a call, and if the IMS core is not able to handle them the client may tear down the call.

An IMS core must be highly scalable, and as an NFV function it will be elastically scaled by an NFV orchestrator running on top of OpenStack. The requirements that such an orchestrator places on OpenStack are not addressed in this use case.


Although this user story is specifically about deploying the Project Clearwater virtual IMS core, it is more generally representative of the issues involved in deploying in OpenStack any scalable Telco-grade control plane Virtual Network Function (VNF) deployed as a series of load-balanced stateless N+k pools.

Requirements Specification

Use Cases

  • As a communication service provider, I want to deploy a highly available, high scale, high performance virtual IMS core on OpenStack to provide my core Voice-over-IP service.

Usage Scenario Examples

Project Clearwater [3] is an open-source implementation of an IMS core designed to run in the cloud and be massively scalable. It provides P/I/S-CSCF functions together with a BGCF and an HSS cache, and includes a WebRTC gateway providing interworking between WebRTC & SIP clients.


The problem statement above leads to the following requirements.

  • Compute application

    OpenStack already provides everything needed; in particular, there are no requirements for an accelerated data plane, nor for core pinning nor NUMA.

  • HA

    Project Clearwater itself implements HA at the application level, consisting of a series of load-balanced N+k pools with no single points of failure [4].

    To meet typical SLAs, it is necessary that the failure of any given host cannot take down more than k VMs in each N+k pool. More precisely, given that those pools are dynamically scaled, it is a requirement that at no time is there more than a certain proportion of any pool instantiated on the same host.

    That by itself is insufficient for offering an SLA, though: to be deployable in a single OpenStack cloud (even spread across availability zones or regions), the underlying cloud platform must be at least as reliable as the SLA demands. Those requirements will be addressed in a separate use case.

  • Elastic scaling

    An NFV orchestrator must be able to rapidly launch or terminate new instances in response to applied load and service responsiveness. This is basic OpenStack nova function.

  • Placement zones

    In the IMS architecture there is a separation between access and core networks, with the P-CSCF component (Bono - see [4]) bridging the gap between the two. Although Project Clearwater does not yet support this, it would in future be desirable to support Bono being deployed in a DMZ-like placement zone, separate from the rest of the service in the main MZ.

Rejected User Stories / Usage Scenarios



  • NFV - Networks Functions Virtualisation, see
  • IMS - IP Multimedia Subsystem
  • SIP - Session Initiation Protocol
  • P/I/S-CSCF - Proxy/Interrogating/Serving Call Session Control Function
  • BGCF - Breakout Gateway Control Function
  • HSS - Home Subscriber Server
  • WebRTC - Web Real-Time-Collaboration