Distributed SFC control for fog environments

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Table of Contents

1. Introduction

Virtualization of functions provides operators with tools to deploy new services much faster, as compared to the traditional use of monolithic and tightly integrated dedicated machinery. As a natural next step, mobile network operators need to re-think how to evolve their existing network infrastructures and how to deploy new ones to address the challenges posed by the increasing customers' demands, as well as by the huge competition among operators. All these changes are triggering the need for a modification in the way operators and infrastructure providers operate their networks, as they need to significantly reduce the costs incurred in deploying a new service and operating it. Some of the mechanisms that are being considered and already adopted by operators include: sharing of network infrastructure to reduce costs, virtualization of core servers running in data centers as a way of supporting their load-aware elastic dimensioning, and dynamic energy policies to reduce the monthly electricity bill. However, this has proved to be tough to put in practice, and not enough. Indeed, it is not easy to deploy new mechanisms in a running operational network due to the high dependency on proprietary (and sometime obscure) protocols and interfaces, which are complex to manage and often require configuring multiple devices in a decentralized way.¶

Service Functions are widely deployed and essential in many networks. These Service Functions provide a range of features such as security, WAN acceleration, and server load balancing. Service Functions may be instantiated at different points in the network infrastructure such as data center, the WAN, the RAN, and even on mobile nodes.¶

Service functions (SFs), also referred to as VNFs, or just functions, are hosted on compute, storage and networking resources. The hosting environment of a function is called Service Function Provider or NFVI-PoP (using ETSI NFV terminology).¶

Services are typically formed as a composition of SFs (VNFs), with each SF providing a specific function of the whole service. Services also referred to as Network Services (NS), according to ETSI terminology.¶

With the arrival of virtualization, the deployment model for service function is evolving to one where the traffic is steered through the functions wherever they are deployed (functions do not need to be deployed in the traffic path anymore). For a given service, the abstracted view of the required service functions and the order in which they are to be applied is called a Service Function Chain (SFC). An SFC is instantiated through selection of specific service function instances on specific network nodes to form a service graph: this is called a Service Function Path (SFP). The service functions may be applied at any layer within the network protocol stack (network layer, transport layer, application layer, etc.).¶

The concept of fog computing has emerged driven by the Internet of Things (IoT) due to the need of handling the data generated from the end-user devices. The term fog is referred to any networked computational resource in the continuum between things and cloud. A fog node may therefore be an infrastructure network node such as an eNodeB or gNodeB, an edge server, a customer premises equipment (CPE), or even a user equipment (UE) terminal node such as a laptop, a smartphone, or a computing unit on-board a vehicle, robot or drone.¶

In fog computing, the functions composing an SFC are hosted on resources that are inherently heterogeneous, volatile and mobile [I-D.bernardos-sfc-fog-ran]. This means that resources might appear and disappear, and the connectivity characteristics between these resources may also change dynamically. These scenarios call for distributed SFC control solutions, where there are SFC pseudo controllers, enabling autonomous SFC self-orchestration capabilities. This document introduces this concept and presents first ideas on mechanisms to select and initialize a service-specific SFC pseudo controller among host nodes which are participating in the SFC.¶

2. Terminology

The following terms used in this document are defined by the IETF in [RFC7665]:¶

• Service Function (SF): a function that is responsible for specific treatment of received packets (e.g., firewall, load balancer).¶
• Service Function Chain (SFC): for a given service, the abstracted view of the required service functions and the order in which they are to be applied. This is somehow equivalent to the Network Function Forwarding Graph (NF-FG) at ETSI.¶
• Service Function Forwarder (SFF): A service function forwarder is responsible for forwarding traffic to one or more connected service functions according to information carried in the SFC encapsulation, as well as handling traffic coming back from the SF.¶
• SFI: SF instance.¶
• Service Function Path (SFP): the selection of specific service function instances on specific network nodes to form a service graph through which an SFC is instantiated.¶

3. Problem statement

[RFC7665] describes an architecture for the specification, creation, and ongoing maintenance of Service Function Chains (SFCs) in a network. It includes architectural concepts, principles, and components used in the construction of composite services through deployment of SFCs.¶

The SFC architecture assumes there is a control plane that configures and manages the SFC components. This role is typically assumed to be played by a centralized controller/orchestrator. This implies that dynamic changes on the SFC (composition, function migration, scaling, etc) can only be performed by the centralized controller, which needs to always have connectivity with the functions, and have updated information on the status of all the nodes hosting the functions. Also, multiple services are managed by the same controller/orchestrator, even if they provide different functionalities with disparate requirements.¶

In a fog environment, with current management and orchestration solutions, SFCs cannot operate if the nodes hosting the functions get disconnected from the infrastructure. This implies that the lifecycle management of an SFC cannot be managed if disconnected from the centralized controller, which means that important actions (e.g., scaling, migrating a function or updating the data plane) might not take place due to the lack of connectivity with the controller/orchestrator (even if connectivity issues are just temporal ones). Additionally, lifecycle management of SFCs require up-to-date monitoring information, with a refresh frequency that is service-specific and might involve a very high overhead with the controller. This severely limits the capability of fast reacting to events local to the nodes hosting the functions, as the SFC cannot autonomously self-orchestrate (decisions can only be taken by the centralized controller/orchestrator).¶

Figure 1 shows an exemplary scenario where a drone makes use of a network service composed of the chain of functions F1-F2-F3. F1 runs on the drone itself (node A), F2 runs in another drone (node B) and F3 runs in a gNB on the ground (node D). The service might be, for example, an autonomous video surveillance activity in which a couple of drones with different types of cameras make use of image recognition to decide where to go next. If the drones move out of the coverage of the node D, the service chain needs to be reconfigured (for example migrating F3 to node C) so it can remain operative (as node D is hosting one function of the SFC). Since node D is also providing the drones with connectivity to the network infrastructure where the SFC controller is located, this type of events cannot be resolved by the SFC controller, as the nodes hosting the functions would be disconnected from the controller.¶

                        node B
                      /       \          F1+-·-·-+F2+-·-·-+F3 SFC
              <====  /\       /\
                       \-----/
                       |     |
                 +-·-·-·-+F2 |
                /      /---+-\                       __________
  /       \    ·     \/   /   \/                   _(          )_
 /\       /\  /       \  ·    /                  _( +----------+ )_
   \-----/   ·          /       ( (oo) )        (_  | SFC ctrl |  _)
   |   +-·-·/          ·           /\             (_+----------+_)
   |  F1 |             |          /\/\ (F3)         (__________)
   /-----\             ·         /\/\/\
 \/       \/      /    |  \     /\/  \/\
  \       /      /\    ·  /\     node D
   node A          \---|-/
                   |   + |
                   |  F3 |
                   /-----\
          <====  \/       \/
                  \       /
                    node C

Another scenario that cannot be properly tackled with current SFC orchestration approaches control appears with highly mobile/volatile environments and/or latency-demanding services, in which centralized lifecycle management is unfeasible due to its high signaling cost (e.g., they require frequent measurements sent from remote nodes to a centralized controller, generating too much signaling, and involving a delay that might be too long to meet the service requirements).¶

4. Distributed SFC control

The fact that -- in fog computing environments -- SFC functions are hosted in heterogeneous, volatile and mobile resources, calls for new orchestration solutions able to cope with dynamic changes to the resources in runtime or ahead of time (in anticipation through prediction) as opposed to today's solutions which are inherently reactive and static or semi-static.¶

These new orchestration solutions have to enable SFCs to autonomously self-orchestrate without having to rely on a centralized controller. The idea introduced in this draft is to enable one of the nodes involved in a service function chain (i.e., in a specific service) to be prepared to take over the control of the SFC and perform network service lifecycle management decisions, replacing at least temporary and at least partially the centralized SFC controller.¶

This draft proposes a new logical entity, complementing the SFC controller/orchestrator found in current architectures and deployments. We refer to this new entity as SFC pseudo controller, and it is characterized by the following:¶

• It is service-specific, meaning that it is defined and meaningful in the context of a given network service. Compared to existing SFC controllers/orchestrators, which manage multiple SFCs instantiated over a common infrastructure, pseudo controllers are constrained to service specific lifecycle management. These SFC pseudo controllers synchronize with the SFC centralized controller/orchestrator to ensure proper resource orchestration.¶
• Potentially, any node involved in a network service might play the role of SFC pseudo controller. But note that it is not mandatory that all nodes are willing/capable to play that role. Therefore, we consider that on a given deployment, only a subset of all the involved nodes are willing or capable of doing so. We refer to these nodes as candidate pseudo controllers. During the initialization phase, out of the candidates, one will be chosen by the SFC controller as selected SFC pseudo controller. Each candidate pseudo controller maintains a local copy of the information required to properly perform lifecycle management of the service. This includes not only information about the network service (e.g., the Network Service Descriptor, NSD, and the Virtual Network Function Descriptors, VNFDs, as defined by ETSI NFV, and the characterization of the resource capabilities of the nodes hosting the functions), but also the information related to performing an efficient monitoring of the service. We refer to this new descriptor as Operations, Administration and Maintenance Descriptor (OAMD).¶
• From the set of available candidate SFC pseudo controllers, one is chosen as selected pseudo controllers for a network service. This active pseudo controller performs monitoring activities at service and resource level and synchronizes periodically with the centralized SFC controller/orchestrator. Note that this is performed in an opportunistic way, if connectivity is available, and that disconnected operation is possible.¶

Candidate pseudo controller instances might be located at any node hosting a service function. The SFC controller typically runs in the network core, at a server (either as a physical or virtual function). The SFC controller and the candidate pseudo controller instances exchange signaling to initialize the selected SFC pseudo controller. This includes signaling to describe the service, signaling to express the readiness and preference from the candidates to play the role of pseudo controller, and the signaling from the SFC controller to indicate the selected one. This is explained in more detail next.¶

+--------+    +--------+    +--------+    +--------+     +----------+
| node A |    | node C |    | node B |    | node D |     | SFC ctrl |
+--------+    +--------+    +--------+    +--------+     +----------+
    |             |             |             |                |
    | F1@A<->F2@B<->F3@C SFC instance traffic |                |
    |<-·-·-·-·-·-·-·-·-·-·-·-·->|<-·-·-·-·-·->|                |
    |             |             |             |                |
    |             |             |1. Candidate pseudo-controller adv.
    |             |             |----------------------------->|
    |--------------------------------------------------------->|
    |             |             |             |                |
    |             |             |       2. Service description |
    |             |             |<-----------------------------|
    |<---------------------------------------------------------|
    |             |             |             |                |
    |             |             |3. Candidate pseudo-controller req.
    |             |             |----------------------------->|
    |--------------------------------------------------------->|
    |             |             |             |                |
    |             |             |             |        (4. Selection)
    |             |             |             |                |
    |             |             5. Candidate pseudo-controller resp.
    |<---------------------------------------------------------|
    |             |             |             |                |

5. IANA Considerations

N/A.¶

6. Security Considerations

TBD.¶

7. Acknowledgments

The work in this draft has been developed under the framework of the H2020 5G-DIVE project (Grant 859881).¶

Authors' Addresses