An Architecture for Use of PCE and PCEP in a Network with Central Control

The Path Computation Element (PCE) was developed to offload path computation function from routers in an MPLS traffic engineered network. Since then, the role and function of the PCE has grown to cover a number of other uses (such as GMPLS ) and to allow delegated control and PCE-initiated use of network resources . According to , Software Defined Networking (SDN) refers to a separation between the control elements and the forwarding components so that software running in a centralized system called a controller, can act to program the devices in the network to behave in specific ways. A required element in an SDN architecture is a component that plans how the network resources will be used and how the devices will be programmed. It is possible to view this component as performing specific computations to place flows within the network given knowledge of the availability of network resources, how other forwarding devices are programmed, and the way that other flows are routed. This is the function and purpose of a PCE, and the way that a PCE integrates into a wider network control system including SDN is presented in . In early PCE implementations, where the PCE was used to derive paths for MPLS Label Switched Paths (LSPs), paths were requested by network elements and the results of the path computations were supplied to network elements using the Path Computation Element Communication Protocol (PCEP) . This protocol was later extended to allow a PCE to send unsolicited requests to the network for LSP establishment . SDN has a far broader applicability than just signaled MPLS or GMPLS traffic engineered networks. The PCE component in an SDN system may be used to determine paths in a wide range of use cases including static LSPs, segment routing , service function chaining (SFC) , and indeed any form of routed or switched network. It is, therefore reasonable to consider PCEP as a general southbound control protocol for use in these environments to allow the PCE to be fully enabled as a central controller. This document introduces the architecture for PCE as a central controller, examines the motivations and applicability for PCEP as a southbound interface, and introduces the implications for the protocol. This document dos not describe the use cases in detail and does not define protocol extensions: that work is left for other documents.

The architecture for the use of PCE within centralized control of a network is based on the understanding that a PCE can determine how connections should be placed and how resources should be used within the network, and that the PCE can then cause those connections to be established. shows how this control relationship works in a network with an active control plane. This is a familiar view for those who have read and understood and . In this mode of operation, the central controller is asked to create connectivity by a network orchestrator, a service manager, an Operations Support System (OSS), a Network Management Station (NMS), or some other application. The PCE-based controller computes paths with awareness of the network topology, the available resources, and the other services supported in the network. This information is held in the Traffic Engineering Database (TED) and other databases available to the PCE. Then the PCE sends a request using PCEP to one of the Network Elements (NEs), and that NE uses a control plane to establish the requested connections and reserve the network resources.

| NE |<--->| NE |<--->| NE | ---- Control ---- ---- ---- Plane ]]> Although the architecture shown in represents a form of SDN, one objective of SDN in some environments is to remove the dependency on a control plane. A transition architecture toward this goal is presented in and is shown in . In this case, services are still requested in the same way, and the PCE-based controller still requests use of the network using PCEP. The main difference is that the consumer of the PCEP messages is a Network Controller that provisions the resources and instructs the data plane using Southbound Interface (SBI) that provides an interface to each NE.

The approach in delivers the SDN functionality but is overly complicated and insufficiently flexible. The complication is created by the use of two controllers in a hierarchical organization, and the resultant use of two protocols in a southbound direction. The lack of flexibility arises from the assumed or required lack of a control plane. This document describes an architecture that reduces the number of components and is flexible to a number of deployment models and use cases. In this hybrid approach (shown in ) the network controller is PCE-enabled and can also speak PCEP as the SBI (i.e., it can communicate with each node along the path using PCEP). That means that the controller can communicate with a conventional control plane-enabled NE using PCEP and can also use the same protocol to program individual NEs. In this way the PCE-based controller can control a wider range of networks and deliver many different functions as described in . PCEP is essentially already capable of acting as an SBI and only small, use case- specific modifications to the protocol are needed to support this architecture. The implications for the protocol are discussed further in .

| NE |...| NE |<....: Control Plane ---- ---- ]]>

Systems with central controllers are vulnerable to two problems: failure or overload of the single controller. These concerns are not unique to the use of a PCE-based controller but need to be addressed in this document before the PCE-based controller architecture can be considered for use in all but the smallest networks. There are three architectural mechanisms that can be applied to address these issues. The mechanisms are described separately for clarity, but a deployment use may any combination of the approaches. For simplicity of illustration, these three approaches are shown in the sections that follow without a control plane. However, the general, hybrid approach of is applicable in each case.

The first and simplest approach to handling controller overload or scalability is to use multiple controllers, each responsible for a part of the network. We can call the resultant areas of control "domains." This approach is shown in . It can clearly address some of the scaling and overload concerns since each controller now only has responsibility for a subset of the network elements. But this comes at a cost because end-to-end connections require coordination between the controllers. Furthermore, this technique does not remove the single-point-of-failure concern even if it does reduce the impact on the network of the failure of a single controller. Note that PCEP is designed to work as a PCE-to-PCE protocol as well as a PCE-to-PCC protocol, so it should be possible to use it to coordinate between PCE-based controllers in this model.

| PCE-based |<-------->| PCE-based |<---| TED | ----- | Controller | | Controller | ----- | | | | /------------ ------------\ / ^ ^ ^ ^ \ / | | | | \ | | | | | | v v v :: v v v ---- ---- ---- :: ---- ---- ---- | NE | | NE | | NE | :: | NE | | NE | | NE | ---- ---- ---- :: ---- ---- ---- :: Domain 1 :: Domain 2 :: ]]>

Multiple parallel controllers may be deployed as shown in . Each controller is capable of controlling all of the network elements thus the failure of any one controller will not leave the network unmanageable and, in normal circumstances, the load can be distributed across the controllers. To achieve full redundancy and to be able to continue to provide full function in the event of the failure a controller, the controllers must synchronize with each other. This is nominally a simple task if there are just two controllers, but can actually be quite complex if state changes in the network are not to be lost. Furthermore, if there are more than two controllers, the synchronization between controllers can become a hard problem. Synchronization issues are often off-loaded as "database synchronization" problems because distributed database packages have already had to address these challenges. In networking the problem may also be addressed by collecting the state from the network (effectively using the network as a database) using normal routing protocols such as OSPF, IS-IS, and BGP.

| PCE-based | | Controller | ----- | Controller | | |__ ...........| | ------------\ \_:__ :------------ ^ ^ \___: \ .....: ^ ^ | | .....:\ \_:___ ..: : | |__:___ \___:_ \_:___ : | ....: | .....: | ..: | : | : | : | : v v v v v v v v ---- ---- ---- ---- | NE | | NE | | NE | | NE | ---- ---- ---- ---- ]]>

shows an approach with hierarchical controllers. This approach was developed for PCEs in and appears in various SDN architectures where a "parent PCE", an "orchestrator", or "super controller" takes responsibility for a high-level view of the network before distributing tasks to lower level PCEs or controllers. On its own, this approach does little to protect against the failure of a controller, but it can make significant improvements in loading and scaling of the individual controllers. It also offers a good way to support end-to-end connectivity across multiple administrative or technology-specific domains. Note that this model can be arbitrarily recursive with one PCE-based controller acting as the parent of of another set of PCE-based controllers.

| PCE-based | | PCE-based |<---| TED | ----- | Controller | | Controller | ----- /| | | |\ / ------------ ------------ \ / ^ ^ ^ ^ \ / | | | | \ / | | | | \ | | | :: | | | v v v :: v v v ---- ---- ---- :: ---- ---- ---- | NE | | NE | | NE | :: | NE | | NE | | NE | ---- ---- ---- :: ---- ---- ---- :: Domain 1 :: Domain 2 :: ]]>

This section gives a very high-level introduction to the applicability of a PCE-based centralized controller. There is no attempt to explain each use case in detail, and the inclusion of a use case is not intended to suggest that deploying a PCE-based controller is a mandatory or recommended approach. The sections below are provided as a stimulus to discussion of the applicability of a PCE-based controller and it is expected that separate documents will be written to develop the use cases in which there is interest for implementation and deployment. As described in specific enhancements to PCEP may be needed for some of these use cases and it is expected that the documents that develop each use case will also address any extensions to PCEP. The rest of this section is divided into two sub-sections. The first approaches the question of applicability from a consideration of the network technology. The second looks at the high-level functions that can be delivered by using a PCE-based controller. As previously mentioned, this section is intended to just make suggestions. Thus the material supplied is very brief. The omission of a use case is in no way meant to imply some limit on the applicability of PCE-based control.

This section provides a list of use cases based on network technology.

This mode of operation is the common approach for an active, stateful PCE to control a traffic engineered MPLS or GMPLS network . Note that the PCE-based controller determines what LSPs are needed and where to place them. PCEP is used to instruct the head end of each LSP, and the head end signals in the control plane to set up the LSP.

Static LSPs are provisioned without the use of a control plane. This means that they are established using management plane or "manual" configuration. Static LSPs can be provisioned as 1-hop, micro-LSPs at each node along the path of an end-to-end path LSP. Each router along the path must be told what label forwarding instructions to program and what resources to reserve. The PCE-based controller keeps a view of the network and determines the paths of the end-to-end LSPs just as it does for the use case described in , but the controller uses PCEP to communicate with each router along the path of the end-to-end LSP. In this case the PCE-based controller will take responsibility for managing some part of the MPLS label space for each of the routers that it controls, and may taker wider responsibility for partitioning the label space for each router and allocating different parts for different uses communicating the ranges to the router using PCEP.

Multicast LSPs may be provisioned with a control plane or as static LSPs. No extra considerations apply above those in and except, of course, to note that the PCE must also include the instructions about where the LSP branches, i.e., where packets must be copied.

Transport SDN (T-SDN) is the application of SDN techniques to transport networks. In this respect a transport network is a network built from any technology below the IP layer and designed to carry traffic transparently in a connection-oriented way. Thus, an MPLS traffic engineering network is a transport network although it is more common to consider technologies such as Time Division Multiplexing (TDM) and Optical Transport Networks (OTN). Transport networks may be operated with or without a control plane and may have point-to-point or point-to-multipoint connections. Thus, all of the considerations in , , and apply. It may be the case that additional technology- specific parameters are needed to configure the NEs and these parameters will need to be carried in the PCEP messages.

Segment routing is described in . It relies on a series of forwarding instructions being placed in the header or a packet: at each hop in the network a router looks at the first instruction and may continue to forward the packet unchanged, strip the top instruction and forward the packet, or strip the top instruction, insert some additional instructions, and forward the packet. The segment routing architecture supports operations that can be used to steer packet flows in a network thus providing a form of traffic engineering. A PCE-based controller can be responsible for computing the paths for packet flows in a segment routing network, for configuring the forwarding actions on the routers, and for telling the edge routers what instructions to attach to packets as they enter the network. These last two operations can be achieved using PCEP and the PCE-based controller will assume responsibility for managing the space of labels or path identifiers used to determine how packets are forwarded.

Service Function Chaining (SFC) is described in . It is the process of directing traffic in a network such that it passes through specific hardware devices or virtual machines (known as service function nodes) that can perform particular desired functions on the traffic. The set of functions to be performed and the locations at which they are to be performed is known as service function chain. Each packet is marked as belonging to a specific chain and that marking lets each successive service function node know which functions to perform and to which service function node to send the packet next. To operate an SFC network the service function nodes must be configured to understand the packet markings and the edge nodes must be told how to mark packets entering the network. Additionally it may be necessary to establish tunnels between service function nodes to carry the traffic. Planning an SFC network requires load balancing between service function nodes and traffic engineering across the network that connects them. These are operations that can be performed by a PCE-based controller, and that controller can use PCEP to program the network and install the service function chains and any required tunnels.

This section provides a list of the high-level functions that can be delivered by using a PCE-based controller.

According to , Traffic Engineering (TE) is concerned with performance optimization of operational networks. In general, it encompasses the application of technology and scientific principles to the measurement, modeling, characterization, control of Internet traffic, and the application of such knowledge and techniques to achieve specific performance objectives. From a practical point of view this involves having an understanding of the topology of the network, the characteristics of the nodes and links in the network, and the traffic demands and flows across the network. It also requires that actions can be taken to ensure that traffic follows specific paths through the network. PCE was specifically developed to address TE in an MPLS network, and so a PCE-based controller is well suited to analyze TE problems and supply answers that can be installed in the network using PCEP. PCEP can be responsible for initiating paths across the network through a control plane, or for installing state in the network node by node such as in a Segment Routed network (see ) or by configuring IGP metrics.

Traffic classification is an important part of traffic engineering. It is the process of looking at a packet to determine how it should be treated as it is forwarded through the network. It applies in many scenarios including MPLS traffic engineering (where it determines what traffic is forwarded onto which LSPs), segment routing (where it is used to select which set of forwarding instructions to add to a packet), and service function chaining (where it indicates along which service function chain a packet should be forwarded). Traffic classification is closely linked to the computational elements of planning for the network functions just listed because it determines how traffic load is balanced and distributed through the network. Therefore, selecting what traffic classification should be performed by a router is an important part of the work done by a PCE-based controller. Instructions can be passed from the controller to the routers using PCEP. These instructions tell the routers how to map traffic to paths or connections. The instructions may use the concept of a Forwarding Equivalence Class (FEC).

Various network services may be offered over a network. These include protection services (including end-to-end protection , restoration after failure, and fast reroute ), Virtual Private Network (VPN) service (such as Layer 3 VPNs or Ethernet VPNs ), or Pseudowires . Delivering services over a network in an optimal way requires coordination in the way that network resources are allocated to support the services. A PCE-based central controller can consider the whole network and all components of a service at once when planning how to deliver the service. It can then use PCEP to manage the network resources and to install the necessary associations between those resources.

PCEP is push-pull protocol that is designed to move requests and responses between a server (the PCE) and Path Computation Clients (PCCs - the network elements). In particular, it has a message (PCInitiate ) that can be sent by the PCE to install state or cause actions at the PCC, and a response message (PCRpt) that is used to confirm the request. As such, there is an expectation that only relatively minor changes to PCEP are required to support the concept of a PCE-based controller. The only work expected to be needed is small extensions to carry additional or specific information elements for the individual use cases. Where possible, consistent with the general principles of how protocols are extended, any additions to the protocol should be made in a generic way such that they are open to use in a range of applications. It is anticipated that new documents will be produced for each use case dependent on support and demand. Such documents will explain the use case and define the necessary protocol extensions.

Security considerations for a PCE-based controller are little different from those for any other PCE system. That is, the operation relies heavily on the use and security of PCEP and so consideration should be given to the security features discussed in and the additional mechanisms described in . It should be observed that the trust model of a network that operates with out a control plane is different from one with a control plane. The conventional "chain of trust" used with a control plane is replaced by individual trust relationships between the controller and each individual NE. This model may be considerably easier to manage and so is more likely to be operated with a high level of security. However debate will rage over overall system security and the opportunity for attacks in an architecture with a central controller since the network can be vulnerable to denial of service attacks on the controller, and the forwarding system may be harmed by attacks on the messages sent to individual routers. In short, while the interactions with a PCE-based controller are not substantially different from those in any other SDN architecture, the security implications of SDN are still open for discussion. The IRTF's SDN Research Group (SDNRG) continues to discuss this topic. It is expected that each new document that is produced for a specific use case will also include considerations of the security impacts of the use of a PCE-based central controller on the network type and services being managed.

The architecture described in this document is a management architecture: the PCE-based controller is a management component that controls the network through a southbound management protocol (PCEP). RFC 5440 contains a substantive manageability considerations section that examines how a PCE-based system and a PCE-enabled system may be managed. A MIB module for PCEP was published as RFC 7420 and a YANG module for PCEP has also been proposed .

This document makes no requests for IANA action.

The following people contributed to discussions that led to the development of this document:

The ideas in this document owe a lot to the work started by the authors of and . The authors of this document fully acknowledge the prior work and thank those involved for opening the discussion. The individuals concerned are: King Ke, Luyuan Fang, Chao Zhou, Boris Zhang, Zhenbin Li. This document has benefited from the discussions within a small ad hoc design team the members of which are listed as document contributors. Thanks to Michael Scharf and Andy Malis for a lively discussion of this document.