Path Computation Element in Advanced SDN Operations
Explore the role of Path Computation Element (PCE) as a centralized controller in SDN operations, its history, functionality, and evolving developments over time. Understand the implications of leveraging PCE for network automation, resource partitioning, and network convergence. Discover the potential benefits and considerations of implementing a PCE-based Centralized Controller in Software Defined Networking.
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Net-Centric 2016 Net-Centric 2016 The Path Computation Element as a Centralized Controller in advanced SDN Operations Adrian Farrel : Old Dog Consulting adrian@olddog.co.uk www.isocore.com/2016
Net-Centric 2016 Agenda Remember - What is a PCE? What does SDN look like? The role of PCE in SDN The implications of putting PCE at the centre What could we do with a PCE-Based Centralized Controller What would it take? Why would we do it? Who is doing it? 2
Net-Centric 2016 PCE Refresher Well, do we really need a PCE refresher? History Origins A mechanism to provide inter-domain paths A tool to offload path computation in MPLS systems Rapid developments Path computation for all connection oriented systems MPLS-TE, GMPLS, MPLS-TP, Server functionality Receives path computation requests Sends responses 3
Net-Centric 2016 A Functional Entity PCE is a functional component It can be embedded in other components It can be a stand-alone sever It s just a things that computes paths LSR PCE server NMS LSR PCC TED TED TED PCE NMS PCE PCE PCC Signalling engine PCE server PCE co-located in the LSR PCC PCE in the NMS PCE in a dedicated server 4
Net-Centric 2016 Developments Over Time It was a request/response server Please compute a path / Here is your answer Stateful PCE is valuable Coordinate with other LSPs in the system Delegated control PCE sends suggested updates when the network changes PCE-Initiated LSPs PCE sends unsolicited LSP setup requests All these features become important for SDN 5
Net-Centric 2016 What is Software Defined Networking? Let s not re-open this debate (please!) Centralized control Especially of end-to-end services Network convergence Converged services and infrastructure Network abstraction Partitioning of resources Network automation Application-to-network relationship Provides access to the forwarding plane of network devices Momentum being driven for commercial benefits Rapidly provide cool services at lower prices Reduce OPEX and simplify network operations Enable better monitoring and diagnostics 6
Net-Centric 2016 Some Popular SDN Architectures Application/Service Requester PCE Orchestrator TED Applications PCE Application Service Coordinator PCE Controller Controller Controller Controller TED TED PCE ABNO Controller Choices I2RS OAM Handler Provisioning Manager Lots of pictures Most show some form of path computation being performed Choices Controllers PCEP 7
Net-Centric 2016 Why is PCE at the Centre? This is really simple Its all about coordination of network resources Centralization avoid contention And its all about traffic engineering Centralization aids planning and optimization And it uses complex and intensive algorithms May be best to offload from the dumb network And it can benefit from large databases Network capabilities Traffic demands and intentions Telemetrics 8
Net-Centric 2016 What Does it Mean To Be At the Centre? PCE-Based Centralized Control PCE is there in all the pictures That means PCE is central to the decision-making processes From there on it is a labelling function Controllers/Orchestrators/OSS use PCE PCE-enabled Controllers/Orchestrators/OSS PCE-based Controllers/Orchestrators/OSS PCE-based Central Controller PCE as a Central Controller This is mainly semantics A lot of it is about how you build software and how you market product 9
Net-Centric 2016 Some Useful Functions Answer computation enquiries (already supported) Modify an LSP that is already in place (stateful PCE) Cause an LSP to be set up (PCE-initiated LSPs) Say what an LSP is for Apply to other technologies Deterministic networking Time-sliced channel hopping (IEEE 802.15.4e) Segment Routing Service Function Chaining PCEP as an SDN Southbound Interface Static LSPs LSP stitching as a hybrid use case LSP stitching as a hybrid use case Answer computation enquiries (already supported) Modify an LSP that is already in place (stateful PCE) Cause an LSP to be set up (PCE-initiated LSPs) Say what an LSP is for Apply to other technologies Deterministic networking Time-sliced channel hopping (IEEE 802.15.4e) Segment Routing Service Function Chaining PCEP as an SDN Southbound Interface Static LSPs 10
Net-Centric 2016 Traffic Classification What is The LSP For? When a TE-LSP is set up, the head end needs to know how to use it What traffic to send on the LSP Whether it is a virtual link Whether to advertise it in the IGP What bits of this information to signal to the tail end What to do with traffic received on an LSP PCEP allows an Active PCE to set up or modify LSPs But we have no way to tell the head end how to use the LSP This is because of history It used to be the LER that made the request of the PCE, so it knew why it wanted the LSP This function is presumably necessary But it is currently missing 11
Net-Centric 2016 But LSPs Are Used Today How? There are several possibilities No-one uses Active PCE The problem doesn t arise Active PCE is used only in controlled environments Head end always knows what the LSP is for Active PCE is used in conjunction with configuration The LSP is set up using PCEP Some other mechanism tells the head end what to do Active PCE is used in conjunction with BGP Flowspec Possibly not what BGP Flowspec was designed for But it works Note that the last two of these seem a waste Why separate the functions? Could use one protocol for everything 12
Net-Centric 2016 What Features for LSP Purposing ? Describe: How to use the LSP What traffic to place on the LSP We already have ways of describing traffic flows e.g., BGP Flowspec How to advertise the LSP Does it appear as a link in a topology? Which instance, what metric, what bandwidth? Compare with RFC 6107 Extra signalling information Other stuff gets signalled to coordinate LSP end points Features of LSP usage 13
Net-Centric 2016 Segment Routing Path Programming and Classification Segment routing is a form of source routing Payload header 1st hop 2nd hop 3rd hop nth hop Payload A central controller can Compute paths through the network Instruct the head end about what stack to impose This is like Normal stateful path computation Stateful path programming Traffic classification 14
Net-Centric 2016 Further Proposed SR Task Labels have to be selected and assigned Selection benefits from centralization Assignment has competing protocol solutions PCE allocates Labels (SID) for node, Adjacency. PCEP for label mapping for node and adjacency. 1417 4106 payload 4106 payload 15
Net-Centric 2016 Static LSPs A TE-LSP is a series of cross connects and resource reservations Familiar in legacy transport networks Popular with MPLS-TP MPLS-TE without a control plane Order of programming is not important Can program in parallel Incoming interface/label Outgoing interface/label 16
Net-Centric 2016 Static LSPs PCEP as an SDN SBI PCEP allows an active PCE to install LSPs to be signalled in a TE network The cross-connects are indicated by the ERO An ERO can include label information (GMPLS) Incoming label and outgoing label LSPs can be short A single hop ERO can be just one cross-connect PCEP is already an SBI for static LSPs 17
Net-Centric 2016 SBI Thoughts The ERO approach is a little ugly It might trigger the signalling component to attempt to do work We haven t worked much on upstream interface for head-end LER in GMPLS or PCEP We could add to PCEP specifically for this function Not a lot of work 18
Net-Centric 2016 Claimed Benefit of Central Control and SBI Recovery from error cases can result in contention for resources and signalling storms Central control enables prioritisation and optimization Comparing apples with apples? Central control of signalled LSPs Central control with programming of cros-connects Contrast with single point of failure Consider congestion of management plane Convergence Time Number of LSPs 19 Source: Huawei Technologies IETF-94
Net-Centric 2016 LSP Stitching LSPs or pseudowires are stitched to form end-to-end services Stitching normally uses configuration Some signalling extensions exist This is an additional static LSP feature Adding cross connect LSPs to normal LSP setup ASBR3 ASBR1 CE2 CE1 PE2 PE1 ASBR2 ASBR4 Signaled LSP Signaled LSP Signaled LSP 20
Net-Centric 2016 Modifications to PCE and PCEP Adding traffic classification to an LSP message is just another Object We already have experience with BGP Flowspec Segment Routing paths are just hop-by-hop paths Each hop TLV needs to encode a SID Static LSPs are just very short LSPs with no signalling Should be possible to use existing messages and objects Could add specialist messages Bottom line Only tweaking PCEP not making radical changes 21
Net-Centric 2016 Competition with Other Protocols Need to be aware of other competing protocols OpenFlow is an SBI Netconf/RESTconf with YANG is ubiquitous BGP is used in many places Proprietary and legacy protocol usages XMPP, TL1, CLI, So why use PCEP? Should this talk be about a PCEP-based Centralized Controller ? It s a question still to be answered, but PCE is at the centre It makes sense to add these functions to PCE PCE talks PCEP Why would you want a PCE to talk multiple protocols? But PCE implementations may be configured using Netconf/YANG 22
Net-Centric 2016 Work in Progress The IETF is looking at this stuff TEAS working group Responsible for traffic Engineering Architecture draft-ietf-teas-pce-central-control An Architecture for Use of PCE and PCEP in a Network with Central Control PCE working group Responsible for extensions to PCEP draft-ietf-pce-segment-routing PCEP Extensions for Segment Routing draft-li-pce-pcep-flowspec PCEP Extension for Flow Specification 23
Net-Centric 2016 Questions adrian@olddog.co.uk 24
