1. Adrian Farrel : Old Dog Consulting
Net-Centric 2016
The Path Computation Element as a Centralized Controller in advanced SDN Operations
Adrian Farrel : Old Dog Consulting

2. 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?

3. 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

4. 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
NMS
PCE server
LSR
TED
PCC
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

5. 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

6. 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

7. Some Popular SDN Architectures
PCE
TED
Orchestrator
Application/Service Requester
Controller
Lots of pictures
Most show some form of path computation being performed
Applications
Application Service Coordinator
PCE
ABNO Controller
Choices
I2RS
OAM
Handler
Provisioning Manager
Choices
Controllers
PCEP

8. 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

9. 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

10. 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 e)
Segment Routing
Service Function Chaining
PCEP as an SDN Southbound Interface – Static LSPs
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 e)
Segment Routing
Service Function Chaining
PCEP as an SDN Southbound Interface – Static LSPs
LSP stitching as a hybrid use case

11. 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

12. 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

13. 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”

14. Segment Routing Path Programming and Classification
Segment routing is a form of source routing
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
1st hop
2nd hop
3rd hop
nth hop
Payload header
Payload

15. 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 traffic classification and installing label stack
PCEP for label mapping for node and adjacency.
1417
4106
payload
4106
payload

16. 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

17. 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

18. 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

19. 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
Source: Huawei Technologies – IETF-94

20. 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
ASBR1
ASBR3
CE2
CE1
PE2
PE1
ASBR2
ASBR4
Signaled LSP
Signaled LSP
Signaled LSP

21. 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

22. 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

23. 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

24. Questions adrian@olddog.co.uk