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Path Computation Element (PCE) Architecture
(draft-ash-pce-architecture-01.txt)
Jerry Ash
AT&T
[email protected]
Adrian Farrell
Old Dog Consulting
[email protected]
JP Vasseur
Cisco Systems, Inc.
[email protected]
Outline
 PCE architecture summary
 provided in backup slides
 you read the draft
 I-D updates based on comments raised on list
 next step: working group draft
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PCE Architecture Summary
(see backup slides attached)
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terminology
assumptions
motivation for PCE architecture
PCE architectural considerations
security & confidentiality
PCE evaluation metrics
PCE architecture overview
 composite PCE
 external PCE
 multiple PCE path computation
 multiple PCE path computation with inter-PCE
communication
 architecture non-goal to specify protocols
 protocol extensions will be worked out in other IDs
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Composite PCE Node
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--------| Routing
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| | Protocol |
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TED
|<-+----------+->
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| Adjacent |
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PCE
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Node
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|Request |
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|Response|
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Service | |
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Request | |Signaling| | Protocol |
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------+->| Engine |<-+----------+->
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External PCE Node
---------| ----|
| | TED |<-+------------>
| ----| TED synchronization
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| mechanism (for example, routing protocol)
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v
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| | PCE | |
| ----|
---------^
| Request/
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Service ---------- Signaling
---------Request| Head-End | Protocol
| Adjacent |
---->| Node
|<---------->|
Node
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-------------------
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Multiple PCE Path Computation
----------
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PCE
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PCE
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----- |
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----- |
| | TED | |
| | TED | |
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----- |
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----- |
------------------^
^
| Request/
| Request/
| Response
| Response
v
v
Service ---------- Signaling
------------- Signaling -----------Request| Head-End | Protocol
|Intermediate | Protocol |Intermediate|
---->| Node
|<---------->|
Node
|<--------->|
Node
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---------------------------------
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Multiple PCE Path Computation
with Inter-PCE Communication
----------
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Inter-PCE Request/Response
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PCE
|<--------------------------------->|
PCE
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----- |
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| | TED | |
| | TED | |
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------------------^
| Request/
| Response
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Service ---------- Signaling
---------- Signaling
---------Request| Head-End | Protocol
| Adjacent | Protocol
| Adjacent |
---->| Node
|<---------->|
Node
|<---------->|
Node
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----------------------------
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I-D Updates Based on
Issues Raised on List
 PCE should advertise its capabilities, for example
 set of constraints it can account for (diversity, SRLGs, optical
impairments, wavelength continuity, etc.)
 text added to Section 6.4
 path computation request include if near-disjoint paths
acceptable
 text added to Section 6.6
 TED information can include info from sources other than IGP
(e.g. LSP routes, reserved bandwidth, measured traffic volume)
 needed to perform LSP re-optimization
 needed to reconfigure virtual network topology (VNT) lower
layer (e.g., optical) paths
 text added to Section 6.7
 elaborate on advantages of stateful PCE & pitfalls of using
stateful PCE in a distributed PCE environment
 text added to Section 6.8
 evaluation metrics should include TED synchronization speed &
impact on the data flows
 text added to Section 7
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I-D Updates Based on
Issues Raised on List
 identify areas for standardization
 added Section 5.5 “Areas for Standardization”
– based on PCE Charter
 other editorial changes
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Next Steps
 propose draft as PCE WG draft
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Backup Slides
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Terminology
 path computation element (PCE)
 entity (component, application or network node) capable of
computing a network path based on network graph & computational
constraints
 e.g., PCE computes path of a TE LSP by using TED &
bandwidth/other constraints
 path computation client (PCC)
 any client application requesting a path computation by the PCE
 domain
 any collection of network elements within a common sphere of
address management or path computational responsibility
 e.g., IGP areas, AS, multiple ASs within a SP network, multiple ASs
across multiple SP networks
 single PCE path computation: single PCE computes a path in a
domain
 multiple PCE path computation: multiple PCEs compute a path in a
domain
 centralized computation model: all paths in a domain computed by a
single, centralized PCE
 distributed computation model: computation of paths in a domain
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shared among multiple PCEs
Assumptions
 PCE may or may not be located at head-end
 e.g. nodes on path contribute to path computation (e.g., loose
hops) making them PCEs
 path computation may be made by PCE physically distinct from the
computed path
 path computed by PCE may be
 complete: full explicit path of strict hops
 partial: mix of strict & loose hops (may be an abstract node such as
an AS)
 PCE path computation can be used in conjunction with other path
computation models
 e.g., inter-AS TE LSP may be computed using PCE in some
domains but not others
 no assumptions made about PCE implementation
 e.g., could be implemented on a router, LSR, dedicated network
server, etc.
 PCE function independent of forwarding capability of node on which
it is implemente
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Motivation for PCE Architecture
 inter-area/AS optimal path computation (node has partial
visibility)
 computation of inter-area/AS diverse path (node has partial
visibility)
 CPU-intensive path computation/global optimization
 backup path computation for bandwidth protection with backup
capacity optimization
 multi-layer networks e.g. TDM network provides connectivity for
client-layer (IP, MPLS, L2, etc.)
 absence of TED or use of non-TE-enabled IGP
 node outside routing domain (e.g., CE to PE path computation)
 network element lacks control plan or routing capability
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PCE Architectural Considerations
 synchronization
 non-synchronized (e.g., PCE makes multiple individual path
computations to generate set of paths)
 synchronized (e.g., single PCE invokes computations by
other PCEs before supplying result to PCC
 PCE discovery & load balancing
 detecting PCE liveness
 PCC-PCE & PCE-PCE communication
 PCE TED synchronization
 stateful vs. stateless PCEs
 monitoring
 policy & confidentiality
 must preserve confidentiality across multiple SPs
 must ensure confidentiality & security of PCC-PCE & PCEPCE messages
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Security & Confidentiality
 PCC-PCE communication
 subject to "usual" security issues
 snooping not a significant issue
– might want to encrypt
 spoofing is very serious
– must offer strong authentication
– protocol is P2P so this is relatively easy
 DoS important because of 'centralized' nature of PCE
 PCE-PCE communication
 same as for PCC-PCE, but add confidentiality
 confidentiality (protection of domain topology information)
 use loose routes
 PCE encrypts ERO segments
– decrypt on entry to domain
 replace ERO segment with cookie
– entry point to domain consults local PCE using cookie to
retrieve next ERO segment
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PCE Evaluation Metrics
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optimality
scalability
load sharing
multiple path computation
reoptimization
path computation time
network stability
synchronization
 between TED & network topology/resource states
 speed of TED synchronization
 impact of synchronization on data flows
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