Applicability of Path Computation Element (PCE) for Abstraction and Control of TE Networks (ACTN)Huawei TechnologiesDivyashree Techno Park, WhitefieldBangaloreKarnataka560066Indiadhruv.ietf@gmail.comHuawei Technologies5340 Legacy Drive, Building 3PlanoTX75023USAleeyoung@huawei.comEricssonTorshamnsgatan,48StockholmSwedendaniele.ceccarelli@ericsson.com
Routing
PCE Working Group
Abstraction and Control of TE Networks (ACTN) refers to the set of virtual network (VN) operations needed to
orchestrate, control and manage large-scale multi-domain TE networks
so as to facilitate network programmability, automation, efficient
resource sharing, and end-to-end virtual service aware connectivity
and network function virtualization services.The Path Computation Element Communication Protocol (PCEP) provides
mechanisms for Path Computation Elements (PCEs) to perform path
computations in response to Path Computation Clients (PCCs) requests.This document examines the applicability of PCE to the ACTN framework. The Path Computation Element Communication Protocol (PCEP) provides
mechanisms for Path Computation Elements (PCEs) to perform path
computations in response to Path Computation Clients (PCCs) requests.The ability to compute shortest constrained TE LSPs in Multiprotocol
Label Switching (MPLS) and Generalized MPLS (GMPLS) networks across
multiple domains has been identified as a key motivation for PCE
development.A stateful PCE is capable of considering, for the purposes of
path computation, not only the network state in terms of links and
nodes (referred to as the Traffic Engineering Database or TED) but
also the status of active services (previously computed paths,
and currently reserved resources, stored in the Label Switched
Paths Database (LSP-DB). describes general considerations for
a stateful PCE deployment and examines its applicability and
benefits, as well as its challenges and limitations through a number
of use cases. describes a set of extensions to PCEP to
provide stateful control. A stateful PCE has access to not only the
information carried by the network's Interior Gateway Protocol (IGP),
but also the set of active paths and their reserved resources for its
computations. The additional state allows the PCE to compute
constrained paths while considering individual LSPs and their
interactions. describes the setup,
maintenance and teardown of PCE-initiated LSPs under the stateful PCE
model. also describes the active stateful PCE.
The active PCE functionality allows a PCE to reroute an existing
LSP or make changes to the attributes of an existing LSP, or a PCC to delegate
control of specific LSPs to a new PCE.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. It is concluded in , that this is the same function that a PCE
might offer in a network operated using a dynamic control plane.
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 Application-Based Network Operation (ABNO) .
Computing paths across large multi-domain environments
require special computational components and cooperation between
entities in different domains capable of complex path computation.
The PCE provides an architecture and a set of functional components
to address this problem space. A PCE may be used to compute
end-to-end paths across multi-domain
environments using a per-domain path computation technique .
The Backward recursive PCE based path computation (BRPC) mechanism
defines a PCE-based path computation procedure to compute
inter-domain constrained MPLS and
GMPLS TE networks. However,
both per-domain and BRPC techniques assume that the sequence of
domains to be crossed from source to destination is known, either
fixed by the network operator or obtained by other means. describes a Hierarchical PCE (H-PCE)
architecture which can be used for computing end-to-end paths for
inter-domain MPLS Traffic Engineering (TE) and GMPLS Label Switched
Paths (LSPs) when the domain sequence is not known.
Within the Hierarchical PCE (H-PCE) architecture,
the Parent PCE (P-PCE) is used to compute a multi-domain
path based on the domain connectivity information. A Child PCE
(C-PCE) may be responsible for a single domain or multiple domains,
it is used to compute the intra-domain path based on its domain
topology information. state the considerations for stateful PCE(s) in
hierarchical PCE architecture. In particular, the behavior changes
and additions to the existing stateful PCE mechanisms (including PCE-
initiated LSP setup and active PCE usage) in the context of networks
using the H-PCE architecture. describes a framework for applying the PCE-based
architecture to inter-layer to (G)MPLS TE. It provides
suggestions for the deployment of PCE in support of multi-layer
networks. It also describes the
relationship between PCE and a functional component in charge of the
control and management of the Virtual Network Topology (VNT) , called the VNT Manager (VNTM). introduces the architecture for PCE as a central
controller (PCECC), it further examines the motivations and applicability for PCEP as a
southbound interface, and introduces the implications for the
protocol. The section 2.1.3 of
describe an hierarchy of PCE-based controller as per the Hierarchy of PCE
framework defined in . describes the high-level
ACTN requirements.
describes the architecture
model for ACTN including the entities (Customer Network
Controller(CNC), Multi-domain Service Coordinator(MDSC), and Provisioning Network Controller (PNC) and their interfaces.The ACTN reference architecture identified a three-tier control
hierarchy as depicted in :The two interfaces with respect to the MDSC, one north of the MDSC
(CMI CNC-MDSC Interface) and one south (MPI MDSC-PNC Interface). A hierarchy of MDSC is
possible with a recursive MPI interface. provides an information model
for ACTN interfaces.This document examines the PCE and ACTN architecture and describes how the PCE architecture
is applicable to ACTN. It also lists the PCEP extensions that are needed to use PCEP as an ACTN interface.
This document also identifies any gaps in PCEP, that exist at the time of publication of this document.Further, ACTN, Stateful H-PCE, and PCECC are based on the same basic hierarchy
framework and thus compatible with each other.ACTN architecture
is based on hierarchy and recursiveness of controllers. It defines
three types of controllers (depending on the functionalities they implement).
The main functionalities are -
Multi domain coordinationAbstractionCustomer mapping/translationVirtual service coordinationSection 3 of describes these functions.It should be noted that, this document lists all possible ways in which PCEP could be used
for each of the above functions, but all functions are not required to be implemented via PCEP. Operator
may choose to use the PCEP for multi domain coordination via stateful H-PCE
but use RESTCONF or BGP-LS to get the topology and support abstraction function.
With the definition of domain being "everything that is under the control of the single logical
controller", as per ,
it is needed to have a control entity that oversees
the specific aspects of the different domains and to build a
single abstracted end-to-end network topology in order to
coordinate end-to-end path computation and path/service
provisioning.
The MDSC in ACTN framework realizes this function by coordinating
the per-domain PNCs in a hierarchy of controllers. It also needs to detach
from the underlying network technology and express customer concerns by
business needs. and
describes a hierarchy of PCE with Parent PCE coordinating multi-domain path
computation function between Child PCE(s). It is easy to see how these
principles align, and thus how stateful H-PCE architecture can be used to realize ACTN.The Per domain stitched LSP in the Hierarchical stateful PCE architecture,
described in Section 3.3.1 of
is well suited for multi-domain coordination function. This includes domain sequence
selection; E2E path computation; Controller (PCE) initiated path setup and reporting.
This is also applicable to multi-layer coordination in case of IP+optical networks.
" describes the procedures to allow a
stateful communication between PCEs for various use-cases. The
procedures and extensions are also applicable to Child and
Parent PCE communication and thus useful for ACTN as well.To realize ACTN, an
abstracted view of the underlying network resources needs to be built.
This includes global network-wide abstracted topology based
on the underlying network resources of each domain. This also
include abstract topology created as per the customer service
connectivity requests and represented as a network slice allocated
to each customer. In order to compute and provide optimal paths, PCEs
require an accurate and timely Traffic Engineering
Database (TED). Traditionally this TED has been obtained from a link
state (LS) routing protocol supporting traffic engineering
extensions. PCE may construct its TED by participating in the
IGP ( and for MPLS-TE; and
for GMPLS). An alternative is offered by BGP-LS .In case of H-PCE , the parent PCE needs to
build the domain topology map of the child domains and
their interconnectivity. and
suggest that BGP-LS could be used as a "northbound" TE advertisement from
the child PCE to the parent PCE. proposes another approaches for learning and maintaining
the Link-State and TE information as an alternative to IGPs and BGP flooding, using PCEP itself.
The child PCE can use this mechanism to transport Link-State and
TE information from child PCE to a Parent PCE using PCEP.
In ACTN, there is a need to control the level of abstraction based on
the deployment scenario and business relationship between the controllers.
The mechanism used to disseminate information from PNC (child PCE) to
MDSC (parent PCE) should support abstraction.
describes a few
alternative approaches of abstraction. The resulting abstracted topology
can be encoded using the PCEP-LS mechanisms
and its optical network extension .
PCEP-LS is an attractive option when the operator
would wish to have a single control plane protocol (PCEP) to
achieve ACTN functions. discusses two ways to build abstract topology from an MDSC standpoint
with interaction with PNCs. The primary method is called automatic generation of abstract topology by configuration. With this
method, automatic generation is based on the abstraction/summarization of
the whole domain by the PNC and its advertisement on the MPI. The secondary method is called
on-demand generation of supplementary topology via Path Compute
Request/Reply. This method may be needed to obtain further complementary information such as
potential connectivity from child PCEs in order to facilitate an end-to-end path provisioning.
PCEP is well suited to support both methods.In ACTN, there is a need to map customer virtual network (VN) requirements into
network provisioning request to the PNC. That is, the
customer requests/commands are mapped into network provisioning requests
that can be sent to the PNC. Specifically, it provides mapping and
translation of a customer's service request into a set of
parameters that are specific to a network type and technology
such that network configuration process is made possible. describes the setup, maintenance and
teardown of PCE-initiated LSPs under the stateful PCE model, without
the need for local configuration on the PCC, thus allowing for a
dynamic network that is centrally controlled and deployed. To
instantiate or delete an LSP, the PCE sends the Path Computation LSP
Initiate Request (PCInitiate) message to the PCC. As described in
, for inter-domain LSP in
Hierarchical PCE architecture, the initiation
operations can be carried out at the parent PCE. In which case after
parent PCE finishes the E2E path computation, it can send the
PCInitiate message to the child PCE, the child PCE
further propagates the initiate request to the LSR. The customer request
is received by the MDSC (parent PCE) and based on the business logic,
global abstracted topology, network conditions and local policy, the
MDSC (parent PCE) translates this into per domain LSP initiation request that a
PNC (child PCE) can understand and act on. This can be done via the PCInitiate message.PCEP extensions for associating opaque policy between PCEP peer
can be used.Virtual service coordination
function in ACTN incorporates customer service-related
information into the virtual network service operations in order to
seamlessly operate virtual networks while meeting customer's
service requirements. describes the
need for associating a set of LSPs with a VN "construct" to
facilitate VN operations in PCE architecture. This association
allows the PCEs to identify which LSPs belong to a certain VN. This association based on VN is useful for various optimizations
at the VN level which can be applied to all the LSPs that
are part of the VN slice. During path computation, the impact of a path for an LSP
is compared against the paths of other LSPs in the VN. This is to make sure
that the overall optimization and SLA of the VN rather than of a single LSP.
Similarly, during re-optimization, advanced path computation algorithm
and optimization
technique can be considered for all the LSPs belonging to a VN/customer
and optimize them all together.
As per ,
to allow virtualization and multi domain coordination, the network
has to provide open, programmable interfaces, in which customer
applications can create, replace and modify virtual network
resources and services in an interactive, flexible and dynamic
fashion while having no impact on other customers. The two ACTN
interfaces are -
The CNC-MDSC Interface (CMI) is an interface
between a Customer Network Controller and a Multi Domain
Service Coordinator. It requests the creation of the network
resources, topology or services for the applications. The
MDSC may also report potential network
topology availability if queried for current capability from
the Customer Network Controller.The MDSC-PNC Interface (MPI) is an interface
between a Multi Domain Service Coordinator and a Provisioning
Network Controller. It communicates the creation request, if
required, of new connectivity of bandwidth changes in the
physical network, via the PNC. In multi-domain environments,
the MDSC needs to establish multiple MPIs, one for each PNC, as
there are multiple PNCs responsible for its domain control.In case of hierarchy of MDSC, the MPI is applied recursively.
From an abstraction point of view, the top level MDSC which
interfaces the CNC operates on a higher level of abstraction (i.e.,
less granular level) than the lower level MSDCs.PCEP is especially suitable on the MPI as it meets the requirement
and the functions as set out in the ACTN framework
. Its recursive nature is well suited
via the multi-level hierarchy of PCE. PCEP can also be applied to the CMI as the
CNC can be a path computation client while the MDSC can be a path
computation server. The describe how PCE and PCEP could help
realize ACTN on the MPI.As per the example in the , there are 4 domains,
each with its own PNC and a MDSC at top. The PNC and MDSC need PCE as
a important function. The PNC (or child PCE) already uses PCEP to communicate to
the network device. It can utilize the PCEP as the MPI to communicate
between controllers too.Building Domain Topology at MDSC: PNC (or child PCE) needs to have the TED to compute path in its domain.
As described in , it can learn the topology
via IGP or BGP-LS. PCEP-LS is also a proposed mechanism to carry link state and
traffic engineering information within PCEP. A mechanism to carry abstracted
topology while hiding technology specific information between PNC and MDSC is
described in . At the end of this
step the MDSC (or parent PCE) has the abstracted topology from each of its PNC
(or child PCE). This could be as simple as a domain topology map as described
in or it can have full topology information of all
domains. The latter is not scalable and thus an abstracted topology of each domain
interconnected by inter-domain links is the most common case.
Topology Change: When the PNC learns of any topology change, the PNC needs
to decide if the change needs to be notified to the MDSC. This is dependent on
the level of abstraction between the MDSC and the PNC.VN Instantiate: MDSC is requested to instantiate a VN, the minimal information
that is required would be a VN identifier and a set of end points. Various path
computation, setup constraints and objective functions may also be provided.
In PCE terms, a VN Instantiate can be considered as a set of paths belonging to the same VN.
As described in and
the VN association can help in identifying the set of paths that belong to a VN.
The rest of the information like the endpoints, constraints and objective function (OF) is
already defined in PCEP in terms of a single path.
Path Computation: As per the example in the ,
the VN instantiate requires two end to end paths between (A in Domain 1 to B in Domain 3)
and (A in Domain 1 to C in Domain 4). The MDSC (or parent PCE) triggers the
end to end path computation for these two paths. MDSC can do path computation based on the abstracted domain topology that
it already has or it may use the H-PCE procedures () using the PCReq and PCRep messages to get the
end to end path with the help of the child PCEs (PNC).
Either way, the resulted E2E paths may be broken into per-domain paths.A-B: (A-B13,B13-B31,B31-B) A-C: (A-B13,B13-B31,B34-B43,B43-C)Per Domain Path Instantiation: Based on the above path computation, MDSC
can issue the path instantiation request to each PNC via PCInitiate message (see
and ).
A suitable
stitching mechanism would be used to stitch these per domain LSPs.
One such mechanism is described in , where
PCEP is extended to support stitching in stateful H-PCE context.
Per Domain Path Report: Each PNC should report the status of
the per-domain LSP to the MDSC via PCRpt message, as per the Hierarchy of stateful
PCE (). The status
of the end to end LSP (A-B and A-C) is made up when all the per domain
LSP are reported up by the PNCs.Delegation: It is suggested that the per domain LSPs are delegated to
respective PNC, so that they can control the path and attributes based
on each domain network conditions.State Synchronization: The state needs to be synchronized between the parent PCE
and child PCE. The mechanism described in
can be used.VN Modify: MDSC is requested to modify a VN, for example the bandwidth for
VN is increased. This may trigger path computation at MDSC as described in the
previous step and can trigger an update to existing per-intra-domain path (via PCUpd message)
or creation (or deletion) of a per-domain path (via PCInitiate message). As described in , this should be done
in make-before-break fashion.VN Delete: MDSC is requested to delete a VN, in this case, based on the E2E paths and
the resulting per-domain paths need to be removed (via PCInitiate message).VN Update (based on network changes): Any change in the per-domain LSP
are reported to the MDSC (via PCRpt message) as per .
This may result in changes in the E2E path or VN status. This may also trigger a re-optimization
leading to a new per-domain path, update to existing path, or deletion of the path.VN Protection: The VN protection/restoration requirements, need to applied to each E2E path
as well as each per domain path. The MDSC needs to play a crucial role in coordinating the
right protection/restoration policy across each PNC. The existing protection/restoration mechanism
of PCEP can be applied on each path. In case PNC generates an abstract topology to the MDSC, the PCInitiate/PCUpd messages from the
MDSC to a PNC will contain a path with abstract nodes and links. PNC would need to take that as an
input for path computation to get a path with physical nodes and links. Similarly PNC would convert
the path received from the device (with physical nodes and links) into abstract path (based on the
abstract topology generated before with abstract nodes and links) and reported to the MDSC.This is an informational document and thus does not have any
IANA allocations to be made. The ACTN framework described in defines key components
and interfaces for managed traffic engineered networks. It also list various security considerations
such as
request and control of resources, confidentially of the information,
and availability of function which should be taken into consideration. When PCEP is used on the MPI, this interface needs to be secured, use of is
RECOMENDED. Each PCEP extension listed in this document, presents its individual security considerations, which continue to apply.The authors would like to thank Jonathan Hardwick for the inspiration behind this document.
Further thanks to Avantika for her comments with suggested text.