Internet Engineering Task Force (IETF) D. King
Request for Comments: 8694
Old Dog Consulting
Category: Informational 郑好棉 (H. Zheng)
ISSN: 2070-1721 华为技术有限公司 (Huawei Technologies)
Applicability of the Path Computation Element to Inter-area and Inter-AS
MPLS and GMPLS Traffic Engineering
The Path Computation Element (PCE) may be used for computing services
that traverse multi-area and multi-Autonomous System (multi-AS)
Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS)
Traffic-Engineered (TE) networks.
This document examines the applicability of the PCE architecture,
protocols, and protocol extensions for computing multi-area and
multi-AS paths in MPLS and GMPLS networks.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8694
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents 1.
Path Computation 1.2.1.
PCE-Based Path Computation Procedure 1.3.
Traffic Engineering Aggregation and Abstraction 1.4.
Traffic-Engineered Label Switched Paths 1.5.
Inter-area and Inter-AS-capable PCE Discovery 1.6.
Objective Functions 2.
Issues and Considerations 3.1.
Destination Location 3.3.
Domain Confidentiality 4.
Domain Topologies 4.1.
Selecting Domain Paths 4.2.
Domain Sizes 4.3.
Domain Diversity 4.4.
Synchronized Path Computations 4.5.
Domain Inclusion or Exclusion 5.
Applicability of the PCE to Inter-area Traffic Engineering 5.1.
Inter-area Routing 5.1.1.
Area Inclusion and Exclusion 5.1.2.
Strict Explicit Path and Loose Path 5.1.3.
Inter-Area Diverse Path Computation 6.
Applicability of the PCE to Inter-AS Traffic Engineering 6.1.
Inter-AS Routing 6.1.1.
AS Inclusion and Exclusion 6.2.
Inter-AS Bandwidth Guarantees 6.3.
Inter-AS Recovery 6.4.
Inter-AS PCE Peering Policies 7.
Multi-domain PCE Deployment Options 7.1.
Traffic Engineering Database and Synchronization 7.1.1.
Applicability of BGP-LS to PCE 7.2.
Pre-planning and Management-Based Solutions 8.
Domain Confidentiality 8.1.
Loose Hops 8.2.
Confidential Path Segments and Path-Keys 9.
Point to Multipoint 10.
Optical Domains 10.1.
Abstraction and Control of TE Networks (ACTN) 11.
Manageability Considerations 12.1.
Control of Function and Policy 12.2.
Information and Data Models 12.3.
Liveness Detection and Monitoring 12.4.
Verifying Correct Operation 12.5.
Impact on Network Operation 13.
Security Considerations 13.1.
Multi-domain Security 14.
IANA Considerations 15.
Normative References 15.2.
Computing paths across large multi-domain environments may require
special computational components and cooperation between entities in
different domains capable of complex path computation.
Issues that may exist when routing in multi-domain networks include
* There is often a lack of full topology and TE information across
* No single node has the full visibility to determine an optimal or
even feasible end-to-end path across domains.
* Knowing how to evaluate and select the exit point and next domain
boundary from a domain.
* Understanding how the ingress node determines which domains should
be used for the end-to-end path.
An information exchange across multiple domains is often limited due
to the lack of trust relationship, security issues, or scalability
issues, even if there is a trust relationship between domains.
The Path Computation Element (PCE) [RFC4655
] provides an architecture
and a set of functional components to address the problem space and
the issues highlighted above.
A PCE may be used to compute end-to-end paths across multi-domain
environments using a per-domain path computation technique [RFC5152
The so-called backward recursive PCE-based computation (BRPC)
] defines a path computation procedure to compute
inter-domain constrained Multiprotocol Label Switching (MPLS) and
Generalized MPLS (GMPLS) Traffic-Engineered (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.
In more advanced deployments (including multi-area and multi-
Autonomous System (multi-AS) environments), the sequence of domains
may not be known in advance, and the choice of domains in the end-to-
end domain sequence might be critical to the determination of an
optimal end-to-end path. In this case, the use of the hierarchical
] architecture and mechanisms may be used to discover the
intra-area path and select the optimal end-to-end domain sequence.
This document describes the processes and procedures available when
using the PCE architecture and protocols for computing inter-area and
inter-AS MPLS and GMPLS Traffic-Engineered paths.
The scope of this document does not include discussions of deployment
scenarios for stateful PCE, active PCE, remotely initiated PCE, or
PCE as a central controller (PCECC).
Generally, a domain can be defined as a separate administrative,
geographic, or switching environment within the network. A domain
may be further defined as a zone of routing or computational ability.
Under these definitions, a domain might be categorized as an
Autonomous System (AS) or an Interior Gateway Protocol (IGP) area (as
] and [RFC4655
For the purposes of this document, a domain is considered to be a
collection of network elements within an area or AS that has a common
sphere of address management or path computational responsibility.
Wholly or partially overlapping domains are not within the scope of
In the context of GMPLS, a particularly important example of a domain
is the Automatically Switched Optical Network (ASON) subnetwork
[G-8080]. In this case, computation of an end-to-end path requires
the selection of nodes and links within a parent domain where some
nodes may, in fact, be subnetworks. Furthermore, a domain might be
an ASON routing area [G-7715]. A PCE may perform the path
computation function of an ASON Routing Controller as described in
It is assumed that the PCE architecture is not applied to a large
group of domains, such as the Internet.
1.2. Path Computation
For the purpose of this document, it is assumed that path computation
is the sole responsibility of the PCE as per the architecture defined
]. When a path is required, the Path Computation Client
(PCC) will send a request to the PCE. The PCE will apply the
required constraints, compute a path, and return a response to the
PCC. In the context of this document, it may be necessary for the
PCE to cooperate with other PCEs in adjacent domains (as per BRPC
]) or with a parent PCE (as per [RFC6805
It is entirely feasible that an operator could compute a path across
multiple domains without the use of a PCE if the relevant domain
information is available to the network planner or network management
platform. The definition of what relevant information is required to
perform this network planning operation and how that information is
discovered and applied is outside the scope of this document.
1.2.1. PCE-Based Path Computation Procedure
As highlighted, the PCE is an entity capable of computing an inter-
domain TE path upon receiving a request from a PCC. There could be a
single PCE per domain or a single PCE responsible for all domains. A
PCE may or may not reside on the same node as the requesting PCC. A
path may be computed by either a single PCE node or a set of
distributed PCE nodes that collaborate during path computation.
According to [RFC4655
], a PCC should send a path computation request
to a particular PCE using [RFC5440
] (PCC-to-PCE communication). This
negates the need to broadcast a request to all the PCEs. Each PCC
can maintain information about the computation capabilities of the
PCEs it is aware of. The PCC-PCE capability awareness can be
configured using static configurations or by automatic and dynamic
PCE discovery procedures.
If a network path is required, the PCC will send a path computation
request to the PCE. A PCE may then compute the end-to-end path if it
is aware of the topology and TE information required to compute the
entire path. If the PCE is unable to compute the entire path, the
PCE architecture provides cooperative PCE mechanisms for the
resolution of path computation requests when an individual PCE does
not have sufficient TE visibility.
End-to-end path segments may be kept confidential through the
application of Path-Keys to protect partial or full path information.
A Path-Key is a token that replaces a path segment in an explicit
route. The Path-Key mechanism is described in [RFC5520
1.3. Traffic Engineering Aggregation and Abstraction
Networks are often constructed from multiple areas or ASes that are
interconnected via multiple interconnect points. To maintain network
confidentiality and scalability, the TE properties of each area and
AS are not generally advertised outside each specific area or AS.
TE aggregation or abstraction provide a mechanism to hide information
but may cause failed path setups or the selection of suboptimal end-
to-end paths [RFC4726
]. The aggregation process may also have
significant scaling issues for networks with many possible routes and
multiple TE metrics. Flooding TE information breaks confidentiality
and does not scale in the routing protocol.
The PCE architecture and associated mechanisms provide a solution to
avoid the use of TE aggregation and abstraction.
1.4. Traffic-Engineered Label Switched Paths
This document highlights the PCE techniques and mechanisms that exist
for establishing TE packet and optical Label Switched Paths (LSPs)
across multiple areas (inter-area TE LSP) and ASes (inter-AS TE LSP).
In this context and within the remainder of this document, we
consider all LSPs to be constraint based and traffic engineered.
Three signaling options are defined for setting up an inter-area or
inter-AS LSP [RFC4726
* Contiguous LSP
* Stitched LSP
* Nested LSP
All three signaling methods are applicable to the architectures and
procedures discussed in this document.
1.5. Inter-area and Inter-AS-capable PCE Discovery
When using a PCE-based approach for inter-area and inter-AS path
computation, a PCE in one area or AS may need to learn information
related to inter-AS-capable PCEs located in other ASes. The PCE
discovery mechanism defined in [RFC5088
] and [RFC5089
the discovery of PCEs and disclosure of information related to inter-
area and inter-AS-capable PCEs.
1.6. Objective Functions
An Objective Function (OF) [RFC5541
] or a set of OFs specifies the
intentions of the path computation and so defines the "optimality" in
the context of the computation request.
An OF specifies the desired outcome of a computation. It does not
describe or specify the algorithm to use. Also, an implementation
may apply any algorithm or set of algorithms to achieve the result
indicated by the OF. A number of general OFs are specified in
Various OFs may be included in the PCE computation request to satisfy
the policies encoded or configured at the PCC, and a PCE may be
subject to policy in determining whether it meets the OFs included in
the computation request or whether it applies its own OFs.
During inter-domain path computation, the selection of a domain
sequence, the computation of each (per-domain) path fragment, and the
determination of the end-to-end path may each be subject to different
OFs and policies.
This document also uses the terminology defined in [RFC4655
]. Additional terminology is defined below:
ABR: IGP Area Border Router -- a router that is attached to more
than one IGP area.
ASBR: Autonomous System Border Router -- a router used to connect
together ASes of a different or the same Service Provider via
one or more inter-AS links.
Inter-area TE LSP: A TE LSP whose path transits through two or more
Inter-AS MPLS TE LSP: A TE LSP whose path transits through two or
more ASes or sub-ASes (BGP confederations)
SRLG: Shared Risk Link Group.
TED: Traffic Engineering Database, which contains the topology and
resource information of the domain. The TED may be fed by
Interior Gateway Protocol (IGP) extensions or potentially by
3. Issues and Considerations
Networks constructed from multi-areas or multi-AS environments may
have multiple interconnect points (multihoming). End-to-end path
computations may need to use different interconnect points to avoid a
single-point failure disrupting both the primary and backup services.
3.2. Destination Location
A PCC asking for an inter-domain path computation is typically aware
of the identity of the destination node. If the PCC is aware of the
destination domain, it may supply the destination domain information
as part of the path computation request. However, if the PCC does
not know the destination domain, this information must be determined
by another method.
3.3. Domain Confidentiality
When the end-to-end path crosses multiple domains, it may be possible
that each domain (AS or area) is administered by separate Service
Providers. Thus, if a PCE supplies a path segment to a PCE in
another domain, it may break confidentiality rules and could disclose
AS-internal topology information.
If confidentiality is required between domains (ASes and areas)
belonging to different Service Providers, then cooperating PCEs
cannot exchange path segments; otherwise, the receiving PCE or PCC
will be able to see the individual hops through another domain.
This topic is discussed further in Section 8
of this document.
4. Domain Topologies
Constraint-based inter-domain path computation is a fundamental
requirement for operating traffic-engineered MPLS [RFC3209
] and GMPLS
] networks in inter-area and inter-AS (multi-domain)
environments. Path computation across multi-domain networks is
complex and requires computational cooperational entities like the
4.1. Selecting Domain Paths
Where the sequence of domains is known a priori, various techniques
can be employed to derive an optimal multi-domain path. If the
domains are connected to a simple path with no branches and single
links between all domains or if the preferred points of
interconnection are also known, the per-domain path computation
] technique may be used. Where there are multiple
connections between domains and there is no preference for the choice
of points of interconnection, BRPC [RFC5441
] can be used to derive an
When the sequence of domains is not known in advance or the end-to-
end path will have to navigate a mesh of small domains (especially
typical in optical networks), the optimum path may be derived through
the application of a hierarchical PCE [RFC6805
4.2. Domain Sizes
Very frequently, network domains are composed of dozens or hundreds
of network elements. These network elements are usually
interconnected in a partial-mesh fashion to provide survivability
against dual failures and to benefit from the traffic-engineering
capabilities of MPLS and GMPLS protocols. Network operator feedback
in the development of the document highlighted that the node degree
(the number of neighbors per node) typically ranges from 3 to 10 (4-5
is quite common).
4.3. Domain Diversity
Domain and path diversity may also be required when computing end-to-
end paths. Domain diversity should facilitate the selection of paths
that share ingress and egress domains but do not share transit
domains. Therefore, there must be a method allowing the inclusion or
exclusion of specific domains when computing end-to-end paths.
4.4. Synchronized Path Computations
In some scenarios, it would be beneficial for the operator to rely on
the capability of the PCE to perform synchronized path computation.
Synchronized path computations, known as Synchronization VECtors
(SVECs), are used for dependent path computations. SVECs are defined
], and [RFC6007
] provides an overview of the use of the
PCE SVEC list for synchronized path computations when computing
In hierarchical PCE (H-PCE) deployments, a child PCE will be able to
request both dependent and synchronized domain-diverse end-to-end
paths from its parent PCE.
4.5. Domain Inclusion or Exclusion
A domain sequence is an ordered sequence of domains traversed to
reach the destination domain. A domain sequence may be supplied
during path computation to guide the PCEs or are derived via the use
of hierarchical PCE (H-PCE).
During multi-domain path computation, a PCC may request specific
domains to be included or excluded in the domain sequence using the
Include Route Object (IRO) [RFC5440
] and Exclude Route Object (XRO)
]. The use of Autonomous Number (AS) as an abstract node
representing a domain is defined in [RFC3209
new subobjects to include or exclude domains such as an IGP area or a
4-byte AS number.
An operator may also need to avoid a path that uses specified nodes
for administrative reasons. If a specific connectivity service is
required to have a 1+1 protection capability, two separate disjoint
paths must be established. A mechanism known as Shared Risk Link
Group (SRLG) information may be used to ensure path diversity.
5. Applicability of the PCE to Inter-area Traffic Engineering
As networks increase in size and complexity, it may be required to
introduce scaling methods to reduce the amount of information flooded
within the network and make the network more manageable. An IGP
hierarchy is designed to improve IGP scalability by dividing the IGP
domain into areas and limiting the flooding scope of topology
information to within area boundaries. This restricts visibility of
the area to routers in a single area. If a router needs to compute
the route to a destination located in another area, a method would be
required to compute a path across area boundaries.
In order to support multiple vendors in a network in cases where data
or control-plane technologies cannot interoperate, it is useful to
divide the network into vendor domains. Each vendor domain is an IGP
area, and the flooding scope of the topology (as well as any other
relevant information) is limited to the area boundaries.
Per-domain path computation [RFC5152
] exists to provide a method of
inter-area path computation. The per-domain solution is based on
loose hop routing with an Explicit Route Object (ERO) expansion on
each Area Border Router (ABR). This allows an LSP to be established
using a constrained path. However, at least two issues exist:
* This method does not guarantee an optimal constrained path.
* The method may require several crankback signaling messages, as
], increasing signaling traffic and delaying the LSP
PCE-based architecture [RFC4655
] is designed to solve inter-area path
computation problems. The issue of limited topology visibility is
resolved by introducing path computation entities that are able to
cooperate in order to establish LSPs with the source and destinations
located in different areas.
5.1. Inter-area Routing
An inter-area TE-LSP is an LSP that transits through at least two IGP
areas. In a multi-area network, topology visibility remains local to
a given area for scaling and privacy purposes. A node in one area
will not be able to compute an end-to-end path across multiple areas
without the use of a PCE.
5.1.1. Area Inclusion and Exclusion
The BRPC method [RFC5441
] of path computation provides a more optimal
method to specify inclusion or exclusion of an ABR. Using the BRPC
procedure, an end-to-end path is recursively computed in reverse from
the destination domain towards the source domain. Using this method,
an operator might decide if an area must be included or excluded from
the inter-area path computation.
5.1.2. Strict Explicit Path and Loose Path
A strict explicit path is defined as a set of strict hops, while a
loose path is defined as a set of at least one loose hop and zero or
more strict hops. It may be useful to indicate whether a strict
explicit path is required during the path computation request. An
inter-area path may be strictly explicit or loose (e.g., a list of
ABRs as loose hops).
A PCC request to a PCE does allow indication of whether a strict
explicit path across specific areas ([RFC7897
]) is required or
desired or whether the path request is loose.
5.1.3. Inter-Area Diverse Path Computation
It may be necessary to compute a path that is partially or entirely
diverse from a previously computed path to avoid fate sharing of a
primary service with a corresponding backup service. There are
various levels of diversity in the context of an inter-area network:
* Per-area diversity (the intra-area path segments are a link, node,
or SRLG disjoint).
* Inter-area diversity (the end-to-end inter-area paths are a link,
node, or SRLG disjoint).
Note that two paths may be disjointed in the backbone area but non-
disjointed in peripheral areas. Also, two paths may be node
disjointed within areas but may share ABRs, in which case path
segments within an area are node disjointed but end-to-end paths are
not node disjointed. Per-domain [RFC5152
], BRPC [RFC5441
], and H-PCE
] mechanisms all support the capability to compute diverse
paths across multi-area topologies.
6. Applicability of the PCE to Inter-AS Traffic Engineering
As discussed in Section 5
(Applicability of the PCE to Inter-area
Traffic Engineering), it is necessary to divide the network into
smaller administrative domains, or ASes. If an LSR within an AS
needs to compute a path across an AS boundary, it must also use an
inter-AS computation technique. [RFC5152
] defines mechanisms for the
computation of inter-domain TE LSPs using network elements along the
signaling paths to compute per-domain constrained path segments.
The PCE was designed to be capable of computing MPLS and GMPLS paths
across AS boundaries. This section outlines the features of a PCE-
enabled solution for computing inter-AS paths.
6.1. Inter-AS Routing
6.1.1. AS Inclusion and Exclusion
] allows the specification of AS or ASBR inclusion or
exclusion. Using this method, an operator might decide whether an AS
must be included or excluded from the inter-AS path computation.
Exclusion and/or inclusion could also be specified at any step in the
LSP path computation process by a PCE (within the BRPC algorithm),
but the best practice would be to specify them at the edge. In
opposition to the strict and loose path, AS inclusion or exclusion
doesn't impose topology disclosure as ASes and their interconnection
are public entities.
6.2. Inter-AS Bandwidth Guarantees
Many operators with multi-AS domains will have deployed the MPLS-TE
Diffserv either across their entire network or at the domain edges on
CE-PE links. In situations where strict QoS bounds are required,
admission control inside the network may also be required.
When the propagation delay can be bounded, the performance targets,
such as maximum one-way transit delay, may be guaranteed by providing
bandwidth guarantees along the Diffserv-enabled path. These
requirements are described in [RFC4216
One typical example of the requirements in [RFC4216
] is to provide
bandwidth guarantees over an end-to-end path for VoIP traffic
classified as an EF (Expedited Forwarding) class in a Diffserv-
enabled network. In cases where the EF path is extended across
multiple ASes, an inter-AS bandwidth guarantee would be required.
Another case for an inter-AS bandwidth guarantee is the requirement
to guarantee a certain amount of transit bandwidth across one or
6.3. Inter-AS Recovery
During a path computation process, a PCC request may contain the
requirement to compute a backup LSP for protecting the primary LSP,
such as 1+1 protection. A single LSP or multiple backup LSPs may
also be used for a group of primary LSPs; this is typically known as
Other inter-AS recovery mechanisms include [RFC4090
], which adds Fast
Reroute (FRR) protection to an LSP. So, the PCE could be used to
trigger computation of backup tunnels in order to protect inter-AS
Inter-AS recovery clearly requires backup LSPs for service
protection, but it would also be advisable to have multiple PCEs
deployed for path computation redundancy, especially for service
restoration in the event of catastrophic network failure.
6.4. Inter-AS PCE Peering Policies
Like BGP peering policies, inter-AS PCE peering policies are required
for an operator. In an inter-AS BRPC process, the PCE must cooperate
in order to compute the end-to-end LSP. Therefore, the AS path must
not only follow technical constraints, e.g., bandwidth availability,
but also the policies defined by the operator.
Typically, PCE interconnections at an AS level must follow the agreed
contract obligations, also known as peering agreements. The PCE
peering policies are the result of the contract negotiation and
govern the relation between the different PCEs.
7. Multi-domain PCE Deployment Options
7.1. Traffic Engineering Database and Synchronization
An optimal path computation requires knowledge of the available
network resources, including nodes and links, constraints, link
connectivity, available bandwidth, and link costs. The PCE operates
on a view of the network topology as presented by a TED. As
discussed in [RFC4655
], the TED used by a PCE may be learned by the
relevant IGP extensions.
Thus, the PCE may operate its TED by participating in the IGP running
in the network. In an MPLS-TE network, this would require OSPF-TE
] or ISIS-TE [RFC5305
]. In a GMPLS network, it would utilize
the GMPLS extensions to OSPF and IS-IS defined in [RFC4203
]. Inter-AS connectivity information may be populated via
] and [RFC5392
An alternative method to providing network topology and resource
information is offered by [RFC7752
], which is described in the
7.1.1. Applicability of BGP-LS to PCE
The concept of the exchange of TE information between Autonomous
Systems (ASes) is discussed in [RFC7752
]. The information exchanged
in this way could be the full TE information from the AS, an
aggregation of that information, or a representation of the potential
connectivity across the AS. Furthermore, that information could be
updated frequently (for example, for every new LSP that is set up
across the AS) or only at threshold-crossing events.
In an H-PCE deployment, the parent PCE will require the inter-domain
topology and link status between child domains. This information may
be learned by a BGP-LS speaker and provided to the parent PCE.
Furthermore, link-state performance, including delay, available
bandwidth, and utilized bandwidth, may also be provided to the parent
PCE for optimal path link selection.
7.2. Pre-planning and Management-Based Solutions
Offline path computation is performed ahead of time before the LSP
setup is requested. That means that it is requested by or performed
as part of an Operation Support System (OSS) management application.
This model can be seen in Section 5.5 of [RFC4655
The offline model is particularly appropriate for long-lived LSPs
(such as those present in a transport network) or for planned
responses to network failures. In these scenarios, more planning is
normally a feature of LSP provisioning.
The management system may also use a PCE and BRPC to pre-plan an AS
sequence, and the source domain PCE and per-domain path computation
to be used when the actual end-to-end path is required. This model
may also be used where the operator wishes to retain full manual
control of the placement of LSPs, using the PCE only as a computation
tool to assist the operator and not as part of an automated network.
In environments where operators peer with each other to provide end-
to-end paths, the operator responsible for each domain must agree on
the extent to which paths must be pre-planned or manually controlled.
8. Domain Confidentiality
This section discusses the techniques that cooperating PCEs can use
to compute inter-domain paths without each domain disclosing
sensitive internal topology information (such as explicit nodes or
links within the domain) to the other domains.
Confidentiality typically applies to inter-provider (inter-AS) PCE
communication. Where the TE LSP crosses multiple domains (ASes or
areas), the path may be computed by multiple PCEs that cooperate
together, with each local PCE responsible for computing a segment of
the path. With each local PCE responsible for computing a segment of
In situations where ASes are administered by separate Service
Providers, it would break confidentiality rules for a PCE to supply
path segment details to a PCE responsible for another domain, thus
disclosing AS-internal or area topology information.
8.1. Loose Hops
A method for preserving the confidentiality of the path segment is
for the PCE to return a path containing a loose hop in place of the
segment that must be kept confidential. The concept of loose and
strict hops for the route of a TE LSP is described in [RFC3209
] supports the use of paths with loose hops; whether it
returns a full explicit path with strict hops or uses loose hops is a
local policy decision at a PCE. A path computation request may
require an explicit path with strict hops or may allow loose hops, as
detailed in [RFC5440
8.2. Confidential Path Segments and Path-Keys
] defines the concept and mechanism of a Path-Key. A Path-Key
is a token that replaces the path segment information in an explicit
route. The Path-Key allows the explicit route information to be
encoded and is contained in the Path Computation Element
Communication Protocol (PCEP) ([RFC5440
]) messages exchanged between
the PCE and PCC.
This Path-Key technique allows explicit route information to be used
for end-to-end path computation without disclosing internal topology
information between domains.
9. Point to Multipoint
For inter-domain point-to-multipoint application scenarios using
MPLS-TE LSPs, the complexity of domain sequences, domain policies,
and the choice and number of domain interconnects is magnified
compared to point-to-point path computations. As the size of the
network grows, the number of leaves and branches increases, further
increasing the complexity of the overall path computation problem. A
solution for managing point-to-multipoint path computations may be
achieved using the PCE inter-domain point-to-multipoint path
10. Optical Domains
The International Telecommunication Union (ITU) defines the ASON
architecture in [G-8080]. [G-7715] defines the routing architecture
for ASON and introduces a hierarchical architecture. In this
architecture, the Routing Areas (RAs) have a hierarchical
relationship between different routing levels, which means a parent
(or higher level) RA can contain multiple child RAs. The
interconnectivity of the lower RAs is visible to the higher-level RA.
In the ASON framework, a path computation request is termed a route
query. This query is executed before signaling is used to establish
an LSP, which is termed a Switched Connection (SC) or a Soft
Permanent Connection (SPC). [G-7715-2] defines the requirements and
architecture for the functions performed by Routing Controllers (RC)
during the operation of remote route queries. An RC is synonymous
with a PCE.
In the ASON routing environment, an RC responsible for an RA may
communicate with its neighbor RC to request the computation of an
end-to-end path across several RAs. The path computation components
and sequences are defined as follows:
* Remote route query. An operation where a Routing Controller
communicates with another Routing Controller, which does not have
the same set of layer resources, in order to compute a routing
path in a collaborative manner.
* Route query requester. The connection controller or RC that sends
a route query message to a Routing Controller that requests one or
more routing paths satisfying a set of routing constraints.
* Route query responder. An RC that performs the path computation
upon reception of a route query message from a Routing Controller
or connection controller, and sends a response back at the end of
When computing an end-to-end connection, the route may be computed by
a single RC or multiple RCs in a collaborative manner, and the two
scenarios can be considered a centralized remote route query model
and a distributed remote route query model. RCs in an ASON
environment can also use the hierarchical PCE [RFC6805
] model to
fully match the ASON hierarchical routing model.
10.1. Abstraction and Control of TE Networks (ACTN)
Where a single operator operates multiple TE domains (including
optical environments), an Abstraction and Control of TE Networks
(ACTN) framework [RFC8453
] may be used to create an abstracted
(virtualized network) view of underlay-interconnected domains. This
underlay connectivity is then exposed to higher-layer control
entities and applications.
ACTN describes the method and procedure for coordinating the underlay
per-domain Provisioning Network Controllers (PNCs), which may be
PCEs, via a hierarchical model to facilitate setup of end-to-end
connections across interconnected TE domains.
Policy is important in the deployment of new services and the
operation of the network. [RFC5394
] provides a framework for PCE-
based policy-enabled path computation. This framework is based on
the Policy Core Information Model (PCIM) as defined in [RFC3060
further extended by [RFC3460
When using a PCE to compute inter-domain paths, policy may be invoked
by specifying the following:
* Each PCC must select which computations it will request from a
* Each PCC must select which PCEs it will use.
* Each PCE must determine which PCCs are allowed to use its services
and for what computations.
* The PCE must determine how to collect the information in its TED,
whom to trust for that information, and how to refresh/update the
* Each PCE must determine which objective functions and algorithms
12. Manageability Considerations
General PCE management considerations are discussed in [RFC4655
the case of multi-domains within a single service provider network,
the management responsibility for each PCE would most likely be
handled by the same service provider. In the case of multiple ASes
within different service provider networks, it will likely be
necessary for each PCE to be configured and managed separately by
each participating service provider, with policy being implemented
based on a previously agreed set of principles.
12.1. Control of Function and Policy
As per [RFC5440
], PCEP implementation allows the user to configure a
number of PCEP session parameters. These are detailed in Section 8.1
In H-PCE deployments, the administrative entity responsible for the
management of the parent PCEs for multi-areas would typically be a
single service provider. In multiple ASes (managed by different
service providers), it may be necessary for a third party to manage
the parent PCE.
12.2. Information and Data Models
A PCEP MIB module is defined in [RFC7420
], which describes managed
objects for modeling PCEP communication, including:
* PCEP client configuration and status.
* PCEP peer configuration and information.
* PCEP session configuration and information.
* Notifications to indicate PCEP session changes.
A YANG module for PCEP has also been proposed [PCEP-YANG].
An H-PCE MIB module or YANG data model will be required to report
parent PCE and child PCE information, including:
* Parent PCE configuration and status.
* Child PCE configuration and information.
* Notifications to indicate session changes between parent PCEs and
* Notification of parent PCE TED updates and changes.
12.3. Liveness Detection and Monitoring
PCEP includes a keepalive mechanism to check the liveliness of a PCEP
peer and a notification procedure allowing a PCE to advertise its
overloaded state to a PCC. In a multi-domain environment, [RFC5886
provides the procedures necessary to monitor the liveliness and
performance of a given PCE chain.
12.4. Verifying Correct Operation
It is important to verify the correct operation of PCEP. [RFC5440
specifies the monitoring of key parameters. These parameters are
detailed in [RFC5520
12.5. Impact on Network Operation
] states that in order to avoid any unacceptable impact on
network operations, a PCEP implementation should allow a limit to be
placed on the number of sessions that can be set up on a PCEP speaker
and that it may also be practical to place a limit on the rate of
messages sent by a PCC and received by the PCE.
13. Security Considerations
PCEP security considerations are discussed in [RFC5440
]. Potential vulnerabilities include spoofing, snooping,
falsification, and using PCEP as a mechanism for denial of service
As PCEP operates over TCP, it may make use of TCP security encryption
mechanisms, such as Transport Layer Security (TLS) and TCP
Authentication Option (TCP-AO). Usage of these security mechanisms
for PCEP is described in [RFC8253
], and recommendations and best
current practices are described in [RFC7525
13.1. Multi-domain Security
Any multi-domain operation necessarily involves the exchange of
information across domain boundaries. This represents a significant
security and confidentiality risk.
It is expected that PCEP is used between PCCs and PCEs that belong to
the same administrative authority while also using one of the
aforementioned encryption mechanisms. Furthermore, PCEP allows
individual PCEs to maintain the confidentiality of their domain path
information using path-keys.
14. IANA Considerations
This document has no IANA actions.
15.1. Normative References
] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209
, DOI 10.17487/RFC3209
, December 2001,
] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473
, January 2003,
] Zhang, R., Ed. and J.-P. Vasseur, Ed., "MPLS Inter-
Autonomous System (AS) Traffic Engineering (TE)
Requirements", RFC 4216
, DOI 10.17487/RFC4216
] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655
, August 2006,
] Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A Framework
for Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726
, DOI 10.17487/RFC4726
] Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
Per-Domain Path Computation Method for Establishing Inter-
Domain Traffic Engineering (TE) Label Switched Paths
(LSPs)", RFC 5152
, DOI 10.17487/RFC5152
, February 2008,
] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440
, March 2009,
] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441
, April 2009,
] Bradford, R., Ed., Vasseur, JP., and A. Farrel,
"Preserving Topology Confidentiality in Inter-Domain Path
Computation Using a Path-Key-Based Mechanism", RFC 5520
, April 2009,
] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC 5541
, June 2009,
] King, D., Ed. and A. Farrel, Ed., "The Application of the
Path Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS and GMPLS", RFC 6805
, November 2012,
15.2. Informative References
] Moore, B., Ellesson, E., Strassner, J., and A. Westerinen,
"Policy Core Information Model -- Version 1
Specification", RFC 3060
, DOI 10.17487/RFC3060
] Moore, B., Ed., "Policy Core Information Model (PCIM)
Extensions", RFC 3460
, DOI 10.17487/RFC3460
, January 2003,
] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630
, September 2003,
] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090
, May 2005,
] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 4203
, DOI 10.17487/RFC4203
, October 2005,
] Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita, N.,
and G. Ash, "Crankback Signaling Extensions for MPLS and
GMPLS RSVP-TE", RFC 4920
, DOI 10.17487/RFC4920
, July 2007,
] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "OSPF Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5088
, DOI 10.17487/RFC5088
January 2008, <https://www.rfc-editor.org/info/rfc5088
] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "IS-IS Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5089
, DOI 10.17487/RFC5089
January 2008, <https://www.rfc-editor.org/info/rfc5089
] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305
, DOI 10.17487/RFC5305
] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307
, DOI 10.17487/RFC5307
, October 2008,
] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5316
, DOI 10.17487/RFC5316
December 2008, <https://www.rfc-editor.org/info/rfc5316
] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392
, DOI 10.17487/RFC5392
January 2009, <https://www.rfc-editor.org/info/rfc5392
] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394
, December 2008,
] Oki, E., Takeda, T., and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP) for
Route Exclusions", RFC 5521
, DOI 10.17487/RFC5521
] Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A Set of
Monitoring Tools for Path Computation Element (PCE)-Based
Architecture", RFC 5886
, DOI 10.17487/RFC5886
, June 2010,
] Nishioka, I. and D. King, "Use of the Synchronization
VECtor (SVEC) List for Synchronized Dependent Path
Computations", RFC 6007
, DOI 10.17487/RFC6007
[G-8080] ITU-T, "Architecture for the automatically switched
optical network", ITU-T Recommendation G.8080/Y.1304,
[G-7715] ITU-T, "Architecture and requirements for routing in the
automatically switched optical networks", ITU-T
Recommendation G.7715/Y.1706, June 2002.
[G-7715-2] ITU-T, "ASON routing architecture and requirements for
remote route query", ITU-T
Recommendation G.7715.2/Y.1706.2, February 2007.
] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952
, DOI 10.17487/RFC6952
, May 2013,
] Zhao, Q., Dhody, D., King, D., Ali, Z., and R. Casellas,
"PCE-Based Computation Procedure to Compute Shortest
Constrained Point-to-Multipoint (P2MP) Inter-Domain
Traffic Engineering Label Switched Paths", RFC 7334
, August 2014,
] Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
Hardwick, "Path Computation Element Communication Protocol
(PCEP) Management Information Base (MIB) Module", RFC 7420
, DOI 10.17487/RFC7420
, December 2014,
] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525
, DOI 10.17487/RFC7525
] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752
, March 2016,
] Dhody, D., Palle, U., and R. Casellas, "Domain Subobjects
for the Path Computation Element Communication Protocol
(PCEP)", RFC 7897
, DOI 10.17487/RFC7897
, June 2016,
] Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
"PCEPS: Usage of TLS to Provide a Secure Transport for the
Path Computation Element Communication Protocol (PCEP)", RFC 8253
, DOI 10.17487/RFC8253
, October 2017,
] Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
Abstraction and Control of TE Networks (ACTN)", RFC 8453
, August 2018,
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
The author would like to thank Adrian Farrel for his review and Meral
Shirazipour and Francisco Javier Jiménez Chico for their comments.
Divyashree Techno Park, Whitefield
125 Nagog Technology Park
Acton, MA 01719
United States of America
2, avenue Pierre-Marzin
22307 Lannion Cedex
2, avenue Pierre-Marzin
22307 Lannion Cedex
100 Church Street
Óscar González de Dios
Emilio Vargas 6
Old Dog Consulting
H1, Huawei Xiliu Beipo Village, Songshan Lake
Additional contact information: