Internet DRAFT - draft-ietf-pce-inter-area-as-applicability
draft-ietf-pce-inter-area-as-applicability
PCE Working Group D. King
Internet Draft Old Dog Consulting
Intended status: Informational H. Zheng
Expires: January 9, 2020 Huawei Technologies
July 8, 2019
Applicability of the Path Computation Element to Inter-Area and
Inter-AS MPLS and GMPLS Traffic Engineering
draft-ietf-pce-inter-area-as-applicability-08
Abstract
The Path Computation Element (PCE) may be used for computing services
that traverse multi-area and 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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 9, 2020.
Copyright Notice
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
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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.
1. Introduction.................................................3
1.1. Domains.................................................4
1.2. Path Computation........................................4
1.2.1 PCE-based Path Computation Procedure.................5
1.3. Traffic Engineering Aggregation and Abstraction.........6
1.4. Traffic Engineered Label Switched Paths.................6
1.5. Inter-area and Inter-AS Capable PCE Discovery...........6
1.6. Objective Functions.....................................6
2. Terminology..................................................7
3. Issues and Considerations....................................7
3.1 Multi-homing.............................................7
3.2 Destination Location.....................................8
3.3 Domain Confidentiality ..................................8
4. Domain Topologies............................................8
4.1 Selecting Domain Paths...................................8
4.2 Domain Sizes.............................................9
4.3 Domain Diversity.........................................9
4.4 Synchronized Path Computations...........................9
4.5 Domain Inclusion or Exclusion............................9
5. Applicability of the PCE to Inter-area Traffic Engineering...10
5.1. Inter-area Routing......................................11
5.1.1. Area Inclusion and Exclusion..........................11
5.1.2. Strict Explicit Path and Loose Path...................11
5.1.3. Inter-Area Diverse Path Computation...................11
6. Applicability of the PCE to Inter-AS Traffic Engineering.....12
6.1. Inter-AS Routing........................................12
6.1.1. AS Inclusion and Exclusion............................12
6.2. Inter-AS Bandwidth Guarantees...........................12
6.3. Inter-AS Recovery.......................................13
6.4. Inter-AS PCE Peering Policies...........................13
7. Multi-Domain PCE Deployment..................................13
7.1 Traffic Engineering Database.............................13
7.1.1. Applicability of BGP-LS to PCE........................14
7.2 Pre-Planning and Management-Based Solutions..............14
8. Domain Confidentiality.......................................15
8.1 Loose Hops...............................................15
8.2 Confidential Path Segments and Path Keys.................15
9. Point-to-Multipoint..........................................16
10. Optical Domains.............................................16
10.1 Abstraction and Control of TE Networks (ACTN)...........17
11. Policy......................................................17
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12. Manageability Considerations................................18
12.1 Control of Function and Policy...........................18
12.2 Information and Data Models..............................18
12.3 Liveness Detection and Monitoring........................19
12.4 Verifying Correct Operation..............................19
12.5 Impact on Network Operation..............................19
13. Security Considerations.....................................19
13.1 Multi-domain Security....................................19
14. IANA Considerations.........................................20
15. Acknowledgements............................................20
16. References..................................................20
16.1. Normative References....................................20
16.2. Informative References..................................21
17. Contributors................................................24
18. Author's Addresses..........................................25
1. Introduction
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:
o Often there is a lack of full topology and TE information across
domains;
o No single node has the full visibility to determine an optimal or
even feasible end-to-end path across domains;
o How to evaluate and select the exit point and next domain boundary
from a domain?
o How might the ingress node determine which domains should be used
for the end-to-end path?
Often information exchange across multiple domains is 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
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 path computation (BRPC) mechanism
[RFC5441] defines a PCE-based path computation procedure to compute
inter-domain constrained Multiprotocol Label Switching (MPLS) and
Generalized MPLS (GMPLS) Traffic Engineered (TE) networks. However,
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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 PCE
[RFC6805] 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.
This document scope does not include discussion on stateful PCE,
active PCE, remotely initiated PCE, or PCE as a central controller
(PCECC) deployment scenarios.
1.1 Domains
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 per [RFC4726] 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 this document.
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 [G-7715-2].
It is assumed that the PCE architecture is not applied to a large
group of domains, such as the Internet.
1.2 Path Computation
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For the purpose of this document, it is assumed that the
path computation is the sole responsibility of the PCE as per the
architecture defined in [RFC4655]. When a path is required the Path
Computation Client (PCC) will send a request to the PCE. The PCE
will apply the required constraints and compute a path and return a
response to the PCC. In the context of this document it may be
necessary for the PCE to co-operate with other PCEs in adjacent
domains (as per BRPC [RFC5441]) or cooperate 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 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.
[RFC4655] defines that 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 co-operative 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 that is a token that replaces a path segment
in an explicit route. The path key mechanism is described in
[RFC5520]
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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 TE properties of each area
and AS are not generally advertized outside each specific area or AS.
TE aggregation or abstraction provide 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 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]:
o Contiguous LSP
o Stitched LSP
o 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] facilitates
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 set of OFs, specifies the
intentions of the path computation and so defines the "optimality",
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in the context of the computation request.
An OF specifies the desired outcome of a computation. An OF 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
[RFC5541].
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 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 policy.
2. Terminology
This document also uses the terminology defined in [RFC4655] and
[RFC5440]. 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
IGP areas.
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 other means.
3. Issues and Considerations
3.1 Multi-homing
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Networks constructed from multi-areas or multi-AS environments
may have multiple interconnect points (multi-homing). End-to-end path
computations may need to use different interconnect points to avoid
a single point failure disrupting both primary and backup services.
3.2 Destination Location
The 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
Where the end-to-end path crosses multiple domains, it may be
possible that each domain (AS or area) are administered by separate
Service Providers, it would break confidentiality rules for a PCE
to supply a path segment to a PCE in another domain, thus disclosing
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 or else 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 [RFC3473] networks, in inter-area and inter-AS (multi-domain)
environments. Path computation across multi-domain networks is
complex and requires computational co-operational entities like the
PCE.
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 is also known, the Per-Domain Path Computation
[RFC5152] technique may be used. Where there are multiple connections
between domains and there is no preference for the choice of points
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of interconnection, BRPC [RFC5441] can be used to derive an optimal
path.
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
from MPLS and GMPLS protocols. Network operator feedback in the
development of the document highlighted that 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 in [RFC5440] and [RFC6007] provides an overview for the
use of the PCE SVEC list for synchronized path computations when
computing dependent requests.
In 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 derived via the use of
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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) [RFC5521]. The use of Autonomous Number (AS) as an
abstract node representing a domain is defined in [RFC3209].
[RFC7897] specifies new sub-objects 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, or if a specific connectivity
service required to have a 1+1 protection capability, two
completely 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:
o This method does not guarantee an optimal constrained path.
o The method may require several crankback signaling messages, as per
[RFC4920], increasing signaling traffic and delaying the LSP setup.
The PCE-based architecture [RFC4655] is designed to solve inter-area
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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 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, during the
path computation request, if a strict explicit path is required or
not. 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 the indication of whether a strict
explicit path across specific areas ([RFC7897]) is required or
desired, or if 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:
o Per-area diversity (intra-area path segments are link, node or
SRLG disjoint.
o Inter-area diversity (end-to-end inter-area paths are link,
node or SRLG disjoint).
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Note that two paths may be disjoint in the backbone area but non-
disjoint in peripheral areas. Also, two paths may be node disjoint
within areas but may share ABRs, in which case path segments within
an area is node disjoint, but end-to-end paths are not node-disjoint.
Per-Domain [RFC5152], BRPC [RFC5441] and H-PCE [RFC6805] 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 4 (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
[RFC5441] allows the specifying of inclusion or exclusion of an AS
or an ASBR. Using this method, an operator might decide if an AS
must be include or exclude 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 are public
entity as well as their interconnection.
6.2 Inter-AS Bandwidth Guarantees
Many operators with multi-AS domains will have deployed 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].
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One typical example of the requirements in [RFC4216] is to provide
bandwidth guarantees over an end-to-end path for VoIP traffic
classified as EF (Expedited Forwarding) class in a DiffServ-enabled
network. In the case where the EF path is extended across multiple
ASes, inter-AS bandwidth guarantee would be required.
Another case for inter-AS bandwidth guarantee is the requirement for
guaranteeing a certain amount of transit bandwidth across one or
multiple ASes.
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,
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 m:n
protection.
Other inter-AS recovery mechanisms include [RFC4090] which adds fast
re-route (FRR) protection to an LSP. So, the PCE could be used to
trigger computation of backup tunnels in order to protect Inter-AS
connectivity.
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 is a
requirement for operator. In inter-AS BRPC process, PCE must
cooperate in order to compute the end-to-end LSP. So, the AS path
must not only follow technical constraints, e.g. bandwidth
availability, but also policies defined by the operator.
Typically PCE interconnections at an AS level must follow 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 PCE.
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,
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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 learnt
by the relevant IGP extensions.
Thus, the PCE may operate its TED is by participating
in the IGP running in the network. In an MPLS-TE network, this
would require OSPF-TE [RFC3630] or ISIS-TE [RFC5305]. In a GMPLS
network it would utilize the GMPLS extensions to OSPF and IS-IS
defined in [RFC4203] and [RFC5307]. Inter-as connectivity
information may be populated via [RFC5316] and [RFC5392].
An alternative method to provide network topology and resource
information is offered by [RFC7752], which is described in the
following section.
7.1.1 Applicability of BGP-LS to PCE
The concept of 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 learnt 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 to 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
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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,
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
to what extent paths must be pre-planned or manually controlled.
8. Domain Confidentiality
This section discusses the techniques that co-operating 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.
In situations where ASes are administered by separate Service
Providers, it would break confidentiality rules for a PCE to supply
a path segment details to a PCE responsible 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].
[RFC5440] supports the use of paths with loose hops, and it is a
local policy decision at a PCE whether it returns a full explicit
path with strict hops or uses loose hops. 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
[RFC5520] defines the concept and mechanism of 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 in the PCEP ([RFC5440]) messages exchanged between the
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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,
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 increase, 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
computation [RFC7334] procedure.
10. Optical Domains
The International Telecommunications 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 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:
o 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.
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o Route query requester. The connection controller or RC that sends a
route query message to a routing controller requesting for one or
more routing paths that satisfy a set of routing constraints.
o Route query responder. An RC that performs path computation upon
reception of a route query message from a routing controller or
connection controller, sending a response back at the end of
computation.
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 distributed remote route query model. RCs in an ASON environment
can also use the hierarchical PCE [RFC6805] model to match fully 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) then 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 then be exposed to higher-layer control
entities and applications.
ACTN describes the method and procedure for coordinating the
underlay per-domain Physical Network Controllers (PNCs), which may
be PCEs, via a hierarchical model to facilitate setup of
end-to-end connections across inter-connected TE domains.
11. Policy
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] and
further extended by [RFC3460].
When using a PCE to compute inter-domain paths, policy may be
invoked by specifying:
o Each PCC must select which computations will be requested to a PCE;
o Each PCC must select which PCEs it will use;
o Each PCE must determine which PCCs are allowed to use its services
and for what computations;
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o The PCE must determine how to collect the information in its TED,
whom to trust for that information, and how to refresh/update the
information;
o Each PCE must determine which objective functions and which
algorithms to apply.
12. Manageability Considerations
General PCE management considerations are discussed in [RFC4655].
In 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 PCEP [RFC5440] implementation allow the user to configure
a number of PCEP session parameters. These are detailed in section
8.1 of [RFC5440].
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 the 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] that describes managed
objects for modeling of PCEP communication including:
o PCEP client configuration and status,
o PCEP peer configuration and information,
o PCEP session configuration and information,
o 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:
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o parent PCE configuration and status,
o child PCE configuration and information,
o notifications to indicate session changes between parent PCEs and
child PCEs, and
o 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
performances 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
[RFC5440] 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, it may also be practical to place a limit on the rate
of messages sent by a PCC and received my the PCE.
13. Security Considerations
PCEP Security considerations are discussed in [RFC5440] and [RFC6952]
Potential vulnerabilities include spoofing, snooping, falsification
and using PCEP as a mechanism for denial of service attacks.
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 in [RFC7525].
13.1 Multi-domain Security
Any multi-domain operation necessarily involves the exchange of
information across domain boundaries. This does represent
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significant security and confidentiality risk.
It is expected that PCEP is used between PCCs and PCEs belonging to
the same administrative authority, and using one of the
aforementioned encryption mechanisms. Furthermore, PCEP allows
individual PCEs to maintain confidentiality of their domain path
information using path-keys.
14. IANA Considerations
This document makes no requests for IANA action.
15. Acknowledgements
The author would like to thank Adrian Farrel for his review, and
Meral Shirazipour and Francisco Javier Jimenex Chico for their
comments.
16. References
16.1. Normative References
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
3473, January 2003.
[RFC4216] Zhang, R., Ed., and J.-P. Vasseur, Ed., "MPLS Inter-
Autonomous System (AS) Traffic Engineering (TE)
Requirements", RFC 4216, November 2005.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework
for Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726, November 2006.
[RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain
Path Computation Method for Establishing Inter-Domain
Traffic Engineering (TE) Label Switched Paths (LSPs)",
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RFC 5152, February 2008.
[RFC5440] Ayyangar, A., Farrel, A., Oki, E., Atlas, A., Dolganow,
A., Ikejiri, Y., Kumaki, K., Vasseur, J., and J. Roux,
"Path Computation Element (PCE) Communication Protocol
(PCEP)", RFC 5440, March 2009.
[RFC5441] Vasseur, J.P., Ed., "A Backward Recursive PCE-based
Computation (BRPC) procedure to compute shortest inter-
domain Traffic Engineering Label Switched Paths",
RFC5441, April 2009.
[RFC5520] 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.
[RFC5541] Le Roux, J., Vasseur, J., Lee, Y., "Encoding
of Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC5541, December 2008.
[RFC6805] King, D. and A. Farrel, "The Application of the Path
Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS & GMPLS", RFC6805, July
2010.
16.2. Informative References
[RFC3060] Moore, B., Ellesson, E., Strassner, J., and A.
Westerinen, "Policy Core Information Model -- Version 1
Specification", RFC 3060, February 2001.
[RFC3460] Moore, B., Ed., "Policy Core Information Model (PCIM)
Extensions", RFC 3460, January 2003.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic
Engineering (TE) Extensions to OSPF Version 2", RFC
3630, September 2003.
[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
2005.
[RFC4203] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
Extensions in Support of Generalized Multi-
Protocol Label Switching (GMPLS)", RFC
4203, October 2005.
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[RFC4920] Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita,
N., and G. Ash, "Crankback Signaling Extensions for MPLS
and GMPLS RSVP-TE", RFC 4920, July 2007.
[RFC5088] Le Roux, JL., Vasseur, JP., Ikejiri, Y., and R. Zhang,
"OSPF Protocol Extensions for Path Computation Element
(PCE) Discovery", RFC 5088, January 2008.
[RFC5089] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "IS-IS Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5089, January 2008.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, October 2008.
[RFC5307] Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
Extensions in Support of Generalized Multi-Protocol
Label Switching (GMPLS)", RFC 5307,
October 2008.
[RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", December 2008.
[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, January 2009.
[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 2008.
[RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP)
for Route Exclusions", RFC 5521, April 2009.
[RFC5886] Vasseur, JP., Le Roux, JL., and Y. Ikejiri, "A Set of
Monitoring Tools for Path ComputationElement (PCE)-Based
Architecture", RFC 5886, June 2010.
[RFC6007] Nishioka, I., King, D., "Use of the Synchronization
VECtor (SVEC) List for Synchronized Dependent Path
Computations", RFC6007, September 2010.
[G-8080] ITU-T Recommendation G.8080/Y.1304, Architecture for
the automatically switched optical network (ASON).
[G-7715] ITU-T Recommendation G.7715 (2002), Architecture
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and Requirements for the Automatically Switched
Optical Network (ASON).
[G-7715-2] ITU-T Recommendation G.7715.2 (2007), ASON routing
architecture and requirements for remote route query.
[RFC6952] 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, May 2013.
[RFC7334] Zhao, Q., Dhody, D., Ali Z., King, D.,
Casellas, R., "PCE-based Computation
Procedure To Compute Shortest Constrained
P2MP Inter-domain Traffic Engineering Label Switched
Paths", August 2014.
[RFC7420] Stephan, E., Koushik, K., Zhao, Q., King, D., "PCE
Communication Protocol (PCEP) Management Information
Base", December 2014.
[RFC7525] 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, May 2015.
[RFC7752] Gredler, H., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", March 2016.
[RFC7897] Dhody, D., Palle, U., and R. Casellas, "Domain Subobjects
for the Path Computation Element Communication Protocol
(PCEP)", June 2016.
[RFC8253] 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, October 2017.
[RFC8453] Ceccarelli, D., Lee, Y. et al., "Framework for
Abstraction and Control of TE Networks (ACTN)", RFC8453,
August 2018.
[PCEP-YANG] Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", work in progress,
October 2018.
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17. Contributors
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
Email: dhruv.ietf@gmail.com
Quintin Zhao
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
US
Email: qzhao@huawei.com
Julien Meuric
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
Email: julien.meuric@orange-ftgroup.com
Olivier Dugeon
France Telecom
2, avenue Pierre-Marzin
22307 Lannion Cedex
Email: olivier.dugeon@orange-ftgroup.com
Jon Hardwick
Metaswitch Networks
100 Church Street
Enfield, Middlesex
United Kingdom
Email: jonathan.hardwick@metaswitch.com
Oscar Gonzalez de Dios
Telefonica I+D
Emilio Vargas 6, Madrid
Spain
Email: ogondio@tid.es
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18. Author's Addresses
Daniel King
Old Dog Consulting
UK
Email: daniel@olddog.co.uk
Haomian Zheng
Huawei Technologies
F3 R&D Center, Huawei Industrial Base, Bantian, Longgang District
Shenzhen, Guangdong 518129
P.R.China
Email: zhenghaomian@huawei.com
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