PCE Working Group D. King
Internet Draft Old Dog Consulting
Intended status: Informational J. Meuric
Expires: June 16, 2012 O. Dugeon
France Telecom
Q. Zhao
Huawei Technologies
Oscar Gonzalez de Dios
Francisco Javier Jimenex Chico
Telefonica I+D
January 16, 2012
Applicability of the Path Computation Element to Inter-Area and
Inter-AS MPLS and GMPLS Traffic Engineering
draft-ietf-pce-inter-area-as-applicability-02
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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This Internet-Draft will expire on October 18, 2012.
D. King, et al. June 16, 2012. [Page 1]
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
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............5
1.4. Traffic Engineered Label Switched Paths....................6
1.5. Inter-area and Inter AS Connectivity Discovery.............6
2. Terminology..................................................6
3. Issues and Considerations....................................7
3.1 Multi-homing.............................................7
3.2 Domain Confidentiality ..................................7
3.3 Destination Location.....................................7
4. Domain Topologies............................................8
4.1 Selecting Domain Paths...................................8
4.2 Multi-Homed Domains......................................8
4.3 Domain Meshes............................................8
4.4 Domain Diversity.........................................8
4.5 Synchronized Path Computations...........................9
5. Applicability of the PCE to Inter-area Traffic Engineering...9
5.1. Inter-area Routing......................................10
5.1.1. Area Inclusion and Exclusion..........................10
5.1.2. Strict Explicit Path and Loose Path...................11
5.1.3. Inter-Area Diverse Path Computation...................11
5.2. Control and Recording of Area Crossing..................11
6. Applicability of the PCE to Inter-AS Traffic Engineering.....11
6.1. Inter-AS Routing........................................12
6.1.1. AS Inclusion and Exclusion............................12
6.1.2. Strict Explicit Path and Loose Path...................12
6.1.3. 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
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7. Multi-Domain PCE Deployment..................................13
7.1 Traffic Engineering Database.............................14
7.2 Provisioning Techniques..................................15
7.3 Pre-Planning and Management-Based Solutions..............15
7.4 Per-Domain Computation...................................15
7.5 Cooperative PCEs.........................................15
7.6 Hierarchical PCEs ......................................16
8. Domain Confidentiality.......................................16
8.1 Loose Hops...............................................16
8.2 Confidential Path Segments and Path Keys.................16
9. Point-to-Multipoint..........................................17
10. Optical Domains.............................................17
10.1. PCE applied to the ASON Architecture......................17
11. Manageability Considerations................................18
12. Security Considerations.....................................18
13. IANA Considerations.........................................18
14. References..................................................18
14.1. Normative References......................................18
14.2. Informative References....................................18
15. Acknowledgements............................................20
16. Author's Address............................................20
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.
The Path Computation Element (PCE) [RFC4655] provides an architecture
and a set of functional components to address this problem space.
Computing optimal routes for LSPs that cross domains in MPLS-TE and
GMPLS networks presents a problem because no single point of path
computation is aware of all of the links and resources in each
domain. A solution may be achieved using the PCE architecture
[RFC4655].
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 Antonymous System
(AS) or an Interior Gateway Protocol (IGP) area ( as per [RFC4726]
and [RFC4655]).
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 (G)MPLS TE LSPs. However, both per-domain
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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-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 [H-PCE] architecture and
mechanisms may be used to discovery the intra-area path and select
the optimal end-to-end domain sequence.
This document examines the applicability and describes the processes
and procedures available when using the PCE architecture, protocols
and protocol extensions for computing inter-area and inter-AS MPLS
Traffic Engineered paths.
1.1 Domains
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 should be applied to small
inter-domain topologies and not to solve route computation issues
across large groups of domains, I.E. the entire Internet.
1.2 Path Computation
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 maybe
necessary for the PCE to co-operate with other PCEs in adjacent
domains (as per BRPC [RFC5441]) or cooperate with the Parent PCE
(as per [H-PCE]).
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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 discussed, 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
configured using static configuration or by listening to
the periodic advertisements generated by PCEs.
One a path computation request is received, the PCC will send a
request to the PCE. A PCE may 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.
A PCE may cooperate with other PCEs to determine intermediate loose
hops. 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]
1.3 Traffic Engineering Aggregation and Abstraction
Networks are often constructed from multiple areas or ASs 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
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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 ASs (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 Connectivity 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 ASs. The PCE
discovery mechanism defined in [RFC5088] and [RFC5089] allow
the discovery of PCEs and disclosure of information related to
inter-area and inter-AS capable PCEs across area and AS boundaries.
2. Terminology
Terminology used in this document.
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 ASs of a different or the same Service Provider via one or
more inter-AS links.
CSPF: Constrained Shortest Path First.
Inter-area TE LSP: A TE LSP whose path transits through two or more
IGP areas.
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Inter-AS MPLS TE LSP: A TE LSP whose path transits through two or
more ASs or sub-ASs (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.
This document also uses the terminology defined in [RFC4655] and
[RFC5440].
3. Issues and Considerations
3.1 Multi-homing
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
single point failures disrupting primary and backup services.
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.
3.2 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 PCC will be
able to see the individual hops through another domain.
3.3 Destination Location
The PCC asking for an inter-domain path computation is typically
aware of the identity of the destination node. Additionally, if the
PCC is aware of the destination domain, it can supply this
information as part of the path computation request. However,
if the PCC does not know the egress domain this information must
be determined by another method.
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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 cooperational 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 simply-connected, or if the preferred points of
interconnection are also known, the Per-Domain Path Computation
[RFC5152] technique can 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 optimal
path.
When the sequence of domains is not known in advance, the optimum
end-to-end path can be derived through the use of a hierarchical
relationship between domains [H-PCE].
4.2 Multi-Homed Domains
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
single point failures disrupting primary and backup services.
4.3 Domain Meshes
Very frequently network domains are composed by dozens or hundreds of
network elements. These network elements are usually interconnected
between them in a partial-mesh fashion, to provide survivability
against dual failures, and to benefit from the traffic engineering
capabilities from MPLS and GMPLS protocols. A typical node degree
ranges from 3 to 10 (4-5 is quite common), being the node degree the
number of neighbors per node.
Networks are sometimes divided into domains. Some reasons for it
range from manageability to separation into vendor-specific domains.
The size of the domain will be usually limited by control plane, but
it can also be stated by arbitrary design constraints.
4.4 Domain Diversity
Whenever an specific connectivity service is required to have 1+1
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protection feature, two completely disjoint paths must be
established on an end to end fashion. In a multi-domain environment
without, this can be accomplished either by selecting domain
diversity, or by ensuring diverse connection within a domain. In
order to compute the route diversity, it could be helpful to have
SRLG information in the domains.
4.5 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. [RFC6007] provides
an overview for the use of the PCE SVEC list for synchronized path
computations when computing dependent requests.
A non-comprehensive list of synchronized path computations include
the following examples:
o Route diversity: computation of two disjoint paths from a source to
a destination (as drafted in the previous section).
o Synchronous restoration: joint computation of a set of alternative
paths for a set of affected LSPs as a consequence of a failure
event. Note that in this case, the requests will potentially
involve different source-destination pairs. In this scenario, the
different path computation requests may arrive at different time
stamps.
o Batch provisioning: It is common that the operator sends a set of
LSPs requests together, e.g in a daily of weekly basis, mainly in
case of long lived LSPs. In order to optimize the resource usage,
a synchronized path computation is needed.
o Network optimization: After some time of operation, the
distribution of the established LSP paths results in a non optimal
use of resources. Also, inter-domain policies/agreements may have
been changed. In such cases, a full (or partial) network planning
action regarding the inter-domain connections will be triggered.
This will involve the request of potentially a big amount of
connections.
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 information flooded
within the network and make the network more manageable. An IGP
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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 a
route to destination located in another area a method is required to
compute a path across area boundaries.
In order to support multiple vendors in a network, in cases where
data and/or control plane technologies cannot interoperate, it is
useful to divide the network in 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
increasing signaling traffic and delaying the LSP setup.
The 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 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, and 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
[RFC5152] provides the mechanisms to compute an inter-area
path. It uses loose hop routing with an ERO expansion on each ABR.
This allows the end-to-end path to be set up following a constrained
path, but faces two major limitations:
- The method does not guarantee the use of an optimal constrained
path.
- This may lead to several crankback signaling messages and hence
delay the path setup.
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[RFC5441] provides a more optimal method to specify inclusion or
exclusion of an ABR. Using this method, an operator might decide if
an area must be include or exclude 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,
one 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 if a strict
explicit path across specific areas is required or desired, or if
the path request is loose.
5.1.3. Inter-Area Diverse Path Computation
It may be neccessary (for protection or load-balancing) to compute
a path that is diverse, from a previously computed path. There are
various levels of diversity in the context of an inter-area network:
- Per-area diversity (intra-area path segments are link, node or
SRLG disjoint.
- Inter-area diversity (end-to-end inter-area paths are link,
node or SRLG disjoint).
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 are node disjoint but end-to-end paths are not node-disjoint.
Both Per-Domain [RFC5152] and BRPC [RFC5441] mechanisms support the
capability to compute diverse across multi-area topologies.
5.2. Control and Recording of Area Crossing
In some environments it be useful for the PCE to provide a PCC the
set of areas crossed by the end-to-end path. Additionally the PCE
can provide the path information and mark each segment so the PCC
has visibility of which piece of the path lies within which area.
Although by implementing Path-Key, the hop-by-hop (area topology)
information is kept confidential.
6. Applicability of the PCE to Inter-AS Traffic Engineering
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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 ASs. 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] a method to specify inclusion or exclusion of an ASBR.
Using this method, an operator might decide if an AS must be include
or exclude from the inter-AS path computation.
6.1.2. Strict Explicit Path and Loose Path
During path computation, the PCE architecture and BRPC algorithm
allow operators to specify if the resultant LSP must follow a strict
or a loose path. By explicitly specify the path, the operator
request a strict explicit path which must pass through one or many
LSR. If this behaviour is well define and appropriate for inter-area,
it implies some topology discovery for inter-AS. So, this feature
when the operator owns several ASs (and so, knows the topology of
its ASs) or restricts to the well-known ASBR to avoid topology
discovery between operators. The loose path, even if it does not
allow granular specification of the path, protects topology
disclosure as it not obligatory for the operator to disclose
information about its networks.
6.1.3. AS Inclusion and Exclusion
Like explicit and loose path, [RFC5441] allows to specify inclusion
or exclusion of respectively 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 ASs are
public entity as well as their interconnection.
6.2 Inter-AS Bandwidth Guarantees
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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.
One typical example of this requirement 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
ASs, 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 ASs.
6.3 Inter-AS Recovery
During path computation, a PCC request may contain backup LSP
requirements in order to setup in the same time the primary and
backup LSPs. It is also possible to request a backup LSP for a group
of primary LSPs. [RFC4090] adds fast re-route protection to LSP. So,
the PCE could be used to trigger computation of backup tunnels in
order to protect Inter-AS connectivity. Inter-AS recovery
requirements needs not only PCE protection and redundancy but also
LSP tunnels protection through FRR mechanisms. Inter-AS PCE
computation must support the FRR mechanisms and the patch computation
for backup tunnels for protection and fast recovery.
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 define by the operator.
Typically PCE interconnections at an AS level must follow 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
The PCE provides the architecture and mechanisms to compute
inter-area and inter-AS LSPs. The objective of this document is not
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to reprint the techniques and mechanisms available, but to highlight
their existence and reference the relevant documents that introduce
and describe the techniques and mechanisms necessary for computing
inter-area and inter-AS LSP based services.
An area or AS may contain multiple PCEs:
- The path computation load may be balanced among a set of PCEs to
improve scalability.
- For the purpose of redundancy, primary and backup PCEs may be used.
- PCEs may have distinct path computation capabilities (P2P or P2MP).
Discovery of PCEs and capabilities per area or AS is defined in
[RFC5088] and [RFC5089].
Each PCE per domain can be deployed in a centralized or distributed
architecture, the latter model having local visibility and
collaborating in a distributed fashion to compute a path across the
domain. Each PCE may collect topology and TE information from the
same sources as the LSR, such as the IGP TED.
When the PCC sends a path computation request to the PCE, the PCE
will compute the path across a domain based on the required
constraints. The PCE will generate the full set of strict hops from
source to destination. This information, encoded as an ERO, is then
sent back to the PCC that requested the path. In the event that a
path request from a PCC contains source and destination nodes that
are located in different domains the PCE is required to co-operate
between multiple PCEs, each responsible for its own domain.
Techniques for inter-domain path computation are described in
[RFC5152] and [RFC5441], both techniques assume that the sequence of
domains to be crossed from source to destination is well known. In
the event that the sequence of domains is not well known, [H-PCE]
might be used. The sequence could also be retrieve locally from
information previously stored in the PCE database (preferably in
the TED) by OSS management or other protocols.
7.1 Traffic Engineering Database
TEDs are automatically populated by the IGP-TE like IS-IS-TE or
OSPF-TE. However, no information related to AS path are provided
by such IGP-TE. It could be helpful for BRPC algorithm as AS path
helper, to populate a TED with suitable information regarding
inter-AS connectivity. Such information could be obtain from
various sources, such as BGP protocol, peering policies, OSS of the
operator or from neighbor PCE. In any case, no topology disclosure
must be impose in order to provide such information.
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In particular, for both inter-area and inter-AS, the TED must be
populated with all boundary node information suitable to
establish PCEP protocol with the next PCE in the path.
7.2 Provisioning Techniques
As PCE algorithms rely on information contained in the TED, it
is possible to populate TED information by means of provisioning. In
this case, the operator must regularly update and store all suitable
information in the TED in order for the PCE to correctly compute LSP.
Such information range from policies (e.g. avoid this LSR, or use
this ASBR for a specific IP prefix) up to topology information (e.g.
AS X is reachable trough a 100 Mbit/s link on this ASBR and 30 Mbit/s
are reserved for EF traffic). Operators may choose the type and
amount of information they can use to manage their traffic engineered
network.
However, some LSPs might be provisioned to link ASs or areas. In this
case, these LSP must be announced by the IGP-TE in order to
automatically populate the TED.
7.3 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, a 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.
This model may also be used where the network 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.
The management based solutions could also be used in conjunction
with the BRPC algorithm. Operator just computes the AS-Path as
parameter for the inter-AS path computation request and let each
PCE along the AS path compute the LSP part on its own domain.
7.4 Per-Domain Computation
[RFC5152] defines the mechanism to compute per-domain path and must
be used in that condition. Otherwise, BRPC [RFC5441] will be used.
7.5 Cooperative PCEs
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When PCE cooperate to compute an inter-area or inter-AS LSP, both
[RFC5152] and [RFC5441] could be used.
7.6 Hierarchical PCEs
The [H-PCE] draft defines how a hierarchy of PCEs may be used. An
operator must define a parent PCE and each child PCE. A parent PCE
can be announced in the other areas or ASs in order for the parent
PCE to contact remote child PCEs. Reciprocally, child PCEs are
announced in remote areas or ASs in order to be contacted by a
remote parent PCE. Parent and each child PCE could also be
provisioned in the TED if they are not announced.
8. Domain Confidentiality
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. However, in some cases (e.g., when ASes 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 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 request 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 enformation to be
encoded and in the PCEP ([RFC5440]) messages exchanged between the
PCE and PCC.
This Path-Key technique allows explicit route information to used
for end-to-end path computation, without disclosing internal topology
information between domains.
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9. Point-to-Multipoint
For the Point-to-Multipoint application scenarios for MPLS-TE LSP,
the complexity of domain sequences, domain policies, choice and
number of domain interconnects is magnified comparing to
P2P path computations. Also as the size of the network grows,
the number of leaves and branches increase and it in turn puts the
scalability of the path computation and optimization into a bigger
issue. A solution for the point-to-multipoint path computations may
be achieved using the PCEP protocol extension for P2MP
[RFC6006] and using the PCEP P2MP procedures defined in
[PCEP-P2MP-INTER-DOMAIN].
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.
10.1. PCE applied to the ASON Architecture
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, a 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.
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 path that satisfies 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
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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 [H-PCE] model to fully match the
ASON hierarchical routing model.
11. Manageability Considerations
This document does not describe any specific protocol,
protocol extensions, or protocol usage, therefore no manageability
considerations need to be discussed here.
12. Security Considerations
This document is informational and does not describe any new
specific protocol, protocol extensions, or protocol usage. As such,
it introduces no new security concerns.
13. IANA Considerations
This document makes no requests for IANA action.
13. References
14.1. Normative References
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[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.
14.2. Informative 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.
D. King, et al. June 16, 2012. [Page 18]
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[RFC4090] Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May
2005.
[RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework
for Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726, November 2006.
[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.
[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)",
RFC 5152, February 2008.
[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.
[RFC6006] Takeda, T., Chaitou M., Le Roux, J.L., Ali Z.,
Zhao, Q., King, D., "Extensions to the Path Computation
Element Communication Protocol (PCEP) for
Point-to-Multipoint Traffic Engineering Label Switched
Paths", RFC6006, September 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
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.
D. King, et al. June 16, 2012. [Page 19]
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[PCEP-P2MP-INTER-DOMAIN] Ali Z., Zhao, Q., King, D., "PCE-based
Computation Procedure To Compute Shortest Constrained
P2MP Inter-domain Traffic Engineering Label Switched
Paths",
draft-zhao-pce-pcep-inter-domain-p2mp-procedures-07.txt,
work in progress, Januaury, 2011.
[H-PCE] King, D. and A. Farrel, "The Application of the Path
Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS & GMPLS", July
2010.
15. Acknowledgements
The author would like to thank Adrian Farrel for his review and
Meral Shirazipour for his comments.
16. Author's Address
Daniel King
Old Dog Consulting
UK
Email: daniel@olddog.co.uk
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
Quintin Zhao
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
US
Email: qzhao@huawei.com
Oscar Gonzalez de Dios
Telefonica I+D
Emilio Vargas 6, Madrid
Spain
Email: ogondio@tid.es
D. King, et al. June 16, 2012. [Page 20]
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Francisco Javier Jimenex Chico
Telefonica I+D
Emilio Vargas 6, Madrid
Spain
Email: fjjc@tid.es
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