PCE Working Group D. King
Internet Draft Old Dog Consulting
Intended status: Informational J. Meuric
Expires: December 3, 2014 O. Dugeon
France Telecom
Q. Zhao
Huawei Technologies
Oscar Gonzalez de Dios
Telefonica I+D
June 3, 2014
Applicability of the Path Computation Element to Inter-Area and
Inter-AS MPLS and GMPLS Traffic Engineering
draft-ietf-pce-inter-area-as-applicability-04
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
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on December, 2014.
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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
1.6. Objective Functions.....................................6
2. Terminology..................................................7
3. Issues and Considerations....................................7
3.1 Multi-homing.............................................7
3.2 Domain Confidentiality ..................................8
3.3 Destination Location.....................................8
4. Domain Topologies............................................8
4.1 Selecting Domain Paths...................................9
4.2 Multi-Homed Domains......................................9
4.3 Domain Meshes............................................9
4.4 Domain Diversity.........................................9
4.5 Synchronized Path Computations...........................9
4.6 Domain Inclusion or Exclusion............................10
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...................12
5.2. Control and Recording of Area Crossing..................12
6. Applicability of the PCE to Inter-AS Traffic Engineering.....12
6.1. Inter-AS Routing........................................13
6.1.1. AS Inclusion and Exclusion............................13
6.1.2. Strict Explicit Path and Loose Path...................13
6.1.3. AS Inclusion and Exclusion............................13
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6.2. Inter-AS Bandwidth Guarantees...........................13
6.3. Inter-AS Recovery.......................................14
6.4. Inter-AS PCE Peering Policies...........................14
7. Multi-Domain PCE Deployment..................................14
7.1 Traffic Engineering Database.............................14
7.2 Provisioning Techniques..................................14
7.3 Pre-Planning and Management-Based Solutions..............16
7.4 Per-Domain Computation...................................16
7.5 Cooperative PCEs.........................................16
7.6 Hierarchical PCEs ......................................16
8. Domain Confidentiality.......................................17
8.1 Loose Hops...............................................17
8.2 Confidential Path Segments and Path Keys.................17
9. Point-to-Multipoint..........................................17
10. Optical Domains.............................................18
10.1. PCE applied to the ASON Architecture....................18
11. Policy......................................................19
12. TED Topology and Synchronization............................19
12.1. Applicability of BGP-LS to PCE..........................20
13. Manageability Considerations................................20
13.1 Control of Function and Policy...........................20
13.2 Information and Data Models..............................21
13.3 Liveness Detection and Monitoring........................21
13.4 Verifying Correct Operation..............................21
13.5 Impact on Network Operation..............................21
14. Security Considerations.....................................21
15. IANA Considerations.........................................22
16. Acknowledgements............................................22
17. References..................................................22
17.1. Normative References....................................22
17.2. Informative References..................................22
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.
The Path Computation Element (PCE) [RFC4655] provides an architecture
and a set of functional components to address this problem space.
A PCE may be used to compute end-to-end paths across multi-domain
environments using a per-domain path computation technique [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,
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.
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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 [RFC6805] architecture and
mechanisms may be used to discovery 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, protocols and protocol extensions for
computing inter-area and inter-AS MPLS and GMPLS Traffic TE paths.
1.1 Domains
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]).
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 only 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 a Parent PCE
(as per [RFC6805]).
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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 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
configured using static configuration or by listening to the
periodic advertisements generated by PCEs.
Once 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 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
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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]:
- 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 ASes. 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.
1.6 Objective Functions
An Objective Function (OF) [RFC5541], or set of OFs, specify the
intentions of the path computation and so define the "optimality"
in the context of that computation request.
An OF specifies the desired outcome of a computation. An OF does not
describe or specify the algorithm to use, and an implementation may
apply any algorithm or set of algorithms to achieve the result
indicated by the OF. [RFC5541] provides the following OFs when
computing inter-domain paths:
o Minimum Cost Path (MCP);
o Minimum Load Path (MLP);
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o Maximum residual Bandwidth Path (MBP);
o Minimize aggregate Bandwidth Consumption (MBC);
o Minimize the Load of the most loaded Link (MLL);
o Minimize the Cumulative Cost of a set of paths (MCC).
OFs can be included in the PCE computation requests 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.
CSPF: Constrained Shortest Path First.
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
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.
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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.
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 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.
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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 [RFC6805].
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
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. 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.
A non-comprehensive list of synchronized path computations include
the following examples:
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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.
4.6 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 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) [RFC5521]. The use of Autonomous Number (AS) as an
abstract node representing a domain is defined in [RFC3209],
[DOMAIN-SEQ] specifies new sub-objects to include or exclude domains
such as an IGP area or an Autonomous Systems.
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
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.
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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.
[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
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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 necessary (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
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.
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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 ASes (and so, knows the topology of
its ASes) 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 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.
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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
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 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
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.
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- 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, [RFC6805]
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.
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
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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 ASes 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
When PCE cooperate to compute an inter-area or inter-AS LSP, both
[RFC5152] and [RFC5441] could be used.
7.6 Hierarchical PCEs
The [RFC6805] draft defines how a hierarchy of PCEs may be used. An
operator must define a parent PCE and each child PCE. A parent PCE
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can be announced in the other areas or ASes in order for the parent
PCE to contact remote child PCEs. Reciprocally, child PCEs are
announced in remote areas or ASes 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 information 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.
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
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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
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
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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 fully match the
ASON hierarchical routing model.
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:
- Each PCC must select which computations will be delegated to a PCE;
- 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,
who to trust for that information, and how to refresh/update the
information;
- Each PCE must determine which objective functions and which
algorithms to apply.
Finally, due to the nature of inter-domain (and particularly using
H-PCE based) path computations, deployment of policy should also
consider the need to be sensitive to commercial and reliability
information about domains and the interactions of services crossing
domains.
12. TED Topology and Synchronization
The PCE operates on a view of the network topology as presented by a
Traffic Engineering Database. 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].
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An alternative method to provide network topology and resource
information is offered by [BGP-LS], which is described in the
following section.
12.1 Applicability of BGP-LS to PCE
The concept of exchange of TE information between Autonomous Systems
(ASes) is discussed in [BGP-LS]. 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.
There are a number of discussion points associated with the use of
[BGP-LS] concerning the volume of information, the rate of churn of
information, the confidentiality of information, the accuracy of
aggregated or potential-connectivity information, and the processing
required to generate aggregated information. The PCE architecture and
the architecture enabled by [BGP-LS] make different assumptions about
the operational objectives of the networks, and this document does
not attempt to make one of the approaches "right" and the other
"wrong". Instead, this work assumes that a decision has been made to
utilize the PCE architecture.
Indeed, [BGP-LS] may have some uses within the PCE model. For
example, [BGP-LS] could be used as a "northbound" TE advertisement
such that a PCE does not need to listen to an IGP in its domain, but
has its TED populated by messages received (for example) from a
Route Reflector. Furthermore, the inter-domain connectivity and
connectivity capabilities that is required optional information for
a parent PCE could be obtained as a filtered subset of the
information available in [BGP-LS].
13. 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 an a previously agreed set of principles.
13.1 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] and will not be repeated here.
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13.2 Information and Data Models
A PCEP MIB module is defined in [PCEP-MIB] 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.
13.3 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.
13.4 Verifying Correct Operation
In order to verify the correct operation of PCEP, [RFC5440] specifies
the monitoring of key parameters. These parameters are detailed in
section 8.4 of [RFC5440] and will not be repeated here.
13.5 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.
14. Security Considerations
PCEP security is defined [RFC5440]. Any multi-domain operation
necessarily involves the exchange of information across domain
boundaries. This does represent a significant security and
confidentiality risk. PCEP allows individual PCEs to maintain
confidentiality of their domain path information using path-keys
[RFC5520].
For further considerations of the security issues related to inter-
domain path computation, see [RFC5376].
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15. IANA Considerations
This document makes no requests for IANA action.
16. Acknowledgements
The author would like to thank Adrian Farrel for his review, and
Meral Shirazipour and Francisco Javier Jimenex Chico for their
comments.
17. References
17.1. Normative References
17.2. Informative References
[RFC3060] Moore, B., Ellesson, E., Strassner, J., and A.
Westerinen, "Policy Core Information Model -- Version 1
Specification", RFC 3060, February 2001.
[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.
[RFC3460] Moore, B., Ed., "Policy Core Information Model (PCIM)
Extensions", RFC 3460, January 2003.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation
Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
3473, 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.
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
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[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.
[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.
[RFC5376] Bitar, N., et al., "Inter-AS Requirements for the Path
Computation Element Communication Protocol (PCECP)", RFC
5376, November 2008.
[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
December 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.
[RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the
Path Computation Element Communication Protocol (PCEP)
for Route Exclusions", RFC 5521, April 2009.
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[RFC5541] Le Roux, J., Vasseur, J., Lee, Y., "Encoding
of Objective Functions in the Path Computation Element
Communication Protocol (PCEP)", RFC5541, December 2008.
[RFC5886] Vasseur, JP., Le Roux, JL., and Y. Ikejiri, "A Set of
Monitoring Tools for Path ComputationElement (PCE)-Based
Architecture", RFC 5886, June 2010.
[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.
[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.
[PCEP-P2MP-INTER-DOMAIN] 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", work in progress.
[BGP-LS] Gredler, H., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and TE
Information using BGP", work in progress.
[PCEP-MIB] Stephan, E., Koushik, K., Zhao, Q., King, D., "PCE
Communication Protocol (PCEP) Management Information
Base", work in progress.
[DOMAIN-SEQ] Dhody, D., Palle, U., and R. Casellas, "Standard
Representation Of Domain Sequence", work in progress.
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18. Author's Addresses
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
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