Network Working Group D. King (Ed.)
Internet-Draft Old Dog Consulting
Intended Status: Informational A. Farrel (Ed.)
Expires: October 18, 2011 Old Dog Consulting
April 18, 2011
The Application of the Path Computation Element Architecture to the
Determination of a Sequence of Domains in MPLS and GMPLS
draft-king-pce-hierarchy-fwk-06.txt
Abstract
Computing optimum routes for Label Switched Paths (LSPs) across
multiple domains in MPLS Traffic Engineering (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 Path Computation
Element (PCE) architecture.
Where the sequence of domains is known a priori, various techniques
can be employed to derive an optimum path. If the domains are
simply-connected, or if the preferred points of interconnection are
also known, the Per-Domain Path Computation technique can be used.
Where there are multiple connections between domains and there is
no preference for the choice of points of interconnection, the
Backward Recursive Path Computation Procedure (BRPC) can be used to
derive an optimal path.
This document examines techniques to establish the optimum path when
the sequence of domains is not known in advance. The document
shows how the PCE architecture can be extended to allow the optimum
sequence of domains to be selected, and the optimum end-to-end path
to be derived through the use of a hierarchical relationship between
domains.
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Contents
1. Introduction..................................................3
1.1 Problem Statement.........................................4
1.2 Definition of a Domain............. ......................5
1.3 Assumptions and Requirements..............................5
1.3.1 Metric Objectives...................................6
1.3.2 Domain Diversity....................................6
1.3.3 Existing Traffic Engineering Constraints............7
1.3.4 Commercial Constraints..............................7
1.3.5 Domain Confidentiality..............................7
1.3.6 Limiting Information Aggregation....................7
1.3.7 Domain Interconnection Discovery....................7
1.4 Terminology...............................................7
2. Per Domain Path Computation...................................8
3. Backward Recursive Path Computation...........................9
3.1. Applicability of BRPC when the Domain Path is not Known..10
4. Hierarchical PCE..............................................10
5. Hierarchical PCE Procedures...................................11
5.1 Objective Functions and Policy............................11
5.2 Maintaining Domain Confidentiality........................12
5.3 PCE Discovery.............................................12
5.4 Parent Domain Traffic Engineering Database................13
5.5 Determination of Destination Domain ......................14
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5.6 Hierarchical PCE Examples.................................14
5.6.1 Hierarchical PCE Initial Information Exchange.......17
5.6.2 Hierarchical PCE End-to-End Path Computation
Procedure Example.........................................17
5.7 Hierarchical PCE Error Handling...........................17
5.8 Hierarchical PCEP Protocol Extensions.....................18
5.8.1 PCEP Request Qualifiers.............................18
5.8.2 Indication of H-PCE Capability......................18
5.8.3 Intention to Utilize Parent PCE Capabilities........19
5.8.4 Communication of Domain Connectivity Information....19
5.8.5 Domain Identifiers..................................19
6. Hierarchical PCE Applicability................................20
6.1 Antonymous Systems and Areas..............................20
6.2 ASON architecture (G-7715-2)..............................20
6.2.1 Implicit Consistency Between Hierarchical PCE and
G.7715.2..................................................21
6.2.2 Benefits of Hierarchical PCEs in ASON...............23
7. Management Considerations ....................................23
7.1 Control of Function and Policy............................23
7.1.1 Child PCE...........................................23
7.1.2 Parent PCE..........................................23
7.1.3 Policy Control......................................24
7.2 Information and Data Models...............................24
7.3 Liveness Detection and Monitoring.........................24
7.4 Verifying Correct Operation...............................24
7.5. Impact on Network Operation..............................25
8. Security Considerations ......................................25
9. IANA Considerations ..........................................25
10. Acknowledgements ............................................25
11. References ..................................................26
11.1. Normative References....................................26
11.2. Informative References .................................26
12. Authors' Addresses ..........................................27
1. Introduction
The capability to compute the routes of end-to-end inter-domain MPLS
Traffic Engineering (TE) and GMPLS Label Switched Paths (LSPs) may be
provided by a Path Computation Element (PCE). The PCE architecture is
defined in [RFC4655]. The methods for establishing and controlling
inter-domain MPLS-TE and GMPLS LSPs are documented in [RFC4726].
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 [RFC4726] and
[RFC4655]. Domains are connected through ingress and egress
boundary nodes (BNs). A more detailed definition is given in
Section 1.2.
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In a multi-domain environment, the determination of an end-to-end
traffic engineered path is a problem because no single point of path
computation is aware of all of the links and resources in each
domain. PCEs can be used to compute end-to-end paths using a per-
domain path computation technique [RFC5152]. Alternatively, the
backward recursive path computation (BRPC) mechanism [RFC5441]
allows multiple PCEs to collaborate in order to select an optimal
end-to-end path that crosses multiple domains. Both mechanisms
assume that the sequence of domains to be crossed between ingress
and egress in known in advance.
This document examines techniques to establish the optimum path when
the sequence of domains is not known in advance. It shows how the PCE
architecture can be extended to allow the optimum sequence of domains
to be selected, and the optimum end-to-end path to be derived.
The model described in this document introduces a hierarchical
relationship between domains. It is applicable to environments with
small groups of domains where visibility from the ingress Label
Switching Router (LSR) is limited. Applying the hierarchical PCE
model to large groups of domains such as the Internet, is not
considered feasible or desirable, and is out of scope for this
document.
1.1 Problem Statement
Using a PCE to compute a path between nodes within a single domain is
relatively straightforward. Computing an end-to-end path when the
source and destination nodes are located in different domains
requires co-operation between multiple PCEs, each responsible for
its own domain.
Techniques for inter-domain path computation described so far
([RFC5152] and [RFC5441]) assume that the sequence of domains to be
crossed from source to destination is well known. No explanation is
given (for example, in [RFC4655]) of how this sequence is generated
or what criteria may be used for the selection of paths between
domains. In small clusters of domains, such as simple cooperation
between adjacent ISPs, this selection process is not complex. In more
advanced deployments (such as optical networks constructed from
multiple sub-domains, or multi-AS environments) the choice of domains
in the end-to-end domain sequence can be critical to the
determination of an optimum end-to-end path.
This document introduces the concept of a hierarchical PCE
architecture and shows how to coordinate PCEs in peer domains in
order to derive an optimal end-to-end path.
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The work is currently scoped to operate with a small group of domains
and there is no intent to apply this model to a large group of
domains, e.g., to the Internet.
1.2 Definition of a Domain
A domain is defined in [RFC4726] as any collection of network
elements within a common sphere of address management or path
computational responsibility. Examples of such domains include
IGP areas and Autonomous Systems. 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].
See Section 6.2 for a further discussion of the applicability to the
ASON architecture.
This document assumes that the selection of a sequence of domains for
an end-to-end path is in some sense a hierarchical path computation
problem. That is, where one mechanism is used to determine a path
across a domain, a separate mechanism (or at least a separate set
of paradigms) is used to determine the sequence of domains.
1.3 Assumptions and Requirements
Networks are often constructed from multiple domains. These
domains are often interconnected via multiple interconnect points.
Its assumed that the sequence of domains for an end-to-end path is
not always well known; that is, an application requesting end-to-end
connectivity has no preference for, or no ability to specify, the
sequence of domains to be crossed by the path.
The traffic engineering properties of a domain cannot be seen from
outside the domain. Traffic engineering aggregation or abstraction,
hides information and can lead to failed path setup or the selection
of suboptimal end-to-end paths [RFC4726]. The aggregation process
may also have significant scaling issues for networks with many
possible routes and multiple TE metrics. Flooding TE information
breaks confidentiality and does not scale in the routing protocol.
The primary goal of this document is to define how to derive optimal
end-to-end, multi-domain paths when the sequence of domains is not
known in advance. The solution needs to be scalable and to maintain
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internal domain topology confidentiality while providing the optimal
end-to-end path. It cannot rely on the exchange of TE information
between domains, and it cannot utilise a computation element that has
universal knowledge of TE properties and topology of all domains.
The sub-sections that follow set out the primary objectives and
requirements to be satisfied by a PCE solution to multi-domain path
computation.
1.3.1 Metric Objectives
The definition of optimality is dependent on policy, and is based on
a single objective or a group objectives. An objective is expressed
as an objective function [RFC5541] and may be specified on a path
computation request. The following objective functions are identified
in this document. They define how the path metrics and TE link
qualities are manipulated during inter-domain path computation. The
list is not proscriptive and may be expanded in other documents.
o Minimize the cost of the path [RFC5541]
o Select a path using links with the minimal load [RFC5541]
o Select a path that leaves the maximum residual bandwidth [RFC5541]
o Minimize aggregate bandwidth consumption [RFC5541]
o Minimize the Load of the most loaded Link [RFC5541]
o Minimize the Cumulative Cost of a set of paths [RFC5541]
o Minimize the number of boundary nodes used
o Limit the number of domains crossed
o Disallow domain re-entry
See Section 5.1 for further discussion of objective functions.
1.3.2 Domain Diversity
A pair of paths are domain-diverse if they do not transit any of the
same domains. A pair of paths that share a common ingress and egress
are domain-diverse if they only share the same domains at the ingress
and egress (the ingress and egress domains). Domain diversity may be
maximized for a pair of paths by selecting paths that have the
smallest number of shared domains. (Note that this is not the same
as finding paths with the greatest number of distinct domains!)
Path computation should facilitate the selection of paths that share
ingress and egress domains, but do not share any transit domains.
This provides a way to reduce the risk of shared failure along any
path, and automatically helps to ensure path diversity for most of
the route of a pair of LSPs.
Thus, domain path selection should provide the capability to include
or exclude specific domains and specific boundary nodes.
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1.3.3 Existing Traffic Engineering Constraints
Any solution should take advantage of typical traffic engineering
constraints (hop count, bandwidth, lambda continuity, path cost,
etc.) to meet the service demands expressed in the path computation
request [RFC4655].
1.3.4 Commercial Constraints
The solution should provide the capability to include commercially
relevant constraints such as policy, SLAs, security, peering
preferences, and dollar costs.
Additionally it may be necessary for the service provider to
request that specific domains are included or excluded based on
commercial relationships, security implications, and reliability.
1.3.5 Domain Confidentiality
A key requirement is the ability to maintain domain confidentiality
when computing inter-domain end-to-end paths. When required by local
policy, a PCE should not need to disclose to any other PCE the intra-
domain paths it computes or the internal topology of the domain it
serves.
1.3.6 Limiting Information Aggregation
It is important to minimise the amount of aggregation within the
solution. There should be no associated computation burden or
requirement to aggregate and abstract traffic engineering link
information.
1.3.7 Domain Interconnection Discovery
To support domain mesh topologies, the solution should allow the
discovery and selection of domain inter-connections. Pre-
configuration of preferred domain interconnections should also be
supported for network operators that have bilateral agreement, and
preference for the choice of points of interconnection.
1.4 Terminology
This document uses PCE terminology defined in [RFC4655], [RFC4875],
and [RFC5440]. Additional terms are defined below.
Domain Path: The sequence of domains for a path.
Ingress Domain: The domain that includes the ingress LSR of a path.
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Transit Domain: A domain that has an upstream and downstream
neighbor domain for a specific path.
Egress Domain: The domain that includes the egress LSR of a path.
Boundary Nodes: Each Domain has entry LSRs and exit LSRs that could
be Area Border Routers (ABRs) or Autonomous System Border Routers
(ASBRs) depending on the type of domain. They are defined here more
generically as Boundary Nodes (BNs).
Entry BN of domain(n): a BN connecting domain(n-1) to domain(n)
on a path.
Exit BN of domain(n): a BN connecting domain(n) to domain(n+1)
on a path.
Parent Domain: A domain higher up in a domain hierarchy such
that it contains other domains (child domains) and potentially other
links and nodes.
Child Domain: A domain lower in a domain hierarchy such that it has
a parent domain.
Parent PCE: A PCE responsible for selecting a path across a parent
domain and any number of child domains by coordinating with child
PCEs and examining a topology map that shows domain inter-
connectivity.
Child PCE: A PCE responsible for computing the path across one or
more specific (child) domains. A child PCE maintains a relationship
with at least one parent PCE.
OF: Objective Function: A set of one or more optimization
criteria used for the computation of a single path (e.g., path cost
minimization), or the synchronized computation of a set of paths
(e.g., aggregate bandwidth consumption minimization). See [RFC4655]
and [RFC5541].
2. Per-Domain Path Computation
The per-domain path computation method for establishing inter-domain
TE-LSPs [RFC5152] defines a technique whereby the path is computed
during the signalling process on a per-domain basis. The entry BN of
each domain is responsible for performing the path computation for
the section of the LSP that crosses the domain, or for requesting
that a PCE for that domain computes that piece of the path.
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During per-domain path computation, each computation results in the
best path across the domain to provide connectivity to the next
domain in the domain sequence (usually indicated in signalling by an
identifier of the next domain or the identity of the next entry BN).
Per-domain path computation may lead to sub-optimal end-to-end paths
because the most optimal path in one domain may lead to the choice of
an entry BN for the next domain that results in a very poor path
across that next domain.
In the case that the domain path (in particular, the sequence of
boundary nodes) is not known, the PCE must select an exit BN based on
some determination of how to reach the destination that is outside
the domain for which the PCE has computational responsibility.
[RFC5152] suggest that this might be achieved using the IP shortest
path as advertise by BGP. Note, however, that the existence of an IP
forwarding path does guarantee the presence of sufficient bandwidth,
let alone an optimal TE path. Furthermore, in many GMPLS systems
inter-domain IP routing will not be present. Thus, per-domain path
computation may require a significant number of crankback routing
attempts to establish even a sub-optimal path.
Note also that the PCEs in each domain may have different computation
capabilities, may run different path computation algorithms, and may
apply different sets of constraints and optimization criteria, etc.
This can result in the end-to-end path being inconsistent and sub-
optimal.
Per-domain path computation can suit simply-connected domains where
the preferred points of interconnection are known.
3. Backward Recursive Path Computation
The Backward Recursive Path Computation (BRPC) [RFC5441] procedure
involves cooperation and communication between PCEs in order to
compute an optimal end-to-end path across multiple domains. The
sequence of domains to be traversed can either be determined before
or during the path computation. In the case where the sequence of
domains is known, the ingress Path Computation Client (PCC) sends a
path computation request to the PCE responsible for the ingress
domain. This request is forwarded between PCEs, domain-by-domain, to
the PCE responsible for the egress domain. The PCE in the egress
domain creates a set of optimal paths from all of the domain entry
BNs to the egress LSR. This set is represented as a tree of potential
paths called a Virtual Shortest Path Tree (VSPT), and the PCE passes
it back to the previous PCE on the domain path. As the VSPT is passed
back toward the ingress domain, each PCE computes the optimal paths
from its entry BNs to its exit BNs that connect to the rest of the
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tree. It adds these paths to the VSPT and passes the VSPT on until
the PCE for the ingress domain is reached and computes paths from the
ingress LSR to connect to the rest of the tree. The ingress PCE then
selects the optimal end-to-end path from the tree, and returns the
path to the initiating PCC.
BRPC may suit environments where multiple connections exist between
domains and there is no preference for the choice of points of
interconnection. It is best suited to scenarios where the domain
path is known in advance, but can also be used when the domain path
is not known.
3.1. Applicability of BRPC when the Domain Path is Not Known
As described above BRPC can be used to determine an optimal inter-
domain path when the sequence is known. Even when the sequence of
domains is not known BRPC could be used as follows.
o The PCC sends a request to the PCE for the ingress domain (the
ingress PCE).
o The ingress PCE sends the path computation request direct to the
PCE responsible for the domain containing the destination node (the
egress PCE).
o The egress PCE computes an egress VSPT and passes it to a PCE
responsible for each of the adjacent (potentially upstream)
domains.
o Each PCE in turn constructs a VSPT and passes it on to all of its
neighboring PCEs.
o When the ingress PCE has received a VSPT from each of its
neighboring domains it is able to select the optimum path.
Clearly this mechanism (which could be called path computation
flooding) has significant scaling issues. It could be improved by
the application of policy and filtering, but such mechanisms are not
simple and would still leave scaling concerns.
4. Hierarchical PCE
In the hierarchical PCE architecture, a parent PCE maintains a domain
topology map that contains the child domains (seen as vertices in the
topology) and their interconnections (links in the topology). The
parent PCE has no information about the content of the child domains;
that is, the parent PCE does not know about the resource availability
within the child domains, nor about the availability of connectivity
across each domain. The parent PCE is aware of the TE capabilities of
the interconnections between child domains as these interconnections
are links in its own topology map.
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Note that in the case that the domains are IGP areas, there is no
link between the domains (the ABRs have a presence in both
neighboring areas). The parent domain may choose to represent this in
its TED as a virtual link that is unconstrained and has zero cost,
but this is entirely an implementation issue.
Each child domain has at least one PCE capable of computing paths
across the domain. These PCEs are known as child PCEs and have a
relationship with the parent PCE. Each child PCE also knows the
identity of the domains that neighbor its own domain. A child PCE
only knows the topology of the domain that it serves and does not
know the topology of other child domains. Child PCEs are also not
aware of the general domain mesh connectivity (i.e., the domain
topology map) beyond the connectivity to the immediate neighbor
domains of the domain it serves.
The parent PCE builds the domain topology map either from
configuration or from information received from each child PCE. This
tells it how the domains are interconnected including the TE
properties of the domain interconnections. But the parent PCE does
not know the contents of the child domains. Discovery of the domain
topology and domain interconnections is discussed further in Section
5.3.
When a multi-domain path is needed, the ingress PCE sends a request
to the parent PCE (using the path computation element protocol, PCEP
[RFC5440]). The parent PCE selects a set of candidate domain paths
based on the domain topology and the state of the inter-domain links.
It then sends computation requests to the child PCEs responsible for
each of the domains on the candidate domain paths.
Each child PCE computes a set of candidate path segments across its
domain and sends the results to the parent PCE. The parent PCE uses
this information to select path segments and concatenate them to
derive the optimal end-to-end inter-domain path. The end-to-end path
is then sent to the child PCE which received the initial path request
and this passes the path on to the PCC that issues the original
request.
5. Hierarchical PCE Procedures
5.1 Objective Functions and Policy
Deriving the optimal end-to-end domain path sequence is dependent on
the policy applied during domain path computation. An Objective
Function (OF) [RFC5541], or set of OFs, may be applied to define the
policy being applied to the domain path computation.
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The OF specifies the desired outcome of the computation. It does
not describe the algorithm to use. When computing end-to-end inter-
domain paths, required OFs may include (see Section 1.3.1):
o Minimum cost path
o Minimum load path
o Maximum residual bandwidth path
o Minimize aggregate bandwidth consumption
o Minimize the number of boundary nodes used
o Minimize the number of transit domains
o Disallow domain re-entry
The objective function may be requested by the PCC, the ingress
domain PCE (according to local policy), or maybe applied by the
parent PCE according to inter-domain policy.
5.2 Maintaining Domain Confidentiality
Information about the content of child domains is not shared for
scaling and confidentiality reasons. This means that a parent PCE is
aware of the domain topology and the nature of the connections
between domains, but is not aware of the content of the domains.
Similarly, a child PCE cannot know the internal topology of another
child domain. Child PCEs also do not know the general domain mesh
connectivity, this information is only known by the parent PCE.
As described in the earlier sections of this document, PCEs can
exchange path information in order to construct an end-to-end inter-
domain path. Each per-domain path fragment reveals information about
the topology and resource availability within a domain. Some
management domains or ASes will not want to share this information
outside of the domain (even with a trusted parent PCE). In order to
conceal the information, a PCE may replace a path segment with a
path-key [RFC5520]. This mechanism effectively hides the content of a
segment of a path.
5.3 PCE Discovery
It is a simple matter for each child PCE to be configured with the
address of its parent PCE. Typically, there will only be one or two
parents of any child.
The parent PCE also needs to be aware of the child PCEs for all child
domains that it can see. This information is most likely to be
configured (as part of the administrative definition of each
domain).
Discovery of the relationships between parent PCEs and child PCEs
does not form part of the H-PCE architecture. Mechanisms that rely on
advertising or querying PCE locations across domain or provider
boundaries are undesirable for security, scaling, commercial, and
confidentiality reasons.
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The parent PCE also needs to know the inter-domain connectivity.
This information could be configured with suitable policy and
commercial rules, or could be learned from the child PCEs as
described in Section 4.
In order for the parent PCE to learn about domain interconnection
the child PCE will report the identity of its neighbor domains. The
IGP in each neighbor domain can advertise its inter-domain TE
link capabilities [RFC5316], [RFC5392]. This information can be
collected by the child PCEs and forwarded to the parent PCE, or the
parent PCE could participate in the IGP in the child domains.
5.4 Parent Domain Traffic Engineering Database
The parent PCE maintains a domain topology map of the child domains
and their interconnectivity. Where inter-domain connectivity is
provided by TE links the capabilities of those links must also be
known to the parent PCE. Furthermore the parent domain
may contain nodes and links in its own right. Therefore, the
parent PCE maintains a traffic engineering database (TED) for
the parent domain in the same way that any PCE does.
The parent domain may just be the collection of child domains and the
inter-domain links, or it may contain nodes and links in its own
right.
The mechanism for building the parent TED is likely to rely heavily
on administrative configuration and commercial issues because the
network was probably partitioned into domains specifically to address
these issues.
In practice, certain information may be passed from the child domains
to the parent PCE to help build the parent TED. In theory, the parent
PCE could listen to the routing protocols in the child domains, but
this would violate the confidentiality and scaling issues that may be
responsible for the partition of the network into domains. So it is
much more likely that a suitable solution will involve specific
communication from an entity in the child domain (such as the child
PCE) to convey the necessary information. As already mentioned, the
"necessary information" relates to how the child domains are inter-
connected. The topology and available resources within the child
domain do not need to be communicated to the parent PCE: doing so
would violate the PCE architecture. Mechanisms for reporting this
information are described in the examples in Section 5.6 in abstract
terms as "a child PCE reports its neighbor domain connectivity to its
parent PCE"; the specifics of a solution are out of scope of this
document, but the requirements are indicated in Section 5.8.
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In models such as ASON (see Section 6.2), it is possible to consider
a separate instance of an IGP running within the parent domain where
the participating protocol speakers are the nodes directly present in
that domain and the PCEs (routing controllers) responsible for each
of the child domains.
5.5 Determination of Destination Domain
The PCC asking for an inter-domain path computation is aware of the
identity of the destination node by definition. If it knows the
egress domain it can supply this information as part of the path
computation request. However, if it does not know the egress domain
this information must be determined by the parent PCE.
In some specialist topologies the parent PCE could determine the
destination domain based on the destination address, for example from
configuration. However, this is not appropriate for many multi-domain
addressing scenarios. In IP-based multi-domain networks the
parent PCE may be able to determine the destination domain by
participating in inter-domain routing. Finally, the parent PCE could
issue specific requests to the child PCEs to discover if they contain
the destination node, but this has scaling implications.
5.6 Hierarchical PCE Examples
The following example describes the hierarchical domain topology.
Figure 1 (sample hierarchical domain topology) demonstrates four
interconnected domains within a fifth parent domain. Each domain
contains a single PCE:
o Domain 1 is the ingress domain and child PCE 1 is able to compute
paths within the domain. Its neighbors are Domain 2 and Domain 4.
The domain also contains the source LSR (S) and three egress
boundary nodes (BN11, BN12, and BN13).
o Domain 2 is served by child PCE 2. Its neighbors are Domain 1 and
Domain 3. The domain also contains four boundary nodes (BN21, BN22,
BN23, and BN24).
o Domain 3 is the egress domain and is served by child PCE 3. Its
neighbors are Domain 2 and Domain 4. The domain also contains the
destination LSR (D) and three ingress boundary nodes (BN31, BN32,
and BN33).
o Domain 4 is served by child PCE 4. Its neighbors are Domain 2 and
Domain 3. The domain also contains two boundary nodes (BN41 and
BN42).
All of these domains are encompassed within Domain 5 which is served
by the parent PCE (PCE 5).
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-----------------------------------------------------------------
| Domain 5 |
| ----- |
| |PCE 5| |
| ----- |
| |
| ---------------- ---------------- ---------------- |
| | Domain 1 | | Domain 2 | | Domain 3 | |
| | | | | | | |
| | ----- | | ----- | | ----- | |
| | |PCE 1| | | |PCE 2| | | |PCE 3| | |
| | ----- | | ----- | | ----- | |
| | | | | | | |
| | ----| |---- ----| |---- | |
| | |BN11+---+BN21| |BN23+---+BN31| | |
| | - ----| |---- ----| |---- - | |
| | |S| | | | | |D| | |
| | - ----| |---- ----| |---- - | |
| | |BN12+---+BN22| |BN24+---+BN32| | |
| | ----| |---- ----| |---- | |
| | | | | | | |
| | ---- | | | | ---- | |
| | |BN13| | | | | |BN33| | |
| -----------+---- ---------------- ----+----------- |
| \ / |
| \ ---------------- / |
| \ | | / |
| \ |---- ----| / |
| ----+BN41| |BN42+---- |
| |---- ----| |
| | | |
| | ----- | |
| | |PCE 4| | |
| | ----- | |
| | | |
| | Domain 4 | |
| ---------------- |
| |
-----------------------------------------------------------------
Figure 1 : Sample Hierarchical Domain Topology
Figure 2, shows the view of the domain topology as seen by the parent
PCE (PCE 5). This view is an abstracted topology; PCE 5 is aware of
domain connectivity, but not of the internal topology within each
domain.
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----------------------------
| Domain 5 |
| ---- |
| |PCE5| |
| ---- |
| |
| ---- ---- ---- |
| | |---| |---| | |
| | D1 | | D2 | | D3 | |
| | |---| |---| | |
| ---- ---- ---- |
| \ ---- / |
| \ | | / |
| ----| D4 |---- |
| | | |
| ---- |
| |
----------------------------
Figure 2 : Abstract Domain Topology as Seen by the Parent PCE
5.6.1 Hierarchical PCE Initial Information Exchange
Based on the Figure 1 topology, the following is an illustration of
the initial hierarchical PCE information exchange.
1. Child PCE 1, the PCE responsible for Domain 1, is configured
with the location of its parent PCE (PCE5).
2. Child PCE 1 establishes contact with its parent PCE. The parent
applies policy to ensure that communication with PCE 1 is allowed.
3. Child PCE 1 listens to the IGP in its domain and learns its
inter-domain connectivity. That is, it learns about the links
BN11-BN21, BN12-BN22, and BN13-BN41.
4. Child PCE 1 reports its neighbor domain connectivity to its parent
PCE.
5. Child PCE 1 reports any change in the resource availability on its
inter-domain links to its parent PCE.
Each child PCE performs steps 1 through 5 so that the parent PCE can
create a domain topology view as shown in Figure 2.
5.6.2 Hierarchical PCE End-to-End Path Computation Procedure
The procedure below is an example of a source PCC requesting an
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end-to-end path in a multi-domain environment. The topology is
represented in Figure 1. It is assumed that the each child PCE has
connected to its parent PCE and exchanged the initial information
required for the parent PCE to create its domain topology view as
described in Section 5.6.1.
1. The source PCC (the ingress LSR in our example), sends a request
to the PCE responsible for its domain (PCE1) for a path to the
destination LSR.
2. PCE 1 determines the destination, is not in domain 1.
3. PCE 1 sends a computation request to its parent PCE (PCE 5).
4. The parent PCE determines that the destination is in Domain 3.
(See Section 5.5).
5. PCE 5 determines the likely domain paths according to the domain
interconnectivity and TE capabilities between the domains. For
example, three domain paths (S-BN11-BN21-D2-BN23-BN31-D, S-BN11-
BN21-D2-BN24-BN32-D, and S-BN13-BN41-D4-BN42-BN33-D) are
determined (assuming the link BN12-BN22 is not suitable for the
requested path).
6. PCE 5 sends edge-to-edge path computation requests to PCE 2
which is responsible for Domain 2 (e.g., BN21-BN23 and BN21-BN24)
and to PCE 4 for Domain 4 (e.g., BN41-BN42).
7. PCE 5 sends source-to-edge path computation requests to PCE 1
which is responsible for Domain 1 (e.g., S-BN11 and S-BN13).
8. PCE 5 sends edge-to-egress path computation requests to PCE3
which is responsible for Domain 3 (e.g., BN31-D, BN32-D, and
BN33-D).
9. PCE 5 correlates all the computation responses from each child
PCE, adds in the information about the inter-domain links, and
applies any requested and locally configured policies.
10. PCE 5 then selects the optimal end-to-end multi-domain path
that meets the policies and objective functions, and supplies the
resulting path to PCE 1.
11. PCE 1 forwards the path to the PCC (the ingress LSR).
5.7 Hierarchical PCE Error Handling
In the event that a child PCE in a domain cannot find a suitible
path to the egress. The child PCE should return the relevent
error notifying the parent PCE. Depending on the error response the
parent PCE can elect to:
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o Cancel the request and send the relevent response back to the
intial child PCE requesting an end-to-end path.
o Relax the contraints associated with the intial path request;
o Select another candidate domain and send the path request to the
child PCE responsible for the domain.
If the parent PCE does not recieve a response from a child PCE within
an alloted time period. The parent PCE can either:
o Send the path request to another child PCE in the same domain, if a
secoundary child PCE exists;
o Select another candidate domain and send the path request to the
child PCE responsible for that domain.
5.8 Requirements for Hierarchical PCEP Protocol Extensions
This section lists the high-level requirements for extensions to the
PCEP to support the hierachical PCE model.
[Editors Note: This section may be expanded as work progresses.]
5.8.1 PCEP Request Qualifiers
PCEP request (PCReq) messages are used by a PCC or a PCE to make a
computation request or enquiry to a PCE. The requests are qualified
so that the PCE knows what type of action is required.
Support of the H-PCE architecture will introduce two new
qualifications as follows:
o It must be possible for a child PCE to indicate that the request it
sends to a parent PCE shold be satisfied by a domain sequence only,
that is, not by a full end-to-end path. This allows the child PCE
to initiate per-domain or backward recursive path computation.
o A parent PCE needs to be able to ask a child PCE whether a
particular node address (the destination of an end-to-end path) is
present in the domain that the child PCE serves.
In PCEP, such request qualifications are carried as bit-flags in the
RP object carried within the PCReq message.
5.8.2 Indication of H-PCE Capability
Although parent/child PCE relationships are likely configured, it
assist network operations if the parent PCE is able to indicate to
the child that it really is capable of acting as a parent PCE. This
will help to trap misconfigurations.
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A parent PCE needs a way to indicate that is capable of acting as a
parent PCE, and should also be able to indicate the identity of the
parent domain. This informaiton is most obviously carried in the Open
Object within the Open message.
5.8.3 Intention to Utilize Parent PCE Capabilities
A PCE that is capable of acting as a parent PCE might not be
configured or willing to act as the parent for a specific child PCE.
This fact could be determined when the child sends a PCReq that
requires parental activity (such as querying other child PCEs), and
could result in a negative response in a PCEP Error (PCErr) message.
However, the expense of a poorly targetted PCReq can be avoided if
the child PCE indicates that it might wish to use the parent as a
parent (for example, on the Open message), and if the parent
determines at that time whether it is willing to act as a parent to
this child.
5.8.4 Communication of Domain Connectivity Information
Section 5.4 describes how the parent PCE needs a parent TED and
indicates that the information might be supplied from the child PCEs
in each domain. This requires a mechanism whereby information about
inter-domain links can be supplied by a child PCE to a parent PCE,
for example on a PCEP Notify (PCNtf) message.
The information that would be exchanged includes:
o Identifier of advertising child PCE
o Identifier of PCE's domain
o Identifier of the link
o TE properties of the link (metrics, bandwidth)
o Other properties of the link (technology-specific)
o Identifier of link end-points
o Identifier of adjacent domain
It may be desirable for this information to be periodically updated,
for example, when available bandwidth changes. In this case, the
parent PCE might be given the ability to configure thresholds in the
child PCE to prevent flapping of information.
5.8.5 Domain Identifiers
Domain identifiers are already needed to allow a PCE to indicate
which domains it serves, and to allow the representation of domains
as abstract nodes in paths. The wider use of domains in the context
of this work on H-PCE will require that domains can be identified in
more places within objects in PCEP messages. This should pose no
problems.
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However, more attention may need to be applied to the precision of
domian identifier definitions.
6. Hierarchical PCE Applicability
As per [RFC4655], PCE can inherently support inter-domain path
computation for any definition of a domain as set out in Section 1.2.
Hierarchical PCE can be applied to inter-domain environments,
including Antonymous Systems and IGP areas. The hierarchical PCE
procedures make no distinction between, Antonymous Systems and IGP
area applications, although it should be noted that the TED
maintained by a parent PCE must be able to support the concept of
child domains connected by inter-domain links or directly connected
at boundary nodes (see Section 4).
This section sets out the applicability of hierarchical PCE to three
environments:
o MPLS traffic engineering across multiple Autonomous Systems
o MPLS traffic engineering across multiple IGP areas
o GMPLS traffic engineering in the ASON architecture
6.1 Antonymous Systems and Areas
Networks are comprised of domains. A domain can be considered to be
a collection of network elements within an AS or area that has a
common sphere of address management or path computational
responsibility.
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 AS or area a method is
required to compute a path across teh AS and area boundaries.
When an LSR within an AS or area needs to compute a path across an
area or AS boundary it must also use an inter-AS computation
technique. Hierachical PCE is equally applicable to computing
inter-area and inter-AS MPLS and GMPLS paths across domain
boundaries.
6.2 ASON Architecture
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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.
Note that the RA hierarchy can be recursive.
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. For an end-to-end connection, the route may be computed
by a single RC or multiple RCs in a collaborative manner (i.e., RC
federations). In the case of RC federations, [G-7715-2] describes
three styles during remote route query operation:
o Step-by-step remote path computation
o Hierarchical remote path computation
o A combination of the above.
In a hierarchical ASON routing environment, a child RC may
communicate with its parent RC (at the next higher level of the ASON
routing hierarchy) to request the computation of an end-to-end path
across several RAs. It does this using a route query message (known
as the abstract message RI_QUERY). The corresponding parent RC may
communicate with other child RCs that belong to other child RAs at
the next lower hierarchical level. Thus, a parent RC can act as
either a Route Query Requester or Route Query Responder.
It can be seen that the hierarchical PCE architecture fits the
hierarchical ASON routing architecture well. It can be used to
provide paths across subnetworks, and to determine end-to-end paths
in networks constructed from multiple subnetworks or RAs.
When hierarchical PCE is applied to implement hierarchical remote
path computation in [G-7715-2], it is very important for operators to
understand the different terminology and implicit consistency
between hierarchical PCE and [G-7715-2].
6.2.1 Implicit Consistency Between Hierarchical PCE and G.7715.2
This section highlights the correspondence between features of the
hierarchical PCE architecture and the ASON routing architecture.
(1) RC (Routing Controller) and PCE (Path Computation Element)
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[G-8080] describes the Routing Controller Component as an
abstract entity, which is responsible for responding to requests
for path (route) information and topology information. It can be
implemented as a single entity, or as a distributed set of
entities that make up a cooperative federation.
[RFC4655] describes PCE (Path Computation Element) is an entity
(component, application, or network node) that is capable of
computing a network path or route based on a network graph and
applying computational constraints.
Therefore, in the ASON architecture, a PCE can be regarded as a
realizations of the RC.
(2) Route Query Requester/Route Query Responder and PCC/PCE
[G-7715-2] describes the Route Query Requester as a Connection
Controller or Routing Controller that sends a route query message
to a Routing Controller requesting for one or more paths that
satisfy a set of routing constraints. The Route Query Responder
is a Routing Controller that performs path computation upon
receipt of a route query message from a Route Query Requester,
sending a response back at the end of the path computation.
In the context of ASON, a signaling controller initiates and
processes signaling messages and closely coupled to a signaling
protocol speaker. A routing controller makes routing decisions
and is usually coupled to configuration entities and/or routing a
protocol speaker.
It can be seen that a PCC corresponds to a Route Query Requester,
and a PCE corresponds to a Route Query Responder. A PCE/RC can
also act as a Route Query Requester sending requests to another
Route Query Responder.
The PCEP path computation request (PCReq) and path computation
reply (PCRep) messages between PCC and PCE correspond to the
RI_QUERY and RI_UPDATE messages in [G-7715-2].
(3) Routing Area Hierarchy and Hierarchical Domain
The ASON routing hierarchy model is shown in Figure 6 of
[G-7715] through an example that illustrates routing area levels.
If the hierarchical remote path computation mechanism of
[G-7715-2] is applied in this scenario, each routing area should
have at least one RC for route query function and there is a
parent RC for the child RCs in each routing area.
According to [G-8080], the parent RC has visibility of the
structure of the lower level, so it knows the interconnectivity
of the RAs in the lower level. Each child RC can compute edge-to-
edge paths across its own child RA.
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Thus, an RA corresponds to a domain, and the hierarchical
relationship between RAs corresponds to the hierarchical
relationship between domains. Furthermore, a parent PCE in a
parent domain can be regarded as parent RC in a higher routing
level, and a child PCE in a child domain can be regarded as child
RC in a lower routing level.
6.2.2 Benefits of Hierarchical PCEs in ASON
RCs in an ASON environment can use the hierarchical PCE model to
fully match the ASON hierarchical routing model, so the hierarchical
PCE mechanisms can be applied to fully satisfy the architecture and
requirements of [G-7715-2] without any changes. If the hierarchical
PCE mechanism is applied in ASON, it can be used to determine end-to-
end optimized paths across sub-networks and RAs before initiating
signaling to create the connection. It can also improve the
efficiency of connection setup to avoid crankback.
7. Management Considerations
General PCE management considerations are discussed in [RFC4655]. In
the case of the hierarchical PCE architecture, there are additional
management considerations.
The administrative entity responsible for the management of the
parent PCEs must be determined. In the case of multi-domains (e.g.,
IGP areas or multiple ASes) within a single service provider
network, the management responsibility for the parent PCE would most
likely be handled by the service provider. In the case of multiple
ASes within different service provider networks, it may be necessary
for a third-party to manage the parent PCEs according to commercial
and policy agreements from each of the participating service
providers.
7.1 Control of Function and Policy
7.1.1 Child PCE
Support of the hierarchical procedure will be controlled by the
management organization responsible for each child PCE. A child PCE
must be configured with the address of its parent PCE in order for
it to interact with its parent PCE. The child PCE must also be
authorized to peer with the parent PCE.
7.1.2 Parent PCE
The parent PCE must only accept path computation requests from
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authorized child PCEs. If a parent PCE receives requests from an
unauthorized child PCE, the request should be dropped.
This means that a parent PCE must be configured with the identities
and security credentials of all of its child PCEs, or there must be
some form of shared secret that allows an unknown child PCE to be
authorized by the parent PCE.
7.1.3 Policy Control
It may be necessary to maintain a policy module on the parent PCE
[RFC5394]. This would allow the parent PCE to apply commercially
relevant constraints such as SLAs, security, peering preferences, and
dollar costs.
It may also be necessary for the parent PCE to limit end-to-end path
selection by including or excluding specific domains based on
commercial relationships, security implications, and reliability.
7.2 Information and Data Models
A PCEP MIB module is defined in [PCEP-MIB] that describes managed
objects for modeling of PCEP communication. An additional PCEP MIB
will be required to report parent PCE and child PCE information,
including:
o Parent PCE configuration and status,
o Child PCE configuration and information,
o Notifications to indicate session changes between parent PCEs and
child PCEs.
o Notification of parent PCE TED updates and changes.
7.3 Liveness Detection and Monitoring
The hierarchical procedure requires interaction with multiple PCEs.
Once a child PCE requests an end-to-end path, a sequence of events
occurs that requires interaction between the parent PCE and each
child PCE. If a child PCE is not operational, and an alternate
transit domain is not available, then a failure must be reported.
7.4 Verifying Correct Operation
Verifying the correct operation of a parent PCE can be performed by
monitoring a set of parameters. The parent PCE implementation should
provide the following parameters:
Parameters monitored by the parent PCE:
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o Number of child PCE requests.
o Number of successful hierarchical PCE procedures completions on a
per-PCE-peer basis.
o Number of hierarchical PCE procedure completion failures on a per-
PCE-peer basis.
o Number of hierarchical PCE procedure requests from unauthorized
child PCEs.
7.5. Impact on Network Operation
The hierarchical PCE procedure is a multiple-PCE path computation
scheme. Subsequent requests to and from the child and parent PCEs do
not differ from other path computation requests and should not have
any significant impact on network operations.
8. Security Considerations
The hierarchical PCE procedure relies on PCEP and inherits the
security requirements defined [RFC5440]. Any multi-domain
operation necessarily involves the exchange of information across
domain boundaries. This is bound to represent a significant
security and confidentiality risk especially when the child
domains are controlled by different commercial concerns.
The hierarchical PCE architecture makes use of PCE policy
[RFC5394] and the security aspects of the PCE communication protocol
documented in [RFC5440]. It is expected that the parent PCE will
require all child PCEs to use full security when communicating with
the parent and that security will be maintained by not supporting the
discovery by a parent of child PCEs.
Confidentiality may be enhanced by the use of Path Keys [RFC5520].
Further considerations of the security issues related to inter-AS
path computation see [RFC5376].
9. IANA Considerations
This document makes no requests for IANA action.
10. Acknowledgements
The authors would like to thank David Amzallag, Oscar Gonzalez de
Diosm and Franz Rambach for their comments and suggestions.
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11. References
11.1 Normative References
[RFC4655] Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain
Path Computation Method for Establishing Inter-Domain
Traffic Engineering (TE) Label Switched Paths (LSPs)",
RFC 5152, February 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", RFC
5441, April 2009.
[RFC5520] Brandford, R., Vasseur J.P., and Farrel A., "Preserving
Topology Confidentiality in Inter-Domain Path
Computation Using a Key-Based Mechanism
RFC5520, April 2009.
[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.
11.2. Informative References
[RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework
for Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726, November 2006.
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[RFC4875] Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
"Extensions to Resource Reservation Protocol - Traffic
Engineering (RSVP-TE) for Point-to-Multipoint TE Label
Switched Paths (LSPs)", RFC 4875, May 2007.
[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.
[RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5316, December 2008.
[RFC5376] Bitar, N., et al., "Inter-AS Requirements for the
Path Computation Element Communication Protocol
(PCECP)", RFC 5376, November 2008.
[RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5392, January 2009.
[RFC5541] Roux, J., Vasseur, J., and Y. Lee, "Encoding
of Objective Functions in the Path
Computation Element Communication
Protocol (PCEP)", RFC5541, December 2008.
[PCEP-MIB] Stephan, E., K. Koushik, Q. Zhao, and D. King, "PCE
communication protocol (PCEP) Management Information
Base", Work in Progress, June 2010
12. Authors' Addresses
Daniel King
Old Dog Consulting
Email: daniel@olddog.co.uk
Adrian Farrel
Old Dog Consulting
Email: adrian@olddog.co.uk
Quintin Zhao
Huawei Technology
125 Nagog Technology Park
Acton, MA 01719
US
Email: qzhao@huawei.com
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Fatai Zhang
Huawei Technologies
F3-5-B R&D Center, Huawei Base
Bantian, Longgang District
Shenzhen 518129 P.R.China
Email: zhangfatai@huawei.com
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